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Application of laser-induced breakdown spectroscopyfor detection of elements in flowback water samplesfrom shale gas wells

Haebum Lee, Gibaek Kim, Hyun-A Kim, Hyunok Maeng, Heewon Park, and Kihong Park

DOI: 10.1364/AO.381687 Received 05 Nov 2019; Accepted 19 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: Laser-induced breakdown spectroscopy (LIBS) was applied to rapidly detect elements in flowback water samplesfrom shale gas wells in Oklahoma, USA. Two types of LIBS systems (aerosolization and collection on a substrate)were used. The LIBS with aerosolization system provided rapid determination of elements in flowback water, butmoisture present in the chamber and variation in the water droplet size could make quantification difficult. In thesubstrate collection system, a comparison among substrate types showed that a hydrophilic cellulose filter gavethe most homogeneous sample distribution after drying and provided the best performance. The elements inflowback water samples were also determined by inductively coupled plasma-optical emission spectroscopy (ICPOES).ICP-OES data showed spatial variations for the elements among the different wells. Among the elements, Kshowed the highest variation (relative standard deviation = 62.8%) and Mg the lowest (relative standard deviation= 39.1%). Good correlations (r = 0.98–0.99) were observed between Ca, K, Mg, and Na LIBS peak areas determinedusing the cellulose filter and their mass concentrations (ppm) measured by ICP-OES for aqueous solutions. Thelimits of detection for Ca, K, Mg, and Na by the LIBS were 122 ppm, 68 ppm, 36 ppm, and 142 ppm, respectively.Both the LIBS and ICP-OES data showed that element concentrations in the flowback water samples were in theorder Na, Ca, Mg, and K from highest to lowest. Our data suggest that the LIBS technique could rapidly detectelements in flowback water samples on site. However, accurate quantification of elements present in lowconcentrations in water samples is limited.

Compressive spectral imaging approach usingadaptive coded apertures

Zhang Hao, Xu Ma, and Gonzalo Arce

DOI: 10.1364/AO.382854 Received 20 Nov 2019; Accepted 18 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: Coded aperture snapshot spectral imager (CASSI) acquires three-dimensional (3D)spectral images with two-dimensional (2D) coded projection measurements. This paper proposesan adaptive design method of the coded apertures according to the a-priori knowledge of thetarget scene, to improve the sensing efficiency and imaging performance of the super-resolutionCASSI system. The adaptive coded apertures are constructed from the nonlinear thresholding ofthe grey-scale map of the scene. Theoretical proof is provided to demonstrate the superiority ofthe adaptive coded apertures over the traditional random coded apertures. The improvement ofthe reconstruction performance is also verified by a set of simulations based on different spectraldata.

An optical asymmetric cryptosystem for multi-imagein CMYK color space

Yuxin Shen, Chen Tang, Lixiang Zhou, and Zhenkun Lei

DOI: 10.1364/AO.381723 Received 04 Nov 2019; Accepted 18 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: In this paper, we propose an optical single-channel asymmetric cryptosystem for multi-image in Cyan-Magenta-Yellow-Black (CMYK) color space. To the best of our knowledge, this is the first time that multiple images in CMYKcolor space are directly encrypted. The proposed optical asymmetric cryptosystem is based on the quick response(QR) encoding process and the designed Fresnel-Linear canonical-Fractional Fourier transforms (FLFT) encryptionprocess. Each FLFT encryption process consists of phase truncated FLFT and random amplitude-phase masks. Theproposed cryptosystem without color space conversion can improve the quality of the decrypted images and avoidlosing information. In addition, by utilizing the QR codes, the crosstalk and quality loss problems can be reducedefficiently. Numerical simulation results demonstrate that the proposed cryptosystem possesses high robustnessagainst various types of attacks, high security for encrypting multiple color images, and fast encryption efficiency.Furthermore, the proposed cryptosystem outperforms the other relevant cryptosystems, and can be extended toencrypt multiple color images in a straightforward way.

Phase-modulation combined deflectometry for smalldefect detection

zhao shuai qi, Zhao Wang, Junhui Huang, qiongqiong duan, Chao Xing, and Jianmin Gao

DOI: 10.1364/AO.382104 Received 04 Nov 2019; Accepted 17 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: Deflectometry has been widely used in detecting defects on specular surfaces. However, it is still verychallenging to detect defects on semi-specular or diffuse surfaces, for the low contrast and low signal to noise ratio(SNR). To address this challenge, we proposed a phase-modulation combined method for accurate defect detection.Based on the phase and modulation of captured fringes, a dual-branch convolutional neural network (DB-CNN) isemployed to simultaneously extract geometric and photometric features from the phase-shifting pattern sequenceand modulation, which improves the defect detection performance significantly. Compared with state-of-the-artmethods, the results demonstrated the proposed method’s effectiveness and impressive capability to reduce falsepositives (FPs).

Extending the imaging range through scattering layersto entire correlation range

Yaoyao Shi, Youwen Liu, Wei Sheng, and Dabo Zhu

DOI: 10.1364/AO.376902 Received 02 Sep 2019; Accepted 17 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: A method of extending the imaging range through scattering layers around a reference point (RP) is realized.Objects within the entire correlation range of the RP can be totally recovered. By scanning the light source, objectswithin the memory effect (ME) range of the RP is completely recovered with high quality. By combining the shift ofcamera to move the object to the center of observation view, objects far away from the RP is retrieved withimproved signal to noise ratio. The extended imaging range is about 3.5 times of the ME range and more than 16times of the imaging range with normal static illumination. The RP can be imprecisely placed at a distance from theobjects instead of precisely replacing them owing to the extended imaging range. This simple system methodforcefully breaks the limitation of the ME range and is very easy to implement in practical applications, which ismeaningful for the researches of scattering imaging.

Edge Re-projection Method for High Quality EdgeReconstruction in Non-Line-of-Sight Imaging

Zhengyang Shi, Xia Wang, Yang Li, Zhi-Bin Sun, and wenxi zhang

DOI: 10.1364/AO.376403 Received 27 Aug 2019; Accepted 17 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: How to image scenes or detect objects hidden from view has been an increasing interest in recent years. Previousstudies have demonstrated non-line-of-sight object reconstruction by using time-resolved detectors and a backprojectionalgorithm. Whereas filtered back-projection method reconstructs high frequency spatial information,such as the edge of an object, with poor quality. Here, we propose an optimized back-projection algorithm toimprove the object edge reconstruction quality. We base our method on the observation that the spatial frequencyand geometric information required to reconstruct an edge is distributed unevenly across scanning positions of therelay wall. Our method extracts edge voxels from the first projection result, correcting the signal response weightat different scanning positions according to their relative contribution to the object edge reconstruction, and thenre-projects data. Simulations and experiments show that compared to the filtered back-projection algorithm, ourmethod achieves better reconstruction results for the object edge, which makes it easier to distinguish the objectshape.

Noise effects and the impact of detector responses onthe characterization of extreme ultraviolet attosecondpulses

Shuyuan Hu, Maximilian Hartmann, Anne Harth, Christian Ott, and Thomas Pfeifer

DOI: 10.1364/AO.379562 Received 06 Nov 2019; Accepted 17 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: We employ numerical simulations to study the effects of noise on the reconstruction of the duration and satelliteintensity ratio for transform-limited 200-as-duration single and double pulses. The forms of noise we implementare delay jitters between the attosecond pulse and the near-infrared laser field, energy resolution of thephotoelectron detector and Poisson noise in streaking spectrograms with different count levels. We use thestreaking method to characterize the pulse and the extended ptychographic iterative engine (ePIE) retrievalalgorithm to reconstruct the pulse from the simulated streaking spectrogram. We found that, for practicalpurposes, implementing a combination of all three mentioned noise contributions, the attosecond pulse durationwill be overestimated when the photoelectron count level is low. Furthermore, the satellite pulse amplitude of theattosecond double pulse can be retrieved within 10% accuracy.

A high precision laser spot center positioning methodfor weak light conditions

Ruihong Gao, Heshan Liu, ya zhao, Ziren Luo, and Gang Jin

DOI: 10.1364/AO.381626 Received 30 Oct 2019; Accepted 17 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: In the acquisition stage of many space applications,such as the Taiji program, the spot center of weak laserlight needs to be determined accurately. Under weaklight conditions, the precision of most traditional positioningmethods is greatly affected. In this paper, wepresent a high precision laser spot center positioningmethod based on the theoretical analysis of influencefactors of precision. It is shown through experimentalstudy that the precision of the method can fulfilled therequirement of the Taiji program.

Volatile and permanent optical gratings recorded inBi2TeO5 photorefractive crystal under high cw-intensity

Ivan Oliveira and Danilo Capovilla

DOI: 10.1364/AO.381665 Received 30 Oct 2019; Accepted 17 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: We report the recording of optical gratings on photorefractive Bi2TeO5 crystals using l=532 nm wavelengthlight. We studied the behavior of this material under high light intensity and found out the presenceof a fast and a slow gratings, both of photorefractive nature and exhibiting quite significant lightintensity dependence for 1-13 kW/m2 range. Permanent grating was found out after the complete erasureof fast and slow holograms recorded at room temperature. The experimental results show that the diffractionefficiency of permanent grating increase with the recorded light intensity. The permanent gratingperformance as an optical Bragg filter was characterized by measuring the angular selectivity approximately1.0 mrad. We also show that the diffraction efficiency of permanent grating is quite dependent onthe direction of light polarization.

Single-shot measurements by Fresnel diffraction ofdivergent waves from a phase plate

Masoud Ghoorchi-Beygi and Masoomeh Dashtdar

DOI: 10.1364/AO.381429 Received 29 Oct 2019; Accepted 16 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: Recently, Fresnel diffraction (FD) of a plane wave from phase steps has been studied and applied for precisemeasurements of the light wavelength, and height and refractive index of the step, by changing the angle ofincidence or step height to induce phase shifts. In this study we formulate FD of cylindrical and sphericalwavefronts as 1D and 2D divergent waves from a phase plate. Since the phase difference of the divergent wavevaries continuously along the edge of the phase plate, it can be applied for single-shot measurements. It is shownthat the diffracted intensity distribution is a periodic function along the lines parallel to the plate edge. The phasedistribution in this direction is a linearly varying function of the position squared, with a slope depends on the lightwavelength, the plate thickness and refractive index, and the radius of wavefront curvature (RWC) on theobservation plane. The diffraction patterns are simulated and experimentally verified. Also, the RWC anddisplacement are determined as examples of applications in the experimental part of the report.

Low-Noise and High-Sensitivity Φ-OTDR Basedon Optimized Dual-pulse Heterodyne DetectionScheme

Zewu Ju, Zhijie Yu, Qingkai Hou, Kang Lou, Mo Chen, Yang Lu, and Zhou Meng

DOI: 10.1364/AO.383303 Received 15 Nov 2019; Accepted 16 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: In this paper, a novel phase-sensitive optical time-domain reflectometry (Φ-OTDR)based on optimized dual-pulse heterodyne detection scheme (DHDS) is proposed, which isdesigned to implement distributed vibration sensing with low phase noise and high sensitivity.The optimized DHDS employs an unbalanced interferometer to separate a light pulse intodual probe pulses so that they are generated by the laser at the same time, which ensures thatthe measurement sensitivity of a phase-interrogation based Φ-OTDR can be improved simplyby increasing the space interval of the dual probe pulses while the phase noise of theΦ-OTDR does not deteriorate. In addition, the proposed DHDS utilizes only oneacousto-optic modulator (AOM) to shift the frequencies of dual probe pulses so as toeliminate the effects of frequency shift jitters, and thus guarantees low phase noise level of aΦ-OTDR. The distributed vibration sensing performances of the Φ-OTDR with proposedDHDS are theoretically and experimentally studied in terms of multi-event signal restorationand phase noise level. The proposed approach solves the contradiction between themeasurement sensitivity and phase noise of a Φ-OTDR, and promotes the Φ-OTDR to theapplications of distributed weak vibration sensing.

Towards free-running operation of dual-comb fibrelasers for methane sensing

Philippe Guay, Alex Tourigny-Plante, Nicolas Bourbeau Hebert, Vincent Michaud-Belleau, Steeve Larouche, Khaoula Fdil, and Jérôme Genest

DOI: 10.1364/AO.379043 Received 30 Sep 2019; Accepted 16 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: The phase information provided by the beat notebetween frequency combs and two continuous-wavelasers is used to extrapolate the phase evolution ofcomb modes found in a spectral region obtained vianonlinear broadening. This thereafter enables usinginterferogram self-correction to fully retrieve the coherenceof a dual-comb beat note between two independentfiber lasers. This approach allows to forego thef 􀀀2 f self-referencing of both combs, which is a significantsimplification. Broadband near-infrared methanespectroscopy has been conducted as a demonstration ofthe simplified system’s preserved performance.

Accurate quantification of alkalinity ofsintered ore by random forest model based onPCA and variable importance (PCA-VI-RF)

xinxin deng, yang guang, hong zhang, and guanyu chen

DOI: 10.1364/AO.382102 Received 31 Oct 2019; Accepted 16 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: The alkalinity of sintered ore has an important impact on the quality, output and energyconsumption of blast furnace smelting, and there is an urgent need for a method for accuratelyquantifying of the alkalinity of sintered ore. The present work explores the combination of thelaser-induced breakdown spectroscopy (LIBS) technique and random forest (RF) based onprincipal component analysis (PCA) and variable importance for the quantitative analysis of thealkalinity of sintered ore. 16 sintered ore samples were used in this study, and the characteristiclines of LIBS spectra for sintered ore samples can be identified based on the National Institute ofStandards and Technology (NIST) database. At first, abnormal spectra are identified and rejectedby PCA coupled with Mahalanobis distance (MD). And then, the input variable for RF calibrationmodel is optimized according to the variable importance threshold obtained by the RF model,and two RF model parameters of ntree and mtry are determined by out-of-bag (OOB) estimate.Finally, the PCA-VI-RF model is built under the optimal model parameters. In order to verify thepredictive ability of the quantitative model, the PCA-VI-RF model prediction results werecompared with random forest (RF) model, partial least squares (PLS) model and least squaressupport vector machine (LSSVM) model, the result demonstrated that PCA-VI-RF shows a betteranalytical performance than other methods. Compared with the RF model with the originalspectrum as input, the averaged relative errors (ARE) of test results decreased from 5.82% to3.94%, coefficients of determination (R2) of the test set increased from 0.8957 to 0.9814, andthe root mean square error (RMSE) decreased from 0.1502% to 0.0860%. The speed of modelingand prediction has also been greatly improved, and the modeling time was reduced from 4675.56s to 16.86 s. The stability of the PCA-VI-RF model was verified by the relative standard deviation(RSD) of the test data prediction results, and the RSD reached below 4.74%. This study showsLIBS combining PCA-VI-RF is an effective method for accurate quantification of the alkalinity ofsintered ore. It has great significance for the potential application of real-time online analysis ofthe alkalinity of sintered ore.

Microstructure and spectral characteristics ofspectrum-fingerprint fiber with double luminouscenters for anti-counterfeiting application

Jishu Zhang, Youjiang Wang, Yonggui Li, and Yanan Zhu

DOI: 10.1364/AO.383997 Received 21 Nov 2019; Accepted 16 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: Spectrum-fingerprint anti-counterfeiting fiber with double luminous centers was tentatively prepared usingSrAl2O4:Eu2+,Dy3+, Sr2MgSi2O7:Eu2+,Dy3+ and PAN powder as main raw materials by wet spinning. Themicrostructure and spectral properties of the fiber were studied by means of SEM, XRD and Fluorescencespectrophotometer. The results showed that the two rare earth luminous materials were randomly dispersed onthe interior and surface of the fiber. Due to the spinning process, the luminescent materials were agglomerated infiber and there were many voids in the fiber. Compared with pure rare earth luminous materials, the emissionwavelength of the spectrum-fingerprint anti-counterfeiting fiber has no obvious shift, but the addition proportionand amount of two rare earth luminous materials have great influence on the spectral curve of the fiber. This fiberwith two luminous centers maintains the basic characteristics of spectrum-fingerprint anti-counterfeiting fiber andis a new type of anti-counterfeiting fiber with high anti-counterfeiting application potential.

Transport-of-intensity phase imaging with polarizationdirected flat lenses

Shunsuke Kakei, Koshi Komuro, and Takanori Nomura

DOI: 10.1364/AO.386020 Received 13 Dec 2019; Accepted 16 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: A phase imaging technique based on the transport of intensity equation with polarization directed flatlenses is demonstrated. Transport-of-intensity phase imaging enables ones to obtain a phase distributionfrom through-focus intensity distributions by solving the transport of intensity equation. In general,the through-focus intensity distributions are obtained by redmechanical scanning of an image sensor ortarget object. Therefore, a precise alignment of an optical system is required. To solve this issue, theintroduction of polarization directed flat lenses is red presented. In the proposed method, two intensitydistributions at different depth positions on the optical axis are obtained without mechanical scanningby changing polarization states of incident light. The feasibility of the proposed method is confirmed byan optical experiment.

Designing with Very Thin Optical Films

Ronald Willey, Audrius Valavičius, and Fred Goldstein

DOI: 10.1364/AO.383929 Received 20 Nov 2019; Accepted 16 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: There is increasing interest in the design of films with thicknesses on the order of10 nm and less for a variety of applications: nano particles, plasmonics, quantum dots, solarreflectors, black mirrors, etc. The indices of refraction (n and k) for the effective media ofsuch coatings depend on the materials with which such “layers” interface and the specificprocess parameters used to produce those films. The structures may typically be nucleatingisland structures and may also be continuous films. A key factor is that the n- and k-valuesvary with thickness until some thickness is obtained, usually greater than 20 nm. Heretofore,films have not taken into account index variations with thickness during the design process.Software has now been developed wherein the index at a given thickness is computed at eachiteration of the design optimization process. This allows more realistic design results utilizingthe full representation of the behavior of the layers in question; the resulting coatings, whenproduced, are in better agreement with the designs. Including n and k versus wavelength andthickness in the design process is referred to here as Double Dispersion.

Fast modal analysis for Hermite-Gaussianbeams via deep learning

Yi An, Tianyue Hou, Jun Li, Liangjin Huang, Jinyong Leng, Lijia Yang, and Pu Zhou

DOI: 10.1364/AO.377189 Received 05 Sep 2019; Accepted 16 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: The eigenmodes of Hermite-Gaussian (HG) beams emitting from solid-statelasers make up a complete and orthonormal basis, which are gaining increasing interest inrecent years. Here, we demonstrate deep learning-based mode decomposition (MD) schemeof HG beams for the first time, to the best of our knowledge. We utilize large amounts ofsimulated samples to train a convolutional neural network (CNN) and then use this trainedCNN to perform MD. The results of simulated testing samples have shown that our schemecan achieve an averaged prediction error of 0.013 when 6 eigenmodes are involved. Thescheme takes only about ms to perform MD for one beam pattern, indicating promisingreal-time MD ability. When larger number of eigenmodes are involved, the method can alsosucceed with slightly larger prediction error. The robustness of the scheme is also investigatedby adding noise to the input beam patterns and the prediction error is smaller than 0.037 forheavily noisy patterns. This method offers a fast, economic and robust way to acquire both themode amplitude and phase information through a single-shot intensity image of HG beams,which will be beneficial to the beam shaping, beam quality evaluation, studies of resonatorperturbations, and adaptive optics for resonators of solid-state lasers.

Generation of homogeneous 3D Gaussian noisewith spherically symmetric covariance

Daniel Wegner and Endre Repasi

DOI: 10.1364/AO.381334 Received 29 Oct 2019; Accepted 15 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: In this paper an approach for 3D noise generation is presented. The proposed algorithm might bea useful tool for the generation of correlated phase screens. These phase screens can be usedfor the simulation and modelling of optical wave propagation through atmospheric turbulence.Arbitrary user-defined covariance functions between voxel pairs can be achieved. Correlated 3Dnoise is formed by superposition of multiple uncorrelated 3D Gaussian noise patterns. Theseuncorrelated input noise patterns are of different dimensions. They are upsampled to same targetdimensions by linear interpolation. Each input pattern then contributes to total covariance ondifferent spatial scales. The covariances between different voxels are expressed analytically bypropagation of error. For a subset of randomly chosen voxels in the entire voxel space relativedeviations between the analytical and user-defined covariances are calculated. A sum of squaresof these relative deviations is then minimized by machine learning methods. The optimizedparameters are the weighting factors of individual uncorrelated 3D noise patterns. Correspondingcovariance functions are numerically evaluated for two current atmospheric turbulence spectra.The first one is the Generalized Modified Atmospheric Spectrum (GMAS). The second one is theGeneralized Modified Von Karman Spectrum (GMVK). Based on these covariance functions,optimal superpositions are calculated. Finally statistical properties of these patterns are validatedby ensemble sample covariance analysis.

Optimized Reconstruction with Noise Suppression forInterferenceless Coded Aperture CorrelationHolography

Chao Liu, Tianlong Man, and Yuhong Wan

DOI: 10.1364/AO.381635 Received 29 Oct 2019; Accepted 15 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: A modified non-linear reconstruction technique with noise modulation parameter is proposed for theinterferenceless coded aperture correlation holography (I-COACH), thus the signal-to-noise ratio of reconstructedimage is improved without sacrifice of the field of view and temporal resolution of the system. In order to obtainthe optimal reconstructed image, the no reference structural sharpness (NRSS) is introduced as the evaluationmetric of reconstructed image quality during non-linear reconstruction. On the other hand, the noise modulationfunction is built in order to analyze the effect of phase on noise when the amplitude of point spread hologram andobject hologram is unity of 1. Both the NRSS and noise modulation function are combined with non-linearreconstruction in I-COACH for improving the imaging performances. The validities of the proposed method underdifferent experimental conditions have been demonstrated by experiments.

Theoretical and experimental comparison of adistributed acoustic sensor at 850 nm and1550 nm wavelengths

Agnese Coscetta, Ester Catalano, Enis Cerri, Luigi Zeni, and Aldo Minardo

DOI: 10.1364/AO.382907 Received 13 Nov 2019; Accepted 15 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: In this paper, we analyze the performance of a distributed acoustic sensor at twodifferent interrogation wavelengths. We show theoretically that, in a coherent OTDRoperating at 850 nm, the dynamic signal-to-noise ratio (SNR) is enhanced, compared to anidentical configuration operating at 1550 nm. Such enhancement is maximum at theinterrogating pulse input section, while decreasing along the fiber in virtue of the higher loss.Experimental tests, carried out using two heterodyne C-OTDR detection schemes operating atthe analyzed wavelengths, confirm the SNR improvement.

Experimental demonstration of SSB NPAM-4/DDdegenerate MDM transmission based on MIMOnonlinear equalizer

Xiang Gao, Bo Xu, Pan Wu, Yuancheng Cai, and Kun Qiu

DOI: 10.1364/AO.384338 Received 27 Nov 2019; Accepted 15 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: We experimentally demonstrate a degenerate mode division multiplexing (MDM) transmission system with singlesideband Nyquist pulse shaped PAM-4 modulation and direct detection techniques. Hilbert superpositioncancellation (HSC) processing is applied to cancel the 1-st order crosstalk at the receiver side. A LMS-Volterrabased MIMO nonlinear equalizer (NLE) is utilized to compensate the 2-nd order distortion. Experimental resultsshow that compared with the MIMO linear equalizer without HSC processing, the MIMO NLE with HSC processingcan help improving the BER performance by almost one order of magnitude at the ROP of -10 dBm.

Thin fiber-based Mach-Zehnderinterferometric sensor for measurement ofliquid-level, refractive index, temperature andaxial strain

Wei Liu, Xuqiang Wu, Gang Zhang, Shili Li, Cheng Zuo, Wujun Zhang, and Benli Yu

DOI: 10.1364/AO.378873 Received 26 Sep 2019; Accepted 14 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: An all-fiber Mach-Zehnder interferometric sensor capable of measuring liquidlevel,refractive index (RI), temperature, and axial strain is proposed and experimentallydemonstrated. The proposed sensor is based on a fiber ball-thin fiber (TF)-core-offsetstructure sandwiched between two standard single mode fibers (SMFs). The variations ofambient liquid-level, RI, temperature, and axial strain cause the change of phase differencebetween the cladding modes and the core mode, which leads to the shift of interferencespectrum. The wavelength shifts of three resonant dips in the transmission spectrum are usedto investigate the sensing characteristics of the sensor. Experimental results show that thesensor with TF length of 20mm exhibits high RI and liquid-level sensitivities of -131.7092nm/RIU and -120.7 pm/mm at a wavelength of 1589.5nm. Meanwhile, the sensor isinsensitive to temperature and axial strain, and the maximum sensitivities are 0.0390 nm/°Cand -4.84×10-4 nm/με, respectively. In addition, the sensor shows superiority in measuringmultiple parameters simultaneously.

Development of an Auto-Focusing System UsingElectrically Tunable Lens in Large Area HolographicLithography

Rui Hou, JIA YU, Yanyan Huang, Huaheng Ke, and Huiping Liu

DOI: 10.1364/AO.379853 Received 10 Oct 2019; Accepted 14 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: Due to the limited processing accuracy of the platform and unevenness of the glass substrate itself, holographiclithography system is prone to out-of-focus imaging problem, therefore the real-time focusing components areextremely important for holographic lithography systems. In this paper, a real-time focus monitoring and adjustingsystem using an Electrically Tunable Lens (ETL) for large area lithography is introduced. Combined with the ETL,the limited depth of field of the microscopic objective has been effectively expanded, and the automatic focusingevaluation and adjustment are achieved. The development including simulation using Zemax, optics system designand implementation, experiments and evaluation are demonstrated in this paper. The results show that the out-offocusproblem in our large area holographic lithography system has been significantly alleviated.

Fundamental design parameters of twocomponentoptical systems – theoreticalanalysis

Antonin Miks and Petr Pokorny

DOI: 10.1364/AO.383985 Received 22 Nov 2019; Accepted 14 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: The paper presents a detailed theoretical analysis of two-components opticalsystems of Petzval objective, tele-objectives, reverse tele-objectives and objectives ofanallactic type. This type of optical systems is very popular in practice, especially in the fieldof photographic technologies and surveying devices (theodolites, levelling devices, etc.)where anallactic telescopes with inner focusing are used. The paper presents methods ofdesigning of fundamental parameters of the objective, i.e. focal distances of the objective’scomponents and their mutual distance, and radii of curvatures of individual surfaces if thecomponents are cemented doublets. Further, a detailed analysis of aberration properties ofthose optical systems is presented.

Ultra-low Stress SiO2 Coatings By Ion BeamSputtered Deposition

Aaron Davenport, Emmett Randel, and Carmen Menoni

DOI: 10.1364/AO.380844 Received 17 Oct 2019; Accepted 14 Jan 2020; Posted 16 Jan 2020  View: PDF

Abstract: High mechanical stress can affect the performance of multilayer thin film opticalcoatings, causing wavefront aberrations. This is particularly important if the multilayer stack isdeposited onto thin substrates, such as those used in adaptive optics. Stress in thin film coatingsis dependent on the deposition process, and ion beam sputtered (IBS) thin films are known tohave high compressive stress. In the present work, we show that stress in IBS SiO2 thin filmscan be reduced from 490 MPa to 48 MPa using high energy O2 assist ion bombardment duringdeposition while maintaining high optical quality. A comparison on the reduction of stress inSiO2 deposited from oxide and metal targets is provided.

A simulation framework for evaluating lightweightspectral cameras in drone-based aquatic sensingapplications

Ryan O'Shea and Samuel Laney

DOI: 10.1364/AO.381564 Received 04 Nov 2019; Accepted 14 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: Optical remote sensing of aquatic environments using aerial drones is becoming more feasible aslightweight, low-power, multi- and hyperspectral cameras increase in availability. Use of these cameras insuch applications involves complex trade-offs in optical design and in deployment strategies, and simulationsprovide a means to examine this multidimensional design space to identify specific limitations onperformance for a given measurement scenario. In this paper, such a simulation framework is developedand its use in two realistic aquatic remote sensing scenarios is explored. Such a framework can provideinsight not only into uses of existing camera systems, but also aspects of optical design or hardware thatwould lead to improved accuracy when using such cameras aerially over natural water bodies.

Ultracompact polarization-insensitive power splitterusing subwavelength-grating-based MMI couplers ona SOI platform

Wanqin Zhong and Jinbiao Xiao

DOI: 10.1364/AO.382097 Received 31 Oct 2019; Accepted 14 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: An ultracompact and polarization-insensitive power splitter using a subwavelength-grating-based multimodeinterference (MMI) coupler on a SOI platform is proposed and analyzed in detail. By properly tailoring the structuralparameters of the subwavelength gratings (SWGs), embedded in the center of the MMI coupler, the effective reflectiveindices for TE and TM modes supported by this MMI coupler can be engineered, leading to equal coupling lengths for thetwo polarizations and an efficient reduction in length for the used MMI coupler. As a result, an ultracompactpolarization-insensitive power splitter can be realized. Moreover, to effectively minimize the loss, tapered waveguides areused and two right angles are cut at both corners of the used MMI coupler. Results show that a footprint of 2.2×3.8 μm2 forthe MMI region is achieved with an insertion loss (IL) of 0.07 dB for both TE and TM modes (polarization dependent loss(PDL) ~ 0 dB) and a reflection loss (RL) of -28.29 dB (-31.25 dB) for TE (TM) mode at the wavelength of 1.55 μm. Insertionloss below 0.3 dB is obtained over the bandwidth of 200 nm, covering the C-band. In addition, fabrication tolerances to thestructural parameters are analyzed and the injected light propagating through the power splitter is also presented.

Phase-sensitive amplification of QPSK signal using adispersion engineered silicon-graphene oxide hybridwaveguide

Zhihua Chen, Hongjun Liu, Nan Huang, and Zhaolu Wang

DOI: 10.1364/AO.382778 Received 08 Nov 2019; Accepted 14 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: We numerically investigate phase-sensitive amplification of quadrature phase shift keying (QPSK) signal in a 35μm dispersion engineered silicon-graphene oxide hybrid waveguide. The four-wave mixing efficiency is effectivelyenhanced by exploiting the ultrahigh Kerr nonlinearity and low loss of graphene oxide in ultra-wide wavelengthrange. A new structure of dispersion flat silicon-graphene oxide hybrid waveguide is proposed and used to achievethe phase regeneration of QPSK signal using dual-conjugated-pump degenerate scheme. The phase-dependent gainand phase-to-phase transfer functions are calculated to analyze the properties of phase-sensitive amplifier (PSA).The constellation diagrams of QPSK signal and the error vector magnitude are used to assess the regenerationcapacity. The simulation results show that the proposed PSA with a good phase noise squeezing capability haspotential applications in all-optical signal processing.

Effects of fixatives on myelin molecular orderprobed with RP-CARS microscopy

Giuseppe de Vito, Paola Parlanti, Roberta Cecchi, Stefano Luin, Valentina Cappello, Ilaria Tonazzini, and Vincenzo Piazza

DOI: 10.1364/AO.384662 Received 02 Dec 2019; Accepted 13 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: When live imaging is not feasible, sample fixation allows preserving the ultrastructure ofbiological samples for subsequent microscopy analysis. This process could be performed withvarious methods, each one affecting differently the biological structure of the sample. Whilethese alterations were well-characterized using traditional microscopy, little information isavailable about the effects of the fixatives on the spatial molecular orientation of thebiological tissue. We tackled this issue by employing Rotating-Polarization Coherent Anti-Stokes Raman Scattering (RP-CARS) microscopy to study the effects of different fixatives onthe myelin sub-micrometric molecular order and micrometric morphology. RP-CARS is anovel technique derived from CARS microscopy that allows probing spatial orientation ofmolecular bonds while maintaining the intrinsic chemical selectivity of CARS microscopy.By characterizing the effects of the fixation procedures, the present work represents a usefulguide for the choice of the best fixation technique(s), in particular for polarisation-resolvedCARS microscopy. Finally, we show that the combination of paraformaldehyde andglutaraldehyde can be effectively employed as a fixative for RP-CARS microscopy, as long asthe effects on the molecular spatial distribution, here characterized, are taken into account.

Propagation characteristics of a partiallycoherent self-shifting beam in random media

Yuyan Wang, Zhangrong Mei, Ming Zhang, and yonghua mao

DOI: 10.1364/AO.384851 Received 03 Dec 2019; Accepted 13 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: In recent publication [Opt. Lett. 43, 4727 (2018)], a novel class of partiallycoherent source with complex degree of coherence was introduced. In this paper, we get theexpression of the cross-spectral density function of the self-shifting beam generated from thelight source propagating in random media. And then we calculated and simulated thebehaviors of the spectral density and the spectral degree of coherence in the propagation. Theresults show that there will be a phenomenon of self-shifting in propagation and thecoherence of the beam is Gaussian when it is far enough from the light source. The lightintensity is weakened with increase of turbulence, while the wander of the center of thespectral density remains unchanged in different media.

A passively Q-switched Nd:YVO4 laseroperating at 1.3 μm with graphene oxide andferroferric-oxide nanoparticles hybrid assaturable absorber

Huaiwei Zhang, Jiying Peng, Xiangpeng Yang, chao ma, Qianqian Zhao, guoliang Chen, Xinyang Su, Decai Li, and Yi Zheng

DOI: 10.1364/AO.383013 Received 14 Nov 2019; Accepted 13 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: A self-made saturable absorber (SA) based on hybridized graphene oxide (GO) andFe3O4 nanoparticles (FONP) was inserted into the linear cavity to generate a passively Qswitchedsolid-state Nd:YVO4 laser operating at 1.3 μm waveband. The laser had a minimumpulse width of 163 ns and a maximum repetition rate of 314 kHz. This experiment is the firstto demonstrate that hybridized GO and FONP (GO-FONP) can be used as an SA in passivelyQ-switched pulse lasers. Results show that GO-FONP has the potential to be used forpassively Q-switched laser generation.

Single longitudinal mode GaAs-based quantumdot laser with refractive index perturbation

Yunyun Ding, Ren Lv, Zhong-Kai Zhang, Hui-Hong Yuan, and Tao Yang

DOI: 10.1364/AO.383089 Received 13 Nov 2019; Accepted 13 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: We report on the design, fabrication and characterization of single longitudinalmode InAs/GaAs quantum dot lasers emitting at 1.3 μm communication band. The influenceof simply etched surface high-order gratings in the ridge of the Fabry-Perot lasers has beenstudied. A 35th-order surface grating is fabricated by standard photolithography to introducethe refractive index perturbation, which leads to the reduced mirror-loss at the desiredwavelength and thus realizing single longitudinal mode lasing. Stable single mode operationsare maintained at the injection current range of 45-100mA with a side-mode-suppression-ratioup to 33 dB.

Three-dimensional detection andquantification of defects in SiC by opticalcoherence tomography

Pei Ma, Jiajie Ni, jiawei sun, Xuedian Zhang, Hui Chen, and Junying Li

DOI: 10.1364/AO.384174 Received 02 Dec 2019; Accepted 13 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Silicon carbide (SiC) is widely used in high power electronic devices. However,defects on the SiC significantly reduce the yield and decrease the performance of SiC.Accurate detection of the defects is essential in the process control. We demonstrated anon-invasive three-dimensional (3D) defect detection method for SiC using optical coherencetomography (OCT). Defects including the triangular defects, hexagonal voids, grainboundaries and carrot defects were inspected and analyzedon SiC wafers. The 3D images ofdefects acquired with OCT provided detailed information on the 3D structures anddimensions of defects, as well as the locations and orientations of the defects inside thewafers. This technique was not only useful for rapid defect screening in the process control, itwas also extremely helpful in understanding the formation mechanism of these defects in SiC.

An absorption path extension TDLAS system with adual fiber loop configuration

cunguang zhu, Chenxi Li, Pengpeng Wang, Feng Peng, Xiaojun Cui, Tongwei Chu, and Bing Wu

DOI: 10.1364/AO.385238 Received 11 Dec 2019; Accepted 13 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Increasing the absorbance by lengthening the absorption path is a direct and effective approach to improve thesignal-to-noise ratio (SNR) of the infrared gas absorption spectroscopy. However, once the absorption path isextended by designing and optimizing the gas cell structure to a certain extent, a bottleneck will appear due to thedifficulties in the optical alignment and the interference effect. A modified tunable diode laser absorptionspectroscopy (TDLAS) system with a dual fiber loop configuration is proposed, which can extend the effectiveabsorption path length of the original multi-pass cell several times. The relevant theoretical model has beenestablished and its effectiveness has been verified through experiments.

Multi-focus image fusion method using energy ofLaplacian and deep neural network

Hao Zhai and Yi Zhuang

DOI: 10.1364/AO.381082 Received 22 Oct 2019; Accepted 13 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Multi-focus image fusion consists in the integration of the focus regions of multiple source images into a singleimage. At present, there are still some common problems in image fusion methods, such as block artifacts, artificialedges, halo effects, and contrast reduction. To address these problems, a novel multi-focus image fusion methodusing energy of Laplacian and deep neural network is proposed in this paper. The deep neural network iscomposed of multiple denoising autoencoders and a classifier. The Laplacian energy operator can effectivelyextract the focus information of source images, and the trained DNN model can establish a valid mappingrelationship between source images and a focus map according to the extracted focus information. First, theLaplacian energy operator is used to perform focus measurement for two source images to obtain thecorresponding focus information maps. Then, the sliding window technology is used to sequentially obtain thewindows from the corresponding focus information map, and all the windows are fed back to the trained deepneural network model to obtain a focus map. After binary segmentation and small region filtering, a final decisionmap with good consistency is obtained. Finally, according to the weights provided by the final decision map,multiple source images are fused to obtain a final fusion image. Experimental results demonstrate that theproposed fusion method is superior to other existing ones in terms of subjective visual effects and objectivequantitative evaluation.

Improved Control of ElectronComputer-Generated Holographic GratingGroove Profiles Using Ion Beam Gas-AssistedEtching

Cameron Johnson, Dylan Bauer, and Benjamin McMorran

DOI: 10.1364/AO.376876 Received 03 Sep 2019; Accepted 13 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: In a transmission electron microscope, electrons are described by matter-waves withwavelengths 5 orders of magnitude smaller than optical electromagnetic waves. Analogous tooptical holography, electron wavefronts can be shaped using nanoscale holographic gratings.Here we demonstrate a novel, scalable nanofabrication method for creating off-axis holographicgratings that demonstrate near ideal diffraction efficiencies for binary, sinusoidal, and blazedgrating groove profiles. We show that this method can produce up to 50 m diameter areagratings that diffract up to 68% of the transmitted electron wave into a desired diffraction orderwith less than 7% into any other order. Additionally, we find that the amount of inelasticallyscattered electrons from the material gratings remaining in the coherent diffraction orders fromthe gratings is negligible in the far-field.

Flat optical frequency comb generation basedon dual-parallel Mach-Zehnder modulator andsingle recirculation frequency shift loop

Dan Li, Shibao Wu, Yu Liu, and Yifeng Guo

DOI: 10.1364/AO.381880 Received 06 Nov 2019; Accepted 12 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: To generate flat optical frequency comb (OFC), a new scheme based on dualparallelMach-Zehnder modulator (DPMZM) and single recirculation frequency shift (RFS)loop is proposed and analyzed. Compared with the traditional single loop recirculationfrequency shift method, the quantity of comb lines is doubled, and the comb flatness is betterwhen the number of cycles is the same. Theoretical analysis model is established, and thesimulation results show that 111-line OFC with frequency spacing of 10 GHz, flatness of1.32dB and optical signal to noise ratio (OSNR) of 27.4dB can be obtained by adopting theproposed scheme.

Design of arbitrary ratio optical power splitterbased on discrete differential multi-objectiveevolutionary algorithm

Yuan Xie, Tianye Huang, Qizheng Ji, Ming Yang, Jin Wang, Xin Tu, zhuo cheng, Guizhen Xu, Qian Wei, Yiheng Wu, and Ping Shum

DOI: 10.1364/AO.382215 Received 31 Oct 2019; Accepted 12 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Traditional photonic integrated devices are designed to predict their opticalresponse by transforming the structure and parameters, and it is often difficult to obtaindevices with excellent performance in all aspects. The nanophotonic computing designmethod based on the optimization algorithm has revolutionized the traditional photonicintegrated devices design technology. Here, we report a discrete differential evolutionalgorithm that simulates a natural selection process to achieve an ultra-compact arbitrarypower ratio splitter. The footprint of the designed splitter is only 2.5×2.5 μm2, the simulatedtotal transmission efficiency is above 90%, the power ratio error is less than 3%, and it canwork normally over the C-band. Our algorithm can provide new ideas for the application ofgenetic algorithms to the automatic optimization of photonic integrated devices.

Simplified unobscured optics design fordiffractive telescope

Chuanwang He, Peng Huang, He Yiwei, xiaochun dong, and fan bin

DOI: 10.1364/AO.379922 Received 11 Oct 2019; Accepted 12 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Unobscured optical system provides a good solution of high-performancediffractive telescope. However, the unobscured diffractive telescope, such as off-axis threemirrordiffractive telescope (OTDT), suffers from the complex calculation of initialparameters, strict tolerance and fabrication of aspheric mirror diffractive optical elements(DOE). In this paper, a simplified unobscured design method that is effective in achievingcompact and loose tolerance is proposed. Combining the deflection of optical path andSchupmann achromatic theory, a novel unobscured diffractive telescope (NUDT) is rapidlyconverted from coaxial transmissive system. Not only the loose tolerance is obtained, but thefabrication of aspheric mirror DOE is avoided. The designed system with focal length of 400mm, F-number of 5, and working waveband is 612.8-652.8 nm is analyzed. The results shownthat the imaging quality is approached the diffraction limit within a field of view (FOV) of0.04°×0.02°. Compared to the OTDT, the designed system has great advantages in designstep, DOE fabricating and system alignment. It provides a reference for unobscureddiffractive telescope development.

Constraints and performance tradeoffs inAuger-suppressed HgCdTe focal plane arrays

Marco Vallone, Michele Goano, Francesco Bertazzi, Giovanni Ghione, Stefan Hanna, Detlef Eich, Alexander Sieck, and Heinrich Figgemeier

DOI: 10.1364/AO.385075 Received 05 Dec 2019; Accepted 12 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Majority carrier depletion has been proposed as a method to suppress the dark current originating from quasineutralregions in HgCdTe infrared focal plane array detectors. However, a very low doping level is usuallyrequired for the absorber layer, a task quite difficult to achieve in realizations. In order to address this point, weperformed combined electromagnetic and electric simulations of a planar 55 pixels miniarray with 5mm-widesquare pixels, assessing the effect of the absorber thickness, its doping level in the interval ND = [1014,1015] cm􀀀3,and temperature in the interval 140K – 0 K, both in dark and under illumination. Looking for a tradeoff,we found that the path towards high-temperature operation has quite stringent requirements on the residualdoping, whereas a reduction of the absorber thickness helps only moderately to reduce the dark current. Underillumination, inter-pixel crosstalk is only slightly cut down by a decrease of temperature or absorber doping inthe considered intervals, whereas it gets more effectively reduced by thinning the absorber.

Concentric reemission pattern from a planarwaveguide with a thin uniform luminescent layer

Ryo Matsumura, Yasuhiro Tsutsumi, and Ichiro Fujieda

DOI: 10.1364/AO.384323 Received 26 Nov 2019; Accepted 11 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: When a beam of light excites a single spot on a thin luminescent layer embedded in a planar waveguide, aconcentric reemission pattern is observed. An analytical expression is formulated by following the series of eventsin the waveguide: generation of angle-dependent photoluminescence spectra, reflection at the waveguide-airboundary, absorption by the luminescent layer and generation of next-generation photoluminescence. Theformula reproduces the peak radii observed in the experiments with some organic dyes. It provides insights forthe reemission events in a luminescent solar concentrator and the crosstalk in an energy-harvesting display basedon photoluminescence.

Experimental analysis of sub-micrometer opticalintensity distributions after an opaque disk

Olivier Emile and janine emile

DOI: 10.1364/AO.387699 Received 08 Jan 2020; Accepted 11 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: Generation of sub-wavelength beam sizes is a fascinating challenge with several implications. The observationof a 120 nm laser spot size in the visible part of the spectrum is here reported. It has a size variationof less than 10 % on a distance of 50 mm along the axis of propagation. This so-called Arago spot resultsfrom the diffraction of the light from a laser diode by the edges of an absorbing disk. Applications arediscussed. Furthermore, hollow beams carrying orbital angular momentum with 400 nm diameter darkspot in the centre are evidenced. This paves the way towards atom lithography via atom guiding or newspectroscopy on forbidden transitions.

Guided Terahertz Pulse Reflectometry withDouble Photoconductive Antenna

Mingming Pan, Quentin Cassar, Frederic Fauquet, Georges Humbert, Patrick Mounaix, and Jean-Paul Guillet

DOI: 10.1364/AO.381646 Received 08 Nov 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Developments towards the implementation of terahertz pulses imaging system within aguided reflectometry configuration are reported. Two photoconductive antennas patterned on thesame LT-GaAs active layer in association with a silica pipe hollow-core waveguide allowed to obtaina guided optics-free imager. Besides working in a pulsed regime, the setup does not requireadditional optics to focus and couple the terahertz pulses into the waveguide core, simplifying theglobal implementation in comparison to other reported guided terahertz reflectometry systems. Thesystem is qualified for imaging purposes by means of a 1951 USAF resolution test chart. An imageresolution, after a 53 mm propagation length, by about 0.707 LP/mm over the 400-550 GHzintegrated frequency band was obtained providing promising basis to pursue the effort towardscompact guided pulse imagers for sample inspection within the terahertz range.

A wide multiwavelength Brillouin-Raman fiber laser assistedby an arc-shaped fiber attenuator

Harith Ahmad and ABDULMOGHNI AL-ALIMI

DOI: 10.1364/AO.380663 Received 18 Oct 2019; Accepted 10 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: A wide bandwidth single-spacing half-open cavity multi-wavelength Brillouin-Raman fiber laser (MWBRFL) isdemonstrated. The laser cavity contains a fiber loop mirror (FLM) with an arc-shaped optical fiber attenuator that isused to control the mirror reflectivity, suppressing gain competition from longitudinal cavity modes. A tuning rangeof 45 nm with 637 lines at Raman and 1525 nm Brillouin pump powers of 1.2 W and 12 dBm can be achieved byutilizing a 10-dB arc-shaped optical fiber attenuator in the cavity. This is in comparison to 433 Stokes linesobtained over a 31-nm tuning range for the half-open MWBRFL cavity without any feedback power optimization.The MWBRFL has low power fluctuations of less than 0.1 dB over a one hour test period. The inclusion of the arcshapedoptical fiber attenuator in the MWBRFL can control the reflectivity of the FLM as well as improving thelaser’s tuning range to generate a high number of Brillouin Stokes signal.

Progress in the measurement and reduction ofthermal noise in optical coatings forgravitational-wave detectors

Massimo Granata, Alex Amato, Gianpetro Cagnoli, Matthieu Coulon, Jerome Degallaix, Daniele Forest, Lorenzo Mereni, Christophe MICHEL, Laurent Pinard, Benoit SASSOLAS, and julien teillon

DOI: 10.1364/AO.377293 Received 16 Sep 2019; Accepted 10 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: Coating thermal noise is a fundamental limit for precision experiments basedon optical and quantum transducers. In this review, after a brief overview of the techniquesfor coating thermal noise measurements, we present the latest world-wide research activityon low-noise coatings, with a focus on the results obtained at the Laboratoire des MatériauxAvancés. We report new updated values for the Ta2O5, Ta2O5-TiO2 and SiO2 coatings ofthe Advanced LIGO, Advanced Virgo and KAGRA detectors, and new results from sputteredNb2O5, TiO2-Nb2O5, Ta2O5-ZrO2, MgF2, AlF3 and silicon nitride coatings. Amorphous silicon,crystalline coatings, high-temperature deposition, multi-material coatings and composite layersare also briefly discussed, together with the latest developments of structural analyses and models.

Rapid 3D measurement technique for colorful objectsemploying RGB color light projection

Jianhua Wang, Yuguo Zhou, and Yanxi Yang

DOI: 10.1364/AO.382302 Received 04 Nov 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Three-dimensional (3D) measurement of colorful objects is challenging. Since different color can absorb different wavelength ofprojected light, the brightness and contrast of the captured fringe are not uniform employing single color light projection, whichwill lead to measurement error. In this paper, we present a rapid 3D measurement technique for colorful objects employing red,green and blue (RGB) light projection. According to the research in this paper, for common color, the pixel with the largestbrightness and contrast can be extracted from the three fringes projected by RGB light. Besides, we introduce the selection methodof exposure time, and then combine the high-speed projection technique with the optimal pixel extraction algorithm to get theoptimal set of fringes for phase calculation. Experiments show that the proposed method improves the measurement accuracy andefficiency.

Design and fabrication of As2Se3 chalcogenidewaveguides with low optical losses

Zhuodong Fan, Kunlun Yan, Limeng Zhang, Jingshuang Qin, Jinbo Chen, Rongping Wang, and Xiang Shen

DOI: 10.1364/AO.386280 Received 19 Dec 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: In this paper, we report the fabrication and characterization of chalcogenide-basedplanar waveguide for the possible applications in broadband light sources and/or biochemicalsensing. Ge11.5As24Se64.5 film as bottom cladding followed by another layer of As2Se3 wasdeposited on thermally oxidized silicon (TOX) wafer using thermal evaporation, and thewaveguides were patterned directly on the As2Se3 layer by UV exposure followed byinductively coupled plasma dry etching. Device structure was optimized by using commercialsoftware "COMSOL Multiphysics" based on complete vector finite components and thefundamental mode of the waveguide was calculated. By optimizing the geometry of thewaveguide, the zero dispersion wavelength was shifted to a short wavelength (at ~2.3 μm),which facilitates SC generation with shorter wavelength pump source. The insertion loss ofthe rib waveguides with different widths was measured using the cut-back method, and thebest propagation loss at 1550 nm was 1.4 dB/cm.

Adjacency Radiance around a Small Island: Implicationsfor System Vicarious Calibrations

Barbara Bulgarelli and Giuseppe Zibordi

DOI: 10.1364/AO.378512 Received 24 Sep 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: The adjacency radiance field surrounding a small island (i.e., the Lampedusa Island in the Central MediterraneanSea) was theoretically analysed to address implications on a hypothetical nearby System Vicarious Calibration(SVC) infrastructure for satellite ocean color sensors. Simulations, performed in the visible and near-infrared(NIR) region for the Ocean Land Color Instrument (OLCI) operated onboard Sentinel-3 satellites, show differentpatterns of adjacency effects (AE) around the island. In the direction of the reflected sunbeam (i.e., in the northwesternregion), AE mainly originate by missing glint contributions from the sea surface masked by the island.These AE are mainly negative, decrease with wavelength, and strongly depend on sea surface anisotropy (i.e., seastate) and illumination conditions; this hinders the capability to provide a general unique description of theirfeatures. In the remaining marine regions, AE are positive and do not exceed the radiometric sensitivity of OLCIdata beyond approximately 14 km from the coast. At shorter distances, uncertainties in satellite radiance due toAE would hence not allow fulfilling requirements for SVC.

A new iterative retrieval method for ocean attenuation profiles measured by airborne lidar

Hang Liu, Peng CHEN, zhihua mao, and Delu PAN

DOI: 10.1364/AO.379406 Received 09 Oct 2019; Accepted 09 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: Measurements of ocean optical properties are of great significance in many fields, such as marine environmental monitoring, marine hydrological surveying, underwater laser communication, and marine carbon cycle monitoring. Currently, lidar remote sensing for ocean optical properties has been increasingly applied because of its ability to provide vertical structure information, which cannot be directly obtained by ocean color remote sensing. However, application of this technology demands an inversion method to infer two quantities, attenuation and backscatter, from a single measurement. In this paper, a new iterative retrieval method is demonstrated to deduce attenuation coefficient from ocean lidar return signals. Using this method, one calculates the logarithmic backscatter-to-attenuation ratio k by using iterative solution based on a bio-optical model. An example of lidar-processing results in each procedure from raw data to attenuation is presented, and the inversion results are compared with in situ measurements. The correlation coefficient R between the lidar-retrieval and in-situ measurements is 0.8, and the root mean squares error is 0.032. We then map the vertical structure of the lidar-retrieved attenuation along airborne lidar flight tracks., and discuss the influences of logarithmic backscatter-to-attenuation ratio k, reference depth z_m and reference value α_m. The results show that the reference value has little influence on the results for high-optical-thickness water, and that k is the main error source in lidar return inversion. It demonstrates that the optimum k obtained by this new iteration method is 1.03 in the sea area of Wuzhizhou Island in the South China Sea. Primary results indicate that this method is effective for providing lidar attenuation coefficient.

On the performance of adaptive hybrid MQAM-MPPMscheme over Nakagami and Log-normal dynamicvisible light communication channels

ABD EL-RAHMAN EL-FIKKY, Abdallah Ghazy, Haitham khallaf, Ehab Mohamed, Hossam Shalaby, and Mostafa Hussein Aly

DOI: 10.1364/AO.379893 Received 09 Oct 2019; Accepted 09 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: In this paper, we introduce the idea of using adaptive hybrid modulation techniques to overcome channelfading effects on visible light communication (VLC) systems. Hybrid M-ary quadrature-amplitude modulation(MQAM) and multipulse pulse-position modulation (MPPM) technique are considered due to itsability to make gradual changes in spectral efficiency to cope with channel effects. First, Zemax optics studiosimulator is used to simulated dynamic VLC channels. The results of Zemax show that Nakagami andLog-normal distributions give the best fitting for simulation results. The performance of MQAM-MPPMis analytically investigated for both Nakagami and Log-normal channels, where we obtained closed formexpressions for the average bit error rate (BER). The optimization problem of evaluating hybrid modulationtechnique settings that lead to highest spectral efficiency under specific channel status and constraintof outage probability is formulated and solved using exhaustive search. Our results reveal that adaptivehybrid scheme improves system spectral efficiency compared to ordinary QAM and MPPM schemes. Ourresults reveal that adaptive hybrid scheme improves system spectral efficiency compare to ordinary QAMand MPPM schemes. Specifically, at low average transmitted power, 􀀀32 dBm, adaptive hybrid schemeshows 280% improvement in spectral efficiency compared to adaptive versions of the ordinary schemes.At higher power, 􀀀20 dBm, 6.5% and 725% improvement are obtained compared to ordinary QAM and ordinaryMPPM, respectively. Also, adaptive hybrid scheme shows high improvement in average BER andoutage probability compared to ordinary schemes. The hybrid scheme shows 28%, 34%, 38% improvementfor m = 1, 2, 3 for Nakagami channels at BER = 10􀀀3. Besides, the outage probability of hybrid schemesof BER = 10􀀀3 shows 30% and 14% better performance than ordinary MQAM and MPPM schemes.

Monitoring of keyhole entrance and moltenpool with quality analysis during AdjustableRing Mode (ARM) laser welding

Lin Wang, Masoud MOHAMMADPOUR, Baixuan Yang, Xiangdong Gao, JEAN-PHILIPPE LAVOIE, Klaus Kleine, Fanrong Kong, and Radovan Kovacevic

DOI: 10.1364/AO.383232 Received 13 Nov 2019; Accepted 09 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: The process monitoring of topside and the evaluation of surface quality wereapplied in Adjustable Ring Mode (ARM) laser welding of 316 stainless steel by using visualmonitoring and confocal sensor technology. Experiments were conducted in different powerarrangements of ARM laser. In this study the process status was evaluated based on the areaof keyhole entrance and width of the pool. The geometric characteristics of the topside weldwas measured by a confocal sensor. The shape of the fusion zone was analyzed based on itscross-section. The process mechanism of a different mode of ARM laser welding wasinvestigated by analyzing the process status and geometry characteristics of the surface bead.Experimental results showed that dual-mode laser welding could stabilize the keyholeentrance with a uniform weld surface formation in comparison to the pure-ring mode laserweld. The dual-mode laser also generated a wider molten pool with a wider fusion zone thanthe pure-center laser weld.

Characterization of a single-frequencyBismuth-doped fiber power amplifier with aCW and modulated seed source at 1687 nm

Grzegorz Gomolka, Aleksandr Khegai, Sergey Alyshev, Aleksey Lobanov, Sergey Firstov, and Michal Nikodem

DOI: 10.1364/AO.384413 Received 26 Nov 2019; Accepted 09 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: In this paper we report the performance of a bismuth-doped fiber amplifier at1687 nm. This wavelength region is particularly interesting for laser-based spectroscopy andtrace gas detection. The active bismuth-doped fiber is pumped at 1550 nm. With less than10 mW of the seed power, more than 100 mW is obtained at the amplifier’s output. We alsoinvestigate the signal at the output when wavelength-modulated seed source is used andpresent wavelength modulation spectroscopy of methane transition near 1687 nm.A significant baseline is observed in the spectra recorded when fiber amplifier is used. Originof this unwanted background signal is discussed and ways of its suppression aredemonstrated.

Computing the relevant colors which describethe color palette of paintings

Juan Luis Nieves, Luis Robledo, Yu‐Jung Chen, and Javier Romero

DOI: 10.1364/AO.378659 Received 24 Sep 2019; Accepted 09 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: In this paper we introduce an innovative parameter which allows us to evaluate theso-called “relevant colors” in a painting, in other words the number of colors that would standout for an observer when just glancing at a painting. These relevant colors allow us tocharacterize the color palette of a scene and on this basis those discernible colors which arecolorimetrically different within the scene. We have tried to carry out this characterization ofthe chromatic range of paints according to authors and styles. We have used a collection of4,266 paintings by 91 painters, from which we have extracted various parameters which areexclusively colorimetric in order to characterize the range of colors. After this refinement ofthe set of selected colors our algorithm obtained an average number of 18 relevant colors,which partially agreed with the total 11 to 15 basic color names usually found in othercategorical color studies.

Improving purity of the radially polarized beamgenerated by a geometric phase retarder with spatiallyvariable retardance

Maksym Ivanov, Aidas Matijosius, and Viktorija Tamuliene

DOI: 10.1364/AO.381007 Received 24 Oct 2019; Accepted 09 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: Geometric phase retarders—such as q-plates and Swaveplates—have found wide applications due to simplicityof operational principles and flexibility forthe generation of azimuthally symmetric polarizationstates and optical vortices. Ellipticity of the polarizationvector and phase of the generated beam stronglydepend on the retardation of the plate. Real devicesusually have retardation value slightly different thanthe nominated one. Previously unattended perturbationof the retardation leads to asymmetry in intensitydistribution and variation of ellipticity of the local polarizationvector of the generated beam. We elucidatethat controlled and intentionally driven azimuthallyvariable, oscillating perturbation of the retardation revealspossibility to avoid distortions in the generatedbeam and leads to the recovery of the symmetricallydistributed intensity and polarization (with zero ellipticity)of the beam. Described recovery of the desiredpolarization state could find application for generationof the high purity beam with azimuthally symmetricpolarization, which local polarization ellipse has zeroellipticity.

Measuring the topological charge of coherencevortices through the geometry of the far-field crosscorrelationfunction

Cleberson Alves, João Paulo de Amaral, Antonio Neto, José Neto, and Alcenisio Silva

DOI: 10.1364/AO.381556 Received 24 Oct 2019; Accepted 09 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: The determination of the orbital angular momentum of optical vortex beams has attracted attention of manyresearchers in last years. For some applications, it is convenient to use a partially coherent vortex beam because ofstrongest robustness. In this work we developed a method to measure the topological charge of a partiallycoherent vortex beam. Our method relies simply in the measurement of the minimum radius of a zero contour ofthe modulus of the cross-correlation function and in the measurement of the full width at half maximum of itscentral spot.

Production of Azimuthally Polarized Beam with HelicalPhase by use of the Axicons and Annular GlassCylinder

Zhenyu Rong, Li Zhang, Xiaoyi Chen, and Yujing Han

DOI: 10.1364/AO.382333 Received 04 Nov 2019; Accepted 09 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: In this paper, we proposed a method for producing the azimuthally polarized vector beam experimentally. Theexperimental setup includes two same axicons and one annular glass cylinder. The top angle of the two axiconswere placed facing each other and the annular cylinder was set among the two axicons. One circular polarizedbeam will passed through the first axicon, the annular cylinder and the second axicon in turn. When the beamincident on the inner surface of the annular cylindrical satisfied the Brewster angle, we can obtain the azimuthallypolarized beam for the reflected light from the annular cylindrical only contains the s-polarization component. Wehave derived that the azimuthally polarized vector beam has the helical phase factor with helical phase factor ofexp(-iφ) for left circularly polarized beam incident and exp(iφ) for right circularly polarized beam incident.

Regularized tomographic PIV for incompressible flowsbased on conservation of mass

Ning Liu and Lin Ma

DOI: 10.1364/AO.380720 Received 07 Nov 2019; Accepted 09 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: 3D3C (three-dimensional and three-component) velocity measurements have long been desired to resolve the 3Dspatial structures of turbulent flows. Recent advancements have demonstrated tomographic particle image velocimetry(tomo-PIV) as a powerful technique to enable such measurements. The existing tomo-PIV technique obtains 3D3Cvelocity field by cross-correlating two frames of 3D tomographic reconstructions of the seeding particles. A mostimportant issue in 3D3C velocity measurement involves uncertainty, as the derivatives of the measurements are usuallyof ultimate interests and uncertainties are amplified when calculating derivatives. To reduce the uncertainties of 3D3Cvelocity measurements, this work developed a regularized tomo-PIV method. The new method was demonstrated toenhance accuracy significantly by incorporating the conservation of mass into the tomo-PIV process. The new methodwas demonstrated and validated both experimentally and numerically. The results illustrated that the new method wasable to enhance the accuracy of 3D3C velocity measurements by 40~50% in terms of velocity magnitude and by 0.6~1.1°in terms of velocity orientation, compared to the existing tomo-PIV technique. These improvements brought about bythe new method are expected to expand the application of tomo-PIV techniques when accuracy and quantitative 3D flowproperties are required.

Diffraction efficiency evaluation for diamond turning ofharmonic diffractive optical elements

Peng Zhou, Changxi Xue, Yang Chao, Chang Liu, and Xingguo Liu

DOI: 10.1364/AO.376978 Received 03 Sep 2019; Accepted 09 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: Single point diamond turning (SPDT) is a commonly used method of manufacturing harmonicdiffractive optical elements (HDOEs). Diffraction efficiency of HDOEs is sensitive to surface-relief profileerror, and half-round tool can reduce the profile error obviously. Furthermore, the diamond tools also bringsurface roughness errors. The two errors will produce shadowing effect and scattering effect. In this paper,the two kinds of errors, especially the surface roughness are described accurately. A mathematical model isproposed to reveal the relationship among diffraction efficiency, cutting tool radius, feed rate, microstructurezone period widths and the refractive index of the substrate material and balance the influence of shadowingeffect and scattering effect. The simulation results show that the model can guide the acquisition ofhigh-precision surface topography and high diffraction efficiency, which improves the imaging quality of theoptical system.

Multiple kinds of pesticide residue detection usingfluorescence spectroscopy combined with partial leastsquares models

RENDONG Ji, Shicai Ma, Hua Yao, Yue Han, Xiao Yang, Ruiqiang Chen, YINSHAN Yu, Xiaoyan Wang, Dongyang Zhang, Tiezhu Zhu, and Hai Bian

DOI: 10.1364/AO.382311 Received 04 Nov 2019; Accepted 08 Jan 2020; Posted 08 Jan 2020  View: PDF

Abstract: Compared with high-performance liquid chromatography and mass spectroscopy, fluorescence spectroscopy hasattracted considerable attention in the field of pesticide residue detection due to its practical advantages ofproviding rapid, simultaneous analysis and non-destructive detection. However, given that the concentration ofpesticide residue detected via fluorescence spectroscopy is calculated in accordance with the Beer-Lambert law,this method can only detect samples containing a single kind of pesticide or several kinds of pesticides withcompletely different fluorescence. Multiple partial least squares (PLS) models were introduced in this work toovercome this disadvantage and achieve the concentration of zhongshengmycin, paclobutrazol, boscalid andpyridaben whose fluorescence are overlapping. The R-square of the models for zhongshengmycin, paclobutrazol,boscalid and pyridaben were 0.9942, 0.9912, 0.9913 and 0.9847, respectively. Results indicated that fluorescencespectroscopy combined with multiple PLS models can be used to detect multiple kinds of pesticides in the water.

A fast and non-iterative zonal estimation for the nonrectangulardata in the transparent surfacereconstruction from polarization analysis

Zhuang Sun, Yang Qiao, Zhaoguo Jiang, Xiping Xu, xuanrui gong, and Jing Zhou

DOI: 10.1364/AO.381416 Received 25 Oct 2019; Accepted 08 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: In the method of surface reconstruction from polarization, the reconstructed area is generally nonrectangularand contains a large number of sampling points. There is a difficulty that the coefficient matrixin front of the height vector changes with the shape of the measured data when using the zonal estimation.The traditional iterative approaches consume more time for the reconstruction of this type of data. Thispaper presents a non-iterative zonal estimation to reduce the computing time and to accurately reconstructthe surface. The index vector is created according to the positions both of the valid and invalid elements inthe difference and gradient matrices. It is used to obtain the coefficient matrix corresponding to the generaldata. The heights in the non-rectangular area are calculated non-iteratively by the least squares method. Atthe same time, the sparse matrix is applied for handling the large-scale data quickly. The simulation andexperiment are designed to verify the feasibility of the proposed method. The results show that the proposedmethod is high-efficiency and accurate in the reconstruction of non-rectangular data.

Real-time reflectance anisotropy spectroscopy ofGaAs homoepitaxy

Alfonso Lastras, LilIana Estela Guevara Macías, Jose Santiago Garcia, Jorge Gallegos, isaac azahel ruiz alvarado, Rodolfo Martínez Espinosa, Augusto Ariza-Flores, Ricardo Castro García, Rosa López Estopier, Raul Balderas, and luis lastras-martinez

DOI: 10.1364/AO.383611 Received 19 Nov 2019; Accepted 08 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: Reflectance anisotropy spectroscopy (RAS) is a highly sensitive optical probe for the real-time study of the epitaxial growthof zincblende semiconductors. Here we report on 1) non-equilibrum RAS spectra acquired in real-time during the homoepitaxialgrowth of GaAs and 2) RAS spectra for GaAs surfaces under equilibrium with several Arsenic overpressures. We show that in bothcases RAS spectra can be decomposed into two basic components, each with a characteristic line shape. We further show that bothdynamic and equilibrium RAS spectra are described by the same pair of basic components. We conclude that the time evolution ofnon-equilibrium RAS spectra acquired during the epitaxial growth can be described in terms of RAS spectra for equilibrium surfaces.The results reported here should be useful for the interpretation of the physics underlying the rapid time-evolution of dynamic RASspectra during the first monolayer growth. Thus, we show that RAS constitutes a valuable tool for the study of epitaxial growthmechanisms.

Optical design and analysis of atwo-spherical-mirror-based multipass cell

Rong Kong, Tao Sun, peng liu, and Xin Zhou

DOI: 10.1364/AO.381632 Received 28 Oct 2019; Accepted 08 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: A systematic method for the optical design and analysis of a multipass cell (MPC) with twospherical mirrors is presented. By varying the parameters, including the distance between themirrors, the position of the incident point and the incident angles, a variety of dense patternscan be formed on the mirrors. Three MPCs with exemplary patterns of seven circles, rotatingtriangles and petals are designed and built. We compare and analyse the parameters of eachMPC with respect to the volume, the optical path length (OPL), the number of reflections and thestability of the final spot position. This new type of MPC exhibits the superior characteristicsof compactness, a long effective OPL and affordable cost, which make it very suitable forapplications to trace gas sensing.

Sodium lidar observation over full diurnalcycles in Beijing, China

Yuan Xia, Xuewu Cheng, Faquan Li, Yong Yang, Xin Lin, Jing Jiao, Lifang Du, Jihong Wang, and Guotao Yang

DOI: 10.1364/AO.382077 Received 04 Nov 2019; Accepted 08 Jan 2020; Posted 10 Jan 2020  View: PDF

Abstract: Due to the severe interference from strong solar background light on the receivedsignal, daytime ground-based lidar observation of sodium (Na) layer is challenging. In thispaper, a Na lidar permitting full-diurnal-cycle observations of metal Na layer over Beijing,China (40.5°N, 116°E) was reported. In order to suppress the skylight background duringdaytime effectively with less signal losses, a dual-channel Faraday filtering unit wasimplemented in the lidar receiver. Based on the diurnal Na lidar system, a good number ofcontinuous observational results lasted even more than 120 hours with good signal-to-noiseratio were obtained, demonstrating its reliability. Considerable variations within the Na layerduring the day were revealed, especially on the layer top and bottom side. In particular, strongsporadic Na layers (Nas) events occurred during the daytime of three consecutive days werealso captured with complete evolution process. These observational results showed theadvantages of the diurnal Na lidar for investigating the metal layer photochemistry anddynamics in the mesosphere and lower thermosphere region. Na layer observations over thewhole diurnal cycle not only benefit to the improvements of current theoretical models, butalso can allow for a specialized analysis of Nas occurred in the daytime and provide valuableobservational supports for investigating the rapid production and disappearance mechanismsof Na atoms.

Static Resource Allocation of Advanced ReservationRequests in Elastic Optical Networks

yi Zhao, Qi Zhang, Xiangjun Xin, Yiqiang Li, Ran Gao, Ying Tao, Qinghua Tian, Feng Tian, Dong Chen, and Guixing Cao

DOI: 10.1364/AO.385140 Received 05 Dec 2019; Accepted 07 Jan 2020; Posted 08 Jan 2020  View: PDF

Abstract: Considering the flexibility characteristic of advanced reservation (AR) requests, the problem of static routing,modulation, spectrum, and time assignment (RMSTA) of AR requests in Elastic Optical Networks (EONs) is studiedin this paper, in order to deploy the spectrum resource economically and enable more requests to be served. Themulti-objective Integer Linear Program (ILP) model, which can minimize the maximum utilized frequency and timeslot indexes as well as find a tradeoff between them, is used to formulate the RMSTA problem. Then the proportionoptimal RMSTA (PO-RMSTA) heuristic algorithm with three sorting strategies is proposed to get the sub-optimalsolutions. The PO-RMSTA algorithm and sorting strategies, AET (ascending order of elastic time), DDV (descendingorder of data volume) and AAS (ascending order of alternative schemes), are simulated in our work and proved toobtain the approximate optimal solutions. The sorting policy AET achieved the best performance when minimizingthe maximum utilized frequency slot index, whereas the sorting policy DDV worked best when minimizing themaximum utilized time slot index. As for the compromise between two indexes, both AET and AAS providedsatisfying results.

CO2 Laser Thermal Reflow Shaped Convex GlassMicrolens Array after Bessel Picosecond LaserInscribing and Hydrofluoric Acid Processing

Shuyu Yang, Kuang Peng, Xin Cao, Wenfeng Wang, Chen Yong, Yuebin Li, Jiang Zhao, and Bo Li

DOI: 10.1364/AO.383189 Received 27 Nov 2019; Accepted 07 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: In this paper, a convex micro glass lens array fabrication processing which utilizes CO2 laser thermal reflow theBessel picosecond laser inscribing and hydrofluoric acid processed micro glass pillars array is presented. TheBessel picosecond laser permits high tolerance and precise micro pillars fabrication. In the thermal reshapeprocess, the CO2 laser power, relative defocus length and scanning velocity are three crucial parameters to themicro lens array’s focal length. By using this method, micro lens arrays with focal length ranging from several tensmicrometers to several hundred micrometers can be created. This research provide another way for fabricatingconvex micro glass lens array with several hundred micrometers focal length in good utility.

High-sensitivity Temperature Sensor based onFano Resonance in an Optofluidic MicrocapillaryResonator

Xiangshang Zhu, Zhijian Zhan, Jun Li, Mi Li, and Yuejiang Song

DOI: 10.1364/AO.383288 Received 15 Nov 2019; Accepted 07 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: In this paper we demonstrate a high-sensitivity temperature sensor based on highorder Fano resonance in an optofluidic microcapillary resonator. High order FR modes(Q~3000) are excited in an ethanol-filled optofluidic microcapillary resonator for temperaturesensing. Due to the high thermo-optic coefficient of ethanol and a large energy fraction ofhigh order modes in the core liquid, the sensitivity as high as -0.402 nm/oC and the detectionlimit of 0.026 oC can be achieved. Also the sensitivity and free spectral range of differentorder radial modes are calculated theoretically. The experimental results agree with thetheoretical results well. Through the comparison between the theoretical and experimentalresults, the radial order number of the mode used for temperature sensing is estimated to be10.

Lensometer with autocollimation and square Ronchi grid

Alberto Cordero-Davila, Santiago Cruz-Ponce, and Jorge González-García

DOI: 10.1364/AO.377172 Received 12 Sep 2019; Accepted 07 Jan 2020; Posted 08 Jan 2020  View: PDF

Abstract: A lensometer based on an autocollimation system and a square Ronchi grid was designed, constructed and tested.Refractive powers of monofocal, astigmatic, bifocal and progressive lenses were measured. Focal plane wasidentified when no spots, or minimum number of fringes, are observed on the bironchigram (pattern with a squareRonchi grid). For cylindrical lenses, the spots were transformed in fringes along X and Y-directions from which thecylindrical and spherical powers were obtained. For the progressive lenses, a zero spots circle moved on thebironchigram plane along the umbilic zone while the square Ronchi grid was moved along the optical axis. Thislensometer is compact, cheap and precise. Our measurements and errors were very similar to those obtained witha commercial lensometer.

Evaluation of quadratic phase hologram calculationalgorithms in the Fourier regime

David Fischer and Stefan Sinzinger

DOI: 10.1364/AO.381547 Received 24 Oct 2019; Accepted 06 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: The display of phase-only holograms with a spatial light modulator (SLM) has many applications due toits potential for dynamic three-dimensional projection of arbitrary patterns. We describe an innovativemodification of the quadratic phase method for hologram calculation which uses error diffusion for initializationof an iterative phase retrieval algorithm. We compare the performance of our algorithm to othervariations of hologram calculation approaches that use the quadratic phase method in the Fourier regime.Parameter variation is conducted for finding the differences and limits of the methods. Experiments withan SLM show the validity of the simulations.

Optoelectronic Properties and Anisotropic Stress ofMo: ZnO Thin Films Deposited on Flexible Substratesby Radio Frequency Magnetron Sputtering

Hsi-Chao Chen, GUAN-TING PENG, and Tan-Fu Liu

DOI: 10.1364/AO.383440 Received 15 Nov 2019; Accepted 06 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: This research investigated the optoelectronic properties and anisotropic stress of Mo doped ZnO films, which weredeposited on PET and PC flexible substrates with radio frequency (RF) magnetron sputtering. The opticalproperties, x-ray diffraction (XRD) spectra, Hall Effect measurements, and self-made phase-shift shadow moiréinterferometer readings were utilized to evaluate the performances of the MZO films. Based on the results, thetransmittance and (002) peak size of the XRD spectra decreased when the substrate temperature increased.However, this took place especially when the oxygen flow was on the increase. Also, carrier mobility, carrierconcentration, and anisotropic stresses increased at higher substrate temperatures but this was not the case whenthe oxygen flow increased. The energy gap (Eg) of the MZO films showed a blue shift with an increase in thesubstrate temperatures, but this rather changed to a red shift when the oxygen flow was observed to be on the rise.

Reference wave source based on silicon nitridewaveguide in point diffraction interferometer

Yuankai Chen, Yongying Yang, Chen Wang, Li Yao, and Jian Bai

DOI: 10.1364/AO.383740 Received 25 Nov 2019; Accepted 06 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: Reference wave source (RWS) is the key component of the point diffraction interferometer (PDI), whichdetermines the quality of the reference wave. The silicon nitride waveguide RWS is now proposed to efficientlyovercome the drawbacks of the existing RWSs, aimed at providing a spherical reference wave with high NA andhigh accuracy. The waveguide RWS consists of the straight waveguide, the bend waveguide and the Y branch edgecoupler. The straight waveguide determines the accuracy and the NA of the reference wave, while the latter twodetermine the light transmittance of the RWS. Simulation results show that the PV and the RMS of the deviationfrom an ideal spherical wave are 2.86×10-4λ (λ=532nm) and 4.83×10-5λ respectively, and the maximum lighttransmittance could reach 24%. Experiment results show that the NA of the reference wave reaches up to 0.58, itsspot has a good circular symmetry and its intensity is Gaussian distribution. Although the light transmittance isonly 0.2%, it is expected to be improved with the development of experimental conditions and waveguidefabrication technology.

Evaluation of glint correction approaches for fine-scaleocean color measurements by lightweighthyperspectral imaging spectrometers

Ryan O'Shea, Samuel Laney, and Zhongping Lee

DOI: 10.1364/AO.377059 Received 01 Oct 2019; Accepted 06 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: Low-power, lightweight, off-the-shelf imaging spectrometers, deployed on above-water fixed platformsor on low-altitude aerial drones, have significant potential for enabling fine-scale assessment ofradiometrically-derived water quality properties (WQPs) in oceans, lakes, and reservoirs. In such applicationsit is essential that the measured water-leaving spectral radiances be corrected for surface-reflectedlight, i.e. glint. However, noise and spectral characteristics of these imagers, and environmental sourcesof fine-scale radiometric variability such as capillary waves, complicate the glint correction problem. Despitehaving a low signal-to-noise ratio, a representative lightweight imaging spectrometer provided accurateradiometric estimates of chlorophyll concentration - an informative WQP - from glint-correctedhyperspectral radiances in a fixed-platform application in a coastal ocean region. Optimal glint correctionwas provided by a spectral optimization algorithm, which outperformed both a hardware solution utilizinga polarizer and a subtractive algorithm incorporating the reflectance measured in the near-infrared.In the same coastal region this spectral optimization approach also provided the best glint correction forradiometric estimates of backscatter at 650 nm, a WQP indicative of suspended particle load.

Characterization of physiological states of thesuspended marine microalgae using polarized lightscattering

Yong Wang, Jincai Dai, Ran Liao, Jin Zhou, Fanqiang Meng, Yue Yao, Huirong Chen, Yi Tao, and Hui Ma

DOI: 10.1364/AO.377332 Received 09 Sep 2019; Accepted 06 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: Physiological states of marine microalgal cells can influence the photosynthesis efficiency which affectsapproximately half of global carbon fixation. The detection of the algae physiological profiles is important for themarine ecology and economy. In this paper, we propose a polarized light scattering method to detect sensitivelychanges in the physiological states of the suspended marine microalgal cells. Our experimental setup is designedto measure the scattered polarization parameters of the cells suspended in the seawater individually. Twospecies of microalgal cells cultured in the laboratory were measured respectively for several days. Experimentalresults showed that both species displays distinctive changes in their polarized photon scattering featurescorresponding to changes in their physiological states. The changes are far more prominent than those displayedin unpolarized light scattering. Microscopy observations, simulations for microspheres of different diametersand refractive indices, or different shapes indicated that the polarization features of the scattered photons aresensitive to the submicron microstructures of the cells. This study demonstrates the potential of the polarizedlight scattering technique to characterize the physiological states of the suspended marine microalgae.

Simultaneous measurement of the DC-Two PhotonAbsorption signal offset and the amplitude of theintensity autocorrelation in the focusing offemtosecond pulses.

Catalina Ramirez-Guerra, Martha Rosete-Aguilar, and Jesus Garduno-Mejia

DOI: 10.1364/AO.377678 Received 13 Sep 2019; Accepted 06 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: In this work, the DC-Two Photon Absorption signal offset (DCTPA) and the amplitude of the autocorrelation (AAC) aremeasured simultaneously around the focal point of an apochromatic microscope objective (AMO) by using the ZscanAutocorrelation technique. The AAC is obtained from the nonlinear sensor response given by the two-photonabsorption(TPA), generated in a GaAsP photodiode, for femtosecond laser pulses. We verify that the change in theDCTPA signal along-z is coincident with the amplitude of the intensity autocorrelation and that the highestamplitude of the AC is reached at the same position as the highest amplitude of the DCTPA signal. The DCTPA signal istypically used as a reference for the alignment in a collinear intensity autocorrelator and we show that can be alsoused as a practical procedure to estimate the depth of focus. The DCTPA signal measurement allows locating theoptimum spatial-temporal coupling given by the highest amplitude of the intensity autocorrelation. Additionally,we find a variation in the pulse duration within the same region due to the radially varying GDD.

Independently Tunable Fano Resonances in a MIMCoupled Cavities System

Yonghao Chen, Li Chen, Kunhua Wen, Yihua Hu, and Weitao Lin

DOI: 10.1364/AO.381381 Received 24 Oct 2019; Accepted 05 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: Herein, multiple Fano resonances with excellent ability to be tuned independently are produced in a subwavelengthmetal-insulator-metal (MIM) system. The input and output waveguides are separated by a metal gap,and a stub and an end-coupled cavity are placed below and right side of the input waveguide as discrete states,respectively. Owing to the mode interferences, double ultra-sharp and asymmetric Fano resonant peaks areobserved in the transmission spectrum. Successfully, the basic structure is extended by two extra rectangularcavities, giving rise to four Fano resonances with high refractive index sensitivity and figure of merit. Due to thediscrete modes of Fano resonances from different coupling cavities, the resonant wavelength of them can becontrolled independently, which can provide greatly flexibility for tuning Fano resonances. The performances ofthe proposed structure are investigated by both the finite-difference time-domain (FDTD) method and themultimode interference coupled-mode theory (MICMT). It is believed that the research can provide importantguidance to design Fano resonance structures, and the proposed structure has a wide application in the sensors,switches and nano-photonic integrated circuits devices.

Simultaneous measurement of temperatureand refractive index based on hybrid SPR-MMIfiber sensor

Yu Zhang, Meijiang Liu, Yaxun Zhang, Zhihai Liu, Xinghua Yang, Jianzhong Zhang, Jun Yang, and Libo Yuan

DOI: 10.1364/AO.382787 Received 11 Nov 2019; Accepted 05 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: We propose and demonstrate a hybrid fiber-based sensor combining a multimodeinterference (MMI) structure and a surface plasmon resonance (SPR) structure forsimultaneous measurement of temperature and refractive index (RI) of a liquid sample. Weconfigure the MMI structure by connecting a single-mode fiber, a no-core fiber, and a singlemodefiber sequentially. We set up the SPR structure by coating a gold film with a thicknessof 50nm on the surface of the no-core fiber. We measure the sensitivity of RI and thetemperature of the MMI and SPR structure, respectively. Then we obtain the coefficientmatrix to simultaneously measure the temperature and RI of a liquid sample and obtain thehighest refractive index sensitivity of 2061.6 nm/RIU and temperature sensitivity of 37.9pm/. We verify the feasibility of the sensor in the liquid alcohol. The testing results indicatethat the proposed sensor and testing method are feasible, accurate, and convenient.

Compact cross-slot waveguide polarization beamsplitter using a sandwich-type coupler

Shengbao Wu, Zhicheng Zhao, Ting Feng, and Steve Yao

DOI: 10.1364/AO.383073 Received 13 Nov 2019; Accepted 05 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: A compact silicon-based cross-slot waveguide (CSW) polarization beam splitter (PBS) utilizing a sandwich-typecoupler is proposed. The coupler consists of two identical CSWs separated by a horizontal slot waveguide (HSW)and S-bend sections at each end of the coupling section are introduced to decouple and separate the light. Owing tostrong birefringence of the HSW, the coupling length ratio between two different polarized modes (TE and TM) isadjustable in a suitable range by simply changing the gap width between the CSW and the HSW. The coupling lengthof TM mode can be easily made half of that of TE mode with a proper gap width, which is a favorable condition torealize a short PBS. Then, by making the length of the coupling section equal to the coupling length of TE mode, TEand TM modes are split. The PBS has the advantages of small footprint and high polarization extinction ratio (PER)and may be useful in the polarization management of a CSW based on-chip optical system. Numerical resultsindicate that a PBS of ~12μm with an insertion loss (IL) of 0.17 (0.53) dB, PER of 24.12 (21.14) dB for TM (TE) modeis achieved at the wavelength of 1.55 μm, with a bandwidth covering the full C-band for keeping PER > 15dB and IL< 1dB.

High-speed 3D digital image correlation for rollingdeformation of tire sidewall and measuring dynamiccontact patch length

Xueliang Gao, Ye Zhuang, Shu Liu, Weiguang Fan, Chengwei Zhu, and Qiang Chen

DOI: 10.1364/AO.377604 Received 13 Sep 2019; Accepted 04 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: The tires played a vital important in the vehicle's operational stability, comfort and security. However, due to theinfluence of test equipment and tire operating conditions, the perception of rolling tire characteristics was still inthe stage of gradual improvement, especially the analysis of sidewall rolling deformation and dynamic contactpeculiarities of tires/road interactions, which restricted the analysis of rolling energy dissipation and the accurateobservation of tire forces. In this paper, the high-speed stereo vision system was created by the high-speedcameras, and the relative rigid and flexible displacement, strain, trajectory trend of marker points and real-timeglobal displacement field of sidewall during tire rolling cycle were acquired utilizing the improved DIC algorithmunder the different rolling velocities. Meanwhile, the periodic dissipation of strain concentration region wasobserved and the phenomenon of strain resonance appeared at the overlap of periods. The relative flexibilitystrain and shear strain of the marker point were obtained on the plane element, which resulted in the relativeflexibility major principle strain of the plane element, and then it achieved that the leading and trailing points ofthe contact patch were recognized and the dynamic contact patch length was accurately measured.

Design of a compact hyperspectral imagingspectrometer with freeform surface based onanastigmatism

JiaLun Zhang, Chao Lin, Zhenhua Ji, Hao Wu, Chengliang Li, Bowen Du, and yuquan zheng

DOI: 10.1364/AO.379390 Received 11 Oct 2019; Accepted 03 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: Hyperspectral imaging spectrometers with a wide field of view(FoV) have significant application values. However,enhancing the FoV will increase the volume of the imaging spectrometer and reduce the imaging quality, so a wideFoV spectrometer system is difficult to design. Based on the theory of off-axis astigmatism, we present a methodwhich including "prism box", "partial anastigmatism" and partial differential equation to solve the parameters of afreeform surface. In this method, a compact wide FoV imaging spectrometer with freeform surface is designed. Thespectrometer is an Offner structure with two curved prisms as the dispersion elements. The primary mirror andtertiary mirror of the Offner spectrometer are an aspheric surface and a freeform surface respectively to correctthe off-axis aberration of wide FoV. The ratio of the slit length to the total length of the spectrometer is close to 0.4.Compared to conventional spectrometers of the same specifications, the total length of the spectrometer is reducedby 40% and the volume by 70%. The compact imaging spectrometer has potential application in the field of spaceremote sensing. Besides, the design method of the spectrometer provides a reference for the design of other opticalsystems with freeform surfaces.

The thermal accumulation effect of three-junctionGaAs cell with multi-pulse laser irradiation

Lei Qi, Yali Liu, Hao Jing, Yanxin Xie, Xiaomin Yang, and Rongzhu Zhang

DOI: 10.1364/AO.383948 Received 22 Nov 2019; Accepted 03 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: A theoretical model on accumulation of temperature and stress in three-junction GaAs solarcell is proposed to analyze its damage characteristics while irradiated by multi-pulse laser. Thedistribution and accumulation effect of temperature and stress with different pulse width are calculated.Specifically, the influences of pulse energy and duty ratio on the accumulation effect are discussed.Results show that the accumulation is weakened as pulse energy and duty ratio decreases and differentwith the different pulse width. The accumulation rate of stress is more rapidly than which oftemperature under nanosecond laser irradiation. Furthermore, tensile stress damage is the main damageform under nanosecond laser irradiation and melting damage will change to the main damage form formillisecond laser.

Single shot 3D profilometry by polarizationpattern projection

Yuuki Maeda, Shuhei Shibata, Nathan Hagen, and Yukitoshi Otani

DOI: 10.1364/AO.382690 Received 11 Nov 2019; Accepted 03 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: We demonstrate a uniaxial 3D profilometry system illuminating the sample with alinear polarization pattern and measuring a polarization camera. The linear polarizationpattern is generated by a spatial light modulator and a quarter wave plate in the opticalsystem. The system can measure 4 different fringe patterns with a phase difference of 90degrees simultaneously in the polarization camera. Therefore, we can measure threedimensionalshapes in a single shot. We present the measurement principles of the system andshow the results of a real-time 3D profilometry experiment.

Fast multiple-scattering holographic tomography based on wave propagation method

Damian Suski, Julianna Winnik, and Tomasz Kozacki

DOI: 10.1364/AO.378907 Received 02 Oct 2019; Accepted 02 Jan 2020; Posted 03 Jan 2020  View: PDF

Abstract: We develop a time-efficient computation scheme for holographic tomography reconstruction technique that accounts for multiple scattering by applying the forward model based on wave propagation method (WPM). The computational efficiency is achieved by employing adjoint equations for calculation of the gradient of the data fidelity term in the gradient descent optimization procedure. In the paper we provide a general computation scheme that is suitable for various forward models that can be represented in the form of the iterative equation. Next, we provide the complete solution for the time-efficient reconstruction utilizing WPM. In the considered reconstruction case, the proposed algorithm enables the 114-fold speed-up of computations with respect to the original tomographic method.

An algorithm based on the optimal block zonal strategy for fast wavefront reconstruction

Zhongye Ji, Xiaofang Zhang, Zhili Zheng, Yan Li, and Jun Chang

DOI: 10.1364/AO.380999 Received 25 Oct 2019; Accepted 02 Jan 2020; Posted 03 Jan 2020  View: PDF

Abstract: Fast wavefront reconstruction is crucial for improving the temporal frequency of adaptive optics (AO) systems which a mass of subapertures are used. In this paper we present a novel block zonal reconstruction algorithm based on Southwell geometry to speed up the wavefront reconstruction from Shack-Hartmann wavefront sensor measurements. Therein, we use the theory of computational complexity to install a novel optimal block zonal strategy to get the best size of the subwavefront and give the verification through simulations. Compared with the classical Southwell entire wavefront reconstruction algorithm, the algorithm based on the optimal block zonal strategy needs only a few of milliseconds to finish the reconstruction process from100×100 subapertures. Moreover, we analyze the superiority to use our algorithm to realize the phase reconstruction of the unconnected subwavefront which cannot be reconstructed by using the existing traditional methods. And the simulation and experiments show that the precision of the algorithm based on the proposed optimal block zonal strategy is comparable to HASO, the time consumption is much less than that of the traditional zonal reconstruction, and it is applicable to the square wavefront, circular wavefront and local unconnected wavefront. Our proposed algorithm can be widely utilized in astronomical observation, laser transmission and remote sensing.

Ultracompact and ultrabroadband mode division multiplexer based on Au nanocubes array assisted directional coupler

Kolsoom Mehrabi and Abbas Zarifkar

DOI: 10.1364/AO.383518 Received 15 Nov 2019; Accepted 02 Jan 2020; Posted 03 Jan 2020  View: PDF

Abstract: An ultracompact and ultrabroadband two-mode (de)multiplexer based on asymmetric directional coupler for mode division multiplexing is proposed. The device structure consists of a pair of silicon waveguides with an array of plasmonic Au nanocubes sandwiched in the coupling region. The coupling region length of the directional coupler is decreased to 1 μm for coupling of the fundamental transverse magnetic (TM) mode to the first order one by excitation of the surface plasmon polaritons (SPPs). This is the shortest length reported for multiplexing of the TM modes until now, to the best of our knowledge. The proposed mode (de)multiplexer has a low loss of 0.72 dB and low crosstalk of -28.3 dB at the communication wavelength of 1.55 μm. Also, the three-dimensional finite-difference time-domain (3D-FDTD) simulation results show that a broad bandwidth of 190 nm is realized with the crosstalk less than -10 dB and the insertion loss lower than 1.29 dB. Furthermore, impact of the fabrication tolerances on the performance of the proposed (de)multiplexer is studied in detail.

Unified Analysis of Coherence Property of Stokes Wave Generated Via Stimulated Raman Process in Optical Fiber

Partha Mondal and Shailendra Varshney

DOI: 10.1364/AO.383849 Received 22 Nov 2019; Accepted 02 Jan 2020; Posted 03 Jan 2020  View: PDF

Abstract: We investigate the evolution of coherence property of noise-seeded Stokes wave in short (< 1 ps) and long pulse (> 1 ps) regimes numerically through set of coupled nonlinear equations. The simulations include quantum noise by incorporating noise seed in the pump field. The spectral phase fluctuations of the Stokes wave for both the regimes are characterized and the degrees of first-order mutual spectral coherence is calculated for different conditions. Statistical analysis yield the effect of spectral coherence of the Stokes wave in optical fiber on pump power, fiber length and pump pulse width for short and long pulse regimes. It is observed that noise-seeded stimulated Raman process causes degradation of spectral coherence with the increase in pump power, fiber length and pulse width of the pump wave. The degradation of the spectral coherence is manifested by the transition of the Stokes wave from quasi-coherent to incoherent spectrum.

General formula to design a freeform singlet free of spherical aberration and astigmatism: Comment

Juan Valencia-Estrada and Jorge Garcia-Marquez

DOI: 10.1364/AO.379238 Received 04 Nov 2019; Accepted 01 Jan 2020; Posted 03 Jan 2020  View: PDF

Abstract: In their article [Appl. Opt. 58, 1010–1015 (2019)] González-Acuña et al claimed: “an analytical closed-form formula for the design of freeform lenses free of spherical aberration and astigmatism.” However, as we show here, their formula can only be applied when the object and image are both real and the image is inversed. Here, we present the complete solution of this particular family of lenses.

Numerically stable formulation of Mie theory for an emitter close to a sphere

matt majic and Eric Le Ru

DOI: 10.1364/AO.379694 Received 04 Oct 2019; Accepted 01 Jan 2020; Posted 03 Jan 2020  View: PDF

Abstract: Numerical implementations of Mie theory make extensive use of spherical Bessel functions.These functions are however known to overflow/underflow (grow too large/small for floating point precision) for orders much larger than the argument. This is not a problem in applications such as plane wave excitation, as the Mie series converge before these numerical problems arise.However, for an emitter close to the surface of a sphere, the scattered field in the vicinity of the sphere is expressed as slowly converging series, with multipoles up to order 1000 required in some cases. These series may be used to calculate experimentally-relevant quantities such as the decay rate of an emitter near a sphere. In these cases, overflow/underflow prevents any calculation in double precision using Mie theory and alternatives are either computationally intensive (for example arbitrary precision calculations) or not accurate enough (for example the electrostatics approximation).We here present a formulation of Mie theory that overcomes these limitations.Using normalized Bessel functions where the large growth/decay is extracted as a prefactor, we re-express the Mie coefficients for scattering by spheres in a normalized form. These normalized expressions are used to accurately compute the series for the electric field and decay rate of a dipole emitter near a spherical surface, in cases where the Mie coefficients would normally overflow before any degree of accuracy can be obtained.

Fiber SPR refractive index sensor with the variable core refractive index

Chunlan Liu, Xin Zhang, Yachen Gao, Yong Wei, Ping Wu, Yudong Su, and Peng Wu

DOI: 10.1364/AO.380665 Received 22 Oct 2019; Accepted 01 Jan 2020; Posted 03 Jan 2020  View: PDF

Abstract: In this paper, a refractive index sensor based on the control of the fiber core refractive index is proposed and described. By employing ultraviolet curable adhesive with various refractive index and hollow capillary fiber, the special fiber with a controllable core refractive index is fabricated, and by this special fiber, we fabricate a novel refractive index SPR sensor. Functional testing of the sensing probes with the core refractive index of 1.590,1.516 and 1.454 are performed respectively, and their average sensitivity are 1580nm/RIU, 2220nm/RIU and 3467nm/RIU respectively. Furthermore, in order to explore the detection effect of the sensing probe with the higher core refractive index, we establish the mathematic model for theoretical calculation. According to the testing and simulating results, with the increase of the core refractive index, the refractive index of the testing solution increases, and the refractive index detecting range shifts to the high refractive index, but the average sensitivity decreases to some degree. This study provides a new method for the detection of high refractive index solutions and a new idea for the fabrication of wavelength division multiplexing distributed SPR sensors.

Entanglement of near-surface optical turbulence to atmospheric boundary layer dynamics and particulate concentration: implications on optical wireless communication systems

Anand Sarma, SUNILKUMAR K, S Satheesh, and Krishnamoorthy K

DOI: 10.1364/AO.381737 Received 29 Oct 2019; Accepted 01 Jan 2020; Posted 03 Jan 2020  View: PDF

Abstract: Localized reduction in optical turbulence due to enhanced atmospheric heating caused by the solar absorption of aerosol Black Carbon is reported. Immediate response of atmospheric turbulence to Black Carbon-induced atmospheric warming strongly depends on the available solar radiation (time of the day), Black Carbon concentration and atmospheric boundary layer dynamics. Besides the significant climate implications of reduction in turbulence kinetic energy, large reduction in the refractive index structure parameter (Cn2) resulting from Black Carbon-induced warming would impact the atmospheric propagation of laser beams. Interestingly, aerosols contribute significantly (up to 25%) to the signal deterioration in optical wireless communication systems during convectively stable atmospheric conditions; when higher signal-to-noise ratios are expected otherwise due to the reduced thermal convection. Competing effects of the fractional contributions of aerosol extinction and scintillations on beam attenuation are reported; daytime being largely dominated by scintillation effects while the nighttime being dependent on the ambient aerosol concentration as well. We put forward the entanglement of optical turbulence to aerosol concentration, atmospheric boundary layer dynamics and surface-reaching solar radiation and discuss the possible implications on optical propagation.

A squared peak-to-peak algorithm for the spectral interrogation of short-cavity fiber-optic Fabry-Perot sensors

Haibin Chen, Yao Zhang, Yang Li, Xin Jing, Suzhe Yuan, Xiongxing Zhang, Wei Wang, Rong Liu, and Quanmin Guo

DOI: 10.1364/AO.382169 Received 04 Nov 2019; Accepted 31 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: The cavity length of short-cavity Fabry-Perot (FP) sensors cannot be effectively interrogated using the conventional peak-to-peak method if the spectrum of the exciting source is not wide enough. In this paper, we propose a squared peak-to-peak algorithm for interrogation of short cavity fiber-optic FP sensors. By squaring the DC-filtered reflection spectrum of an FP sensor in the frequency domain, we produce an additional peak, with which the cavity length of a sensor can be estimated using the same calculations as performed with the conventional peak-to-peak method. For investigation of the feasibility of this technique, we conducted simulations and practical experiments analyzing fiber-optic FP sensors with cavity lengths in the range of 15–25 μm. The maximum error in cavity length estimated using the proposed algorithm in experiments was 0.030 μm.

Wavelength-switchable all-fiber laser emitting radially polarized beams

Fan Shi, Han Yao, Yiping Huang, Wenlin Li, Teng Wang, and Xianglong Zeng

DOI: 10.1364/AO.382496 Received 12 Nov 2019; Accepted 31 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: We present two kinds of mode-selective coupler (MSC) based ultrafast radially polarized lasers delivering switchable wavelength and pulsewidth. One is a linear-cavity fiber laser mode-locked in the 1.5 μm region, which produces not only wavelength-agile radially polarized pulses in the spatial domain, but also switch-able femtosecond and picosecond pulses in the tem-poral domain. The other one is a nonlinear polarization rotation (NPR) technique assisted passively mode-locked fiber ring laser in the 1.0 μm region, presenting an ideal broadband spectral switching with picosecond radially polarized pulses output. The presented fiberlasers would contribute a kind of compact laser sourcewhich enables a flexible option of radially polarized beam in spectral and temporal domains

Variability of relationship between the volume scattering function at 180° and the backscattering coefficient for aquatic particles

Lianbo Hu, Xiaodong Zhang, Yuanheng Xiong, Deric Gray, and Ming-Xia He

DOI: 10.1364/AO.383229 Received 20 Nov 2019; Accepted 31 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: As an active remote sensing technique, lidar (light detection and ranging) has distinct advantages over passive ocean color technique, particularly in its ability of profiling water constituents. Properly interpreting lidar signal for characterizing particle distribution relies on a key parameter, χp(π) that relates the particulate volume scattering function (VSF) at 180° (βp(π)) that a lidar measures to the particulate backscattering coefficient (bbp). However, χp(π) has been seldom studied due to the challenges in accurately measuring βp(π) and bbp concurrently in the field. In this study, χp(π), as well as its spectral dependence, was re-examined using the VSFs measured with high angular resolution in a wide range of waters and a physically sound, well validated VSF-inversion method. The effect of particle shape on estimating βp(π) from measured VSFs was tested using two inversion kernel libraries, one assuming particles of spherical shape and other of hexahedral shape. The reconstructed VSFs using either of the two kernels agreed well with the measured VSFs, both having a mean percentage difference <5%. χp(π) derived using both libraries showed a spatial variation up to a factor of 5 could be induced if a fixed value of χp(π) was to be used to interpret lidar signal collected in different waters. χp(π) showed little dependence on wavelengths. χp(π) derived with the sphere library was smaller than those derived with the hexahedral library but consistent with χp(π) estimated directly from high-spectral-resolution lidar. The possible explanation was that the sphere kernel was able to capture the backscattering enhancement feature of marine particles near 180°, even though the hexahedral kernel seemed to better reproduce the overall scattering.

Study on optical characteristics of oil spill based on polarization scattering rate

jiang xu, xiao wang, and weixian qian

DOI: 10.1364/AO.383418 Received 18 Nov 2019; Accepted 31 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: As a new analytical method for identifying marine oil slicks, the primary function of the polarization scattering model is to determine the intensity of polarized scattered light from different oil spill zones. In the polarized light path, the energy reduction is mainly due to the scattering characteristics of the surface of the sample to be tested. To quantify equivalence, we define the polarized scattering rate (PSR). The PSR describes the probability that linearly polarized incident particles scatter to the unit solid angle in the direction of the target surface. In order to verify the applicability of the model, we applied it to detect the actual oil spill at sea in the case of simulated sunlight finally. The research indicates that PSR only characterizes the amplitude conversion between the polarized scattering wave and the incident wave, and is not affected by the polarization characteristics of the incident wave, thus reflecting the true polarization characteristics of the target itself. The PSR of crude oil and seawater depends not only on the physical properties of the target itself, but also on the observation conditions such as relative attitude orientation, spatial geometric position relationship, and working frequency of equipment and instruments.

Reconfigurable multiwavelength fiber laser based on multimode interference in highly germania-doped fiber

Tianye Huang, dazhong Zhang, Seongwoo Yoo, Qian Wei, Raghuraman Sidharthan, Zhichao Wu, Bin Yan, Chaolong Song, and Zhuo Cheng

DOI: 10.1364/AO.383627 Received 21 Nov 2019; Accepted 31 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: A reconfigurable multiwavelength erbium-doped fiber laser based on all-fiber multimode interferometer (MMI) is proposed and experimentally demonstrated. The interferometer is constructed by sandwiching a section of highly germania-doped fiber (HGDF) between two sections of single mode fiber (SMF). The insertion loss of the interferometer is as low as 2 dB. Due to the polarization-dependent spectral filtering effect formed by MMI, by rotating the intracavity polarization controller (PC), the laser output can be switched among single, dual, and triple wavelengths lasing states with optical single-to-noise ratio (OSNR) up to 50 dB. In Particular, the obtained dual-wavelength state shows high stability with wavelength shift within ±0.04 nm, wavelength spacing variation within ±0.03 nm and power fluctuation within ±0.04 dB by monitoring the output spectra over 8 hours at room temperature. By changing the length of HGDF, the wavelength spacing can also be flexibly manipulated. Taking the advantages of reconfiguration, low cost, and easy fabrication, this fiber laser may have great potential in various optical applications.

Pore size assessment by GASMAS and gas adsorption

Ahmed Alsaudi, Abdulaziz Aljalal, Watheq Al-Basheer, Khaled Gasmi, and Samer Qari

DOI: 10.1364/AO.381730 Received 30 Oct 2019; Accepted 30 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: The line broadening of oxygen gas confined in porous alumina powders is correlated to the average pore size. The broadening of the P9P9 oxygen line at 763.84 nm is measured using gas in scattering media absorption spectroscopy (GASMAS), while the average pore size is determined using the gas adsorption technique. The average pore size of the samples lies within the range 10–40 nm. In this range, the contribution of the wall collision broadening is found to be approximately inversely related to the average pore diameter. Furthermore, the confined oxygen gas absorbance measured by GASMAS is found to be linearly correlated to the effective porosity evaluated by the saturation method.

Efficient depth recovering method free from projector errors by use of pixel cross-ratio invariance in fringe projection profilometry

Huijie Zhu, Shuo Xing, and Hongwei Guo

DOI: 10.1364/AO.383204 Received 13 Nov 2019; Accepted 30 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: In fringe projection profilometry, errors related to the projector are not easy to compensate for, because the projector is much more difficult to calibrate than a camera. Immune to the projector errors, the depth recovering method based on pixel cross-ratio invariance enables circumventing this issue by calculating depth of a point from the shift of its camera pixel, instead of from its fringe phase. With it, however, one has to search three reference phase maps along epipolar lines for the pixels having the same phases. It increases the measurement time significantly. To improve the measurement efficiency, this paper derives, from the pixel cross-ratio invariance, a generic function representing the relationship between the depths and the corresponding pixel shifts, and suggests a calibration method for determining its coefficient matrices. Using this function allows us to recover the object depths just by searching a single reference phase map, thus reducing the time duration for data processing to about one third. Besides, different from the previous method that depends on exactly three reference phase maps, the method proposed in this paper uses a relation function whose coefficients can be determined from more reference phase maps in the least squares sense, thus being helpful for improving measurement accuracy. Experimental results demonstrate this method to be effective in improving both the measurement accuracy and efficiency.

5 x 25 Gbit/s WDM Transmitters Based on Passivated Graphene-Silicon Electro-Absorption Modulators

Chiara Alessandri, Inge Asselberghs, Steven Brems, Cedric Huyghebaert, Joris Van Campenhout, Dries Van Thourhout, and Marianna Pantouvaki

DOI: 10.1364/AO.383462 Received 18 Nov 2019; Accepted 30 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: Today, one of the key challenges for the exploitation of graphene devices is establishing fabrication processes that can ensure performance stability, repeatability and that can eventually enable production in high volumes. In this paper we use up-scalable fabrication processes to demonstrate three 5-channel WDM transmitters, each based on 5 graphene-silicon electro-absorption modulators. A passivation-first approach is used to encapsulate graphene, which results in hysteresis-free and uniform performance across the 5 channels of each WDM transmitter, for a total of 15 modulators. Open eye diagrams are obtained at 25 Gb/s using 2.5 Vpp, thus demonstrating potential for multi-channel data transmission at 5 x 25 Gb/s on each of the three WDM transmitters.

Frequency Tracking Out-of-Lock Control in Resonator Fiber Optic Gyro

Ming Lei, huaiyong Yu, Yuan Fang, Zhe Feng, Yanji Wu, and Lizhe Zhang

DOI: 10.1364/AO.375755 Received 19 Aug 2019; Accepted 30 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: Resonator fiber optic gyro (RFOG) is being pursued because of its theoretical potential to meet navigation grade performance in a small size, high precision, and lower cost. The stability RFOG operation is based on the synchronization of laser frequency to the fiber ring resonator (FRR) resonance frequency. Frequency tracking out-of-lock, will lead to peak pulse and zero bias change at output of the RFOG, which seriously degrade the performance. Firstly, the influence mechanism of frequency tracking out-of-lock is analyzed, the change of current and temperature in frequency tracking, and the symmetry change caused by backscatter, polarization are the main reasons of the peak pulse and zero bias error. Secondly, a scheme of out-of-lock control of RFOG based on temperature closed-loop operation using digital signal processing is proposed. The improved scheme, signal processing and implementation method are investigated in detail. Finally, a RFOG prototype is assembled and tested, 10 min tracking of the laser frequency to FRR’s single resonance frequency is realized by temperature closed-loop operation. The static performance of RFOG over 1h show that the RFOG output errors caused by frequency tracking out-of-lock are successfully eliminated. The output peak pulse is reduce from 3000 deg/h to 200 deg/h, the zero bias is eliminated from 50~600 deg/h to 0, the bias stability of RFOG is improved from 15.2 deg/h to 1.85 deg/h, which indicates a remarkable advance performance of RFOG to satisfy the civil navigation application requirements.

Temperature Dependence of Collisional Broadeningand Shift for the Kr 4p⁶S₀¹ -» 5p[3/2]₂ Electronic Transition

Abinash Sahoo, Dominic Zelenak, and Venkateswaran Narayanaswamy

DOI: 10.1364/AO.380102 Received 15 Oct 2019; Accepted 29 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: Temperature scaling of collisional broadening parameters for krypton (absorber) 4p⁶S₀¹ -» 5p[3/2]₂ electronic transition centered at 107.3 nm in the presence of major combustion species (perturber) is investigated. The absorption spectrum in the vicinity of the transition is obtained from the fluorescence due to the two-photon excitation scan of krypton. Krypton was added in small amounts to major combustion species such as CH₄, CO₂, N₂, and Air which then heated to elevated temperatures when flowed through aset of heated coils. In a separate experimental campaign, laminar premixed flat flame product mixtures of methane combustion were employed to extend the investigations to higher temperature ranges relevant tocombustion. Collisional full width half maximum (wC) and shift (δC) were computed from the absorption spectrum by synthetically fitting Voigt profiles to the excitation scans and their corresponding temperature scaling was determined by fitting power-law temperature dependencies to the wC and δC data for each perturber species. The temperature exponents of wC and δC for all considered combustion species (perturbers) were −0.73 and −0.6 respectively. Whereas the temperature exponents of wC are closer to the value (−0.7) predicted by the dispersive interaction collision theory, the corresponding exponents of δC are in between the dispersive interaction theory and the kinetic theory of hard-sphere collisions. Comparison with existing literature on broadening parameters of NO, OH, and CO LIF spectra reveal interesting contributions from non-dispersive interactions on the temperature exponent.

Characterization of silicon oxynitride filmsdeposited by HIPIMS deposition technique

Bohuei Liao, Chien-Nan Hsiao, Ming-Hua Shiao, and Sheng-Hui Chen

DOI: 10.1364/AO.377983 Received 23 Sep 2019; Accepted 29 Dec 2019; Posted 06 Jan 2020  View: PDF

Abstract: In this research, silicon oxynitride films were prepared by high-power impulse magnetron sputtering with 45/955pulse on/off time. The extinction coefficient was smaller than 1×10-3 from 250 nm to 700 nm after introducing 2.2 sccm O2gas at room temperature. A 3-layer of AlF3/ SiOxNy AR coating was designed and fabricated on double side quartz and ahigh transmittance of 99.2% was attained at 248 nm. The silicon oxynitride films deposited at 350 °C had the bettermechanical and optical properties in the VIS range. The hardness of all deposited films was larger than 19 GPa and thelargest hardness could reach to 29.8 GPa. A film structure of 6-layer transparent hard coating/glass/4-layer AR coating wasdesigned and deposited. Its average transmittance was 96.0 % in the visible range while its hardness was 21 GPa andsurface roughness of 0. nm.

Tellurium nanorods based saturable absorber for ultrafast passive mode-locked erbium-doped fiber laser

Hiba Hassan, MOHAMMED A. MUNSHID, and Abdulhadi Al-Janabi

DOI: 10.1364/AO.383566 Received 20 Nov 2019; Accepted 29 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: In this paper, we propose and demonstrate an ultrafast Te nanorods as saturable absorber (SA) for producing mode-locking from erbium-doped fiber laser, for the first time. The Te nanorods were fabricated by a simple green chemical method with conserving energy and without purification process. The morphology and structure measurements confirm a uniform Te nanorods with a constant aspect ratio. The synthesized SA has a saturation intensity and modulation depth of 25.44 MW/cm² and 4%, respectively. By integrating the proposed SA into an erbium-doped all fiber-based ring cavity, the mode-locked was readily generated. The conventional soliton pulses of ∼3.56 ps pulse width was obtained at 1566.7 nm central wavelength and a pulse repetition rate of 1.87 MHz. The results show that, the moderate saturable absorption characteristics of Te nanorods have superior performance in ultrafast optics field which is eligible in many applications, such as optical communications.

Towards high-optical-strength, fluorine-resistant coatings for intracavity KrF laser optics

Vladimir Zvorykin, Radmir Gaynutdinov, Michail Isaev, Dmitrii Stavrovskii, and Nikolai Ustinovskii

DOI: 10.1364/AO.378061 Received 18 Sep 2019; Accepted 28 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: Intracavity optics of e-beam-pumped high-energy KrF lasers should survive in a hostile environment of simultaneous irradiation by intensive UV laser light, scattered electrons, bremsstrahlung X-ray radiation, and permanent etching by fluorine containing working gas mixture. The present paper is focused on the research and development of high-optical-strength and fluorine-resistant AR coatings capable to protect fused silica windows and HR rare mirrors of these lasers against fluorine attack. The most promising coatings deposited by e-beam evaporation were NdF3-based compositions, which have low water content, well resistance to fluorine and demonstrated the highest damage thresholds for AR coatings 29.8 J/cm2 in a large irradiation spot. Outstanding “water-free” coatings of MgF2 and Al2O3 were deposited by CO2 laser evaporation with preliminary laser treatment of substrates.

A least square fitting demodulation technique for the interrogation of optical fiber Fabry-Perot sensor with arbitrary reflectivity

Wenyi Liu, Qianyu Ren, Ping-gang Jia, yingping hong, Ting Liang, Jia Liu, and Jijun Xiong

DOI: 10.1364/AO.380005 Received 14 Oct 2019; Accepted 28 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: A novel Fabry-Perot (F-P) demodulation technique based on least square fitting for arbitrary reflectivity F-P sensors is proposed. The demodulation method was simulated and analyzed to verify feasibility of the algorithm. Two different finesse F-P interferometers constructed with a reflector bracket was used to make the stability experiments and the stepping experiments. The results show that the demodulation technique can interrogate the cavity length of F-P interferometers with different fineness in a wide range, and the demodulation error is less than 12 nm.

Laser speckle correlation technique application for study of aluminium nanopowder combustion

Fyodor Gubarev, Lin Li, Andrei Mostovshchikov, Alexander Ilyin, and Iulia Sytnik

DOI: 10.1364/AO.380295 Received 14 Oct 2019; Accepted 27 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: This paper presents the results of application of laser speckle correlation (LSC) technique for studying the combustion process of aluminium-based nanopowders. For assessing the results, a combined experimental scheme is proposed with simultaneous application of LSC analyses and another feasible method of nanopowder combustion study – laser monitoring. In the paper, we present the principle of using LSC technique to characterize the surface changes of nanopowder during combustion. Calculating the correlation coefficient of images sequence, is possible to estimate the time parameters of combustion of aluminium nanopowder and aluminium mixture with iron nanopowder. Comparing the results obtained with the LSC method and laser monitoring, we conclude that LSC is quiet acceptable for investigating the combustion process of metal nanopowders. In contrast to laser monitoring, the LSC method allows to determine the preheating period preceding the first combustion wave. In practice, the LSC method application for nanopowder combustion control is perspective because of the simple hardware implementation.

Convolutional-neural-network-based approach to estimate bulk optical properties in diffuse optical tomography

Sohail Sabir, Sanghoon Cho, Yejin Kim, Rizza Pua, Duchang Heo, Kee Hyun Kim, Young-Wook Choi, and Seungryong Cho

DOI: 10.1364/AO.377810 Received 19 Sep 2019; Accepted 26 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: Deep learning has been actively investigated for various applications such as image classification, computer vision, and regression tasks, and it has shown state-of-the-art performance. In diffuse optical tomography (DOT), the accurate estimation of the bulk optical properties of a medium is paramount because it directly affects the overall image quality. In this work, we exploit deep learning to propose a novel convolutional neural network (CNN)-based approach to estimate the bulk optical properties of a highly scattering medium such as biological tissue in DOT. We validated the proposed method by using experimental as well as simulated data. For performance assessment, we compared the results of the proposed method with those of existing approaches. The results demonstrate that the proposed CNN-based approach for bulk optical property estimation outperforms the existing methods in terms of estimation accuracy with lower computation time.

Broadband Astigmatism-free Offner imaging spectrometer with high resolution

Jing Lin, Su Wu, and Lei Yu

DOI: 10.1364/AO.381834 Received 31 Oct 2019; Accepted 26 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: An advanced optical design has been proposed for the astigmatism-free Offner imaging spectrometer with high resolution in broadband. The anastigmatic theory is thoroughly analyzed. Astigmatism is corrected by two pairs of lens-mirror combination. On the basis of the concentric structure, the lens is used to make the meridional image distance equal to sagittal image distance. An example of design has been presented with F number as 3 working in 400-1000 nm according to the optimized theory. By the ray-tracing results, the spectral sampling is 0.6nm/pixel, the RMS spots radii in all fields of view are less than 5μm. The spectral keystone distortion and smile distortion are less than 0.1%. The prototype is manufactured based on theory and tolerance analysis. The achieved prototype has advantages of high spectral resolution(3nm) and compact configuration.

2D Modeling of Silicon Optical PN Phase Shifter

Darpan Mishra and Ramesh Sonkar

DOI: 10.1364/AO.382816 Received 08 Nov 2019; Accepted 25 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: In this paper, a 2D model of a silicon lateral PN optical phase shifter is presented which can be used for multiple parameter study and optimization of the device performance without the need of any commercial numerical tools. The model shows good agreement with TCAD simulation and can be used to calculate the phase shift, absorption loss, modulation efficiency, and insertion loss of the phase shifter. Multiple parameter study includes the waveguide dimensions, operating wavelength, cladding material, doping concentrations, junction offset, and applied voltage. The model employs the effective index method to determine the mode properties and construct the 2D mode field. The PN diode is modeled by taking into account the fringing electric field at the core-cladding interface which results in a wider depletion region near the interface. Multiple loss components are discussed and the scattering loss and free-carrier absorption are modeled using Payne-Lacey and Soref model respectively. The model uses the 2D modal overlap with the 2D carrier distribution across the waveguide to calculate the phase shifter performance metrics. The algorithm used to model the 2D nature of the PN diode depletion region is presented in detail and uses mathematical and analytical formulas instead of numerical methods, making the model faster and easy to implement, with accuracy in par with commercial tools.

Athermalization of dual-waveband infrared systems containing DOEs via optical–digital joint design

Qingfeng Cui, Yang Hu, Lin Sun, and Bo Zhang

DOI: 10.1364/AO.381214 Received 23 Oct 2019; Accepted 25 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: Because of material limitations, achieving an athermal design for dual-waveband infrared systems is difficult. This study integrates single-layer diffractive elements to reduce the volume and weight of such a design and introduces optical–digital joint methods to eliminate the impact of low diffraction imaging efficiency. To enable athermalization, temperature polychromatic integral diffraction efficiency and temperature integral wavelength weight are incorporated in the PSF model. Influence of low diffraction efficiency is eliminated via subsequent algorithm processing. Accordingly, athermal design and processing of a cooled dual-waveband infrared system is achieved and verified via experimental results.

Analysis of Antimagnetic Interference Properties and Structural Optimization of Magneto-Optical Glass Current Sensors

yansong li, Jun Wan, Weiwei Zhang, and Jun Liu

DOI: 10.1364/AO.381295 Received 23 Oct 2019; Accepted 25 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: To solve the issue of external magnetic fields interfering with magneto-optical glass current sensors (MOCS), this paper proposes a strip-like splicing structure. First, a mathematical model for integrating the magnetic field strength along the optical path is constructed, and the relationship between the gap size a, the interference source distance d, the angle of the interference source θ and the relative error ε is analyzed. Then, an optical-electromagnetic COMSOL Multiphysics simulation model is constructed. The simulated results are consistent with the theoretical derivation, verifying the correctness of the latter. Finally, the strip-like splicing MOCS structure is optimized by combining two different MOCSs with a tilt angle of 45°. Thus, their output signals are summed and the adjacent interference phase signals nearly compensate mutually. This new structure facilitates real-world installation, avoiding the angle being limited to a fixed value, and meeting a 0.2 accuracy of the anti-electromagnetic interference properties.

Designing Multi-quantum Dots Doped Wavelength Down-shifting Model of c-Si Solar Cells by Using Machine Learning

Binjie Ren and Chun Jiang

DOI: 10.1364/AO.381558 Received 29 Oct 2019; Accepted 25 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: The solar cells energy conversion efficiency may be significantly improved by better exploitation to the original solar spectrum. In this paper, we propose a multi-quantum dots doped wavelength down-shifting (WDS) model, which can reduce the energy loss resulted from the mismatch between the solar spectrum and the solar cell material’s responsivity. The WDS layer can nearly perfectly absorb the photons in the range of 300 to 700 nanometers and then re-emit them at a longer wavelength. The distribution of quantum dots in WDS layer is provided based on the analysis about its spectral properties with machine learning regression algorithm. And lastly, the relationship between the optimizing ability of the model and Stokes shift of the quantum dots (QDs) is also discussed. Ideally, our model can provide up to 29% improvements to the external quantum efficiency of c-Si solar cells, which fully demonstrates the potential of our model.

Microsphere assisted interferometry with high numerical apertures for 3D topography measurements

Lucie Hüser and Peter Lehmann

DOI: 10.1364/AO.379222 Received 01 Oct 2019; Accepted 25 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: Improving the lateral resolution takes a decisive role in the research on optically measuring systems in order to expand the fields of application of optical metrology. By means of microspheres put on an object in a microscope and therefore used as a near-field support, it has already been shown that a super-resolution of structures below Abbe’s diffraction limit is possible. The following investigations give more detailed theoretical and experimental insight into the physical mechanisms responsible for the transition of near-field information to the far-field. In particular, the effects of microspheres as near-field support on the behavior of phase-evaluating interference microscopes close to the optical resolution limit are studied experimentally as well as with numerical simulations. Special attention is drawn to measured data taken with a Linnik microscope of high numerical aperture. Finally, measurement results of grating structures with a period below Abbe’s diffraction limit are presented.

A common-path based device for magnetomotive OCT noise reduction

Zenhe Ma, Xiaodi Liu, Bin Yin, Yuqian Zhao, Jian Liu, Yao Yu, and Yi Wang

DOI: 10.1364/AO.377118 Received 04 Sep 2019; Accepted 25 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: Magnetomotive optical coherence tomography (MMOCT) is a promising imaging method for noninvasive three-dimensional tracking of magnetic nanoparticles (MNPs) motions in target tissues or organs. The external B-field is the driving force that provides MMOCT contrast. However, B-field modulation also introduces modulation noise thereby decreasing the quality of MMOCT image. In this paper, a common-path based device is designed for modulation noise reduction. The device is capable of adjusting interference distance, reference light intensity and imaging position (X-Y translation). The sensitivity of the MMOCT is increased by ~20 times with the new device. Using the proposed device, the distribution of MNPs injected in zebrafish was imaged.

Effects of working pressure and power onphotovoltaic and defect properties of MagnetronSputtered Sb2Se3 thin-film solar cells

Youyang Wang, Junda Li, Yuanjing Chen, Jianhong Zhou, Jiayan Zhang, Wangchen Mao, Shiqi Zheng, Yuhan Pan, Yiqi Liu, Kaijie Dai, Xiaobo Hu, Jiahua Tao, Guoen Weng, Jinchun Jiang, Shaoqiang Chen, and Junhao Chu

DOI: 10.1364/AO.382805 Received 08 Nov 2019; Accepted 24 Dec 2019; Posted 24 Dec 2019  View: PDF

Abstract: Antimony selenide (Sb2Se3) is an emerging material with potential applications in photovoltaics, while magnetronsputtering is an important method in material growth. In this study,Sb2Se3 thin films prepared by magnetronsputtering technique with varied working pressure and sputtering power, were fabricated into solar cells with astructure of Glass/ITO/CdS/Sb2Se3/Au. The J-V measurements and X-ray diffraction (XRD) were introduced tocompare the photovoltaic and structural properties of the cell samples. Characterization and identification of thedefects in Sb2Se3 thin films were investigated by admittance measurements. The Sb2Se3 cell samples prepared withappropriate sputtering power (about 60 W) or working pressure (about 0.4 Pa) were found to own better crystalqualities and lower defect densities, which may be the reason for better efficiency.

Polarized skylight compass based on the soft-margin support vector machine working in cloudy conditions

Huaju Liang, Hongyang Bai, Ning Liu, and Xiubao Sui

DOI: 10.1364/AO.381612 Received 30 Oct 2019; Accepted 24 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: The skylight polarization pattern, which is a result of the scattering of unpolarized sunlight by particles in the atmosphere, can be used by many insects for navigation. Inspired by insects, several polarization navigation sensors have been designed and combined with various heading determination methods in recent years. However, up until now, few of these studies have fully considered the influences of different meteorological conditions, which play key roles in navigation accuracy, especially in cloudy weather. Therefore, this study makes a major contribution to the study on bioinspired heading determination by designing a skylight compass method to suppress cloud disturbances. The proposed method transforms the heading determination problem into a binary classification problem by segmentation, connected component detection, and inversion. Considering the influences of noise and meteorological conditions, the binary classification is solved by the soft-margin support vector machine (SVM). In addition, to verify this method, a pixelated polarization compass platform is designed, which can take polarization images at four different orientations simultaneously in real time. Finally, field experimental results show that the designed method can more effectively suppress the interference of clouds compared with other methods. Additionally, the maximum error and root-mean-square error of heading determination are -0.78° and 0.215°, respectively, in sunny conditions; –2.25° and 1.026° in cloudy conditions; 5.28° and 2.660° in overcast conditions.

Characterization of fuel-water mixtures andemulsions with ethanol using laser-inducedfluorescence

Matthias Koegl, Christopher Mull, Yogeshwar Nath Mishra, Stefan Will, and Lars Zigan

DOI: 10.1364/AO.380392 Received 17 Oct 2019; Accepted 23 Dec 2019; Posted 02 Jan 2020  View: PDF

Abstract: In charged spark-ignition (SI) engines, additional water injection allows for thereduction of temperature under stoichiometric mixture conditions. However, a highercomplexity of the injection and combustion processes is introduced when a mixture of fueland water (“emulsion”) is injected directly into the combustion chamber using the sameinjector. For this purpose, the mixture must be homogenized before injection so that areproducible composition can be adjusted. In principle, gasoline and water are not miscibleand may form an instable macro-emulsion during mixing. However, addition of ethanol,which is a biofuel component that is admixed to gasoline, can improve the mixing and maylead to a stable micro-emulsion. For the assessment of the distribution of the water and fuelphases in the mixture, a novel imaging concept based on laser-induced fluorescence (LIF) isproposed. In a first spectroscopic study, a fluorescence dye for imaging of the water phase isselected and evaluated. The fluorescence spectra of the dye dissolved in pure water areinvestigated under varied conditions using a simplified pressure cell equipped with a stirrer.The study comprises effects of temperature, dye concentration and photo-dissociation onfluorescence signals. In a second step, fuel is mixed with water (5 vol. % to 10 vol. %)containing the dye, and the water dispersion in fuel is investigated in an imaging study.Additionally, the miscibility of fuel and water is studied for varying ethanol content, and thehomogeneity of the mixture is determined. These first investigations are also essential for theassessment of the potential of the LIF-technique for studying the distribution of the waterphase in IC engine injection systems and sprays.

Demonstration of dual-channel two-dimensionalreflection grating filter

Jianyu Zhou, Xinbin Cheng, Jinlong Zhang, Hongfei Jiao, Zhanyi Zhang, Haoran Li, and Zhanshan Wang

DOI: 10.1364/AO.377898 Received 12 Sep 2019; Accepted 23 Dec 2019; Posted 23 Dec 2019  View: PDF

Abstract: A dual-channel two-dimensional (2-D) reflection grating filter operating around 1.55 μm wavelength region isdemonstrated, exhibiting dual-channel reflection peaks at 1.492 μm and 1.647 μm. The sidebands intrinsic to thiskind of grating are suppressed by appropriately-designed antireflective (AR) thin films, and this can be proved byequivalent medium theory (EMT). Using the modal analysis method, the excitation modes of the dual-channelreflection peaks are determined to be the TM0 (1.490 μm) and TE0 (1.638 μm) modes. The estimated relativeerrors in the wavelength determination of these modes are less than 1%. This is found to be in accord withanalyses of the reflectivity spectra and electromagnetic fields. The dual-channel reflection peaks are sensitive tothe background refractive index and may be useful in biosensing applications.

Simultaneous measurement of the surfaceshape and thickness for an optical flat with awavelength-tuning Fizeau interferometer withsuppression of drift error

Yangjin Kim, YoungHoon Moon, Kenichi Hibino, Naohiko Sugita, and Mamoru Mitsuishi

DOI: 10.1364/AO.379718 Received 04 Oct 2019; Accepted 23 Dec 2019; Posted 23 Dec 2019  View: PDF

Abstract: Two types of phase-shifting algorithms were developed for simultaneousmeasurement of the surface and thickness variation of an optical flat. During wavelengthtuning, phase-shift nonlinearity can cause a spatially non-uniform error and spatially uniformDC drift error. A 19-sample algorithm was developed that eliminates the effect of thespatially uniform error by expanding the 17-sample algorithm with characteristic polynomialtheory. The 19-sample algorithm was then altered to measure the surface shape of the opticalflat by rotation of the characteristic diagram. The surface shape and thickness variation weremeasured with these two algorithms and a wavelength-tuning Fizeau interferometer.

Design and analysis of refractive index sensorsbased on slotted photonic crystal nanobeamcavities with sidewall gratings

Fu Long, Chao-Sheng Deng, and Song-Shan Ma

DOI: 10.1364/AO.380459 Received 16 Oct 2019; Accepted 23 Dec 2019; Posted 23 Dec 2019  View: PDF

Abstract: We propose and numerically investigate a refractive index sensor based on a onedimensionalslotted photonic crystal nanobeam cavity with sidewall gratings for refractive indexsensing in a gaseous environment. By using the three-dimensional finite-difference time-domainmethod, we demonstrate that our proposed sensor simultaneously possesses a high quality factor of3:71106 and a high sensitivity of 508 nm/RIU (refractive index unit) at the resonant wavelengthnear 1583 nm, yielding an detection limit as low as 1:97 10􀀀6 RIU. Moreover, the mode volumeof the cavity’s fundamental resonant mode is found to be as small as 0.022 ¹nº3, resulting in avery compact effective sensing area. We finally study and assess the effect of fabrication disorderon the performances of our proposed sensor. We believe our proposed sensor will be a promisingcandidate for applications not only in multiplexed biochemical sensing and multielement mixturedetection but also in optical trapping of single biomolecules or nanoparticles.

Impact of third-order dispersion and three photon absorption on mid-infrared time magnification via four wave mixing in Si0.8Ge0.2 waveguides

Aihu Zheng, Qibing Sun, Leiran Wang, mulong liu, Chao Zeng, Guoxi Wang, Lingxuan Zhang, Weichen Fan, Wei Zhao, and Wenfu Zhang

DOI: 10.1364/AO.379232 Received 09 Oct 2019; Accepted 23 Dec 2019; Posted 24 Dec 2019  View: PDF

Abstract: We investigate the influence of third-order dispersion of dispersive elements, three photon absorption (3PA) and free carrier effects on mid-infrared time magnification via four wave mixing (FWM) in Si0.8Ge0.2 waveguides. It is found that the magnified waveform is seriously distorted by these factors, and conversion efficiency is decreased a lot mainly caused by the nonlinear absorption. A time lens based on FWM in Si0.8Ge0.2 waveguides is proposed for time magnification of mid-infrared ultrashort pulses, in which the low distortion high magnification in the time domain could be obtained by optimizing system parameters. These results make it possible to analyze the transient dynamic process through oscilloscopes and detectors with GHz bandwidth, and have important applications in ultrafast process analysis, optical pulse sampling, and optical communications.

Enhanced quasi-phased matched second harmonic generation in silicon-organic hybrid microring resonator

mehdi Sadeghi and Babak Janjan

DOI: 10.1364/AO.380046 Received 21 Oct 2019; Accepted 23 Dec 2019; Posted 24 Dec 2019  View: PDF

Abstract: An enhanced second harmonic generation (SHG) process in dually-resonant silicon-organic hybrid micro-ring resonator is presented. The necessary phase matching is fulfilled through quasi-phase-matching (QPM) by periodically altering the spatial distribution of second-order nonlinearity, χ(2), of polymer infiltrated into slot layer along the micro-ring resonator propagation length. Absolute conversion efficiency up to P2ω/Pω = 38% (12%) with only 10 (1) mW pump power at λ = 3.1 μm is obtained in an optimized structure which shows a ~76 (240)-fold improvement compared to that of the straight waveguide. Also, our efficiency is comparable to the previous works even with the much lower pump power. We expect that our device provides a potential application for bridging the mid-infrared and near-infrared bands on the silicon photonics platform.

Exhaustive dithering algorithm for 3-D shapereconstruction by fringe projectionprofilometry

Adriana Silva Mejía, ANTONIO MUÑOZ, Jorge Flores, and Jose de Jesus Villa Hernandez

DOI: 10.1364/AO.381924 Received 30 Oct 2019; Accepted 23 Dec 2019; Posted 24 Dec 2019  View: PDF

Abstract: Three-dimensional shape reconstruction by projection of defocused binary patternsovercomes the non-linearity introduced by the projector. Current patch-based procedures thatgenerate dithered patterns are time-consuming and are affected by the harmonics introducedthrough the tiling process. To overcome this problem, we propose a novel idea to generatedithering patterns using the composition of two-dimensional patches as a stack of onedimensionalarrays obtained through an efficient deterministic approach. This procedure is aone-dimension optimization problem -in the intensity domain- employing only a quarter ofthe fringe pitch. Furthermore, the unwanted distorting harmonics are eliminated using aHilbert transform method. Both numerical simulations and experimental results verify theeffectiveness of the proposal.

Compact and broadband 2×2 optical switch based onhybrid plasmonic waveguides and curved directionalcouplers

Mahsa Babaei, Abbas Zarifkar, and Mehdi Miri

DOI: 10.1364/AO.381281 Received 23 Oct 2019; Accepted 22 Dec 2019; Posted 23 Dec 2019  View: PDF

Abstract: In this paper, a compact, broadband and CMOS compatible 2×2 optical switch based on hybrid plasmonicwaveguides (HPWs) and curved directional coupler is presented. The proposed coupler consists of a combination of straightand curved hybrid plasmonic waveguides. By using the ability of HPW for confining the light wave in a sub-wavelength scaleand curved structure to decrease the wavelength dependence of the directional coupler, a 3dB power splitter with a 3 dB±0.5bandwidth of about 410 nm and footprint of 7.6μm×2μm is achieved. By exploiting this optimal power splitter in a Mach-Zehnder interferometer (MZI) structure, a 2×2 electro-optic (EO) switch with a broad bandwidth of 400 nm and a smallfootprint of 33μm×2.5μm is realized which denotes the widest bandwidth compared to the previously reported similarstructures. The three-dimensional finite-difference time domain (3D-FDTD) simulation results show a switching speed of 0.15THz for the proposed optical switch, while the extinction ratio, power consumption, and insertion loss are 20 dB, 95.2 fJ/bit and4 dB, respectively at the central wavelength of 1550 nm.

Frequency Tuning Ratio Testing of the Laser Via HollowPhotonic Crystal Fiber Resonator

Ming Lei, Yuan Fang, huaiyong Yu, Yi Yang, Qiang Xiang, and Lizhe Zhang

DOI: 10.1364/AO.379900 Received 14 Oct 2019; Accepted 22 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: In order to satisfy the requirement of measurement of laser frequency tuning ratio(FTR), an experimental equipment based on hollow photonic crystal fiber resonator (HPCFR) isproposed in this paper. Firstly, the principle scheme of the equipment consisting of HPCFR isdesigned, the resonance curves of the HPCFR is theoretically analyzed, calculated, and simulated;secondly, the transmissive HPCFR sample is fabricated and resonance curve is obtained; eventually,the experiment results from the established laser FTR experimental setup demonstrate that the FTRof narrow linewidth fiber laser and semiconductor laser are 17.6 MHz/V and 30.9 MHz/mA,respectively, which are basically in accordance with the factory parameters of the lasers. The workshows the FTR experimental equipment via HPCFR has advantages of high precision and goodlong-term stability.

A simulation technique enabling calibration-free frequency-modulation-spectroscopy measurements of gas conditions and lineshapes with modulation frequencies spanning kHz to GHz

Christopher Goldenstein and Garrett Mathews

DOI: 10.1364/AO.380609 Received 16 Oct 2019; Accepted 22 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: A simulation technique enabling calibration-free measurements of gas properties (e.g., temperature, mole fraction) and lineshapes via wavelength- or frequency-modulation spectroscopy (WMS or FMS) is presented. Unlike previously developed models, this simulation technique accurately accounts for: 1) absorption and dispersion physics and 2) variations in the WMS/FMS harmonic signals which can result from intensity tuning induced by scanning the laser’s carrier frequency (e.g., via injection-current tuning of tunable diode lasers (TDLs)). As a result, this approach is applicable to both WMS and FMS experiments employing a wide variety of light sources and any modulation frequency (typically kHz to GHz). The accuracy of the simulation technique is validated via comparison with: 1) simulated signals produced by established WMS and FMS models at conditions where they are accurate and 2) experimental data acquired at conditions where existing models are inaccurate. At conditions where existing WMS and FMS models are accurate, the new simulation technique yields nearly identical (within 0.1%) results. For experimental validation, the wavelength of a TDL near 1392 nm was scanned across a single absorption line of H2O with a half-width at half-maximum of 350 MHz while frequency-modulation was performed at 100 MHz. The best-fit first-harmonic (1f) signal produced by the new simulation technique agrees within 1.6% of the measured 1 f signal, and the H2O mole fraction and transition collisional width corresponding to the best-fit 1 f spectrum agree within 1% of expected values.

Discrimination of hazardous bacteria with combination LIBS andstatistic methods

Yu Zhao, Qianqian Wang, Xutai Cui, Geer Teng, Kai Wei, and Da Hai

DOI: 10.1364/AO.379136 Received 02 Oct 2019; Accepted 21 Dec 2019; Posted 23 Dec 2019  View: PDF

Abstract: Real-time biohazard detectors must be developed to facilitate the rapidimplementation of appropriate protective measures against foodborne pathogens.Laser-induced breakdown spectroscopy (LIBS) is a promising technique for thereal-time detection of hazardous bacteria (HB) in the field. However, distinguishingamong various HBs that exhibit similar C, N, O, H, or trace metal atomic emissionscomplicates HB detection by LIBS. This paper proposes the use of LIBS andchemometric tools to discriminate Staphylococcus aureus, Bacillus cereus, andEscherichia coli on slide substrates. Principle component analysis (PCA) and thegenetic algorithm (GA) were used to select features and reduce the size of spectraldatas. Several models based on artificial neural network (ANN) and support vectormachine (SVM) were built using the feature lines as input data. The proposedPCA-GA-ANN and PCA-GA-SVM discrimination approaches exhibited correctclassification rates of 97.5% and 100%, respectively.

Optimization study of metallic sub-wavelengthgratings as the polarizer in infrared wavelengths

Zongyao Yang, Bo Feng, Bingrui Lu, Yifang Chen, Wenhao Li, Wei Zhang, and Tao Li

DOI: 10.1364/AO.378636 Received 25 Sep 2019; Accepted 20 Dec 2019; Posted 20 Dec 2019  View: PDF

Abstract: Despite the polarimetric detection in the infrared wavelengths of 8–10 μm is of great importance andbroad applications, there has been limited addressing of the grating based polarizers in this band. One of the mainissues lies in the process incompatibility between the conventional nanofabrication technique and the II-VImaterials such as HgCdTe, so that the direct integration of polarizers with sensors still remains a big challenge.This paper reports our recent work on optimizing the grating structures, materials and nanofabrication processesfor enhancing both the transmittance and the extinction ratio of polarizers on Si and/or ZnSe wafers, usingnumerical simulations for the grating design and the electron beam lithography for the nanoscale patterngeneration. By utilizing Finite Difference Time Domain (FDTD) method, both the transmittance and theextinction ratio are maximized by optimizing the grating geometric dimensions and the duty cycle for twodifferent grating materials of Al and Au as comparison. Based on the designed structures, nanofabrications ofsubwavelength gratings in both Al and Au are carried out and the processes are compared for achieving highpolarization performance. Optical characterizations of the fabricated polarizers demonstrate that both hightransmittance and extinction ratio can be achieved in feasible parameters and the nanoprocess developed in thiswork.

Assessment of anisotropy of collagenstructures through spatial frequencies ofMueller matrix images for cervical pre-cancerdetection

Mohammad Zaffar and Asima Pradhan

DOI: 10.1364/AO.377105 Received 03 Sep 2019; Accepted 20 Dec 2019; Posted 20 Dec 2019  View: PDF

Abstract: Analysis of spatial frequency of Mueller matrix (MM) images in the Fourierdomain yields quantifying parameters of anisotropy in the stromal region in normal andprecancerous tissue sections of human uterine cervix. The spatial frequencies of MMelements reveal reliable information of microscopic structural organization arising from thedifferent orientations of collagen fibers in the connective tissue and their randomization withdisease progression. Specifically, the local disorder generated in the normal periodic andregular structure of collagen during the growth of the cervical cancer finds characteristicmanifestation in the Fourier spectrum of the selected Mueller matrix elements encoding theanisotropy effects through retardance and birefringence. In contrast, Fourier spectra ofdifferential polarization gated images are limited to only one orientation of collagen. Fourierspectra of first row elements M11, M12, M13 & M14 and first column elements M11,M21,M31& M41 discriminates CIN-I from normal cervical tissue samples with 95% to 100%sensitivity and specificity. FFT Spectra of first and fourth row elements classify CIN-I andCIN-II grades of cervical cancerous tissues with 90% -100% sensitivity and 87%-100%specificity. Normal and CIN-II grade samples are successfully discriminated through Fourierspectra of every MM elements while that of M31 element arises as the key classifier amongnormal, CIN-I and CIN-II grades of cervical cancer with 100% sensitivity and specificity.These results demonstrate the promise of spatial frequency analysis of Mueller matrix imagesas a potentially novel approach for cancer / pre-cancer detection.

Realization of a compact cross-gratingspectrometer and validating experimentaltests

Matthias Kraus, Erik Foerster, Verena Bagusat, Tobias Hönle, Xavier Uwurukundo, Patrick Bohnert, Robert Brüning, Hartmut Hillmer, and Robert Brunner

DOI: 10.1364/AO.379595 Received 17 Oct 2019; Accepted 20 Dec 2019; Posted 20 Dec 2019  View: PDF

Abstract: Echelle inspired cross-grating spectrometers offer the potential to bridge the gapbetween classical high-end Echelle spectrometers and curved-grating single elementinstruments. In particular, the cross-grating approach offers the possibility to achievesimultaneously a high spectral resolution and a wide accessible spectral range in compactdimensions and without moving parts. We report on the complete realization andimplementation details of an all-reflective cross-grating spectrometer based on a modifiedCzerny–Turner configuration including a folded beam path and a toric-convex mirror foraberration compensation. The applicability of the cross-grating spectrometer is demonstratedby test measurements including the recording of the spectra of different plant leaves. For thecross-grating spectrometer, with an accessible wavelength range between 330 nm and1100 nm, a spectral resolution of 0.6 nm at 589 nm was achieved.

Towards Automatic Surface Inspection forS-PVC Using a Composite Vision-based Method

Qilin Bi, Miaohui Wang, Minling Lai, Jiaxin Lin, Jialing Zhang, and Xiaoguang Liu

DOI: 10.1364/AO.378773 Received 23 Sep 2019; Accepted 19 Dec 2019; Posted 20 Dec 2019  View: PDF

Abstract: Appearance defect inspection is crucial for quality control in the context of Industry4.0. This research introduces a joint surface defect inspection and classification framework forPolyvinyl chloride (PVC) pipe based on the low-cost visual sensors and high-efficient computervision algorithms. Firstly, we build a robust imaging system to acquire the surface of PVC(S-PVC) by considering its characteristics and the illumination condition into the modelingprocess. Secondly, we adopt the region of interest method to eliminate the background interferencecaptured in the S-PVC imaging, and design an efficient S-PVC defect inspection and classificationmethod. Thirdly, we build an automatic machine prototype to evaluate the efficiency of theproposed method. Experimental results demonstrate that our framework has the advantages oflow-latency, high precision, and robustness.

Steady-state frequency-tracking distortion in digitalPound–Drever–Hall technique

Xiao Wang, Li Shuang Feng, Jun Xu, and Peiren Ni

DOI: 10.1364/AO.379557 Received 02 Oct 2019; Accepted 19 Dec 2019; Posted 20 Dec 2019  View: PDF

Abstract: We present a general method for evaluating the steady-state frequency-tracking distortion in digital Pound-Drever-Hall technique with modulation harmonic distortion. The theoretical tracking distortion model isestablished based on the multi-beam interference theory. The effects of the additional harmonic phase shift andthe relative distortion ratio changes in the model are simulated by the Runge-Kutta method. Moreover, wedemonstrate the steady-state frequency-tracking distortion caused by the modulation harmonic distortion in aresonant frequency tracking system with a 35-mm Si3N4 waveguide ring resonator. According to the measured andsimulated results, we obtain the optimal modulation frequency and depth with minimal frequency-trackingdistortion, which are 11.49 MHz and 3.96, respectively.

Design and fabrication of superior non-polarizinglong-wavelength pass edge filter applied in laser beamcombining technology

Hongfei Jiao, Xinshang Niu, Xuemin Zhang, Jinlong Zhang, Xinbin Cheng, and Zhanshan Wang

DOI: 10.1364/AO.378135 Received 17 Sep 2019; Accepted 19 Dec 2019; Posted 20 Dec 2019  View: PDF

Abstract: The wavelength combining technique has been widely used to increase the power scaling of lasers. The edge filterwhich is non-polarizing for wavelength has proven to be a critical component for the combining process, andshould have high spectral efficiency, high laser induced damage threshold and excellent thermal stability. Here, wedesign a superior non-polarizing long-wavelength pass edge filter using a novel depolarized initial film structurebased on a tuned Fabry-Perot interference filter. The coating fabricated by electron beam evaporation canwithstand 130 kW/cm2 of continuous laser energy with high spectral efficiency at 45° and a small temperaturerise.

Correlation of long-duration exposure and acceleratedtesting of protected silver mirrors

Chung-tse Chu, Diana Alaan, Kelsey Folgner, James Barrie, and Peter Fuqua

DOI: 10.1364/AO.377882 Received 16 Sep 2019; Accepted 19 Dec 2019; Posted 07 Jan 2020  View: PDF

Abstract: The stability of two types of protected silver mirrors was studied by both long-termexposure in a clean-room laboratory and the mixed-flowing-gas (MFG) accelerated environmentaltest with two different gas concentrations. The two types of mirrors behaved very differentlywhen exposed to the clean-room air for 6 years. The mirrors that were subjected to a reducedBattelle Class II MFG test protocol for 10 days exhibited similar corrosion mechanisms to thoseexposed to the clean-room air. With further testing of silver mirrors in both field exposures andMFG test conditions, it is feasible that the MFG accelerated test method can be developed toquantitatively assess the durability of protected silver mirrors in ambient applications.

Sapphire AMP: wet etch to expose sub-surfacedamage and increase laser damage resistance andmechanical strength

Tayyab Suratwala, William Steele, Joel Destino, Lana Wong, Mary Norton, Ted Laurence, Chantel Aracne-Ruddle, Philip Miller, Nan Shen, Michael Feit, Nathan Ray, Christopher Carr, Craig Rivers, Vanessa Peters, Spencer Jeppson, Daniel Malone, and Waldi Greene

DOI: 10.1364/AO.381739 Received 08 Nov 2019; Accepted 19 Dec 2019; Posted 09 Jan 2020  View: PDF

Abstract: A novel method of wet chemical etching of sapphire workpieces (such as optics, wafers, windows, and cones),called the Sapphire Advanced Mitigation Process (or Sapphire AMP), has been developed which exposes subsurfacemechanical damage created during the optical fabrication process and significantly enhances the surfacelaser damage resistance (>2x) and mechanical strength (up to ~2.6x). Sapphire AMP involves first treating theworkpiece with a mixture of sulfuric and phosphoric acid ([H2SO4]:[H3PO4]=1:3) at 220oC, followed with phosphoricacid at 160oC, then with sodium hydroxide base (NaOH) & surfactant at 40oC, and finally with a high pressuredeionized water spray rinse. Sapphire AMP has been demonstrated on both A and C plane sapphire workpieces.The mechanism of this etch process involves the reaction of the sapphire (Al2O3) surface with sulfuric acid (H2SO4)forming aluminum sulfate (Al2(SO4)3) which has low solubility. The high phosphoric acid content in the first andsecond steps of Sapphire AMP results in the efficient conversion of Al2(SO4)3 to aluminum phosphate (AlPO4), whichis very soluble, greatly reducing reaction product redeposition on the workpiece surface. Sapphire AMP is shown toexpose sub-surface mechanical damage on the sapphire surface created during the grinding and polishingprocesses, whose etched morphology has either isotropic or anisotropic evolution depending on the nature of theinitial surface damage. Sapphire AMP was also designed to remove the key known surface, laser absorbingprecursors (namely foreign chemical impurities, fracture surface layer of preexisting sub-surface damage, andreaction product or foreign species redeposition or precipitation). Static and sliding indention induced surfacemicrofractures on sapphire are shown after Sapphire AMP to have a significant decrease in the fastphotoluminescence intensity (a known metric for measuring the degree of laser damaging absorbing precursors).In addition, the onset of laser damage (at 351 nm 3 ns) on Sapphire AMP treated workpieces was shown to increasein fluence from ~4 to >9.5 J/cm2. Finally, biaxial ball-on-ring mechanical tests on sapphire disks showed anincrease in the failure stress from 340 MPa (with pre-existing 28 μm flaws) to ~900 MPa after Sapphire AMP, whichis attributed to the blunting of the surface microfractures.

Moon with amethyst hues during full eclipse from tropical region

Marija Strojnik

DOI: 10.1364/AO.382471 Received 11 Nov 2019; Accepted 18 Dec 2019; Posted 18 Dec 2019  View: PDF

Abstract: We report on the first documented observations, to the best of our knowledge, of the amethyst(super wolf) Moon recorded in the region just south of the northern tropic of cancer (latitude21°7.745' N), at about 2,000-m- height above the sea level during the lunar eclipse on Jan. 20, 2019.During the color transition from the brownish red to amethyst blue (a mixture of dark blue and somered), the moon in the center of the Earth shadow (mid-eclipse) was nearly in zenith in Leon,Mexico. We interpret the amethyst color as arising from the inability of red rays to curve into theaxial regions of the lunar spherical surface, while the scarce, randomly distributed blue rays are stillincident there.

All-optical Aggregation and De-aggregationBetween 3×BPSK and 8QAM in HNLF withWavelength Preserved

Zhilai Zhong, Hongxiang wang, and Yuefeng Ji

DOI: 10.1364/AO.379130 Received 01 Oct 2019; Accepted 17 Dec 2019; Posted 17 Dec 2019  View: PDF

Abstract: All optical aggregation and de-aggregation with wavelength preserved play animportant role in flexible optical network. In addition, it is also expected to apply in networknode which connecting different networks and increase the channel utilization by freeing uplow-speed channel. In this paper, we proposed an aggregation and de-aggregation setupbetween three binary phase shift keying signals and 8-ary quadrature amplitude modulationsignal using nonlinear effect in high nonlinear optical fiber. Moreover, the bit-error rate ofsignal is analyzed to evaluate the performance of the system by numerical simulation.

A Zn2+ Responsive FluorescenceEnhancement for Optical Data Storage

Xu Peng Yuan, Miao Zhao, Xinjun Guo, Yao Li, Zongsong Gan, and Hao Ruan

DOI: 10.1364/AO.378204 Received 17 Sep 2019; Accepted 17 Dec 2019; Posted 18 Dec 2019  View: PDF

Abstract: In this letter, we put forward a new application in optical data storage (ODS) oftetraphenylethene (TPE)-doped photopolymer, which has aggregation-induced emissionattribute. The photopolymer host reacted with the excitation light at the focal point of ahigh-numerical-aperture lens to enhance the fluorescence intensity mainly because thefunction of Zn2+ ion. We recorded data inside the photopolymer matrix by using thisproperty and had distinct fluorescence intensity contrast between the photochemicalregions and other regions. This attribute paves a new way for super-resolution ODS andopens the way to exploring the possibility of utilizing TPE-doped photopolymer aschemical sensors in the future.

Calibration Method for Line-Structured Light VisionSensor based on A Single Cylindrical Target

Zhenmin Zhu, Xinyun Wang, Fuqiang Zhou, and Yigang Cen

DOI: 10.1364/AO.378638 Received 30 Sep 2019; Accepted 17 Dec 2019; Posted 18 Dec 2019  View: PDF

Abstract: This paper presents a new calibration method for line structured light vision sensors. Thelaser stripe intersecting the cylindrical target and the line structured light is captured by the camera, andthe light plane parameters of the line structured light are obtained by combining the cross-sectionalcharacteristics of the ellipse. Nonlinear optimization of the light plane parameters using multiplelocations to get the optimal solution of the light plane equation. This method requires only a singlecylindrical target, which in turn greatly simplifies the calibration process. The results of simulationexperiments and physical experiments show that the proposed calibration method can achieve highercalibration accuracy and measurement accuracy. The effectiveness of the new calibration method isverified.

Precision Heterodyne Oxygen-Calibration Spectrometry:Vertical Profiling of Water and Carbon Dioxide in theTroposphere and Lower Stratosphere

David Bomse, Jared Tso, Monica Flores, and Houston Miller

DOI: 10.1364/AO.379684 Received 03 Oct 2019; Accepted 17 Dec 2019; Posted 18 Dec 2019  View: PDF

Abstract: We describe the development of a near-infrared laser heterodyne radiometer (LHR): PrecisionHeterodyne Oxygen-Calibration Spectrometer (PHOCS). The prototype instrument is equipped with two heterodynereceivers for oxygen and water (measured near 1278 nanometers) and carbon dioxide (near 1572 nanometers)concentration profiles, respectively. The latter may be substituted by a heterodyne receiver module equipped with alaser to monitor atmospheric methane near 1651 nanometers.). Oxygen measurements are intended to provide drygas corrections and – more importantly – determine accurate temperature and pressure profiles that, in turn, improvethe precision of the CO2 and H2O column retrievals. Vertical profiling is made feasible by interrogating the verylow-noise absorption lines shapes collected at ≈0.0067 cm-1 resolution. PHOCS complements results from theOrbiting Carbon Observatory (OCO-2), Active Sensing of CO2 Emissions over Nights, Days, and Seasons(ASCENDS), and ground-based Fourier transform spectrometers. In this manuscript we describe the development ofthe instrument by Mesa Photonics and present the results of initial tests in the vicinity of Washington, D.C.

Low cost Hyper-spectral imaging system using LinearVariable Bandpass Filter for Agritech Applications

Shigeng Song, Des Gibson, SAM AHMADZADEH, Hin On Chu, BARRY WARDEN, RUSSELL OVEREND, FRASER MACFARLANE, Paul Murray, Stephen Marshall, MATT AITKENHEAD, DAMIAN BIENKOWSKI5, and RUSSELL ALLISON

DOI: 10.1364/AO.378269 Received 17 Sep 2019; Accepted 16 Dec 2019; Posted 19 Dec 2019  View: PDF

Abstract: Hyperspectral imaging for agricultural applications provides a solution for nondestructive,large-area crop monitoring. However, current products are bulky and expensive due tocomplicated optics and electronics. A linear variable filter was developed for implementation into aprototype hyperspectral imaging camera that demonstrates good spectral performance between450nm – 900nm. Equipped with a feature extraction and classification algorithm, the proposedsystem can be used to determine potato plant health with ~88% accuracy. This algorithm was alsocapable of species identification and is demonstrated as being capable of differentiating betweenrocket, lettuce and spinach. Results are promising for an entry-level, low-cost hyperspectral imagingsolution for agriculture applications.

Design and analysis of reactionless large-aperture faststeering mirror with piezoelectric actuators

Bing Ran, ping yang, Lianghua Wen, Rui Du, Kangjian Yang, Shuai Wang, and Bing Xu

DOI: 10.1364/AO.379344 Received 02 Oct 2019; Accepted 16 Dec 2019; Posted 16 Dec 2019  View: PDF

Abstract: The reaction force of fast steering mirror (FSM) directly transmits to the FSM’s base, leading dynamic couplinginterference to the optical platform and degradation of laser beam quality in adaptive optics (AO) system. In thispaper aΦ320 mm aperture symmetrically arranged reactionless FSM actuated by piezoelectric actuators wasproposed. The corresponding dynamic equation about reaction force was established by rotational equation ofNewton's second law, and the equilibrium condition of reaction force compensation was deduced. Then, the finiteelement (FE) piezoelectric-coupling method was used to simulate dynamic characteristics, which shows that theelimination ratios of reaction force are 99% at low frequency (@50 Hz), and 94.1% at high frequency (@120 Hz).The experimental results show that the first resonance frequency of the FSM is 159.82 Hz, the tilting angle is ±1.3',the reaction force at high frequency (@120 Hz) is 112.95 N, and the elimination ratio of reaction force is 90.14%.The simulation and corresponding test results indicate that the developed reactionless FSM significantly eliminatesreaction force and therefore improve FSM’s stability and pointing accuracy.

Improved adaptive-optics performance usingpolychromatic speckle mitigation

Noah Van Zandt and Mark Spencer

DOI: 10.1364/AO.379972 Received 09 Oct 2019; Accepted 16 Dec 2019; Posted 16 Dec 2019  View: PDF

Abstract: Adaptive-optics (AO) systems correct the optical distortions of atmosphericturbulence to improve resolution over long paths. In applications such as remote sensing,object tracking, and directed energy, the AO system’s beacon is often an extended beaconreflecting off an optically rough surface. This situation produces speckle noise that cancorrupt the wavefront measurements of the AO system, degrading its correction of theturbulence. This work studies the benefits of speckle mitigation via polychromaticillumination. To quantify the benefits over a wide range of conditions, this work uses anumerical wave-optics model with the split-step method for turbulence and the spectralslicingmethod for polychromatic light. It assumes an AO system based on a Shack-Hartmannwavefront sensor. In addition, it includes realistic values for turbulence strength, turbulencedistribution along the path, coherence length, extended-beacon size, and object motion. Theresults show that polychromatic speckle mitigation significantly improves AO systemperformance, increasing the Strehl ratio by 180% (from 0.10 to 0.28) in one case.

Image Correlation by One-DimensionalSignatures Invariant to Rotation, Position, andScale Using Radial Hilbert Transform Optimized

Alfredo Castro Valdez, Josué Borrego, and Selene Solorza-Calderón

DOI: 10.1364/AO.381574 Received 25 Oct 2019; Accepted 16 Dec 2019; Posted 16 Dec 2019  View: PDF

Abstract: This paper presents a new methodology for pattern recognition invariant to rotation, position, and scale. The methoduses the correlation of signatures, where the signatures were created with a new equation called radial Hilberttransform optimized (RHTO) for longer signatures. An analysis with eight non-homogeneous illumination patterns wasperformed with 2000 letters variants and 30 phytoplankton species. The higher confidence level was founded using theradial Hilbert optimized methodology. Also, it is used a correlation called adaptive linear-nonlinear correlation, whichgave a better discrimination performance than the non-linear correlation function.

Development and growth of corrosion features onprotected silver mirrors during acceleratedenvironmental exposure

Kelsey Folgner, Chung-tse Chu, Scott Sitzman, Sean Stuart, Zachary Lingley, and James Barrie

DOI: 10.1364/AO.375891 Received 17 Sep 2019; Accepted 16 Dec 2019; Posted 17 Dec 2019  View: PDF

Abstract: Various layer schemes have been developed to protect thin film silver mirrors from tarnish and corrosion.However, the mechanisms by which these additional layers improve mirror durability are not fully understood.Accelerated environmental exposure testing of protected silver mirrors was used to investigate the effects of layercomposition on the mechanisms of corrosion feature development and growth. Two model mirror coatings wereanalyzed in which the composition of the base layer below the silver and the adhesion layer above the silver werevaried. Large circular corrosion features formed preferentially along the silver-chromium interfaces, regardless ofwhere this interface was located within the layered structure of the mirror. The corrosion features originated atcoating defects in the dielectric protection layer and their growth was likely driven by the oxidation andchloridation of both silver and chromium at the Ag-Cr interfaces.

Optical Interference Coating Design Contest 2019: ANon-polarizing Beamsplitter and a Color-mixingChallenge.

Jennifer Kruschwitz, Vladimir Pervak, Jason Keck, Eric Baltz, Amit Deliwala, Zach Gerig, Fabien Lemarchand, Samuel Pellicori, Kageyuki Sato, William Southwell, Michael Trubetskov, Kunihiko Yano, and Wenjia Yuan

DOI: 10.1364/AO.378119 Received 17 Sep 2019; Accepted 16 Dec 2019; Posted 17 Dec 2019  View: PDF

Abstract: A non-polarizing beamsplitter and a light color-mixing challenge were the topics of the design contestheld in conjunction with the 2019 Optical Interference Coatings topical meeting of the Optical Society ofAmerica. A total of 10 designers from China, France, Germany, Japan, and the United States submittedover 70 designs for problems A and B. The design problems and the submitted solutions are describedand evaluated.

Compensation of general asymmetric static loads for acomplete optical system of freeform mirrors

Johannes Hartung, Sebastian Merx, and Henrik von Lukowicz

DOI: 10.1364/AO.376214 Received 23 Aug 2019; Accepted 16 Dec 2019; Posted 17 Dec 2019  View: PDF

Abstract: Optical systems consisting of freeform metal mirrors are state of the art in optical engineering. Thefreeform shapes allow for more compact designs and offer more degrees of freedom for aberration correction.Themetal components allow to relocate effort from the integration to the fabrication stage. Metalmirrors for spaceborne optical systems experience several loads during orbital commissioning or operation.The present article focuses on describing general static loads within the optical design to close theiteration loop between optical and mechanical design using this knowledge to investigate how to compensateload induced surface shape errors within the optical design. First, this analysis is performed fora two mirror system with an asymmetric force load step. Second, the compensation of this load is discussedby performing another optical design step or a direct mechanical compensation step at mechanicaldesign level. Since thermal loads usually introduce expansion effects, a third point of the article is thediscussion of thermal loads with some general results, to embed them into the shown formalism. Thearticle concludes by showing residual optical errors of the compensated optical system and comparingthem with the nominal design.

A line width study of pixelated aluminum nanowiregratings on polarization performance

Miao Yu, Zhengxun Song, Litong Dong, Li Li, liang cao, Wenjun Li, yingying Song, Li Lei, and Zuobin Wang

DOI: 10.1364/AO.379239 Received 09 Oct 2019; Accepted 15 Dec 2019; Posted 16 Dec 2019  View: PDF

Abstract: Nowadays, nanowire gratings are widely used in various applications such as imaging sensors and high resolutionmicroscopes. Structure parameters are the main factors that affect the optical performance of the gratings. This work aims topresent the influence of the line width of pixelated aluminum nanowire gratings with a fixed period on the transmittance andextinction ratio in the visible region. By controlling the exposure doses of electron beam lithography (EBL), different linewidths of pixelated aluminum nanowire gratings with a period of 170 nm were fabricated. The significant effect of line widthdifference on the polarization performance were verified by the simulations of finite difference time domain (FDTD) software.The simulations were divided into two parts: the discussion of the pure aluminum without considering oxidation and thediscussion of the surface aluminum being oxidized into the aluminum oxide. An optical system was built to evaluate theperformance of the fabricated structures. The results show that the trends of the measurement results are consistent with that ofsimulation. This work will give a guide to the fabrication and evaluation of the nanowire gratings.

A laser speckle projection based handheldanthropometric measurement system withsynchronous redundancy reduction

Xiao Yang, Xiaobo Chen, GUANGKUN ZHAI, and Juntong Xi

DOI: 10.1364/AO.380322 Received 11 Oct 2019; Accepted 15 Dec 2019; Posted 16 Dec 2019  View: PDF

Abstract: Human body measurement is essential in modern rehabilitation medicine, which can be well combined with thetechnology of additive manufacturing. Laser speckle projection based digital image correlation is a single-shot,accurate and robust technique for human body measurement. In this paper, we present a handheldanthropometric measurement system based on laser speckle projection. A flexible retroreflective marker target isdesigned for multi-view data registration. Meanwhile, a synchronous redundancy reduction algorithm based on are-projected global disparity map is proposed. Experiment results validate the proposed system is effective andaccurate for different human body parts measurement. Comparative experiments show that the proposedredundancy reduction algorithm has high efficiency and can well preserve the features of complex shape. Thecomprehensive performance of the algorithm is higher than the other two tested methods.

Dual-projector structured light 3D shape measurement

Ying Yu, Daniel Lau, Matthew Ruffner, and Kai Liu

DOI: 10.1364/AO.378363 Received 19 Sep 2019; Accepted 15 Dec 2019; Posted 16 Dec 2019  View: PDF

Abstract: Structured light illumination is an active 3D scanning technique that uses a projector and camera pairto project and capture a series of stripe patterns, but with a single camera and single projector, structuredlight scanning has issues associated with scan occlusions, multi-path, and weak signal reflections.To address these issues, this paper proposes dual-projector scanning using a range of projector/cameraarrangements. Unlike previous attempts at dual-projector scanning, the proposed scanner drives bothlight engines simultaneously, using temporal-frequency multiplexing to computationally decouple theprojected patterns. Besides presenting the details of how such a system is built, we also present experimentalresults demonstrating how multiple projectors can be used to (1) minimize occlusions; (2) achievehigher signal to noise ratios having twice a single projector’s brightness; (3) reduce the number of componentvideo-frames required for a scan; and (4) detect multi-path interference.

Isosbestic Light Absorption by MetallicDimers: Effect of Interparticle ElectromagneticCoupling

Lanxin Ma and Wang Chengchao

DOI: 10.1364/AO.379021 Received 26 Sep 2019; Accepted 13 Dec 2019; Posted 16 Dec 2019  View: PDF

Abstract: Isosbestic plasmonic nanostructures, which feature an invariance of opticalabsorption and heat generation upon varying the incident light polarization, have broadapplication in many fields such as nanochemistry, optical nanoantennas, and microbubbleformation. In this study, we focus on the isosbestic optical absorption by metallic dimers, andsystematically investigate the coupling between two interacting particles by using both thesuperposition T-matrix method and dipole approximation model. We observe that theinterparticle coupling effects on particle absorption can be both positive and negative,compared with an isolated particle. Meanwhile, the optical absorption properties of sphereswith small size parameters can realize more flexible control through changing the sphere size,interparticle distance and incident light wavelength. For illuminations with incident lightpropagating perpendicularly to the line joining the centers of the two spheres, isosbesticconditions will be satisfied as long as the absorption efficiencies for transverse andlongitudinal illuminations are equal. For transverse illuminations along the dimer axis, theratio of absorption efficiency of the two metallic spheres presents the fluctuation change withthe interparticle distance. Due to the strong interparticle coupling effects, it even leads to thatthe absorption efficiency of the far sphere higher than that of the near sphere. Our results areaimed at expanding our understanding of the interparticle electromagnetic coupling effects onisosbestic light absorption in plasmonic nanoparticle systems.

Dual-camera snapshot spectral imaging with pupil-domain optical diffuser and compressed sensing algorithms

Jonathan Hauser, Michael Golub, Amir Averbuch, Menachem Nathan, Valery Zheludev, and Michael Kagan

DOI: 10.1364/AO.380256 Received 14 Oct 2019; Accepted 13 Dec 2019; Posted 13 Dec 2019  View: PDF

Abstract: We propose a snapshot spectral imaging (SSI) method for the visible spectral range using two digital cameras placed side-by-side: regular RGB camera and a monochromatic camera equipped with a dispersive diffractive diffuser placed at the pupil of the imaging lens. While spectral imaging (SI) was shown to be feasible using a single monochromatic camera with a pupil diffuser [Applied Optics 55 (3), 432 (2016)], adding an RGB camera provides more spatial and spectral information for stable reconstruction of the spectral cube of a scene. Results of optical experiments confirm that the combined data from the two cameras relaxes the complexity of the underdetermined reconstruction problem and improves the reconstructed image quality obtained using compressed sensing (CS)-based algorithms.

Terahertz optical properties of wood-plasticcomposites

Atsushi Nakanishi and Hiroshi Satozono

DOI: 10.1364/AO.379758 Received 07 Oct 2019; Accepted 13 Dec 2019; Posted 16 Dec 2019  View: PDF

Abstract: Terahertz optical materials containing polymeric materials have been useful for terahertz technologies. We investigated the THz optical properties of wood-plastic composites (WPCs), which are composed of polystyrene and wood powder, and their suitability as THz optical materials. We found that the refractive indexes and absorption coefficients of the WPCs increased with increasing wood powder content. WPCs are inexpensive and have tunable THz optical properties. Furthermore, we found that WPCs exhibit polarization-independent THz optical properties. WPCs are promising materials for THz optical components.

Fast template matching method in white-lightscanning interferometry for 3D micro-profilemeasurement

YiLiang Huang, Jian GAO, Lanyu Zhang, Haixiang Deng, and Xin Chen

DOI: 10.1364/AO.379996 Received 09 Oct 2019; Accepted 13 Dec 2019; Posted 16 Dec 2019  View: PDF

Abstract: White-light scanning interferometry (WLSI) is an important measurementtechnique that has been widely used in three-dimensional (3D) profile reconstruction.Because of the effects of environmental noise and phase changes caused by surface reflection,existing WLSI algorithms have problems in measurement accuracy and measurement speed.Addressing these problems, this paper proposes a fast template matching (FTM) method todetermine precisely the zero optical path difference (ZOPD) position in the WLSI. Due to theuniform shape of the interference signals, a template interference signal can be obtained inadvance by performing a least-square fitting or Fourier interpolation on an interference signalof one pixel. In the method, the ZOPD position is be initially obtained by the centroid method.Then, the ZOPD position is determined by a precise matching process through moving thetemplate interference signal on the measured interference signal. Through the two-stepprocesses, the ZOPD position can be obtained precisely with much less time. The methodwas simulated and verified through the measurement of a spherical surface, a 1.8 μm-heightstandard step and a flip-chip substrate. The experimental results show that the proposedalgorithm can achieve both high precision and fast measurement.

Bilayer Synchronous Measuring Method ofCurved Screen Based on Line-structured LightScanning Sensor

Changshuai Fang, Linlin Zhu, ning yan, and Xiaodong Zhang

DOI: 10.1364/AO.380396 Received 15 Oct 2019; Accepted 13 Dec 2019; Posted 16 Dec 2019  View: PDF

Abstract: Curved screens are widely used in smart wearables, optics, aerospace, and otherdisplay fields. To overcome the difficulty of overall quality evaluation of large-area curvedscreen, this paper proposes a novel measuring method to realize the bilayer synchronousmeasurement based on the line-structured light scanning. The integrated shape information ofthe large bilayer curved screen can be obtained by only a single probe in one scanning step.The two-step image processing in the image and 3D data space is proposed to reduce thenoises due to the weak reflection from the bilayer screen. To realize the large area curvedscreen measurement, a splicing algorithm based on the optimization principle is also proposedto rectify the position deviation of each splicing step. A curved mobile phone screen wasmeasured, the experimental results effectively proved the validity of the proposed method. Acamera lens was also measured to prove its potential for other multi-layer parts.

A Novel Method of FTD Measurement Based on PhaseQuantization and Delay Synthesis

Donglin Qin, Xiaofeng Jin, Xianbin Yu, Xiangdong Jin, Shilie Zheng, Xianmin Zhang, Guoyong Wang, and Qinggui Tan

DOI: 10.1364/AO.380136 Received 10 Oct 2019; Accepted 12 Dec 2019; Posted 17 Dec 2019  View: PDF

Abstract: A novel method of fiber transfer delay (FTD) measurement base on phase quantization and delay synthesis isproposed and demonstrated. By detecting the differential phase shifts of a set of frequency-multiplied RF signalstransmission through the fiber link with and without the FTD under the test, the 2π phase ambiguity problem canbe solved. To avoid the phase quantization error near the digital quantization boundary, a self-check and errorcorrectionmethod is proposed so as to greatly improve the reliability of measurement. In the experiment, themeasurement repeatability around 0.018 ps within a period of 80 s is achieved for back-to-back fiber link and thetest resolution of 0.03 ps is proved with a motorized tunable delay line. The system is available for measurement ofa large FTD range up to 100 microseconds with no dead zone.

Matrix algorithm for 3D non-paraxial optical fieldmodeling under arbitrary spatial coherence

Roman Castaneda, Julián Laverde, and Jaime Moreno

DOI: 10.1364/AO.381010 Received 22 Oct 2019; Accepted 11 Dec 2019; Posted 13 Dec 2019  View: PDF

Abstract: Non-paraxial modeling of optical field propagation at distances comparable to the wavelength and under arbitrary spatialcoherence is crucial for micro and nano-optics. Fourier and Fresnel transform-based algorithms are unable to simulate itaccurately because of their paraxial approach. A non-paraxial matrix algorithm, supported by the theoretical model thatcharacterizes the optical field and the setup configuration in terms of sets of real and virtual point emitters, is capable tosimulate the 3D optical field distribution in the volume delimited by the input and the output planes, placed at a very shortdistance to each other, by using experimental data as entries. The algorithm outcomes are accurate predictions of thepower spectrum of interference and diffraction experiments. Simulations of specific experimental situations, includingspeckle phenomena, illustrate the algorithm capabilities.

Faceted Gratings for optical security feature

Qiang Song, Yoran Eli Pigeon, and Kevin Heggarty

DOI: 10.1364/AO.378122 Received 16 Sep 2019; Accepted 11 Dec 2019; Posted 11 Dec 2019  View: PDF

Abstract: A method of optimizing and manufacturing a diffractive blazed grating array (DBA) is proposedto create a visual security feature when illuminated by a divergent Light emitting diode (LED) source. Apure phase grating array serving as the optical security component consists of blazed grating cells with thesame size of 75μm. After a divergent spherical wave is decomposed into harmonic-waves, each grating cellof the DBA deflects locally the harmonic-waves into predefined directions and forms a feature pattern onthe target plane. Particularly, a two-step optimization method is further developed for optimizing the periodand orientation of each grating cell. The DBA sample is fabricated by using our home-built parallel directwritephoto-plotter with a resolution of 0.75μm. Both numerical simulations and optical experiments aredemonstrated to validate the proposed model. Since the optical security component developed is a surfacerelief structure of a single polymer material, it can be replicated for mass production by using standard rollto roll nano-imprint technology. This design algorithm proposed in this work will enable the extension ofthe optical security elements to a broader realm and facilitate extensive developments in other researchfields of the optics community, such as light shaping, specific illumination for lithography and microscopesystems.

Investigation of reaction mechanism and opticaltransparency in nanosecond laser welding of glassesassisted with titanium film

Xin Zhang, Liang Guo, Qingmao Zhang, Jiaming Li, Deqiang Zhao, Hao Wang, and Qitao Lv

DOI: 10.1364/AO.378409 Received 20 Sep 2019; Accepted 11 Dec 2019; Posted 12 Dec 2019  View: PDF

Abstract: The welding of glasses is widely used in many fields, such as optics, microfluidics,microelectromechanical systems (MEMS), and so on. In this paper, two pieces of 1 mm-sodalime glass substrates were welded using a 1064 nm nanosecond laser assisted with a 14nm-titanium thin film coated. Results show that after the laser irradiation, the welded areabecomes highly transparent close to uncoated glass. The maximum change rate oftransmittance of the welded zone is 8.88% in the wavelength range of 400-1800 nm,compared to a piece of 2 mm-glass substrate. The chemical reaction process between thetitanium film and the glass substrate of the highly transparent welded sample was analyzed byX-ray photoelectron spectroscopy. Welded quality and shear strength were characterized byscanning acoustic microscopy and shear tests.

High performance of an all-optical 2-bit analog todigital converter based on kerr effect nonlinearnanocavities

Mojtaba Hosseinzadeh Sani, Saeed Khosroabadi, and Mahshid Nasserian

DOI: 10.1364/AO.379575 Received 09 Oct 2019; Accepted 11 Dec 2019; Posted 12 Dec 2019  View: PDF

Abstract: In this paper, a new configuration for all optical analog to digital convertor (OADC) based onnonlinear materials has been proposed. This structure is the combination of two main parts; Quantization block,followed by an optical coder. The refractive index of the nonlinear composite material varies with the intensityof the optical field. Sampling and quantizing have been performed at central wavelength λ=1550nm by threering resonators which are filled by nonlinear material AlGaAs with linear refractive index of n1=1.4 and Kerrindex of n2=1.5×10-17 m2/W. The maximum sampling rate is 260GS/S. The sampling accuracy of the structure is1040KS. The overall area of the structure is 540μm2. The fast plane waves expansion (PWE) method is used inthe band structure calculations and two-dimensional finite difference time domain (2-D FDTD) methods is usedto calculate the transducer transmission spectrum, their resonant frequencies and quality coefficients, and thetransducer output power at single wavelengths and constant intensities.

Extreme ultraviolet photon conversion efficiency oftetraphenyl butadiene

Joshua Graybill, Chandra Shahi, Michael Coplan, Alan Thompson, Robert Vest, and Charles Clark

DOI: 10.1364/AO.380185 Received 09 Oct 2019; Accepted 10 Dec 2019; Posted 12 Dec 2019  View: PDF

Abstract: Extreme ultraviolet (EUV) radiation can be converted to visible light using tetraphenyl butadiene (TPB)as a phosphor. 1mm films of TPB were prepared using thermal vapor-deposition of the pure material andby spin-coating suspensions of TPB in high molecular weight polystyrene (HMW-PS)/toluene solutions.Calibrated sources and detectors were used to determine the effective photon conversion efficiency ofthe films for incident EUV radiation in the wavelength range 125 nm l 200 nm. After exposureto atmosphere, the efficiency of the vapor-deposited films decreased significantly while the efficiencyof the spin-coated films remained unchanged. The production of TPB films by spin coating offers theadvantages of simplicity and long-term stability.

Ultraviolet trifrequency Rayleigh DWL for stratosphereatmospheric wind measurements during day-timebased on an ultra-narrow bandwidth optical receiver.

Fei Han, Yuli Han, Dongsong sun, Mingyong Hu, hengjia liu, Anran Zhou, Nannan Zhang, shan jiang, jiaqi chu, Jun Zheng, and jiaxin lan

DOI: 10.1364/AO.376887 Received 03 Sep 2019; Accepted 10 Dec 2019; Posted 10 Dec 2019  View: PDF

Abstract: An ultra-narrow bandwidth optical receiver-based ultraviolet trifrequency Rayleigh Doppler wind Lidar (DWL)technology is proposed that is able to simultaneously detect stratosphere wind with high precision duringday-time. The Lidar system is designed, and the principle of wind measurement is analyzed. An ultra-narrowbandwidth element used for suppressing strong background light is designed as an important part of theultra-narrow bandwidth optical receiver. A three-channel Fabry-Perot interferometer (FPI) is capable ofmeasuring wind speed. A non-polarized beam splitter cube optically contacted on three-channel FPI can offer astable splitting ratio. The parameters of the three-channel FPI is optimized. The structure and parameters of theultra-narrow band element is designed and the transmission curve is measured. The transmission curve andstability of the three-channel FPI are validated. The background photon number is collected with the ultra-narrowbandwidth element and with-interference filter (IF) alternately from 8:00 am to 6:00 pm. Based on the selectedsystem parameters and measured background photon number, the detection performance of the proposed Lidar issimulated. Simulation results show that with 200 m range resolution from 15km to 25km, 500m range resolutionfrom 25km to 40km and 30 min total accumulation time for paired line-of-sight (LOS) measurement, within ±100m/s LOS wind speed range, the daytime LOS wind speed errors is below 4.77 m/s from 15km to 40km altitude.Compared with the traditional IF-based dual-FPI Rayleigh Doppler Lidar, the wind speed accuracies are improvedby 1.29 – 16.29 times and the detection altitude are improved from .55 km to 40 km with the same winddetecting precision.

Instrument response effects on the retrieval ofoceanic lidar

Xue Shen, Zhipeng Liu, Yudi Zhou, Qun Liu, Peituo XU, zhihua mao, Chong Liu, Longhua Tang, Na Ying, Miao Hu, and Dong Liu

DOI: 10.1364/AO.382201 Received 06 Nov 2019; Accepted 10 Dec 2019; Posted 10 Dec 2019  View: PDF

Abstract: Seawater properties can be retrieved from the oceanic lidar returns. However, the actual returns include the idealreturns convolved by the instrument response, which inevitably introduces retrieval error. In this paper,instrument response effects on the retrieval of oceanic lidar are analyzed from different aspects. The resultsdemonstrate that the retrieval of the lidar attenuation coefficient near water surface is affected by the instrumentresponse in homogeneous water. Considering the ratio of the signal distortion region (relative error ofattenuation>10%) to the maximum detection depth (3 dynamic ranges) is less than 20%, the pulse width ofinstrument response should be less than 10 0.042(Kd ) 2 0.709(Kd ) 1 1.136 − − + − + ns. In addition, an average relative error of 55%will be introduced to the retrieval of phytoplankton layer thickness in the stratified water, which can be reduced to6% after correcting the influence of instrument response. However, relative error greater than 20% still existswhen the instrument response length is 2 times larger than the layer thickness. These conclusions provideguidelines to a future design of oceanic lidar.

Optoelectronic Optimization of Graded-Bandgap Thin-Film AlGaAs Solar Cells

Faiz Ahmad, Akhlesh Lakhtakia, and Peter Monk

DOI: 10.1364/AO.381246 Received 22 Oct 2019; Accepted 06 Dec 2019; Posted 06 Dec 2019  View: PDF

Abstract: An optoelectronic optimization was carried out for an AlGaAs solar cell containing an n-AlGaAs absorber layer with a graded bandgap, a periodically corrugated Ag backreflector combined withlocalized ohmic Pd-Ge-Au backcontacts. The bandgap of the absorber layer was varied either sinusoidallyor linearly. An efficiency of 33.1% with the 2000-nm-thick n-AlGaAs absorber layer is predicted with linearlygraded bandgap along with silver backreflector and localized ohmic backcontacts, in comparisonto 27.4% efficiency obtained with homogeneous bandgap and a continuous ohmic backcontact. Sinusoidalgrading of the bandgap enhances the maximum efficiency to 34.5%. Thus, grading the bandgap ofthe absorber layer, along with a periodically corrugated Ag backreflector and localized ohmic Pd-Ge-Aubackcontacts can help realize ultrathin and high-efficient AlGaAs solar cells for terrestrial applications.© 2019 Optical Society of America

ABER performance investigation of LDPC-coded multihopparallel underwater wireless opticalcommunication system

Ang Li, Ping Wang, Weina Pang, Wenwen Chen, Sheng Liu, and Lixin Guo

Doc ID: 374039 Received 05 Aug 2019; Accepted 02 Dec 2019; Posted 03 Dec 2019  View: PDF

Abstract: The average bit error rate (ABER) performance of a low-density parity-check (LDPC)-coded multi-hop parallelunderwater wireless optical communication (UWOC) system is investigated with the combined effects ofabsorption, scattering, the misalignment characterized by the beam spread function (BSF) and the oceanturbulence-induced fading modeled by log-normal distribution. With the max-min criterion as the best pathselection scheme, the cumulative distribution function (CDF) for identically and independently distributed (i.i.d)and non-identically and independently distributed (non-i.i.d) links are derived, respectively. Then,the analyticalABER expressions of binary phase shift keying (BPSK) and multiple phase shift keying (MPSK) subcarrier intensitymodulation (SIM) schemes are deduced with the help of the Gauss-Laguerre quadrature rule, and they are alsoconfirmed by Monte Carlo (MC) simulation. In addition, LDPC codes are applied in the simulation to improve thesystem performance.The results show that the combined degrading effects are mainly limited by the link length,especially under coastal ocean condition. And the multi-hop parallel transmission demonstrates good ABERperformance and can expand the communication range in ocean. Furthermore, LDPC codes can significantlyimprove the ABER performance of UWOC system, and the coding gain is strongly affected by channel conditions andthe corresponding parameters of LDPC codes. This work is beneficial for the UWOC system design.

High harmonic generation wave frontdependence on driving infrared wave front

Thomas Wodzinski, Swen Künzel, Jayanath C.P. Koliyadu, Mukhtar Hussain, Barbara Keitel, Gareth Williams, Philippe Zeitoun, Elke Plönjes, and Marta Fajardo

Doc ID: 378843 Received 25 Sep 2019; Accepted 21 Nov 2019; Posted 22 Nov 2019  View: PDF

Abstract: With high harmonic generation (HHG) spatially and temporally coherent XUV to softx-ray (100 nm to 10 nm) table-top sources can be realized by focusing a driving infrared (IR) laseron a gas target. For applications like coherent diffraction imaging, holography, plasma diagnosticsor pump-prope experiments it is desirable to have control over the wave front (WF) of the HHsto maximize the number of XUV photons on target or to tailor the WF. Here, we demonstratecontrol of the XUV WF by tailoring the driving IR WF with a deformable mirror. The WF ofboth IR and XUV beam are monitored with WF sensors. We present a systematic study of thedependence of the aberrations of the HHs on the aberrations of the driving IR laser and explainthe observations with propagation simulations. We show that we can control the astigmatismof the HHs by changing the astigmatism of the driving IR laser without compromising the HHgeneration efficiency with a WF quality from 8 to 13:3. This allows us to shape the XUVbeam without changing any XUV optical element.

Lidar remote sensing of the aquatic environment

James Churnside and Joseph Shaw

Doc ID: 379597 Received 08 Oct 2019; Accepted 07 Nov 2019; Posted 07 Nov 2019  View: PDF

Abstract: This paper is a review of lidar remote sensing of the aquatic environment. The optical properties of seawater relevant to lidar remote sensing are described. The three main theoretical approaches to understanding the performance of lidar are considered (time-dependent radiative transfer equation, Monte Carlo simulations, and quasi-single scattering assumption). Basic lidar instrument design considerations are presented, and examples of lidar studies from surface vessels, aircraft, and satellite are given.

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