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A Bayesian approach to retrievingoptically-relevant properties from hyperspectralwater-leaving reflectances

Zachary Erickson, Paul Jeremy Werdell, and Ivona Cetinic

DOI: 10.1364/AO.398043 Received 21 May 2020; Accepted 10 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: Current methods to retrieve optically-relevant properties from ocean color observationsdo not explicitly make use of prior knowledge about property distributions. Here we implementa simplified Bayesian approach that takes into account prior probability distributions on twosets of five optically-relevant parameters, and conduct a retrieval of these parameters usinghyperspectral simulated water-leaving reflectances. We focus specifically on the ability of themodel to distinguish between two optically-similar phytoplankton taxa, diatoms and Noctilucascintillans. The inversion retrieval gives most-likely concentrations and uncertainty estimates,and we find that the model is able to probabilistically predict the occurrence of Noctilucascintillans blooms using these metrics. We discuss how this method can be expanded to includea priori covariances between different parameters, and show the effect of varying measurementuncertainty and spectral resolution on Noctiluca scintillans bloom predictions.

Chaos synchronization and communication in globalsemiconductor laser network with coupling time delaysignature concealment

Shiqin Liu, Ning Jiang, Anke Zhao, Yiqun Zhang, and Kun Qiu

DOI: 10.1364/AO.400262 Received 16 Jun 2020; Accepted 09 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: Chaos synchronization and pairwise bidirectional communication with coupling time delay signatures (CTDS)concealment in global heterogeneous coupled semiconductor laser (SL) network are achieved by introducingidentical chaotic injections from an external semiconductor laser with self-feedback. The properties of chaossynchronization and CTDS in four indicative cases are comparatively discussed. Moreover, the influences of keyparameters on the quality of chaos synchronization and the CTDS characteristic are thoroughly investigated. Onthe basis of the chaos synchronization, the chaotic communication performance is further analyzed. The numericalresults demonstrate that with the joint contributions of heterogeneous couplings and external identical chaoticinjections, isochronous chaos synchronization can be achieved between two arbitrary semiconductor lasers andsimultaneously the CTDS are suppressed to a distinguishable level close to 0, over a wide parameter range. Besides,bidirectional transmission with a bit rate beyond 6Gbit/s can be achieved between the synchronized SLs.Comparing with the conventional two-user communication system, the proposed SL network with CTDSconcealment supports flexible network-type message exchanges between pairwise SLs.

Analysis of factors affecting delay accuracy ofsub-femtosecond liquid crystal variableretarders

Xuhao Qi, Wei Fan, Dajie Huang, Gang Xia, He Cheng, and Zhi-Dong Shi

DOI: 10.1364/AO.397864 Received 18 May 2020; Accepted 09 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: To obtain a wide range of high-precision time-delay controllable devices, a 100-μm-thick nematic liquid crystal (LC) cell was fabricated using the LC HTD028200-200.Factors influencing the delay calibration accuracy of LC variable retarders (LCVRs) wereanalyzed theoretically and experimentally. Subsequently, within the total voltage tuning rangeof 0–5 V, the LCVR had a delay of 4.977–97.404 fs, total delay range of 90 fs, tuningaccuracy of ± 0.216 fs, and tuning resolution of 0.128 fs. This device will play an importantrole in spectral imaging.

Coupling coefficients for dielectric cuboids located infree space

Alexander Trubin, Anton Kupriianov, Volodymyr Fesenko, and Vladimir Tuz

DOI: 10.1364/AO.399930 Received 10 Jun 2020; Accepted 08 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: Practical formulas are derived for calculating the far-field radiation pattern and coupling coefficient of arectangular dielectric resonator (cuboid) with free space as well as mutual coupling coefficients betweentwo cuboids for their different orientations relative to each other. An approach is developed using the coupled mode theory and the perturbation theory for the Maxwell equations. The correctness of obtained formulas is checked against the full-wave numerical simulations performed by the COMSOL Multiphysicselectromagnetic solver. In particular, the obtained formulas can be used for revealing optical features ofrealistic (i.e., consisting of a finite number of resonators) all-dielectric metasurfaces with arbitrary curvedshapes.

Deformable Deformable mirror resolution matching basedtwo-stage wavefront sensorless adaptiveoptics method

Deen Wang, Xin Zhang, Ying Yang, Xuewei Deng, Wanjun Dai, Chuang Sun, Yamin Zheng, Dongxia Hu, Feng Jing, Qiang Yuan, and Lei Huang

DOI: 10.1364/AO.394621 Received 07 Apr 2020; Accepted 07 Jul 2020; Posted 09 Jul 2020  View: PDF

Abstract: In high-power laser facilities, the application of traditional wavefront controlmethod is limited under the influence of continuous phase plate (CPP). In order to obtainsatisfied far-field intensity distribution at the target of the beamline with the CPP, a noveldeformable mirror (DM) resolution matching based two-stage wavefront sensorlessadaptive optics method is proposed and demonstrated. The principles of the DMresolution matching method and two-stage wavefront sensorless adaptive optics methodare introduced respectively. Based on the numerical model, the matching relationshipbetween the actuator space of the DM and the spatial period of the CPP is investigated. Byusing the resolution matched DM, the feasibility of the two-stage wavefront sensorlessadaptive optics method is numerically and experimentally verified. Both the numericaland the experimental results show that, the presented DM resolution matching based twostage wavefront sensorless adaptive optics method could achieve the target focal spotcontrol under the influence of the CPP, and the profile and the intensity uniformity of thecorrected focal spot are optimized close to the designed ideal focal spot.

Efficient radiation-releasing in device-levelglass ceramics driven by blue-laser

Dong Han, Desheng Li, Li Gang, Edwin Yue Bun Pun, and Lin Hai

DOI: 10.1364/AO.385793 Received 12 Dec 2019; Accepted 07 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: Ce3+ doped M3Al5O12 (MAG, M=Lu, Y) glass ceramics (GCs) have been provedto be shapeable phosphors for white lighting driven by 453 nm laser. Quantitativecharacterization reflects that the net emission powers of 4wt% LuAG doped GC and 4wt%YAG doped GC are 59.99 and 66.22 mW at the pump power of 117.63 mW, and thequantum yields reach up to 71.1% and 78.0%, respectively. Miniaturization of devices canbe achieved for LuAG/YAG-GCs by optimizing sample size and phosphor concentrationwith maintaining fluorescence intensity of the samples. Presupposed color coordinate tracereveals that the high-brightness white fluorescence can be realized when the appropriateintensity ratio is determined between residual laser and samples emission. The tunablewhite fluorescence and the efficient radiation-releasing illustrate that LuAG/YAG-GCs arepotential candidates for application in solid-state laser illumination

Single-shot multispectral birefringence mapping bysupercontinuum vector beams

Misaki Shoji, Toshitaka Wakayama, HIrokazu Ishida, Hiroyuki Kowa, Kazuyuki Sakaue, Taisuke Miura, and Takeshi Higashiguchi

DOI: 10.1364/AO.393419 Received 24 Mar 2020; Accepted 07 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: We demonstrated a single-shot, multispectral birefringence mapping by the supercontinuum vector beam. Thevector beam, which was generated by a pair of axially symmetric waveplate, leads to angular-variant polarizationmodulation to divide birefringence properties of sample substrate into Fourier space. This strategy allowsmultispectral birefringence mapping from a single shot image captured by a multispectral imaging detector. Forsupercontinuum vector beam analysis, we also compensated the retardance error of the axially symmetricwaveplate in the super-broadband spectrum. Resolutions of retardance and azimuthal angle were 0.4° and 0.2°,respectively, and the spatial resolution was 60 μm. Those results are expected to provide us a single shot,multispectral birefringence mapping with high spatial resolution as compared with scanning laser microscope. Ourproposal has extendibility to develop high-speed, high-resolution birefringence imaging spectroscopy.

Impact of frequency-dependent spherical aberration inthe focusing of ultrashort pulses

José Agustín Moreno-Larios, Martha Rosete-Aguilar, Oscar Rodriguez Herrera, and Jesus Garduno-Mejia

DOI: 10.1364/AO.394300 Received 02 Apr 2020; Accepted 07 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: In this paper the temporal and spatial intensity pulse distributions are calculated around the focal regionof an optical system using a combination of ray-tracing and a wave propagation method. We analyse howto measure the width of the intensity pulse distributions to estimate pulse duration and spot size in orderto study the impact of the variation of spherical aberration with frequency in a pulse on the intensity distributions. Two experimental techniques used in the laboratory are also modeled: the knife-edge test tomeasure spatial distribution and the intensity autocorrelation technique to measure the temporal distribution. We use two measuring criteria: the Full-Width Half-Maximum (FWHM) and the standard deviation(σ) to compare between the spatial and temporal intensity distributions of the calculated diffraction patterns and those obtained from the simulated experimental techniques. We will show that the FWHMis not a good criterion since it gives different results in the measured intensity distributions in timeand space when they are measured directly from the theoretical modeling and when they are measuredfrom the modeled experimental techniques used in the laboratory. The standard deviation, however, isa consistent criterion giving the same results for the calculated intensity distributions and the modeledexperiments. ©

Ultra-stable optical components using adjustablecommercial mirror mounts anchored in a ULE spacer

Soham Kulkarni, Ada Uminska, Joseph Gleason, Simon Barke, Reid Ferguson, Josep Sanjuan, Paul Fulda, and Guido Mueller

DOI: 10.1364/AO.395831 Received 22 Apr 2020; Accepted 07 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: This paper describes a novel approach to build ultrastable interferometers using commercial mirror mountsanchored in a ULE base. These components will playa critical role in Any Light Particle Search (ALPS) andwill also be included in ground testing equipmentfor the upcoming Laser Interferometer Space Antenna(LISA) mission. Contrary to the standard ultra-stabledesigns where mirrors are bonded to the spacers, rulingout any later modifications and alignments, our designremains flexible and allows to optimize and change thealignment of optical components at all stages. Herewe present the dimensional stability and angular stability of two commercial mirror mounts characterizedin a cavity setup. The long-term length change in thecavity did not exceed 30 nm and the relative angular stability was within 2 µrad, which meet the requirementsfor ALPS. We were also able to demonstrate 1 pm/√Hzlength noise stability which is a critical requirementfor various sub-systems in LISA. These results have ledus to design similar opto-mechanical structures whichwill be used in ground verification for the testing of theLISA telescope.

Learned Linear Models for Detecting Watercraftin a Maritime Environment

Colin Olson and Jonathan Nichols

DOI: 10.1364/AO.396496 Received 30 Apr 2020; Accepted 07 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: This work provides a new approach to constructing linear models for object detectionin a scene. Specifically, we use representative training data in order to estimate the parametersdescribing a generalized wavelet model for the expressed purpose of detecting the presenceof maritime targets in a scene. The parameter estimates are taken as those that maximize theprobability of detecting the targets for a fixed probability of false alarm. The approach is thendemonstrated on a database of short-wave infrared imagery containing various watercraft. Resultsare then compared to some of the more standard wavelet bases used in detection applications.

Diffractive distortion of pixelated computergenerated hologram with oblique illumination

CHENG-TA MU and Cheng-Huan Chen

DOI: 10.1364/AO.394650 Received 07 Apr 2020; Accepted 07 Jul 2020; Posted 10 Jul 2020  View: PDF

Abstract: Computer generated holograms (CGHs) have their phase and/or amplitudemodulation pattern being calculated rather than recorded as for traditional holograms. Inpractice, the CGH devices are normally pixelated, no matter passive or active ones. In manycases, the reconstruction light illuminates on the CGH devices obliquely, and the patterngenerated on the target plane will be distorted from the originally desired one, even if themodulation on the CGH devices has been calibrated for the corresponding illumination anglein CGH calculation and optimization. The distortion is purely related to the diffractionbehavior resulting from the geometry of the pixel on the CGH, and therefore diffractivedistortion has been coined for this specific phenomenon. In this paper, quantitative analysis ofdiffractive distortion and corresponding scheme for correction have been given based onscalar diffraction theory. The proposed concept is that the distortion of reconstructed image isproportional to the distortion of the signal window. Experiment has been conducted withphase type liquid crystal on silicon spatial light modulator (SLM). Both the distortion analysisand the correction scheme have been verified quantitatively for various illumination angle anddirection of the reconstruction light.

Single-shot common-path off-axis dual-wavelengthdigital holographic microscopy

Manoj Kumar, Xiangyu Quan, Yasuhiro Awatsuji, YOSUKE TAAMADA, and Osamu Matoba

DOI: 10.1364/AO.395001 Received 28 Apr 2020; Accepted 06 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: A single-shot common-path off-axis self-interference dual-wavelength digital holographic microscopic (DHM)system based on a cube beam splitter is presented for some scientific applications. In the system, two laser beams,with wavelengths 532 nm and 632.8 nm are used to expand the phase range and for simultaneous measurement ofthe refractive index and the physical thickness of a specimen. Two laser beams are combined to trans-illuminatethe object under study and then divided the object beam into two beams by using a beam splitter oriented in such away that both the beams propagate in the almost same direction with an appropriate lateral separation betweenthem. One of the object beams is spatially filtered at its Fourier plane by using a pinhole, to generate a referencespherical beam free from the object information. The reference beam interferes with the object beam to form adigital hologram at the faceplate of the image sensor. The phase information is extracted from a single recordeddigital hologram by utilizing the principal component analysis (PCA)-based phase aberration compensationmethod. Owing to the common-path configuration, the system shows high temporal phase stability and it is lessvibration-sensitive compared to its counterparts such as Mach-Zehnder type DHM. The performance of the dualwavelength DHM system is verified by conducting various experiments using microsphere beads and living plantcells.

Analysis and improvements of effective emissivities ofnon-isothermal blackbody cavities

Shufang He, Caihong Dai, Yanfei Wang, Jinyuan Liu, Guojin Feng, and Jinghui Wang

DOI: 10.1364/AO.397229 Received 08 May 2020; Accepted 06 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: The emissivity of the blackbody is a very important parameter in spectral radiance measurement system. In theconventional method, the emissivity is calculated based on the isothermal model. However, the actual temperaturedistribution in the blackbody cavity is always non-isothermal, the emissivity calculated based on the isothermalmodel may not accurately present the radiation characteristic of the blackbody. In this study, the actualtemperature distributions of two blackbodies (one has an extended cone shape, and the other a 65-mm diametercylindrical shape) are measured and the emissivities are calculated accordingly based on the non-isothermalmodel at a certain temperature (873K). The results show there are different tendencies of temperaturedistributions in the two blackbodies. When comparing with the isothermal model, the emissivities in the 873 Ktemperature and 2.0 ~ 20.0 µm wavelength condition are about 1.75% and 0.18% lower at the non-isothermalmodel for the extended cone shape and cylindrical blackbodies, respectively. To improve the emissivity, differenttypes of apertures are customized for the two blackbodies. For the extended cone shape blackbody, the emissivityin the 873 K temperature and 2.0 ~ 20.0 µm wavelength condition increases by 1.12% when using a ring shapegraphite aperture in the cavity; whereas for the cylindrical blackbody, the emissivity in the same conditionincreases by 0.09% when using a high-reflective aperture in front of the cavity opening. Different from previousstudies, this study provides new insight in calculating and improving the effective emissivity of blackbody by usingthe measured temperature in the cavity based on the non-isothermal model.

Analysis and modulation of aberration in EUVlithography projector via rigorous simulation andback propagation neural network

Rongbo Zhao, Lisong Dong, Chunyue Bo, Yayi Wei, and Li-ping Wang

DOI: 10.1364/AO.397250 Received 11 May 2020; Accepted 06 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: Lens aberration is a critical factor affecting lithography, which deteriorates the image fidelity andcontrast. As the critical dimension (CD) decreases, the effect becomes increasingly severe, especially in theextreme ultraviolet lithography (EUVL). Aberration impact on EUVL imaging has been analyzed individuallyfor 36 Zernike terms, and Zernike terms Z9 and Z12 are determined to be the prominent terms affecting theprocess variation band (PVB). Moreover, it is the Zernike Z2 that causes the maximum value of pattern shift(PS) for the CDs of vertical lines (VL) and gaps of tip2tip and tip2line with 14 nm CD. However, there is morethan one Zernike term in the aberration of a real EUV optics. To identify the combined effects among Zerniketerms, the design of experiment (DOE) approach, analysis of variance (ANOVA) as well as F test are adopted.With the help of these statistical analysis methods, the combined Zernike terms leading to prominent PVBand PS for different test patterns are determined. For example, the combined effects of Z2*Z19, Z4*Z9 andZ7*Z10 cause significant PS for VL patterns. To make a reasonable budget for aberration, the definitivescreening design (DSD) and back propagation neural network (BPNN) models of PVB and PS are establishedfor different test patterns respectively. Predicted results demonstrate that the accuracy of BPNN models ofhorizontal lines (HL), VL and gap patterns reaches above 90% and satisfies the requirements of predictingPVB and PS, which is superior to DSD models. As an example, the gap pattern BPNN model is used to findreasonable combinations of Zernike terms which satisfy the requirement that PVB is less than 6 nm. Amongthese combinations, an optimum distribution of Zernike coefficients is determined, which can generateminimum negative impact on imaging quality, and this impact is very close to that of zero aberration. This implies that the BPNN models possess the ability to screen out better aberration combination regardless ofeach individual Zernike term value in the range of -20 mλ to 20 mλ, which will play a guiding role in EUVoptics design.

High contrast multimode fiber imaging based onwavelength modulation

Xuanhao Zhang, Zhong Wen, Yaoguang Ma, Xu Liu, Liqiang Wang, and Qing Yang

DOI: 10.1364/AO.398490 Received 25 May 2020; Accepted 06 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: For the multimode fiber (MMF) having the property of maintaining minimally invasive when performing highresolution observations, MMF imaging is of particular interest in many related fields recently, especially inbioendoscopic imaging. Imaging through point scanning is the most common method of MMF imaging now, whichmeans modulating a scanning focal spot on the end face of fiber by controlling modes in the fiber. However, due tomode interference, there is always a background speckle around the focal spot formed, which affects imagingquality seriously. Increasing controllable modes number can effectively suppress effects of the backgroundspeckle, but it is limited by the number of controllable elements (the elements number of wavefront shapingdevices). In this letter, we propose a new method to increase the contrast-to-noise ratio (CNR) of MMF imagingwithout increasing the number of controllable modes. Wavelength modulation is introduced to suppress thebackground. The background speckles turn to be uncorrelated whereas the signal patterns turn to be strongcorrelated and can be added when 20 different wavelengths of light form a focal spot at same position at the distalend of MMF respectively. Thus, a four-fold enhancement can be gained in CNR at a 200 µm Field-of-View (FOV) bysuppressing background speckles.

Optical Configuration of Modified Fredkin Gate usingLithium Niobate based Mach Zehnder Interferometer

Sanjeev Metya, Shashank Awasthi, Aditya Biswas, and Alak Majumder

DOI: 10.1364/AO.397451 Received 13 May 2020; Accepted 06 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: The continuous quest for reversible computation to be extensively used in applications like digital signal processing, quantum computing, quantum-dot cellular automata and nanotechnology has recentlydiscovered its optical implementation as because light tenders high speed computing with least information loss. In this article, the electro-optic effect of Lithium Niobate based Mach Zehnder Interferometeris explored to configure a 4×4 Modified Fredkin Gate capable of furnishing as many as 16 logical combinations, thus showing potential of curbing the area overhead. The optical design is carried out usingBeam Propagation Method. We have also performed the mathematical modeling and analyzed the resultin MATLAB.

Fast focus-shifter based on a unimorphdeformable mirror

Sven Verpoort, Matthias Bittner, and Ulrich Wittrock

DOI: 10.1364/AO.397495 Received 13 May 2020; Accepted 06 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: On-the-fly remote laser processing plays an increasingly important role in modernfabrication techniques. These processes require guiding of the focus of a laser beam alongthe contours of the workpiece in three dimensions. State-of-the-art galvanometer scannersalready provide highly dynamic and precise transverse x-y beam steering. However, longitudinalfocus-shifting ("z-shifting") relying on conventional optics is restricted to a bandwidth of a fewhundred Hz. We have developed and manufactured a fast piezo-based z-shifting mirror withdiffraction-limited surface fidelity providing a focus shift of ∆z>60 mm with an actuation rate of2 kHz.

Optimal modified lateral shearing interferometer withaxial range extension by using dual optical plate

KWANG-BEOM SEO and Seung Ho Shin

DOI: 10.1364/AO.394757 Received 08 Apr 2020; Accepted 06 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: We present a method to extend the axial range of digital holographic microscopy based on the optimal modifiedlateral shearing interferometer(MLSI). The proposed system can extend the axial range by using dual optical plate.The interference pattern with two spatial wavelengths is generated by dual optical plate with different thicknesses.These spatial wavelengths transfer dual spatial frequency into the Fourier plane by using FFT. Two phases areextracted by dual spatial frequency and combined to create the synthetic wavelength. The synthetic wavelength isapplied to measure the micrometer-scale object without phase unwrapping. Also, the noise-reducing algorithm isused to reduce the phase noise caused by the amplified noise of the synthetic wavelength. The experimental resultconfirms the feasibility of the optimal MLSI by using dual optical plate.

Implementation of a diffuse light spectroscopy basedsix-around-one optical probe for study of cerebralproperties in a murine mouse model of autismspectrum disorder (ASD)


DOI: 10.1364/AO.396681 Received 30 Apr 2020; Accepted 06 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: XLight reflectance spectroscopy (LRS) is a kind of multispectral technique that is sensitive to the absorption andscattering properties of biological molecules in tissue and is used as a noninvasive tool to extract quantitativephysiological information from human tissues and organs. A near-infrared LRS based on single optical probe wasused to monitor changes in optical and hemodynamic parameters in a mouse model of autism. A murine model forautism induced by developmental exposure to valproic acid was used. Since autism attributed to neuroanatomicalchanges, we hypothesize that these changes can be detected using LRS, since spectral properties depend on bothmolecular composition and structural changes. The fiber optic probe in the setup consisting of seven small opticalfibers: six fibers for illumination placed in a circular manner around a central single collection fiber. Overall,measurements demonstrate changes in diffused reflectance spectra, cerebral optical tissue properties (absorptionand scattering) and chromophore levels. Furthermore, we were able to identify differences in male versus femalegroups. Finally, the effectiveness of S-Adenosylmethionine as a drug therapy was studied and found to improvehemodynamic outcome. For the first, to the best of our knowledge, LRS is utilized to study variations in brainparameters in valproic acid autism model mice through an intact scalp.

Exact calculi of conic constants and bafflesfor any two mirrors Aplanatic telescope

Alberto Cordero-Davila, Andrea Rodríguez Cortés, and Sergio Vazquez-Montiel

DOI: 10.1364/AO.396712 Received 11 May 2020; Accepted 06 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: To calculate the conic constants of the primary and secondary mirrors of anyaplanatic two-mirror telescope in the past non-exact equations were used considering thirdorder approximations and it was not considered that there is an infinite number of conicconstants. In this paper exact nonlinear equations are obtained and with them the two conicconstants of the mirrors are calculated without approximations. We also find that the conicconstants depend on the position at the stop where the calculation is carried out, and there arealways residual aberrations. In addition, a procedure is established for the exact calculation ofthe baffles. Finally, examples show the differences between exact and third-order calculations.

A PSF degradation model of a polarization imagingsystem for wide-field subwavelength nanoparticles

qiong wu, Kun Gao, Zizheng Hua, Zhenzhou Zhang, Hanwen Zhao, Jichuan Xiong, and Peilin Yu

DOI: 10.1364/AO.397357 Received 11 May 2020; Accepted 06 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: We propose a comprehensive point spread function (PSF) degradation model,which considers multiple factors consisting of degradation of specimen retardant samplingand polarization angularly anamorphic sampling, to indicate the image degradationcharacteristics of polarization imaging systems. First, a one-layer optical coherencetomography (OCT) model was established to express the retardancy of medium-loadingspecimens. Then, a PSF degradation model of angularly anamorphic polarization samplingwas deduced through the retrieval of Stokes parameters. Finally, maximum a posterioriprobability (MAP) was adopted to assess the distribution of the proposed model. Hypothesistesting using actual data and numerical simulations demonstrated that the error of the systemfollowed an asymmetric generalized Gaussian distribution (AGGD). Finite-difference timedomain (FDTD) simulation results and an actual imaging experiment demonstrate theconsistency of the proposed model and the degradation characteristics of the PSF, whichprovide support for the improved accuracy and enhanced image quality of the optical fieldretrieval of nanoparticles.

Single-shot in-line Fresnel incoherent holographyusing a dual-focus checkerboarded lens

Shota Sakamaki, Naru Yoneda, and Takanori Nomura

DOI: 10.1364/AO.393176 Received 19 Mar 2020; Accepted 05 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: Fresnel incoherent correlation holography (FINCH) is a technology that can acquire three-dimensionalinformation of incoherent objects such as fluorescence with an in-line optical system. However, it is difficult for FINCH to apply to the dynamic phenomena since FINCH has to detect phase-shifted hologramssequentially to eliminate twin and zero-order images. In this paper, a method that the phase-shifted holograms can be simultaneously obtained with in-line setup by using an optimized simulated diffractionoptical element (sDOE), which is realized by a phase-only spatial light modulator, is proposed. The optimized sDOE is an optical device which has a dual-focus lens, a 2D grating, and a spatial phase shifter.Therefore, the sDOE is called a dual-focus checkerboarded lens. The optical experiment confirms thefeasibility of the proposed method.

Real-time Matching Strategy for RotaryObjects using Digital Image Correlation

Da Yang, Zhilong Su, Shuiqiang Zhang, and Dongsheng Zhang

DOI: 10.1364/AO.397655 Received 18 May 2020; Accepted 05 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: Real-time monitoring of structural health conditions for rotary objects is ofimportance for safety assessments. In this work, an efficient algorithm based on digital imagecorrelation is presented to achieve accurate rotational matching in real-time. The proposedalgorithm measures rotation in object motion with an integer pixel search followed by asubpixel correlation refinement. In the integer pixel search, the reference subset is rotatedinversely to facilitate the correlation computation between the reference and target subsets.Then an independent and global integer pixel search for each point of interest is performed byapplying the particle swarm optimization algorithm. Finally, a modified iterative registrationalgorithm is introduced to refine the displacement in the subpixel level, by considering boththe rotation angle and displacement components. Simulation and rotation experimentsdemonstrate that the proposed method achieves rapid and accurate measurements and is aneffective method for retrieving the rotation data of rotating structures.

MoS2 based broadband and highly efficient solarabsorbers

Zhanshan Sun, Fumin Huang, and yunqi fu

DOI: 10.1364/AO.399772 Received 08 Jun 2020; Accepted 05 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: In order to obtain broadband, highly efficient, wide-angle and polarization-insensitive solar absorber, we proposea universal configuration consisted of monolayer molybdenum disulfide (MoS2) and the metal-insulator-metal(MIM) structure which gives rise to significant absorption enhancement of MoS2 layer. Light trapping structureswith silver square, circle and crossed-shaped resonators are investigated. The localized surface plasmonresonances (LSPRs) among the silver resonators induce prominent interaction between incident photon and MoS2layer, contributing to efficient absorption of light energy. Simulation results show that the absorber made of squarepatches enables the best performance and realizes absorptance higher than 90% from 400 nm to 666 nm and anaverage absorptance greater than 91% in the range of 400 nm-700 nm. The average light absorption within theMoS2 layer reaches to 74% in the visible spectrum, which is one of the highest levels for the existing MoS2-basedabsorbers. Meanwhile, the polarization-independent designs exhibit good angle tolerance within 50° incidences.Such a universal structure can also obtain broadband and highly efficient absorption by using other TMDs such asMoSe2, WS2 and WSe2, which indicants that the configuration has great applicability in solar energy absorption of2D materials. The proposed solar absorbers with simple configuration and broadband absorption in wide incidentangles have potential in applications such as solar cells, photovoltaic devices and blackbody materials.

Study on High Efficiency Edge-Coupling based onLithium Niobate on Insulator Wire Waveguide

li ying, Tian Lan, Jing Li, and Zhiyong Wang

DOI: 10.1364/AO.395897 Received 22 Apr 2020; Accepted 05 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: Edge-coupling on single-crystal thin-film lithium niobate on insulator (LNOI)was systematically studied in thispaper. An inverse taper-shaped spot-size converter (SSC) to convert the mode field from the laser chip to nanoscale LNOI waveguide was adopted to improve the coupling efficiency. The structure of edge coupler was fullyinvestigated and optimized by using eigenmode expansion method. The single-mode conditions of LNOI waveguidefor three common communication bands were taken into consideration. And the length and tip width of inversetaper, the cross-section dimensions of SiON waveguide and the sidewall angle were investigated with respect to thecoupling efficiency. As a result, the maximum coupling efficiency from an edge coupler to laser chip can reach 54%,48% and 58% at 1550 nm, 1310 nm and 850 nm in z-cut LNOI for quasi-TM mode, respectively. This proposedwork gives a better understanding of the function of edge coupler based on LNOI material and provides anappropriate method for the design of edge coupler with high efficiency, which could benefit the further applicationof high-density monolithic integrated optical components.

Dislocation Analysis of Germanium Wafers Under1080 nm Laser Ablation

sha chuan, ZhiChao Jia, zewen li, Yunxiang Pan, Nan pengyu, and Xiaowu Ni

DOI: 10.1364/AO.387936 Received 01 Jul 2020; Accepted 05 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: : COMSOL Multiphysics was employed to establish a dislocation model based on the AH model, the heatconduction equation and Hooke’s law for calculating the dislocation distribution of germanium (Ge) underlaser irradiation. The numerical simulation results were obtained. A continuous 1080 nm laser was utilized toablate the monocrystalline Ge wafers to validate the model. The experimental results show that no surfacedamage appears until the irradiances go up to 4 W/cm2 for 100 ms laser ablation. This is consistent with thenumerical findings. The initiation times of surface damage by the experiments at 4 W/cm2 and above agreewell with the numerical results, which means the model can efficiently predict the dislocation field.

Intracavity birefringence controlled GHz-tuning rangepassively harmonic mode locked fiber laser based onNPR

Yuanda Ling, Qianqian Huang, Qingguo Song, Zhijun Yan, Chengbo Mou, Kaiming Zhou, and Lin Zhang

DOI: 10.1364/AO.398960 Received 29 May 2020; Accepted 04 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: We experimentally demonstrate a harmonic order controllable L band Er-doped passively mode locked fiber laserbased on nonlinear polarization rotation (NPR). Distinct from all previous reports, we find that the intracavitybirefringence is able to control the harmonic order accurately of a passively mode locked fiber laser.Experimentally, under a constant pump power of 704 mW, the harmonic order can be tuned from 113th to 39thmonotonically by adjusting polarization controller (PC) orientation only. The corresponding repetition ratechanges from 2.21 GHz to 0.77 GHz simultaneously. Remarkably, the super-mode suppression ratio (SMSR) of eachharmonic order we observed are higher than 29 dB with a maximum of 36.5 dB. Simulated transmission spectra ofNPR prove that the changed transmission plays an important role in controlling harmonic order.

Shear acoustic wave attenuation influence onacousto-optic diffraction in tellurium dioxidecrystal

Sergey Mantsevich and Ekaterina Kostyleva

DOI: 10.1364/AO.399409 Received 03 Jun 2020; Accepted 04 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: The slow shear acoustic wave attenuation in tellurium dioxide crystal (110) plane isexamined. The measurements were carried for two crystal cut-off angles: 7◦and 10.5◦from the[110] axis. The ultrasound attenuation was examined in the frequency range from 80 to 300 MHzby the acousto-optic (AO) method. The attenuation coefficients were determined for both acousticwave propagation directions for the frequency range from 200 to 300 MHz. The influence ofultrasound attenuation on the AO cell transmission functions was examined for the 10.5◦ AO cellboth theoretically and experimentally. It is shown that acoustic wave absorption affects mainlythe AO diffraction efficiency. The attenuation influence on the AO cell transmission functionshape and its half-width is not significant.

Optical Accessible Generic Exhaust Gas Test Bench for the Investigation of Fundamental SCR-relevant Processes

Anna Schmidt, Sani van der Kley, and Steven Wagner

DOI: 10.1364/AO.397574 Received 15 May 2020; Accepted 04 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: In this work, a generic exhaust gas test bench is introduced on which reproducible experiments can be performed to gain a deeper understanding of processes during exhaust gas aftertreatment of internal combustion engines. We present the design, initial flow characterization as well as tomographic measurement results of gaseous water distributions. The aim of the development was to provide a generic geometry as well as highly reproducible process boundary conditions for numerical simulation of exhaust aftertreatment phenomena. The presented initial measurements are intended to demonstrate the qualification of the test bench for extensive experimental characterization ranging from measurements of the spray injection, film evaporation and reaction kinetics to the highly complex multi-phase flow conditions during selective catalytic reduction (SCR) processes which are characterized by high mass flows and temperatures, pronounced transients and a corrosive atmosphere.

A high-speed fiber based spectrometer forplasma Thomson scattering

Drew Elliott and Alexandros Gerakis

DOI: 10.1364/AO.397599 Received 15 May 2020; Accepted 04 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: We present a novel concept for a Thomson scattering diagnostic, based on a highspeed fiber optic spectrometer. The high speed fiber optic spectrometer presented here translatesa spectral measurement from the frequency domain into the time domain, thus only requiring theuse of a single photodetector for spectral acquisition. The high temporal precision offered by theinstrument gives rise to a number of advantages over traditional spectrometers, such as nearlybackground-free measurements and multiple uses of the same injected beam. Multiple usesof the same beam would enable greatly increased measurement rates, in the range of 10 − 100MHz. The spectral range and resolution of the fiber spectrometer can be easily tailored to beoptimized for the light source and experimental conditions by selecting different lengths of fiber,thus allowing for the proposed technique to exhibit high dynamic range when measuring manypoints simultaneously. Finally, due to the temporal separation of the background from the signal,these improvements are possible without the need for increased average input laser power.

Chaotic markers in dynamic diffraction

Jenny Magnes, Harold Hastings, Miranda Hulsey-Vincent, Cheris Congo, Kathleen Raley-Susman, Anshul Singhvi, Tyler Hatch, and Erik Szwed

DOI: 10.1364/AO.397618 Received 14 May 2020; Accepted 04 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: In a dynamic far-field diffraction experiment, we calculate the largest Lyapunovexponent of a time-series obtained from the optical fluctuations in a dynamic diffractionpattern. The time series is used to characterize the locomotory predictability of an oversampledmicroscopic species. We use a live nematode, Caenorhabditis elegans, as a model organismto demonstrate our method. The time-series is derived from the intensity at one point in thediffraction pattern. This single time-series displays chaotic markers in the locomotion of theCaenorhabditis elegans by reconstructing the multidimensional phase space. The average largestLyapunov exponent (base e) associated with the dynamic diffraction of ten adult wildtype (N2)Caenorhabditis elegans is 1.27 ± 0.03 s−1.

Tunable asymmetric transmission acrossstretchable chiral metamaterial

lei zhou, yueke wang, jiaxin zhou, jian ding, mengjia lu, and Tian Sang

DOI: 10.1364/AO.395708 Received 21 Apr 2020; Accepted 03 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: A stretchable chiral metamaterial with L-shaped and T-shaped Au patterns(SCMM-LT) is proposed to generate asymmetric transmission (AT) for circularly polarizedwaves on the Polydimethylsiloxane (PDMS) substrate in mid-infrared region. The peak valueof AT can reach 50.02% at the resonance wavelength of 19.1µm, owing to theenantiomerically sensitive plasmons. With stretching along x-axis and y-axis respectively, theband of AT shifts to longer wavelength, which prove SCMM-LT can be a candidate as thetunable chiral metamaterial. In the future, the proposed stretchable chiral metamaterial couldpotentially possess high applicability for wearable electronic devices in a variety of sensorfields.

A thermal lens with passively tuned focallength formed in a photochromic material

Krzysztof Dobek and Sabina Brazevic

DOI: 10.1364/AO.396837 Received 05 May 2020; Accepted 03 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: A new way of tuning the focal length of a thermal lens is presented. The thermallens is formed in a photochromic thermo-optical material, through illumination of the materialby a heating laser beam of constant power. The tunability of the lens is achieved by changingthe absorption coefficient of the thermo-optical material at the heating laser beamwavelength. This change is obtained by modifying the intensity of the additional, flat inprofile, light of a wavelength different than that of the heating laser beam. The change inintensity of this light may follow from the effects of external factors, as e.g. decrease indaylight intensity. Therefore, the lens may be considered as a passive auto-tuning lens, withfocal length auto-adjusting to the external illumination.

Improvement in optical fiber-bundle basedimaging using synchronized fiber motion

Neil Momsen, Andrew Rouse, and Arthur Gmitro

DOI: 10.1364/AO.391825 Received 10 Mar 2020; Accepted 03 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: Image quality in fiber-bundle based imaging systems is inherently limited by thesize and spacing of the individual fiber cores. The fiber bundle limits the achievable spatialresolution and superimposes a fixed pattern of signal variability across the image. Toovercome these limitations piezoelectric tubes were used to synchronously dither the fiberbundle on both ends. Experimental results using the dithering approach with a commercialfiber bundle showed a substantial decrease in fixed pattern noise and an increase in spatialresolution.

Real-time 3D thermoacoustic imaging andthermometry using a self-calibrationtechnique

Ehab Tamimi, Hao Xin, and Russell Witte

DOI: 10.1364/AO.393083 Received 17 Mar 2020; Accepted 02 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: Thermoacoustic imaging (TA) is a modality where pulsed microwaves are used togenerate ultrasound waves in tissue, which are highly correlated with temperature. This studyuses a self-calibration approach to improve the estimation of temperature using 3D real-timeTA thermometry (TAT) in porcine tissue during localized heating. The self-calibrationmethod estimated temperatures at eight embedded thermocouple locations with a normalizedroot mean square error (NRMSE) of 3.25 ± 2.08%. The results demonstrate that the methodhas the suitable accuracy and resolution to provide feedback control for breast cancer ablationtherapy.

Bullet signature measurement with chromaticconfocal sensor

Hengzheng Wei, Weinong Wang, Xiaolin Wang, and Yiyi Li

DOI: 10.1364/AO.396273 Received 29 Apr 2020; Accepted 02 Jul 2020; Posted 02 Jul 2020  View: PDF

Abstract: A new non-contact three-dimensional (3D) bullet signature measuring system based on chromatic confocal sensoris developed. The system is composed of a precision rotary table and a chromatic confocal sensor. Themeasurement uncertainty of system is less than 1 μm. When measuring the surface topography of the object thesensor acquires the wavelength information reflected from the object instead of the intensity information. Thisadvantage is very suitable to bullet signature measurement. The chromatic confocal sensor works in the pointmeasuring mode and can acquired data continuously with high speed. One round section measurement on thebullet body only takes less than one minute.

Fast estimation method of feedback factor based onderivative of self-mixing signal

Jin-Hyok Kim, Cholhyon Kim, Dong-Kuk Kim, Hyon-Song Ri, and Gwan-Hyok Jo

DOI: 10.1364/AO.398782 Received 29 May 2020; Accepted 02 Jul 2020; Posted 02 Jul 2020  View: PDF

Abstract: The self-mixing (SM) technique has been a promising optical measurement technique for a few decades. Theestimation of the optical feedback factor C is the key to the SM displacement measurement with a better resolutionthan half a wavelength. But this estimation is very complex and tricky, thus leading to the time-consuming nature,which is one of the important causes that increase the cost and labor for the development of a self-mixing sensor.There have been reported many methods for the C estimation but they are very time-consuming or very sensitiveto noise or can merely work in a certain feedback regime. This paper presents a novel approach to the C estimation,which is straightforward and in particular, can work for all feedback regimes including a weak, moderate, andstrong feedback regime. The estimation precision of the proposed method is not only comparable to that of thedata fitting technique but also much simpler than that.

Toward a mini-Scheimpflug Lidar system forall-day atmospheric remote sensing in theboundary layer

Liang Mei, Yichen Li, Zheng Kong, Teng Ma, Zhen Zhang, Ruonan Fei, Yuan Cheng, Zhenfeng Gong, and kun Liu

DOI: 10.1364/AO.396057 Received 27 Apr 2020; Accepted 02 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: Development of a lightweight, low-cost, easy-of-use and low-maintenance lidartechnique has been of great interest for atmospheric aerosol remote sensing in recent yearsand remains a great challenge. In this work, an 808-nm mini-Scheimpflug lidar (SLidar)system with about 450-mm separation between the transmitter and the receiver has beendeveloped by employing a 114-mm aperture Newtonian telescope (F4). The systemperformances such as the beam characteristic, the range resolution, and the signal-to-noiseratio of the lidar signal have been carefully investigated. In spite of employing a smallreceiving aperture, all-day measurements were still feasible with about one-minute signalaveraging for both the horizontal urban area monitoring and the slant atmospheric sounding inthe boundary layer. The lidar signal in the region of 29-50 m with a scattering angle less than179.5° could be slightly underestimated due to the variation of the phase function. Theextinction coefficient evaluated in the region between 29-2000 m according to the Klettmethod agreed well with the concentrations of particulate matters for both horizontal andslant measurements. The promising result demonstrated in this work has shown a greatpotential of employing the robust mini-SLidar system for atmospheric monitoring in theboundary layer.

The Design of Free-Form Surface Microlens ArrayOptical System With High Efficiency and Uniformity

Zhenmin Zhu, Peiqi Yao, and Weihua Zheng

DOI: 10.1364/AO.398971 Received 02 Jun 2020; Accepted 02 Jul 2020; Posted 06 Jul 2020  View: PDF

Abstract: Microlens array has been widely applied in LED lighting source due to special optical properties,but most of the research lack the analysis and optimization of the complete mathematical models. Hence, the newdesign method of free-form surface microlens array optical system is proposed in this paper. Based on thecharacteristics of TIR and the law of refraction, a complete mathematical model of the free-form microlens isestablished. By numerically solving a set of differential equations, the profile of the free-form surface microlens isobtained. Then we rotated the profile to get the free-form surface microlens. Finally, the proposed microlens arrayis simulated and analyzed in the near-field and the far-field situations, respectively. We also discussed theinfluence of microlens array characteristics on the illumination performance. The result shows the uniformity andefficiency have been improved, both of which can reach more than 90%.

Accurate and Efficient Leap-Frog BeamPropagation Method (LF-BPM) for ModelingMicro and NanoPhotonics Structures

Ahmed Bassam Emam and Mohamed Swillam

DOI: 10.1364/AO.398025 Received 19 May 2020; Accepted 02 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: In this paper, a novel and efficient approach for solving the beam propagation method (BPM) governing equationis proposed. The approach is based on the reformulation of the beam propagation equation to solve real systemmatrices only at each propagation step. The reformulated equation utilizes a leap-frog technique to couple the realand imaginary components of the field in an iterative scheme. The method yields higher processing speed by at least30% than the conventional BPM method. To validate the proposed LF-BPM method, different photonic systems,including directional couplers, and multi-mode interferometers (MMI), are simulated. Results have beenexperimentally verified by comparing it to results measured for fabricated micro-photonic structures. A stabilityanalysis was performed to study the effect of the design parameters on the performance of the proposed scheme. Theproposed LF-BPM approach is considered as a promising technique for efficient modeling of optical structures.

Imaging Fourier Spectrometer in Visible Domain.Experimental Results.

Vladimir Protopopov

DOI: 10.1364/AO.400319 Received 16 Jun 2020; Accepted 01 Jul 2020; Posted 07 Jul 2020  View: PDF

Abstract: Performance of a prototype of a compact imaging Fourier spectrometer for visibledomain is reported. In high-frequency applications, up to 100 MHz, optical signal is recordedby a photo-multiplying tube. Selectivity of spectra modulated at 100 kHz on the backgroundof non-modulated light and possibility of measuring 50 ns decay time of molecular transitionsis experimentally demonstrated. On line-type spectra, with the same exposure time andspectral resolution, the imaging Fourier spectrometer may be 10 times more sensitive than thebest compact grating spectrometers. This superiority disappears on smooth spectra with shortcoherence length. In visible and near-infrared domains, a carefully made prototype of animaging Fourier spectrometer with a photo-multiplying tube proved to be sensitive to as lowoptical flux as 6⋅10−13 W/mm2..

Construction of photorefractive photonic quasicrystalmicrostructures by twisted square lattices

Wentao Jin, Meng Song, Yan Ling Xue, Yuanmei Gao, and Liren Zheng

DOI: 10.1364/AO.397622 Received 13 May 2020; Accepted 01 Jul 2020; Posted 01 Jul 2020  View: PDF

Abstract: A convenient method to fabricate two-dimensional photonic quasicrystal microstructures was experimentallydemonstrated by using a rotatable four-wedge prism. Two-dimensional eight-fold symmetric quasicrystalmicrostructures are formed by two groups of twisted square lattices in a photorefractive crystal. The experimentaldevices of this method are simple and stable without complicated optical adjustment equipment. Optical inducedquasicrystal microstructures are analyzed and verified by magnified imaging and far-field diffraction patternimaging. The method can be extended to fabricate more complex quasicrystal and moiré lattice microstructures.We numerically demonstrate that this method can be used to fabricate other complex photonic microstructures byusing different multi-wedge prism and adjusting the rotation angle properly.

Use of Color Information for Structured-light 3D Shape Measurement of Objects with Shiny Surfaces

Zhenmin Zhu, Chao Yang, Hailiang Zhang, FuMin Zhang, and Xuemei Liu

DOI: 10.1364/AO.397393 Received 12 May 2020; Accepted 01 Jul 2020; Posted 02 Jul 2020  View: PDF

Abstract: Objects with shiny surfaces cannot be directly measured using the conventional structured-light method. To cope with this problem, in this paper, we propose a novel method for removing the saturated components in an image. First, the specular pixels in the image are identified using a highlight extraction algorithm. Then, based on the reflection component separation (RCS) theory, the diffuse and specular components of these specular pixels are separated. For objects with shiny surfaces, use of the RCS approach destroys the color information of highlighted pixels with a large specular reflection component. As such, finally, the color information of the highlighted pixels is recovered using an image repair process. Our experimental results indicate that 95% of the highlights in the images were eliminated. We found the color information of images repaired using this method to be consistent with that of real objects. This proposed method effectively overcomes the interference of reflected light and provides a feasible solution to the problems associated with the structured-light measurement of objects with smooth surfaces.

Deep-learning-based single photon countingcompressive imaging via jointly trained subpixel convolution sampling

Qiurong Yan, Wencheng Li, Yanqiu Guan, Shengtao Yang, Cong Peng, and Zheyu Fang

DOI: 10.1364/AO.394410 Received 06 Apr 2020; Accepted 01 Jul 2020; Posted 02 Jul 2020  View: PDF

Abstract: The combination of single pixel imaging and single photon counting technologycan achieve ultra-high sensitivity photon counting imaging. However, its applications in highresolution and real-time scenarios are limited by the long sampling and reconstruction time.Deep learning based compressive sensing provides an effective solution due to its ability toachieve fast and high-quality reconstruction. This paper proposes a sampling andreconstruction integrated neural network (HRSC-Net) for single photon counting compressiveimaging. To effectively remove the blocking artifact, a sub-pixel convolutional layer is jointlytrained with deep reconstruction network to imitate compressed sampling. By modifying theforward and backward propagation of the network, the first layer is trained into a binarymatrix which can be applied to the imaging system. An improved deep reconstructionnetwork based on the traditional Inception network is proposed and the experimental resultsshow that its reconstruction quality is better than existing deep-learning-based compressivesensing reconstruction algorithms.

Extraction Method of Non-Uniform Auxiliary LaserStripe Feature for Three-Dimensional Reconstructionof Large Components

Yang Zhang, Wei Liu, Wenqi Wang, Peng Gao, Hongwen Xing, and Jianwei Ma

DOI: 10.1364/AO.394309 Received 10 Apr 2020; Accepted 30 Jun 2020; Posted 30 Jun 2020  View: PDF

Abstract: The 3D measurement plays an important role in the processing and assembling of large components in the aviation and aerospace industry.However, precision control is a challenging problem due to the complex on-site illumination environment and serious background interference. Forthe binocular stereo vision measurement system based on auxiliary laser scanning, this paper proposed an extraction method of laser stripe for 3Dreconstruction. First, an evaluation method for the laser stripe is proposed by analyzing the features of the stripe image. Then, a laser stripeextraction method based on self-adaptive threshold is proposed. To further improve the efficiency of image processing, an improved Kalman filteralgorithm is adopted to fast track and locate the region of interest (ROI) of laser stripes in the sequence images. At last, measurement experiments fora large-scale aircraft panel are carried out on site. The results show that the center extraction error is less than 0.1pixel and three-dimensionalreconstruction error is less than 0.06mm. The proposed methods improve the efficiency and accuracy of 3D reconstruction of large components, andthe feasibility of on-site application is also verified.

High-sensitivity SPR sensor based on eight-foldeccentric core PQF with locally coated indium tin oxide

Qiang Liu, Jiudi Sun, Yudan Sun, Wei Liu, Jingwei Lv, Chao Liu, Xianli Li, Zonghuan Ren, Famei Wang, wenshu lu, yu jiang, Tao Sun, and Paul Chu

DOI: 10.1364/AO.395605 Received 28 Apr 2020; Accepted 30 Jun 2020; Posted 30 Jun 2020  View: PDF

Abstract: A highly sensitive surface plasmon resonance (SPR) sensor comprising an eccentric core photonic quasi-crystal fiber (PQF)coated with indium tin oxide (ITO) is designed and numerically analyzed. The novel structure with an eccentric core layoutand local coating not only strengthens coupling between the core mode and surface plasmon polariton (SPP) mode, but alsoprovides higher refractive index sensitivity in the near-infrared region. Analysis based on the finite element method toassess the performance of the sensor and optimize the structural parameters reveals that the maximum wavelengthsensitivity and resolution are 96667 nm/RIU and 1.034×10−6 RIU in the sensing range between 1.380 and 1.413, respectively.Meanwhile, the average sensitivity is enhanced to 25458 nm/RIU. The sensor is expected to have broad applications inenvironmental monitoring, biochemical sensing, food safety testing, and related applications due to the ultra-high sensitivityand resolution.

Image based aberration retrieval using helicalpoint spread functions

René Berlich and Sjoerd Stallinga

DOI: 10.1364/AO.396140 Received 24 Apr 2020; Accepted 30 Jun 2020; Posted 30 Jun 2020  View: PDF

Abstract: A practical method for determining wavefront aberrations in optical systems based onthe acquisition of an extended, unknown object is presented. The approach utilizes a conventionalphase diversity approach in combination with a pupil-engineered, helical point-spread-function(PSF) to discriminate the aberrated PSF from the object features. The analysis of the image’spower cepstrum enables an efficient retrieval of the aberration coefficients by solving a simplelinear system of equations. An extensive Monte-Carlo simulation is performed to demonstratethat the approach makes it possible to measure low order Zernike modes including defocus,primary astigmatism, coma and trefoil. The presented approach is tested experimentally byretrieving the two-dimensional aberration distribution of a test setup by imaging an extended,unknown scene.

Chlorophyll absorption and phytoplankton sizeinformation inferred from hyperspectralparticulate beam-attenuation

Henry Houskeeper, David Draper, Raphael Kudela, and Emmanuel Boss

DOI: 10.1364/AO.396832 Received 04 May 2020; Accepted 30 Jun 2020; Posted 30 Jun 2020  View: PDF

Abstract: Electromagnetic theory predicts spectral dependencies in extinction efficiencynear a narrow absorption band for a particle with an index of refraction close to that of themedium in which it is immersed. These absorption band effects are anticipated in oceanographicbeam-attenuation (beam-c) spectra, primarily due to the narrow red peak in absorption producedby the phytoplankton photopigment, chlorophyll a (Chl a). Here we present a method to obtainChl a absorption and size information by analyzing an eigendecomposition of hyperspectralbeam-c residuals measured in marine surface waters by an automatic underway system. We findthat three principal modes capture over 99% of the variance in beam-c residuals at wavelengthsnear the Chl a red absorption peak. The spectral shapes of the eigenvectors resemble extinctionefficiency residuals attributed to absorption band effects. Projection of the eigenvectors ontothe beam-c residuals produces a time series of amplitude functions with absolute values thatare strongly correlated to concurrent Chl a absorption line height (aLH) measurements (r valuesof 0.59 to 0.83) and hence provide a method for estimating Chl a absorption. Multiple linearregression of aLH on the amplitude functions enables an independent estimate of aLH, withRMSE of 3.19 · 10−3 m−1(3.3%) or log10-RMSE of 18.6%, and a raw-scale R2 value of 0.894based on the Tara Oceans Expedition data. Relationships between the amplitude functions andthe beam-c exponential slopes are in agreement with theory relating beam-c to the particle sizedistribution. Compared with multispectral analysis of beam-c slope, hyperspectral analysis ofabsorption band effects is anticipated to be relatively insensitive to the addition of non-pigmentedparticles and to monodispersion.

Clear Sky Blinks

David Lynch and Steven Richtsmeier

DOI: 10.1364/AO.390043 Received 10 Feb 2020; Accepted 30 Jun 2020; Posted 01 Jul 2020  View: PDF

Abstract: Ice blinks and water skies are brightness variations on theundersides of overcasts that allow ground-based observers tojudge the nature of distant surfaces such as water (dark) or ice(bright). The clear sky should also scatter light from distantsurfaces that might be visually detectable. We demonstrate thatclear sky blinks do occur, can be visually discerned and can besuccessfully photographed. We also model them theoreticallyusing Monte Carlo simulations. The presence of atmosphericaerosols significantly enhances clear sky blinks.

Accurate three-dimensional measurement for smallobjects based on the thin-lens model

Zhangji LU and Lilong Cai

DOI: 10.1364/AO.394754 Received 23 Apr 2020; Accepted 30 Jun 2020; Posted 01 Jul 2020  View: PDF

Abstract: Both accuracy and depth of field (DOF) are required for small objects’ measurement in advanced manufacture and accurate robotics industries. In this paper, a stereo vision system with structured-light basedon the thin-lens model is developed to measure small objects with high accuracy and extended DOF athigh magnification. The model of the proposed stereo vision system is built. The new system calibrationscheme and measurement procedures are proposed. The DOF of the used thin-lens model is extended byutilizing autofocus capability. With the thin-lens model, accurate calibration and extended DOF at highmagnification can be achieved. Three measurement experiments are conducted with the proposed systemto evaluate its effectiveness and accuracy. The error of the protrusions’ height on the reconstructed aluminum part is lower than 0.55% and the standard deviation (STD) of a fitting plane reaches 1.7 µm. Lowcost, high accuracy and extended DOF can be simultaneously achieved for measuring small objects withthe proposed system based on the thin-lens model.

Theoretical and Experimental Comprehensive Study ofGHz-Range Passively Mode-Locked Fiber Lasers

Hector Muñoz, Javier Abreu-Afonso, Gaia Sardiello, and Pere Pérez-Millán

DOI: 10.1364/AO.394072 Received 31 Mar 2020; Accepted 29 Jun 2020; Posted 01 Jul 2020  View: PDF

Abstract: A theoretical model based on the Non-LinearSchrödinger Equation to characterize GHz rangepassively mode-locked fiber lasers was developed.The modeled cavities of the lasers are configured bya highly doped and polarization- maintaining singlefiber of a single type. For different pulse repetitionrates, ranging from 1.0 GHz to 10.0 GHz, gain parameters and pump threshold for a stable mode-lockedlaser emission are studied. Pulse time width, spectralwidth and SESAM (Semiconductor Saturable AbsorberMirror) properties are defined to achieve stable emission. To experimentally validate our theoretical model,1.0 GHz and 2.2 GHz laser cavities have been built upand amplified. A stable and robust operation for bothfrequencies was obtained and the experimental measurements have been found to match the theoreticalpredictions. Finally, enhanced environmental stabilityhas been achieved using a cavity temperature controlsystem and an antivibration enclosure.

Fast laser beam steering into multiple diffractionorders with a single digital micromirror devicefor time-of-flight lidar

Joshua Rodriguez, Braden Smith, Brandon Hellman, and Yuzuru Takashima

DOI: 10.1364/AO.393075 Received 17 Mar 2020; Accepted 29 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: The sampling rate and angular resolution of diffraction-based beam steeringemploying a Digital Micromirror Device (DMD) can be simultaneously enhanced by at leastan order-of-magnitude by synchronizing multiple nanosecond laser sources and pulsesduring each DMD actuation. A time-of-flight single-chip DMD lidar with three sourcesmeasures range at a 3.34 kHz sampling rate and a 3.4° angular resolution across a 48° fieldof view. Employing multiple diffraction orders of the DMD improves the sampling rate atleast by a factor two and up to the number of diffraction orders supported by the DMD. Animproved sampling rate of 6.68 kHz with a 9.6° angular resolution is experimentallydemonstrated by illuminating micromirrors multiple times within a single transition periodof the micromirrors.

Objective analysis of collagen organization in thyroidnodule capsules using second harmonic generationmicroscopy images and the Hough Transform

Juan Bueno, Francisco Avila, Radu Hristu, Lucian Eftimie, and Stefan Stanciu

DOI: 10.1364/AO.393721 Received 26 Mar 2020; Accepted 29 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: Papillary carcinoma is the most prevalent type of thyroid cancer. Its diagnosis requires accurate and subjectiveanalyses from expert pathologists. Here we propose a method based on the Hough Transform (HT) to detect andobjectively quantify local structural differences in collagen thyroid nodule capsules. Second Harmonic Generation(SHG) microscopy images were acquired on non-stained histological sections of capsule fragments surrounding thehealthy gland, and benign and tumoral/malignant nodules. The HT was applied to each SHG image to extractnumerical information on the organization of the collagen architecture in the tissues under analysis. Results showthat control thyroid capsule samples present a non-organized structure composed of wavy collagen distributionwith local orientations. On the opposite, in capsules surrounding malignant nodules, a remodeling of the collagennetwork takes place and local undulations disappear, resulting into an aligned pattern with a global preferentialorientation. The HT procedure was able to quantitatively differentiate thyroid capsules and capsules surroundingpapillary thyroid carcinoma (PTC) nodules. Moreover, the algorithm also reveals that the collagen arrangement ofthe capsules surrounding benign nodules significantly differs from both the thyroid control and PTC nodulecapsules. Combining SHG imaging with the HT results thus in an automatic and objective tool to discriminatebetween the pathological modifications that affect the capsules of thyroid nodules across the progressions of PTC,with potential to be used in clinical settings to complement current state-of-the-art diagnostic methods.

Focal field analysis of highly multi-mode fiber beamsbased on modal decomposition

Hao Pang, Tobias Haecker, alexandre bense, Tobias Haist, and Daniel Flamm

DOI: 10.1364/AO.397498 Received 21 May 2020; Accepted 29 Jun 2020; Posted 30 Jun 2020  View: PDF

Abstract: In this work, a numerical modal decomposition approach is applied to model the optical field of laserlight after propagating through a highly multi-modefiber. The algorithm for the decomposition is basedon the reconstruction of measured intensity profilesalong the laser beam caustic with consideration of intermodal degrees of coherence derived from spectralanalysis. To enhance the accuracy of the model, different approaches and strategies are applied and discussed. The presented decomposition into a set of LPmodes enables both the wave-optical simulation of radiation transport by highly multi-mode fibers and, additionally, the analysis of free-space propagation witharbitrarily modified complex amplitude distributions.

Advanced iterative algorithm for phase calibration ofspatial light modulators integrated in opticalinstrumentation in a vibration environment

Manuel Silva-López, Néstor Uribe-Patarroyo, and Alberto Alvarez-Herrero

DOI: 10.1364/AO.391723 Received 11 Mar 2020; Accepted 29 Jun 2020; Posted 30 Jun 2020  View: PDF

Abstract: We present a method to obtain the phase modulation characteristic curve of a spatial light modulator(SLM) under severe vibration conditions. The procedure is based on the well-known advanced iterativealgorithm (AIA), which allows wavefront extraction from unkown phase-shifted interferograms. Generally, AIA is used to determine the wavefront and the determined phase shifts are of little interest. Incontrast, in our method, the main goal of using AIA is to determine the unknown phase shifts inducedby an SLM during the calibration procedure. Using a segmented approach to calibration, AIA enablessuccessful calibration even in presence of additional random phase shifts due to environmental changes.This method has the potential to calibrate SLMs integrated in complex optical instruments with littleto no modifications to the optical setup, no matter the environmental conditions. We demonstrate ourtechnique by calibrating an SLM under vacuum conditions (10−5 mbar) in a common-path configurationcompatible with usage of an SLM as a wavefront modulator at the pupil plane of an instrument. Our technique compensates the vibrations produced by the vacuum pumps and reduces an order of magnitude theroot-mean-squared error of the calibration curve evaluated with vibration errors. Our technique enhancesthe potential use of SLMs in complex optical systems, including aerospace optical instrumentation.

Process regimes during welding of glass byfemtosecond laser pulse bursts

Sebastian Hecker, Markus Blothe, Daniel Großmann, and Thomas Graf

DOI: 10.1364/AO.392702 Received 13 Mar 2020; Accepted 29 Jun 2020; Posted 30 Jun 2020  View: PDF

Abstract: Various process regimes were observed during micro-welding of glass with bursts of ultrashort laser pulses. Twomajor welding regimes and various sub-regimes were identified for two different materials. The radiation emittedby the laser-induced plasma was used to monitor different regimes that characterize glass micro-welding. Acomprehensive understanding of the various process regimes can be exploited to use the regimes according totheir specific advantages, especially for industrial applications.

Light transmission characteristics analyses of laserscreen in clear water based on Monte Carlo method

Fan Wang, Jinping NI, Hui Tian, and Tong Yang

DOI: 10.1364/AO.394947 Received 10 Apr 2020; Accepted 29 Jun 2020; Posted 30 Jun 2020  View: PDF

Abstract: In order to optimize the design of underwater light screen, the transmission process of thelight beam in water is analyzed with Monte Carlo method. The trajectories and power attenuation ofemitted photons, propagating in different types of water from the light source towards the receiver withdifferent initial light powers and transmission distances, are simulated and calculated. The outputpower can then be obtained to calculate the transmission ratio quantitatively. According to thesimulated data, by using curving fitting method, the exact expression of transmission ratio is derived,which is in the same form with Beer-Lambert-Bouguer law. Meanwhile, experiments are conducted ina light screen with a vertical line beam laser in clear water to verify the simulations. The experimentalresults are in agreement with the simulated results and the differences between them are within ±0.04.The presented results give insight into the design of underwater laser screen.

Experimental determination of full matrices ofthe piezo-optic and elasto-optic coefficientsfor Pb5Ge3O11 crystals

Iryna Martynyuk-Lototska, Oksana Mys, Taras Dudok, Bohdan Mytsyk, Natalya Demyanyshyn, Myroslav Kostyrko, Dmitro Adamenko, Mikhaylo TRUBITSYN, and Rostyslav Vlokh

DOI: 10.1364/AO.398218 Received 21 May 2020; Accepted 28 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: We determine full matrices of piezooptic and elastooptic coefficients for leadgermanate crystals, Pb5Ge3O11, using Dixon–Cohen and interferometric methods. Twodifferent types of samples are studied, single- and multi-domain ones. We show that thecorrect data for the Pb5Ge3O11 crystals can be obtained only on single-domain samples, sincethe domain structure induces a number of features that make the final results unreliable. Wedemonstrate that the acousto-optic figures of merit reach high enough values at the isotropicacousto-optic interactions with longitudinal acoustic waves propagating along the principal Xand Z axes (63×10–15 and 37×10–15 s3/kg, respectively). At the same time, the efficiency ofacousto-optic interactions with transverse acoustic waves is low. The same is true concerninganisotropic acousto-optic diffraction, since the appropriate elasto-optic coefficients are smallenough.

Utilization of image phase information to achievesuper-sampling

James Caron

DOI: 10.1364/AO.396437 Received 14 May 2020; Accepted 28 Jun 2020; Posted 09 Jul 2020  View: PDF

Abstract: For image phase-based super-sampling, an image sequence consisting of slightly displaced frames is upsampled, aligned, and averaged into a single larger image that possesses image resolution exceeding thelimitations of the imaging system. This process obtains a significant portion of high-resolution phaseinformation and models the missing magnitude using deconvolution or reconstruction algorithms. Threesimulations are presented in which a 32-frame sequence with size 256 by 256 pixels are processed tocreate a single 4096 by 4096 pixel image with pixel level resolution. An empirical test was also conductedshowing resolution beyond the digital sampling resolution limit of the camera.

Improved difference model applied in Fouriertransform-based integration method based on Taylortheory

xuanrui gong, Zhuang Sun, yaowen lv, Zhaoguo Jiang, and Xiping Xu

DOI: 10.1364/AO.393949 Received 30 Mar 2020; Accepted 27 Jun 2020; Posted 29 Jun 2020  View: PDF

Abstract: The two-dimensional Fourier-transform-based integration algorithm is widely used in shape or wave-frontreconstruction from gradients. However, its reconstruction accuracy is limited by the truncation error of thedifference model. The truncation error is affected by the distribution of the sampling points. It increaseswhen the sampling points are unevenly distributed and arranged irregularly. For improving, a novel way tocalculate the difference is proposed based on Taylor expansion theory of binary functions. The first-orderpartial derivative terms are used to estimate the second- and third-order partial derivative terms forreducing the truncation error. The proposed difference model is applied to Fourier-transform-basedintegration. The reconstruction results show that it can get better results when the sampling points areirregularly distributed.

Reducing field depth: annular Hadamard masks

Cristina Gomez-Sarabia, Luis Ledesma-Carrillo, and Jorge Ojeda-Castaneda

DOI: 10.1364/AO.397862 Received 18 May 2020; Accepted 26 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: For optically narrowing field depth, we explore the use of annular masks that are coded with the elements of theHadamard matrices. We show that with no further reduction of light throughput, these annular masks can enhancethe influence of focus error on the MTF. We report numerical evaluations of the irradiance PSFs, their associatedMTFs and of some digitally generated images. For complementing our assessments, we evaluate the Fisherinformation of these Walsh-Hadamard, annular masks. We suggest an optical technique for breaking 3-D scenesinto several 2-D sliced frames, by translating out-of-focus images into background noise.

Performance optimization of a reservoir computingsystem based on a solitary semiconductor laser underelectrical-message injection

Qing-Qing Zeng, Zheng-Mao Wu, Dian-Zuo Yue, Xiang-Sheng Tan, Jun-Yao tao, and Guang-Qiong Xia

DOI: 10.1364/AO.394999 Received 14 Apr 2020; Accepted 25 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: A simple reservoir computing (RC) system based on a solitary semiconductor laser (SL) under electrical messageinjection is proposed, and the performances of the RC are numerically investigated. Considering the lack ofmemory capacity (MC) in such a system, some auxiliary methods are introduced to enhance the MC and optimizethe performances for processing complex tasks. Except adopting the pre-existing method that the inputinformation is the current input data combined with some past input data in a weighted sum in the input layer(named as M-input), another auxiliary method named as M-output is proposed to introduce into the output layerfor optimizing the performances of the RC system. The simulated results demonstrate that the MC of the systemcan be improved after adopting the auxiliary methods, and the effectiveness under adopting the M-inputintegrated with the M-output (named as M-both) is the most significant. Furthermore, we analyze the systemperformances for processing Santa Fe time series prediction task and nonlinear channel equalization (NCE) taskafter adopting above three auxiliary methods, and the results show that the M-input is the most suitable for theprediction task while the M-both is the most appropriate for the NCE task.

Beam Tracking Algorithm for Marine Applicationusing Visible Light Communication

renhai feng, yanyan Chang, and Xurui Mao

DOI: 10.1364/AO.394597 Received 08 Apr 2020; Accepted 25 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: Visible light communication (VLC) takes pride in its unique advantage of electromagnetic interference immunity and wide bandwidth. It gradually becomes the main candidate in marinecommunication with great potential. As a green communication link, VLC requires reliablebeam tracking as a prerequisite. Therefore, based on the received signal strength (RSS) of a single input multiple output system in the VLC scenario, this paper firstly proposes a geometricalalgorithm for transmitter localization. On this basis, a linear iterative algorithm using Taylor expansion, implicit function theorem, and time-domain expansion is presented to realize onlinebeam tracking. Under the marine VLC system, iterative denoising algorithm based on hiddenMarkov model (HMM) and principal component analysis (PCA) is proposed for denoising. Simulation results show that the proposed algorithms have predominance on high tracking accuracy(within 10cm) and desirable real-time performance.

Correcting lateral chromatic aberration usingorthogonal fringe patterns

Chao Chen and Bing Pan

DOI: 10.1364/AO.394936 Received 15 Apr 2020; Accepted 25 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: In this paper, we analyzed the distribution of lateral chromatic aberration (LCA)and proposed a high-accuracy and flexible LCA correction method by using orthogonal fringepatterns displayed on a liquid crystal display (LCD) screen. In the proposed method, red,green, and blue orthogonal fringe patterns are consecutively displayed on the LCD screen.These fringe patterns are captured by a color camera from the front viewpoint for calculatingthe absolute phase maps. Then, a pixel deviation matrix (PDM) of LCA is determined bycomparing the obtained phase maps of three channels. Finally, LCA among the red, green,and blue channels of the color camera is corrected through the obtained PDM andinterpolation method in application stage. Both simulated and real experiments verified thevalidity of the proposed method.

Large field of view beaconless laser nutation trackingsensor based on MEMS mirror

Chenzhe Lao, Jianfeng Sun, pei hou, Yu Zhou, Zhiyong Lu, Hongyu He, RONGLEI HAN, XINYU CAI, and YUEXIN LI

DOI: 10.1364/AO.396490 Received 01 May 2020; Accepted 25 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: We propose a laser nutation tracking sensor for beaconless laser communication, which uses a MicroElectro-Mechanical System (MEMS) mirror to achieve high-efficiency and large-amplitude nutation at itsresonant frequency. And we derive a new formula for the case of incompletely detectable optical powerin nutation cycle. In the experiment, we measure the performance of the sensor in calculating boresighterror under three different nutation radius. Combining with the proposed algorithm for the new scene,we complete the accurate boresight calculation in the range of ±200urad at the nutation radius of 4.9um. We trust that the receiving field of view (FOV) of this tracking sensor can be further expanded byincreasing the nutation radius. The sensor, as proposed in this paper, will be of constructive help tosimplify tracking system in the future.

Theoretical and experimental study on the testingaccuracy of the image stabilization system of spaceastronomical telescope

ChengHao Li, Xu He, Qi Ji, zhang xiaohui, and Kuo Fan

DOI: 10.1364/AO.388891 Received 24 Jan 2020; Accepted 24 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: Space astronomical telescopes must test the image stabilization accuracy of their image stabilization system prior toentering the orbit. According to the position characteristics of the fine guidance sensor (FGS) of telescopes required bythe China National Space Administration, a simulation model of moving guide stars is proposed in this study to test theimage stabilization accuracy of space telescopes. The simulation model for synchronous moving guide stars is based onthe principle of mutual collimation of the diagonal field of view of the optical system of telescope. Realize the simulationof homologous, multi-field, high synchronization precision of moving guide stars. Compared with traditional methods,this method requires simple manufacturing, and has high flexibility. The error sources affecting the simulation accuracyof the moving guide stars are analyzed, and an error model is established. Analysis results show that the simulationaccuracy of the moving guide stars can reach 0.046”. The simulation model of guide stars is verified throughexperiments in which the influence of the vibration isolation platform is considered. Experiment results show that thesimulation accuracy of motion guide stars is 0.036”, which satisfies the accuracy requirements (< 0.08”) of the accuracytest for the telescope image stabilization of telescopes.

Spectral characteristics and optical temperaturesensing properties of Er3+/Yb3+ co-doped phosphateglasses with GeO2 modification

Bingtao Zhang, Yong Wang, Zigang Zhao, Wei Yan, Mingming Huo, and Guihuan Chen

DOI: 10.1364/AO.394269 Received 15 Apr 2020; Accepted 24 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: The Er3+/Yb3+ co-doped phosphate glasses with GeO2 modification (PLAZGs) were successfully prepared by meltquenching method. The phenomenological intensity parameters Ωt (t=2, 4, 6) of the PLAZGs have been calculated by theJudd-Ofelt theory. Based on the phenomenological intensity parameters, the spectroscopic parameters of Er3+ andfluorescence intensity ratio (FIR) of green upconversion emissions were estimated. It was observed that, under 980 nmexcitation, all samples exhibit green and red upconversion emisssions of Er3+. The 10 mol% GeO2 modified phosphate glasshas the strongest upconversion emission. Additionally, the fluorescence decays of 2F5/2 → 2F7/2 transition of Yb3+ ions weremeasured to evaluate the energy transfer efficiency from Yb3+ to Er3+ ions. Finally, the optical temperature sensingproperties based on upconversion emissions were investigated at temperature from 150 to 600 K. The maximum absolutetemperature sensitivity S value of 6.0×10-3 K-1 at 400 K is obtained, which indicates that the glass is promising fortemperature sensing application based on the FIR technology.

Piston error correction of sparse aperturesystems using meta-heuristic stochasticparallel gradient descent algorithm

Kaiyuan Yang, Zongliang Xie, Haotong Ma, Hongyi Hu, bo qi, Jianliang Shi, and Wenyi Lv

DOI: 10.1364/AO.394501 Received 08 Apr 2020; Accepted 24 Jun 2020; Posted 26 Jun 2020  View: PDF

Abstract: The next generation of optical telescopes will provide high-resolution imaging ofcelestial objects by using the aperture synthesis technique. To preserve the quality of theimage, fast corrections of the pistons among sub-apertures have to be applied, namely the cophasing of the array. The image-based co-phasing method via an optimization procedure hasbeen newly developed. Despite simpleness and strong commonality, when dealing with largepiston errors, this correction method is also faced with a problem that the metric function iseasy to fall into the local convergence especially in the case of broadband imaging with manysub-apertures. In this study, an improved stochastic parallel gradient descent (SPGD)algorithm based on heuristic search is proposed for co-phasing, termed as meta-heuristicSPGD algorithm. The heuristic research scheme assists the original SPGD algorithm ingetting rid of local extremums. By iterations of this algorithm, the synthetic system can be cophased without any additional instruments and operations. The effectiveness of the proposedalgorithm is verified by means of simulation. Given the efficiency and superiority, it isexpected that the method proposed in this study may find wide applications in multi-apertureimaging.

Real-time infrared image detail enhancementbased on fast guided image filter and plateauequalization

Yaohong Chen, Jin Kang, Gaopeng Zhang, Jianzhong Cao, Qingsheng Xie, and Chiman Kwan

DOI: 10.1364/AO.396417 Received 29 Apr 2020; Accepted 24 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: Image detail enhancement is critical to the performance of infrared imagingsystems as the original images generally suffer from low contrast and low signal-to-noise ratio(SNR). Although conventional decomposition-based methods have advantages in enhancingimage details, they also have clear disadvantages such as intensive computations, overenhanced noise, and gradient reversal artifacts. In this paper, we propose to accelerateenhancement processing by using a fast guided filter and plateau equalization. Our methodconsists of image decomposition, base and detail layers processing, and projection of theenhanced image to 8-bit dynamic range. Experimental results demonstrated that our proposedmethod achieves a good balance among detail enhancement performance, noise and gradientreversal artifacts suppression, and computational cost with a frame rate around 30fps for640x512 infrared images, meaning that it will have a promising prospect in real-timeapplications.

Replacing a Thin Lens by a Thick Lens

Antonin Miks and Pavel Novak

DOI: 10.1364/AO.396726 Received 04 May 2020; Accepted 23 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: The paper deals with the problem of replacing a thin lens by the thick lens with approximately the same properties as the thin lens. The equations enabling calculation of the parameters of the thick lens, which has the same value of focal length and one of the Seidel aberration coefficients (either Seidel aberration coefficient of spherical aberration or Seidel aberration coefficient of coma) as the thin lens are derived. Comparison of the proposed method for calculation of parameters of the thick lens with existing methods is given on examples. Furthermore, the problem of replacing one thick lens made of optical glass of a given value of refractive index by another thick lens with different refractive index but with the same value of focal length and same value of the Seidel aberration coefficient of spherical aberration is investigated.

Generalized Circuit Model for Analysis ofExtraordinary Transmission in Multilevel Slits

Seyyed Pouriya Kashfi and Zahra Ghattan Kashani

DOI: 10.1364/AO.393297 Received 20 Mar 2020; Accepted 23 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: We propose a generalized circuit model for accurate analysis of the extra ordinarytransmission phenomenon in multi-level stepped slit structures. The multi-level stepped slitconsists of some continuous sub-wavelength metal slits with different widths. By applying theproposed circuit model, we study the extra ordinary transmission property of this structure forthe two cases which are the ideal and real metal structures. The accuracy of the achieved resultsis validated by a full wave analysis based on the finite element method. Additionally, since themulti-level slit structure is simply adjustable by manipulating the slit parameters such as widthsand heights, various kinds of transmission spectra and a different number of resonance points areinvestigated by the circuit model and the numerical approach. The multilevel stepped slit can bepotentially used as bandpass filters or optical attenuators in photonics integrated circuits.

Characteristics of photonic jets generated by adielectric sphere illuminated by a Gaussian beam

Jianqiang Huang, Yiming Zhao, Hanlin Yang, Jiajie Wang, Paul Briard, and Yiping Han

DOI: 10.1364/AO.393424 Received 23 Mar 2020; Accepted 23 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: Photonic jets (PJs) formed on the shadow side of micro-sized dielectric spheres excited by focused Gaussian beamsare investigated within the framework of the generalized Lorenz-Mie theory (GLMT). The intrinsic advantages ofrapidity and high accuracy of GLMT in calculations enable us a systematic study of PJs at a low cost and a highreliability. To reveal the influence of beam parameters on the properties of PJs, numerical results concerningvariations of key parameters of PJs, including the maximal intensity, the focal distance which is linked to theposition of maximal intensity, longitudinal and transversal dimensions are presented with the change of the beamwaist radius and the focal center location of the Gaussian beam. The results show that as the beam waist radiusapproaches to the radius of the particle, the energy stream of the Gaussian beam contributes more efficiently to theformation of PJs. By properly tuning the location of the beam focal center, the PJ pattern can be efficientlyengineered in a large extent.

Silicon optical filter with multiple semi-rings

Jing Wan, Jiangyun Miao, Jinwen You, Linlin Xu, and Xinyue Xie

DOI: 10.1364/AO.396269 Received 29 Apr 2020; Accepted 23 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: Here a silicon optical filter with multiple semi-rings is proposed. Itutilizes multiple semi-rings to realize the optical filtering. Its structure is simple,adjustable and integrated. By changing the number and spacing distance ofsemi-rings, the properties of the optical filter can be flexibly adjusted to meetdifferent application requirements. The researching results show that the opticalfilter has a 3 dB bandwidth of 2.52 nm, a free spectral range of 11.33 nm and afinesse of 4.50. The silicon optical filter with multiple semi-rings is compared witha Mach-Zehnder-interferometer based optical filter with the traditional structureand shows better filter performance. Our work provides a new method for findingadjustable optical filters with compact structure and good performance.

Using femtosecond laser pulses for ElectronicDistance Meter Calibration

Haitham Hussein, Osama Terra, Hatem Hussein, and Mohammed Abdelrahman

DOI: 10.1364/AO.393852 Received 30 Mar 2020; Accepted 22 Jun 2020; Posted 23 Jun 2020  View: PDF

Abstract: Electronic distance meters (EDMs) are widely used in different applications suchas surveying and civil engineering. In order to calibrate an EDM, different techniques can beused including displacement interferometers and reference baselines. In this paper, an indoorbaseline is designed and then accurately measured using femtosecond laser pulses from anoptical frequency comb to be used for EDM calibration. The baseline consists of 13 fixedbases that cover 58 meters long. In order to accurately measure the distances between thebases, autocorrelation between femtosecond laser pulses is employed. The measurementshows a maximum precision of 14 μm over the 13 bases. Although this deviation isdominated mainly by the placement of the target mirror, the system capability is much moresufficient to safely calibrate the best available commercial EDM. The stability of the baselineis also investigated by measuring the inter-base distances over long periods of times.

Raman mediated ultrafast all-optical NOR gate

TANMOY DATTA and Mrinal Sen

DOI: 10.1364/AO.396859 Received 04 May 2020; Accepted 21 Jun 2020; Posted 25 Jun 2020  View: PDF

Abstract: A new device architecture has been proposed in this paper implementing the all-optical cascadable logicNOR functionality. The device functions based on stimulated Raman scattering (SRS) in silicon nanocrystal embedded slotted photonic crystal waveguide (SPCW). Substantial miniaturizations both in operatingpower and overall footprint of the device have been achieved owing to the ultra-high SRS gain of siliconnanocrystal and strong spatio-temporal confinement of the SPCW. Successful operation of the device hasbeen demonstrated at a pulse rate which is as high as 125 Gbps.

An improved progressive TIN densificationfiltering algorithm for airborne LiDAR data basedon a multiscale cylindrical neighborhood

Xiankun Wang, Xincheng Ma, Fanlin Yang, Dianpeng Su, Chao Qi, and Shaobo Xia

DOI: 10.1364/AO.394341 Received 03 Apr 2020; Accepted 21 Jun 2020; Posted 23 Jun 2020  View: PDF

Abstract: Laser point cloud filtering is a fundamental step in various applications of lightdetection and ranging (LiDAR) data. The progressive triangulated irregular network (TIN)densification (PTD) filtering algorithm is a classic method and is widely used due to itsrobustness and effectiveness. However, the performance of the PTD filtering algorithmdepends on the quality of the initial TIN-based digital terrain model (DTM). The filteringeffect is also limited by the tuning of a number of parameters to cope with various terrains.Therefore, an improved PTD filtering algorithm based on a multiscale cylindricalneighborhood (PTD-MSCN) is proposed and implemented to enhance the filtering effect incomplex terrains. In the PTD-MSCN algorithm, the multiscale cylindrical neighborhood isused to obtain and densify ground seed points to create a high-quality DTM. By linearlydecreasing the radius of the cylindrical neighborhood and the distance threshold, the PTDMSCN algorithm iteratively finds ground seed points and removes object points. To evaluatethe performance of the proposed PTD-MSCN algorithm, it was applied to fifteen benchmarkLiDAR datasets provided by the ISPRS commission. The experimental results indicated thatthe average total error can be decreased from 5.31% when using the same parameter set to3.32% when optimized. Compared with 5 other publicized PTD filtering algorithms, theproposed PTD-MSCN algorithm is not only superior in accuracy but also more robust.

Distortion of twisted beam passing through a plasmalayer

Davod Nobahar and Hossein Akou

DOI: 10.1364/AO.394698 Received 08 Apr 2020; Accepted 21 Jun 2020; Posted 23 Jun 2020  View: PDF

Abstract: In this paper, we explore what happens to intensity profile, phase distribution, and centroid position ofvortex beam (VB) when it passes through a cold collision-less magnetized plasma layer. For this purpose,we utilize angular spectral expansion accompanied by 4×4 matrix method to obtain total transmission coefficient, intensity and phase profiles, and centroid shifts of VB in the output plane. Based on numericalanalyses, it is found that the evolution of transverse intensity as well as the distortion of phase profile oftransmitted VB are greatly affected by variation of radial and angular mode numbers, external magneticfield, plasma number density, and incident angle. In addition, displacement of VB centroid under variation of angular mode number is presented quantitatively. It is expected that the results of this study willgive more insight into VB communication, radar probing, and plasma diagnostics.

Tunable ultra-narrowband near-infrared absorber made of a nanodisk-based metasurface and a phase-change material Ge2Sb2Te5 layer

Shihao Zhang, Kun Zhou, Qiang Cheng, Lu Lu, BoWen Li, Jinlin Song, and Zixue Luo

DOI: 10.1364/AO.395046 Received 14 Apr 2020; Accepted 21 Jun 2020; Posted 23 Jun 2020  View: PDF

Abstract: A tunable absorber made of a nanodisk-based metasurface is proposed to realize an ultra-narrowband near-infrared perfect absorption. By introducing a phase-change material Ge2Sb2Te5 (GST) layer, we produce a selective and active control of the optical response. It is found that the absorption can be improved from about 62.1% for crystalline GST (cGST) to 99.7% for amorphous GST (aGST) at the corresponding resonant wavelength, which is attributed to the strong electric dipole resonance in the germanium nanodisks. Furthermore, under the aGST state, the full width at half-maximum (FWHM) of 7 nm can be acquired for both TM and TE polarizations and such a nanodisk-based absorber enables a tunable operating wavelength by adjusting geometrical parameters to realize the spectral selectivity. In addition, the nanodisk-based metasurface nanostructure combined with a dielectric Bragg reflector (DBR) with alternately stacked SiO2 and TiO2 layers can realize the near-infrared highly efficient dual-band absorption for aGST and single-band absorption for cGST through the adjustment of electric and magnetic resonances. The designed absorbers have the potential applications in tunable absorption filter, thermal sensing, and optical signal processing.

Wavefront-Coding with Jacobi-Fourier phasemasks for retinal imaging

Eva Acosta Plaza, Miguel Olvera-Angeles, Enrique González, Jose Sasian, Jim Schwiegerling, and Justo Arines

DOI: 10.1364/AO.391941 Received 17 Mar 2020; Accepted 20 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: Wavefront coding is a technique that combines optical phase elements and digital signalprocessing in order to increase the effective depth of focus of optical systems. The success ofwavefront coding lies in the design of a suitable phase mask placed at the system’s aperture.This element allows for image formation invariant under the effects of different second orderoptical aberrations. In optical systems limited by temporally or spatially varying high orderaberrations the use of wavefront coding has not been fully demonstrated. Here, we proposethe choice of Jacobi-Fourier shaped phase masks to produce sharp and clear retinal images ofliving eyes. To demonstrate the potential use of the technique, we analyze the performance ofthe Jacobi-Fourier phase masks through experimental simulations to alleviate aberrations fordifferent eye aberrations. We will show the best mask choice is robust to noise while keepingacceptable resolution and reducing image artifacts.

Accuracy improvement of quantitative LIBS analysis of coal properties using hybrid model based on wavelet threshold de-noising and feature selection method

Peng Lu, Zhuang ZHUO, Wenhao Zhang, jing tang, hailong tang, and Jing Qi Lu

DOI: 10.1364/AO.394746 Received 10 Apr 2020; Accepted 20 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: A hybrid model based on wavelet threshold de-noising (WTD) and recursive feature elimination with cross validation (RFECV) method was proposed to improve the measurements in quantitative analysis of coal properties using laser-induced breakdown spectroscopy (LIBS). Firstly, a modified threshold of WTD was proposed based on wavelet coefficient theory. Interference of noise in LIBS spectrum was reduced by using this modified method. Then, RFECV method was applied to extract effective features from the de-noised LIBS spectrum. Finally, support vector regression (SVR) models of coal properties were established by the selected features. A validation set was used to verify the effectiveness and robustness of the hybrid model. The improvement of the hybrid model on the quantitative analysis of each index of coal properties (heat value, ash, volatile content) were studied and discussed. By using the proposed model, the determination coefficient (R2), root mean square error of prediction (RMSEP) and the average relative error (ARE) were all significantly improved over the original spectra model. The results demonstrated that the proposed model could effectively improve the accuracy of LIBS quantitative analysis for coal properties.

Model of a tunable hybrid Tamm mode–liquid crystaldevice

Maxim Pyatnov, R. G. Bikbaev, Ivan Timofeev, and Stepan Vetrov

DOI: 10.1364/AO.395901 Received 22 Apr 2020; Accepted 20 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: A concept of an easily tunable device based on hybrid Tamm modes is proposed. The device can be controlled using a high-sensitivity chiral liquid crystal serving as a mirror. The coupling of the chiral opticalTamm state with the Tamm plasmons is predicted. The Tamm plasmons are excited at different frequencies for the orthogonal linear polarizations, while the chiral Tamm state is excited at only one frequency.The properties of the proposed model are analytically and numerically calculated. The possibility of creating a two- and three-mode laser with tunable characteristics on the basis of the proposed model arediscussed.

Machine-learning informed predictor importancemeasures of environmental parameters in maritimeoptical turbulence

Christopher Jellen, John Burkhardt, Cody Brownell, and Charles Nelson

DOI: 10.1364/AO.397325 Received 15 May 2020; Accepted 20 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: Prediction of the index of refraction structure constant C_n^2 in the low-altitude maritime environment is challenging. To improve predictive models, deeper understanding of the relationships between environmental parameters and optical turbulence is required. To that end, a robust data set of C_n^2 as well as numerous meteorological parameters were collected over the period of approximately 15 months along the Chesapeake Bay adjacent to the Severn River in Annapolis, Maryland. The goal was to derive new insights into the physical relationships affecting optical turbulence in the near-maritime environment. Using data-driven machine learning feature selection approaches, the relative importance of twelve distinct, measurable environmental parameters were analyzed and evaluated. Random forest nodal purity analysis was the primary machine learning approach to relative importance determination. The relative feature importance results indicated that air temperature and pressure were important parameters in predicting C_n^2 in the maritime environment. In addition, the relative importance findings suggest that the air-water temperature difference, temporal hour weight, and time of year, as measured through seasonality, have strong associations with C_n^2 and could be included to improve model prediction accuracy.

Stimulated emission assisted time-gated detection of a solid-state spin

Bowen Zhao, Xiangdong Chen, Enhui Wang, Yu Zheng, Bo Du, Shen Li, dong yang, Guang-can Guo, and Fang-Wen Sun

DOI: 10.1364/AO.397499 Received 15 May 2020; Accepted 19 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: Nitrogen vacancy (NV) center in diamond is widely studied for the magnetic field and temperature sensing at the nanoscale. Spontaneous emission is recorded to estimate the spin state of NV center. To improve the contrast of the spin-dependent fluorescence signal, we applied a time-gating technique for the detection of NV center. Synchronized 532 and 765 nm pulsed lasers were used to pump the spontaneous and stimulated emission of NV center. The stimulated emission would deplete the spontaneous emission after the NIR laser pulse. Then, the spontaneous emission of NV center in a certain time window was extracted by comparing the fluorescence intensity with different NIR laser's delay. The results showed that the spin-dependent fluorescence contrast could be improved approximately 1.8 times with the time-gating. And the background of the environment was eliminated due to temporal filtering. This work demonstrated that the stimulated emission assisted time-gating technique can be used to improve the performance of an NV center sensor in a noisy environment.

Fano Resonance Based on D-shaped Waveguide Structure and Its Application for Human Hemoglobin Detection

Xing Liu, Jina Li, Jianfeng Chen, Rohimah Siti, He Tian, and Jifang Li

DOI: 10.1364/AO.397976 Received 19 May 2020; Accepted 19 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: Fano resonance is a pervasive resonance phenomenon which can be applied to high sensitivity sensing, perfect absorption, electromagnetic induced transparency and slow light photonic devices. In this paper, we propose a metal–insulator–metal (MIM) waveguide structure consisting of a D-shaped cavity and a bus waveguide with a silver-air-silver barrier. The Fano resonance can be achieved by the interaction between the D-shaped cavity and the bus waveguide. The finite element method is used to analyze the transmission characteristics and magnetic-field distributions of the structure in detail. Simulation results show the Fano resonance can be adjusted by altering the geometric parameters of the MIM waveguide structure or the refractive index of the D-shaped cavity. The maximum refractive index sensitivity of the structure can reach up to 1510nm/RIU, and there is a good linear relationship between resonance wavelength and refractive index. Since its good sensitivity and tunability, the MIM waveguide structure can be used in bio-sensing, such as human hemoglobin detection. We show its applicability for the detection of three different human blood groups as well.

Electroluminescence Image Analysis of aPhotovoltaic Module under Accelerated LifecycleTesting

Teh Lai, Kelly Potter, and Barrett Potter

DOI: 10.1364/AO.391957 Received 03 Mar 2020; Accepted 19 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: Electroluminescence (EL) imaging of Si-based photovoltaic (PV) modules iswidely used to spatially detect and characterize electrical defects, including handling anddegradation-induced cracking of the component Si cells that are associated with reductions inmodule performance. In the present study, a commercial polycrystalline silicon PV modulewas subjected to accelerated lifecycle (ALC) test environmental conditions and examined as afunction of environmental exposure time using EL imaging. The approach followed pixelintensity distributions over each individual PV cell and confirmed a positive correlationbetween module conversion efficiency and the results of the image analysis. Overall, anaverage of a 2.5% reduction in normalized EL intensity was correlated to a 0.35% reductionin actual power conversion efficiency (or a 2.3% decrease in relative efficiency). The imaginganalysis technique offers a rapid, unsupervised means to assess EL data in lieu ofconventional visual interpretation.

Efficiently Enhancing Co-Occurring Details WhileAvoiding Artifacts for Light Field Display

Jian Wei, Shigang Wang, Yan Zhao, Mei-Lan Piao, and Chenxi Song

DOI: 10.1364/AO.392152 Received 17 Mar 2020; Accepted 19 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: The ability of the human visual system (HVS) to perceive a three-dimensional (3D) image at once isfinite, but the detail contrast of the light field display (LFD) is typically degraded during both acquisitionand imaging stages. It is consequently difficult for the viewers to rapidly find a region of interest fromthe displayed 3D scene. Existing image detail boosting solutions suffer from noise amplification, overexaggeration, angular variations, or heavy computational burden. In this paper, we propose a selectiveenhancement method for the captured light field image (LFI), that empowers an attention-guiding LFD.It is based on the fact that the visually salient details within a LFI normally co-occur frequently in bothspatial and angular domains. These co-occurrence statistics are effectively exploited. Experimental resultsshow that, the LFDs improved by our efficient method are free from undesirable artifacts and robustto disparity errors while remaining correct parallaxes and occlusion relationships, thus reducing HVS’sefforts to cognitively process the 3D images. Our work is the first in-depth research on computational andcontent-aware LFD contrast editing, and expected to facilitate numerous LFD-based applications.

On-demand inkjet-printed microdisk laser with aircladding by liquid flow microetching

Taku Takagishi, Hiroaki Yoshioka, Yuya Mikami, and Yuji Oki

DOI: 10.1364/AO.396061 Received 30 Apr 2020; Accepted 19 Jun 2020; Posted 22 Jun 2020  View: PDF

Abstract: We have novelly developed the liquid flow microetching method that can treat a single microdisk in amicro region with precise position control for inkjet-printed microdisk lasers. The injection-drain wetetching setup consisted of two micro needles successfully performed a formation of a fine undercut structure of an inkjet-printed microdisk on a pre-pedestal layer through the individual wet etching process.Then measurement of the undercut structure using a scanning electron microscopy and lasing characteristics with whispering gallery modes were carried out to demonstrate performance of the etched microdisks.The measured lasing threshold decreased by half compared with that of the unetched microdisk directlyprinted on a fluorine-type film. A point to be noted is that this etching method exhibits an excellentundercut and lasing characteristics even when using a clad pre-pedestal layer having a refractive indexhigher than that of core microdisks. This technique, combined with inkjet-printing, offers a powerfultool for individually designing a microdisk and can help develop novel devices that comprise severalinkjet-printed microdisks being evanescently coupled.

Single-shot multi-spectral angle-resolved ellipsometry

GARAM CHOI, Seung Woo Lee, Heui Jae Pahk, and Sin Yong Lee

DOI: 10.1364/AO.396907 Received 07 May 2020; Accepted 18 Jun 2020; Posted 18 Jun 2020  View: PDF

Abstract: We describe an instantaneous measurement scheme of multi-spectral angle-resolved ellipsometry with acolor camera. A back focal plane image captured by the color camera enables to simultaneously measurevarious polarization states along incidence angles and a multi-spectral domain. Modified parametersbased on the principle of micro-ellipsometry are derived for an adequate form in an RGB domain byconsidering the wide-band multi-spectral acquisition. The proposed method is validated by the measurement of uniformly deposited films and comparing our results with a commercial ellipsometer. Thecomparison shows that our proposed method enables real-time inspection with high accuracy and precision.

Error analysis and compensation for a discrete dualrotating retarder Mueller matrix polarimeter

Emanuel Chironi and C. Iemmi

DOI: 10.1364/AO.397482 Received 12 May 2020; Accepted 18 Jun 2020; Posted 18 Jun 2020  View: PDF

Abstract: In this work, the error sources that affect a dual rotating retarder polarimeter, working in a discrete rotation scheme are studied. Moreover, those errors not sufficiently analyzed in the literature are addressed indetail. To this end, the equations necessary for characterizing its components, performing its calibrationand carrying out measurements, are deduced. On the other hand, we discuss strategies to perform theexperimental implementation, correct the existing errors and estimate the margin of uncertainty associated with those errors that cannot be corrected. The study developed in this work allows us to generatea polarimeter with an error margin of 0.2 %, almost an order of magnitude below the recently reportedvalues.

Nonnull interferometric testing of spherical gratingsunder Littrow conditions with opposite diffractionorders

weijian liu, Qun Yuan, RUOYAN WANG, wen ji, Yingze Xue, Jun Ma, Lingjie Wang, and Zhishan Gao

DOI: 10.1364/AO.398003 Received 20 May 2020; Accepted 18 Jun 2020; Posted 18 Jun 2020  View: PDF

Abstract: Diffracted wavefront measurements are qualitative and comprehensive verifications for the spherical grating thatwas manufactured to specifications. Direct interferometric testing of the diffracted wavefront is convenient andimplemented by tilting the spherical grating at a Littrow angle to obtain autoreflection, and then results in anonnull interferometric testing configuration. The diffracted wavefront of the spherical grating contains not onlywavefront errors induced by the manufacturing imperfections, but also inherent wavefront contributions from theautoreflection testing setup. The magnitudes of the latter are affected by both the spherical substrate and thegroove pattern. Through the analysis of geometric aberrations of spherical gratings, the groove patterncontributions are demonstrated to be contrary for the opposite diffraction orders. A nonnull interferometrictesting of spherical gratings is proposed without foreknowledge of the groove pattern, in which the wavefronterrors contributed only by the manufacturing imperfections are derived from dual measurements under Littrowconditions with opposite diffraction orders. Simulations are implemented for varied line spacing (VLS) sphericalgratings with F-number slower than 1.5 and groove density varying from 150 to 300 lp/mm, and the residual errorless than 0.004λ RMS is obtained. The residual misalignment error after conventionally removing defocus and tiltis further analyzed and discussed. A VLS grating that NA is 0.13 and groove density is 200lp/mm, is chosen as anexperimental sample, the diffracted wavefront error with 0.018λ RMS is obtained.

SNR dependence of measurement stability ofheterodyne phase-sensitive optical timedomain reflectometry

Yang Lu, Zhijie Yu, Zewu Ju, Xiaoyang Hu, Mo Chen, and Zhou Meng

DOI: 10.1364/AO.394877 Received 09 Apr 2020; Accepted 17 Jun 2020; Posted 18 Jun 2020  View: PDF

Abstract: Phase-sensitive optical time-domain reflectometry (Φ-OTDR) implementsdistributed vibration measurements by demodulation of vibration induced phase of Rayleighbackscattered light-waves (RBL), and suffers from measurement instability. Weak intensitiesof RBLs and resulting low signal-to-noise ratios (SNR) of intensity measurements are thedominating factors that cause the instability of vibration measurements. In this paper,dependence of the measurement stability of heterodyne Φ-OTDR on the SNR of the intensitymeasurement is investigated analytically and experimentally. An analytical solution of theprobability density function of demodulated phase as a function of SNR is obtained throughrigorous derivation, and the dependence of the measurement stability on the SNR isinvestigated by analyzing the probability density function distribution of demodulated phase.Both the theoretical predictions and experimental results reveal the impact of SNR on themeasurement stability of heterodyne Φ-OTDR. This study fulfills the Φ-OTDR theory, andwould lead to an effective approach to stabilizing vibration quantization.

Design of freeforms to uniformly illuminatepolygonal targets from extended sources viaedge ray mapping

Daniel Birch and Matthew Brand

DOI: 10.1364/AO.392193 Received 09 Mar 2020; Accepted 16 Jun 2020; Posted 16 Jun 2020  View: PDF

Abstract: We consider the design of a compact freeform optical surface that uniformly irradiatesan arbitrary convex polygonal region from an extended light source while controlling spill. Thisproblem has attracted a large literature, mainly treating highly symmetric special cases or caseswhere the solution is approximated by a zero-étendue design based on a point source. Practicalversions of this illumination design problem will likely feature large asymmetric LEDs, compactlenses, and irregular targets on angled projection surfaces. For these settings, we develop asolution method based on an edge ray mapping that routes maximally off-axis rays from the edgesof the source through the edge of the optic to the edges of the target polygon. This determinesthe sag and normals along the boundary of the freeform surface. A "spill-free" surface is theninterpolated from the boundary information, and optimized to uniformize the irradiance whilepreserving the polygonal boundary. Highly uniform irradiances (relative standard deviation< .01) can be attained with good control of spill, even when the exit surface is < 3 sourcediameters from the embedded source.

Mitigation of truncation effects in elongatedShack-Hartmann laser guide star wavefrontsensor images

Richard Clare, Stephen Weddell, and Miska Le Louarn

DOI: 10.1364/AO.394751 Received 09 Apr 2020; Accepted 16 Jun 2020; Posted 16 Jun 2020  View: PDF

Abstract: Laser guide star Shack-Hartmann wavefront sensor images on Extremely Large Telescopes(ELT) will be significantly elongated due to the off-axis projection of the laser relative to thesubapertures. The finite number of pixels of the wavefront sensor detector means the mostelongated images will be truncated, introducing errors in the centroid measurements. In thispaper, we propose appending from a high resolution non-truncated reference image, which canbe calculated from all of the low resolution images, the most likely missing tails to the truncatedwavefront sensor image. We show via numerical simulation that we can improve the centroidestimate for the most elongated subapertures on an ELT in the presence of read and photon noise.

Approximate multi-object filter with known SNR information for optical sensor system

Weijian Si, Hongfan Zhu, and ZHIYU QU

DOI: 10.1364/AO.384609 Received 03 Dec 2019; Accepted 16 Jun 2020; Posted 17 Jun 2020  View: PDF

Abstract: In the image plane of an optical sensor, the amplitude information (AI) is very reliable for distinguishing returns generated by actual targets or coming from clutter generators. However, the majority of recently derived multi-object filters based on Mahler’s finite set statistics (FISST) theory have ignored utilizing this information. This paper proposes an approximate multi-object filter with additive AI applied for optical sensor systems. The algorithm is operated on an image plane generated by the optical sensor which has been pre-processed. After each prediction step, we sample multiple particles to approximate the prior multi-object density. Moreover, at the update step, we employ the amplitude feature likelihood for situations where the signal-to-noise ratio (SNR) information of targets is known. The loopy belief propagation (LBP) method with sequentially updated initialization messages is designed to solve the data association problem involved in the update step of the multi-object particle (MOP) filter. We analyze the convergence performance of the LBP algorithm with additive amplitude information and sequentially updated initialization messages, an ad-hoc method for improving the performance of the AI-aided LBP is designed.

Simulations of photothermal effects andthermodynamics induced by opticalresonance in fiber metallic Fabry-Perot cavity

Pengfei Hao, Min Li, Xiaoyan Wen, Ho-Pui Ho, and Haifei Lu

DOI: 10.1364/AO.394144 Received 31 Mar 2020; Accepted 14 Jun 2020; Posted 15 Jun 2020  View: PDF

Abstract: Our simulations revealed that highly localized optic-thermal transformation canlead to high temperature in the Fiber based Metallic Fabry-Perot cavity (FMFP) due to opticalresonance. Both Transfer Matrix Method and FDTD method are used for optical analysis ofFMFP. Empirical formulas of maximum temperature were derived based on superpositionprinciple. Despite the fact that the derivation of resonance condition for FMFP is usuallydiscarded due to complexity, we propose a simple resonance condition for metallic FabryPerot cavity (MFP). In addition, suddenly tuning on the incident light will cause fast-decayingair pressure and velocity, which are also solved from nanosecond-scale to equilibrium. Thispaper is useful for estimating the heat tolerance threshold of nanostructures on fiber endsurface. Photothermal conversion in FMFP provides excellent miniature heat source forapplications that require high efficiency photothermal conversion, and FMFP is particularlysuitable for optofluidics.

Thin-film InAs/GaAs quantum dot solar cell withplanar and pyramidal back reflectors

Timo Aho, Farid Elsehrawy, Antti Tukiainen, Sanna Ranta, Marianna Raappana, Riku Isoaho, Arto Aho, Arttu Hietalahti, Federica Cappelluti, and Mircea Guina

DOI: 10.1364/AO.396590 Received 30 Apr 2020; Accepted 12 Jun 2020; Posted 15 Jun 2020  View: PDF

Abstract: Quantum dot solar cells are promising for next generation photovoltaics owing to their potential for improveddevice efficiency related to bandgap tailoring and quantum confinement of charge carriers. Yet implementingeffective photon management to increase the absorptivity of the quantum dots is instrumental. To this end, theperformance of thin-film InAs/GaAs quantum dot solar cells with planar and structured back reflectors is reported.The experimental thin-film solar cells with planar reflector exhibited a bandgap-voltage offset of 0.3 V with anopen circuit voltage of 0.884 V, which is one of the highest values reported for quantum dot solar cells grown bymolecular beam epitaxy. Using measured external quantum efficiency and current-voltage characteristics, weparametrize a simulation model that was used to design an advanced reflector with diffractive pyramidal gratingsrevealing a 12-fold increase of the photocurrent generation in the quantum dot layers.

Diel variability of bulk optical properties associated with the growth and division of small phytoplankton in the North Pacific Subtropical Gyre

Fernanda Henderikx Freitas, Mathilde Dugenne, Francois Ribalet, Annette Hynes, Benedetto Barone, David Karl, and Angelicque White

DOI: 10.1364/AO.394123 Received 01 Apr 2020; Accepted 05 Jun 2020; Posted 05 Jun 2020  View: PDF

Abstract: Cross-platform observing systems are requisite to capture the temporal and spatial dynamics of particles in the ocean. We present simultaneous observations of bulk optical properties, including the particulate beam attenuation (cp) and backscattering (bbp) coefficients, and particle size distributions collected in the North Pacific Subtropical Gyre. Clear and coherent diel cycles are observed in all bulk and size-fractionated optical proxies for particle biomass. We show evidence linking diurnal increases in cp and bbp to daytime particle growth and division of cells, with particles <7 μm driving the daily cycle of particle production and loss within the mixed layer. Flow cytometry data reveal the nitrogen-fixing cyanobacterium Crocosphaera (~4-7 μm) to be an important driver of cp at the time of sampling, whereas Prochlorococcus dynamics (~0.5 μm) were essential to reproduce temporal variability in bbp. This study is a step towards better characterizing the particle size range represented by in-situ bulk optical properties and understanding the mechanisms that drive variability in particle production in the oligotrophic open ocean.

Ultra-fast All Optical Half-Adder Realized by CombiningAND/XOR Logical Gates Using Nonlinear Nano RingResonator

parisa sami, CHAO SHEN, and Mojtaba Hosseinzadeh Sani

DOI: 10.1364/AO.392428 Received 11 Mar 2020; Accepted 01 Jun 2020; Posted 02 Jun 2020  View: PDF

Abstract: We proposed an optical Half-Adder design using nonlinear materials. Photonic crystal structure shape ofhexagonal lattice layout and silicon dielectric rods in air bed. The optical Half-Adder structure is designed andoptimized by combining the AND and XOR logic gates. The compound AlGaAs is used as a nonlinear material instructure. The linear part of AlGaAs material is n1=1.4 and the non-linear part is n2=1.5×10-17. The mainperformance for all-optical logic gates is set at an operating wavelength of 1.55 μm. The time delay at all opticalgates provided has a 3.1 PS response time with on/off contrast ratio for SUM and CARRY ports of 12.78 dB and12.9 dB, respectively, and a bit rate is 0.322 Tb/sec. In the best case, the 1/0 contrast ratio between Input=1 andInput=0 of the is 4.59 dB. The computations are performed using the finite difference two-dimensional method (2DFDTD).

A Free Space 16-ary OAM Coded OpticalCommunication System Based on Chaotic-Interleavingand CNNs

Shimaa Elmeadawy, Hossam Shalaby, Nabil Ismail, Fathi Abd El-Samie, and Ahmed Farghal

DOI: 10.1364/AO.390931 Received 20 Feb 2020; Accepted 29 May 2020; Posted 01 Jun 2020  View: PDF

Abstract: Recently, Orbital Angular Momentum (OAM) rays passing through free space have extradited tremendous attention. Throughout free space, the OAM states are subject to atmospheric turbulence (AT) distortion leadingto crosstalk and power discrepancies between OAM states. In this paper, a novel chaotic interleaver with LDPCcoded OAM-shift keying through AT channel is used with convolution neural network (CNN) to enhance theperformance of wireless optical system. The detection process with 2D chaotic interleaver using conjugate lightfield method has a better performance compared to CNN adaptive demodulator for different values of propagation distances. Also, the viability of the proposed model is authenticated by convoying a digital image underdistinctive turbulence conditions and diverse codes. The impacts of different turbulence strengths, transmission distances, signal to noise ratios (SNRs), and diverse values of CNN parameters and hyperparameters areinvestigated and taken into consideration. The proposed CNN is chosen with the optimal values of parametersand hyperparameters that yield the highest accuracy, utmost Mean Average Precision (MAP), and the greatestvalue of Area Under Curve (AUC) for the different optimizers. The simulation results affirm that the proposedmodel can achieve better Peak SNR (PSNR) and lower Mean Square Error (MSE) under different AT conditionswhen the CNN classification capability is restricted. By computing accuracy, MAP, and AUC of the proposedmodel, we realize that using Stochastic Gradient Descent with Momentum (SGDM) or Adaptive Moment Estimation (ADAM) optimizers have a supreme performance compared to Root Mean Square Propagation (RMSProp)optimizer by about 3%, 1.5% and 2%, respectively.

Attitude computation algorithm for star camera basedon combining calibration and attitude determinationprocesses

Mi Wang, Zhao ping, Shuying Jin, and Yufeng Cheng

DOI: 10.1364/AO.391448 Received 26 Feb 2020; Accepted 22 May 2020; Posted 25 Jun 2020  View: PDF

Abstract: In this paper, a new approach for calculating star camera attitudes, which can calculate calibration parameters andattitude information simultaneously, in situations where precise camera calibration results are unknown, isproposed. This algorithm combines the calibration and attitude determination processes, achieving significantlyimproved performance as a result. Experiments using 1500 star images from different sky regions show that,compared with the traditional method that separates calibration and attitude determination, the proposedalgorithm not only exhibits more precise and stable results, but also shows improved tolerance for the starmismatching which is inevitable in star sensor data process technology.

Polarizing interferometer for the unambiguousdetermination of the ellipsometric parameters

Arnulf Röseler and Ulrich Schade

DOI: 10.1364/AO.392538 Received 11 Mar 2020; Accepted 19 May 2020; Posted 19 May 2020  View: PDF

Abstract: We report on a polarizing interferometer-ellipsometer arrangement whichovercomes the need for additional measurements with a retarder for the unambiguousdetermination of the ellipsometric parameters in the far infrared spectral range. It consists of aMartin-Puplett interferometer and a wire-grid polarizer as analyzer. The application of suchinterferometer-ellipsometer is experimentally demonstrated on a polyethylene samplederiving the refractive index and the thickness in the spectral range between 15 and 35 cm-1.Based on these results a similar solution without retarder for the mid-infrared spectral regionis additionally proposed.

High Precision THz-TDS via Self ReferencedTransmission Echo Method

Jon Gorecki, Nick Klokkou, Lewis Piper, Sakellaris Mailis, Nikitas Papasimakis, and Vasilis Apostolopoulos

DOI: 10.1364/AO.391103 Received 20 Feb 2020; Accepted 28 Apr 2020; Posted 28 Apr 2020  View: PDF

Abstract: Terahertz time-domain spectroscopy is a powerful characterisation technique whichallows for the frequency dependent complex refractive index of a sample to be determined. This isachieved by comparing the time-domain of a pulse transmitted through air to a pulse transmittedthrough a material sample, however the requirement for an independent reference scan canintroduce errors due to laser fluctuations, mechanical drift, and atmospheric absorption. In thispaper we present a method for determining complex refractive index without an air reference, inwhich the first pulse transmitted through the sample is compared against the ’echo’ pulse wherethe internal reflections delay the transmission of the echo pulse. We present a benchmarkingexperiment in which the echo reference method is compared to the traditional air method, andshow the echo method is able to reduce variation in real refractive index.

Leonardo Spaceborne Infrared Payloads forEarth Observation: SLSTRs for CopernicusSentinel 3 and PRISMA Hyperspectral Camera forPRISMA Satellite

Peter Coppo, Fabio Brandani, Marco Faraci, Francesco Sarti, Michele Dami, Leandro Chiarantini, Beatrice Ponticelli, Lorenzo Giunti, Enrico Fossati, and Massimo Cosi

DOI: 10.1364/AO.389485 Received 10 Feb 2020; Accepted 13 Apr 2020; Posted 17 Jun 2020  View: PDF

Abstract: Leonardo has been involved in the realization of several infrared payloads forEarth Observation since 1990. Among the currently in orbit operative instruments there arethe two SLSTRs and PRISMA. The SLSTRs are high accuracy radiometers which provideSea Surface Temperature (SST) data continuity with respect to previous (A)ATSRs, in orderto serve climatology over the next 20-years, in the frame of the ESA (European SpaceAgency) Sentinel-3 mission, which is part of the Copernicus program. The PRISMA Programis the first ASI (Agenzia Spaziale Italiana) optical hyperspectral mission for Earthobservation. It is based on a high spectral resolution spectrometer operating in the VIS-SWIRchannels optically integrated with a PANchromatic camera (PAN).

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