Accepted papers to appear in an upcoming issue
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Inverse Source in Near Field: the Case of Strip Current
Raffaele Solimene, Maria Maisto, and Rocco Pierri
Doc ID: 308848 Received 10 Oct 2017; Accepted 20 Mar 2018; Posted 21 Mar 2018 View: PDF
Abstract: In this paper we consider an inverse source problem in near zone for a prototype conﬁguration where the ﬁeld radiated by a bounded strip magnetic current is observed over a rectilinear and parallel (to the source) observation domain. The study focuses on how the achievable performance depends on the conﬁguration’s geometric parameters and the noise. In particular, we succeed in working out an approximation for the singular spectrum of the pertinent radiation operator which allows to obtain analytical estimations for the resolution, the number of degrees of freedom and information content. Remarkably, the role of evanescent waves, that become relevant in very near zone, is highlighted in connection to the available signal to noise ratio.
Temporal modes of stationary and pulsed quasistationary electromagnetic beams
Lutful Ahad, Jari Turunen, Ari Tapio Friberg, and Tero Setala
Doc ID: 319842 Received 15 Jan 2018; Accepted 14 Mar 2018; Posted 15 Mar 2018 View: PDF
Abstract: We present a novel time-domain coherent-mode representation for random, stationary electromagneticbeams. We subsequently introduce random, quasistationary pulsed electromagnetic beams and developan analogous (pseudo) mode decomposition for them as well. The former decomposition is valid providedthe time window in which the field is considered is much longer than the coherence time, while thelatter requires the field to vanish outside the window. For stationary beams the theory is demonstratedby an example illustrating the role of polarization in the representation. In both cases the data neededfor the construction of the mode decomposition is straightforward to measure. The formalisms enable totreat random vector-light beams in time domain in terms of deterministic fields. We expect that the modalrepresentations will find a wide range of applications in problems involving spatiotemporal propagationof temporally partially coherent light in optical systems.
Effect of interaction between the internal cavity andexternal cavity on beam properties in a spectrally beamcombined system
Zhen Wu, Takashi Ito, Hidefumi Akiyama, and Bin Zhang
Doc ID: 313256 Received 10 Nov 2017; Accepted 13 Mar 2018; Posted 16 Mar 2018 View: PDF
Abstract: A high-power beam can be achieved in spectrally beam combined (SBC) systems due to the interaction between an internalcavity and an external cavity. In order to investigate effects of the coupled cavity on the combined beam, we started by thederivation of the equivalent reflectance and transmittance of the external cavity based on a multilayer dielectric grating (MDG).Then, photons in active regions of diode lasers with/without feedback were calculated by utilizing Lang-Kobayashi rateequations. The results indicate that the lens and the MDG have obvious impact on both the reflectance and transmittance of theexternal cavity, which determine whether the photons of laser with feedback is larger or smaller than that without feedback. Thephotons of the laser with feedback and the transmittance of the external cavity codetermine the combining efficiency. Impacts ofthe confinement factor and spontaneous emission factor of diode laser emitters on the coupled cavity are similar to those on acommon laser internal cavity. Spectral compositions of the combined beam have different brightness from each other.
Tunable optical properties of plasmonic Au/Al2O3nanocomposite thin films analyzed by spectroscopicellipsometry accounting surface characteristics
jyoti jaiswal, Satyendra Mourya, Gaurav Malik, and Ramesh Chandra
Doc ID: 317883 Received 27 Dec 2017; Accepted 13 Mar 2018; Posted 15 Mar 2018 View: PDF
Abstract: In the present work, we have fabricated the plasmonic gold/alumina nanocomposite (Au/Al2O3 NC) thin films onthe glass substrate at room temperature (RT) by RF magnetron co-sputtering. The influence of the film thickness(~10-40 nm) on the optical and other physical properties of the samples was investigated and correlated with thestructural and compositional properties. The XRD measurement revealed the formation of Au nanoparticles withaverage crystallite size (5-9.2 nm) embedded in an amorphous Al2O3 matrix. The EDX & XPS results confirmed theformation of Au/Al2O3 NC quantitatively & qualitatively and it was observed that atomic% of Au increased byincreasing thickness. The optical constants of the plasmonic Au/Al2O3 NC thin films were examined by variableangle spectroscopic ellipsometry (VASE) in the wide spectral range of 246-1688 nm, accounting the surfacecharacteristics in the optical stack model, and the obtained results are expected to be unique. Additionally, athickness dependent blue-shift (631-590 nm) of surface plasmon resonance (SPR) peak was observed in theabsorption spectra. These findings of the plasmonic Au/Al2O3 NC films may allow the design and fabrication ofsmall, compact, and efficient devices for optoelectronic and photonic applications.
A unified description of three-dimensional optical diffraction microscopy: from transmission microscopy to optical coherence tomography: Tutorial
Anne Sentenac and Jerome Mertz
Doc ID: 313379 Received 13 Nov 2017; Accepted 09 Mar 2018; Posted 13 Mar 2018 View: PDF
Abstract: In this tutorial, we present a general model linking the data provided by any optical diffraction microscope to the sample permittivity. Our analysis is applicable to essentially all microscope configurations, in transmission or reflection mode, using scanning or full-field illumination, with or without interferometric measurements. We include also a generalization of our analysis to vector fields.
Processing of Chromatic Information in a DeepConvolutional Neural Network
Alban Flachot and Karl Gegenfurtner
Doc ID: 312517 Received 02 Nov 2017; Accepted 07 Mar 2018; Posted 08 Mar 2018 View: PDF
Abstract: Deep convolutional neural networks are a class ofmachine learning algorithms capable of solving nontrivialtasks, such as object recognition, with humanlikeperformance. Little is known about the exact computationsthat deep neural networks learn, and to whatextent these computations are similar to the ones performedby the primate brain. Here, we investigate howcolor information is processed in the different layers ofthe AlexNet deep neural network, originally trained onobject classification of over 1.2M images of objects intheir natural contexts. We found that the color responsiveunits in the first layer of AlexNet learned linearfeatures and were broadly tuned to two directions incolor space, analogously to what is known of color responsivecells in the primate thalamus. Moreover, thesedirections are decorrelated and lead to statistically efficientrepresentations, similarly to the cardinal directionsof the second-stage color mechanisms in primates.We also found, in analogy to the early stages of the primatevisual system, that chromatic and achromatic informationwere segregated in the early layers of the network.Units in the higher layers of AlexNet exhibit onaverage a lower responsivity for color than units at earlierstages.
Possible influences on color constancy by motion ofcolor targets and by attention-controlled gaze
Lifang Wan and Keizo Shinomori
Doc ID: 312715 Received 02 Nov 2017; Accepted 07 Mar 2018; Posted 08 Mar 2018 View: PDF
Abstract: We investigated the influence of motion on color constancy using a chromatic stimulus presented in variousconditions (static, motion, and rotation). Attention to the stimulus and background was also controlled in differentgaze modes, constant fixation of the stimulus and random viewing of the stimulus. Color constancy was examinedin six young observers using haploscopic view of a computer monitor. The target and background were illuminatedin simulation by red, green, blue and yellow, shifted from daylight (D65) by specific color differences along L-M orS-(L+M) axes on the equi-luminance plane. The standard pattern (under D65) and test pattern (under the colorilluminant) of a 5-deg. square were presented side by side, consisting of 1.2-deg. square targets with one of 12colors at each center, surrounded by 0 background ellipses consisting of 8 other colors. The central color targetsin both patterns flipped between top and bottom locations at the rate of 3 deg./s in the motion condition. Theresults indicated an average reduction of color constancy over the 12 test colors by motion. The random viewingparameter indicated better color constancy by more attention to the background, although the difference was notsignificant. Color constancy of the four color illuminations was better to worse in green, red, yellow and blue,respectively. The reduction of color constancy by motion could be explained by less contribution of theillumination estimation effect on color constancy. In the motion with constant fixation condition, the retinastrongly adapted to the mean chromaticity of the background. However, motion resulted in less attention to thecolor of the background, causing a weaker effect of the illumination estimation. Conversely, in the static state withrandom viewing condition, more attention to the background colors caused a stronger illumination estimationeffect, and color constancy was improved overall.
Age, pupil size differences and color choices for the“dress” and the “jacket”
kavita vemuri, Akanksha srivastava, Saksham Agrawal, and Mithra Anand
Doc ID: 313221 Received 09 Nov 2017; Accepted 07 Mar 2018; Posted 12 Mar 2018 View: PDF
Abstract: The color identification responses to photographs of #thedress (white/gold and blue/black camp) and a jacket(white/blue and green/black and teal camp) reveal obvious individual differences in color perception. Toexplore possible association between pupil size/ retinal illuminance and color perception, we recorded thepupil diameters of participants shown 22 uniformly colored (generated from the RGB values using a laptop LCDdisplay) screen followed by photographs of #thedress and jacket. We analyzed: a) pupil size difference in thecolor groups and b) age-related pupil size and/or reflex change and its influence on color perception. The dataconfirms that the average pupil size of the white/gold group was significantly less than the blue/black group forthe dress. The pupil size difference between the color groups is slightly higher in the 21-30 year and 31-55 yearage groups but not in the 18-20 year age group, while a similar variance was not observed for the jacket colorgroups. Interestingly, the average pupil size of both color groups was smaller for the dress compared to thebaseline (collected with a gray hue displayed on the screen) whereas an opposite effect was observed for thejacket. The contrasting results for the two photographs do not allow for a strong inference of only pupil sizechanges principal for differences in color perception. But, a probable explanation of the pupil size differencecould be the subjective variation in the perceptual interpretation of illumination cues in the photographs.
Evidence for an Impact of Melanopsin Activation onUnique White Perception
Dingcai Cao, Adam Chang, and Shaoyan Gai
Doc ID: 308900 Received 10 Oct 2017; Accepted 05 Mar 2018; Posted 06 Mar 2018 View: PDF
Abstract: Current models of human color vision only consider cone inputs at photopic light levels, yet it is unclear whetherthe recently discovered melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs)contribute to color perception. Using a lab-made five-primary photostimulator that can independently control thestimulations of rods, cones and ipRGCs in human retina, we determined the observer’s unique white perception, anequilibrium point for signals arising from the opponent mechanisms of color vision, under different levels ofmelanopsin activation. We found changing melanopsin activation levels shifts the equilibrium point in theparvocellular pathway. Our results suggest potential evidence for an impact of melanopsin activation on uniquewhite perception and the existing color vision model for the periphery may need to be revised by incorporatingmelanopsin signaling.
Normative best-corrected values of the visual image quality metric VSX as a function of age and pupil size
Gareth Hastings, Jason Marsack, Larry Thibos, and Raymond Applegate
Doc ID: 312722 Received 09 Nov 2017; Accepted 04 Mar 2018; Posted 05 Mar 2018 View: PDF
Abstract: The visual image quality metric Visual Strehl (VSX) combines a comprehensive description of the optics of an eye (wavefront error) with an estimate of the photopic neural processing of the visual system, and has been shown predictive of subjective best focus and well correlated with change in visual performance. The three-dimensional relation of best-corrected VSX, age, and pupil size is presented and 95% confidence interval norms are provided for age groups between 20 and 80 years and pupil diameters from 3 to 7 mm. These objectively determined benchmarks represent the best theoretical levels of visual image quality achievable with a sphere, cylinder, and axis correction in normal eyes and can be used to evaluate both traditional and wavefront-guided optical corrections provided by refractive surgery, contact lenses, and spectacles.
Turbulence effects on fluctuations of the aperture-averaged orbital angular momentum
Doc ID: 319383 Received 12 Jan 2018; Accepted 03 Mar 2018; Posted 05 Mar 2018 View: PDF
Abstract: A complete asymptotic theory for the aperture-averaged OAM variance is developed for both weak and strong fluctuation conditions, based on the asymptotic expansions of the Feynman path-integral solution for the fourth-order coherence function of a spherical wave propagating through a random inhomogeneous medium. We show that the OAM fluctuation averaging is very different for circular-symmetric and non-circular-symmetric apertures. Aperture averaging of the OAM fluctuations does not obey the standard “square root” law, and the weak/strong fluctuation conditions for the aperture-averaged OAM are not defined solely by the values of the scintillation index of the incident wave.
Color constancy of color reproduction in art paintings
Kinjiro Amano, Joao Linhares, and Sergio Nascimento
Doc ID: 314643 Received 30 Nov 2017; Accepted 27 Feb 2018; Posted 28 Feb 2018 View: PDF
Abstract: Popular color reproductions of art paintings such as postcards are intended to remind viewers of the original works. It is, however, unclear how well the quality of the reproductions is preserved under various illuminations. Color constancy of the reproductions in relation to colors in the original paintings was estimated computationally with hyperspectral images of the 15thc Flemish paintings, the 20thc modern abstract paintings and their corresponding postcards with a series of illuminants: the CIE daylight D65 with correlated color temperature (CCT) 6500 K, daylight D40, fluorescent lamps F2 and F11, a LED lamp designed for museums with CCTs approximately 3500-4000 K. Despite large colorimetric differences between the types of art-paintings and between the illuminants simulated, local areas showed g color constancy: on skin color areas in the Flemish paintings ranged from 0.76 to 0.81, whereas on the non-skin areas ranged from 0.19 to 0.68. This result suggests that viewers may be able to achieve color constancy with the reproduction postcards disregarding inconsistent colors representations from the original paintings caused by changes in illumination conditions.
Phase function of a spherical particle when scattering an inhomogeneous plane electromagnetic wave
Doc ID: 308756 Received 11 Oct 2017; Accepted 27 Feb 2018; Posted 01 Mar 2018 View: PDF
Abstract: In absorbing media, electromagnetic plane waves are most often inhomogeneous. Existing solutions for the scattering of an inhomogeneous plane wave by a spherical particle provide no explicit expressions for the scattering components. In addition, current analytical solutions require evaluation of a series expansion of infinite sums. It is much more computationally attractive to evaluate just a series expansion of finite sums with few terms. In this work, I develop such a new and much simpler solution. It is similar to the solution for homogeneous plane waves but with new explicit expressions for the angular dependency of the scattering components. I include recurrence formulae for practical evaluation and provide numerical examples to verify that the new expressions match the theory of homogeneous waves when the inhomogeneity of the incident wave is set to zero. The predicted difference in the scattering phase function due to inhomogeneity is not negligible for light entering an absorbing medium at an oblique angle. The presented theory could thus be useful for predicting scattering behavior in dye based random lasing and in solar cell absorption enhancement.
Computational Color Prediction versus Least‑Dissimilar Matching
Emitis Roshan and Brian Funt
Doc ID: 312531 Received 02 Nov 2017; Accepted 27 Feb 2018; Posted 01 Mar 2018 View: PDF
Abstract: The performance of color prediction methods CIECAM02, KSM2, Waypoint, Best Linear, MMV center, and relit colorsignal are compared in terms of how well they explain Logvinenko & Tokunaga’s asymmetric color matchingresults (“Colour Constancy as Measured by Least Dissimilar Matching,” Seeing and Perceiving, vol. 24, no. 5, pp. 407-452, 2011). In their experiment, 4 observers were asked to determine (3 repeats) for a given Munsell paper undera test illuminant which of 22 other Munsell papers was the least-dissimilar under a match illuminant. Their use of“least-dissimilar” as opposed to “matching” is an important aspect of their experiment. Their results raise severalquestions. Question 1: Are observers choosing the original Munsell paper under the match illuminant? If they are,then the average (over 12 matches) color signal (i.e., cone LMS or CIE XYZ) made under a given illuminant conditionshould correspond to that of the test paper’s color signal under the match illuminant. Computation shows that themean color signal of the matched papers is close to the color signal of the physically identical paper under thematch illuminant. Question 2: Which color prediction method most closely predicts the observers’ average leastdissimilarmatch? Question 3: Given the variability between observers, how do individual observers compare tothe computational methods in predicting the average observer matches? A leave-one-observer-out comparisonshows that individual observers, somewhat surprisingly, predict the average matches of the remaining observersbetter than any of the above color prediction methods.
Decay of correlations between cross-polarized electromagnetic waves in a two-dimensional random medium
Doc ID: 315821 Received 15 Dec 2017; Accepted 27 Feb 2018; Posted 01 Mar 2018 View: PDF
Abstract: The problem of multiple scattering of polarized light in a two-dimensional medium composed of fiberlike inhomogeneities is studied. The attenuation lengths for the the density matrix elements are calculated. For a highly absorbing medium it is found that, as the sample thickness increases, the intensity of waves polarized along the fibers decays faster than the other density matrix elements. With further increase in the sample thickness, the off-diagonal elements which are responsible for correlation between the cross-polarized waves disappear. In the asymptotic limit of very thick samples the scattered light proves to be polarized perpendicular to the fibers. The difference in the attenuation lengths between the density matrix elements results in a non-monotonic depth dependence of the degree of polarization. In the opposite case of a weakly absorbing medium, the off-diagonal element of the density matrix and, correspondingly, the correlations between the cross-polarized fields are shown to decay faster than and intensity of waves polarized along and perpendicular to the fibers.
How accurate do different computer-based texturecharacterization methods predict material surfacecoarseness? A guideline for effective online inspection
Sajjad Ghodrati, Saeideh Gorji, and Mohsen Mohseni
Doc ID: 309266 Received 16 Oct 2017; Accepted 27 Feb 2018; Posted 05 Mar 2018 View: PDF
Abstract: The growing industrialization has emphasized the need for high-performance computer-based inspectionmethods. Here, we investigated the performance of four major computer-based texture characterization methodsin prediction of visually perceived and actual surface coarseness of real materials. Gray level co-occurrence matrix(GLCM), distance-dependent edge frequency (DDEF), fractal dimension (FD), and histogram skewness (SK) wereused as the methods. A novel collection of real materials consisted of 20 sandpapers with high, medium, and lowcoarseness level was employed. The results revealed that in high coarseness level the most precise prediction ofactual surface coarseness was made by GLCM and SK while in prediction of visual coarseness all the methodsworked similarly effective. Perfect correlations were observed between GLCM, FD, and SK and visual and alsoactual coarseness in medium coarseness level. In low coarseness level, SK and DDEF acceptably predicted visualand actual coarseness respectively. The image resolution impact on performance of the computer-based methodswas found to be substantial. Results of the research present a guideline for choosing the best computer-basedmethod as a viable substitute for the human observer in online inspections of materials’ texture.
Denoising Imaging Polarimetry by Adapted BM3DMethod
Alexander Tibbs, Ilse Daly, Nicholas Roberts, and David Bull
Doc ID: 312417 Received 31 Oct 2017; Accepted 26 Feb 2018; Posted 27 Feb 2018 View: PDF
Abstract: In addition to the visual information contained in intensity and colour, imaging polarimetry allows visualinformation to be extracted from the polarization of light. However, a major challenge of imagingpolarimetry is image degradation due to noise. This paper investigates the mitigation of noise throughdenoising algorithms and compares existing denoising algorithms with a new method, based on BM3D.This algorithm, PBM3D, gives visual quality superior to the state of the art across all images and noisestandard deviations tested. We show that denoising polarization images using PBM3D allows the degreeof polarization to be more accurately calculated by comparing it to spectral polarimetry measurements.
Fast Propagation of Electromagnetic Fields through Graded-Index Media: Part I
Huiying Zhong, site zhang, Rui Shi, Christian Hellmann, and Frank Wyrowski
Doc ID: 314453 Received 29 Nov 2017; Accepted 23 Feb 2018; Posted 23 Feb 2018 View: PDF
Abstract: Graded-index (GRIN) media are widely used for modeling different situations: some components are designed with GRIN modulation in mind, e.g., multi-mode fibers, optical lenses or acousto-optical modulators; on the other hand, there are other components where the refractive-index variation is undesired due to, e.g., stress or heating; and finally, some effects in nature are characterized by a GRIN variation, like turbulence in air, or biological tissues. Modeling electromagnetic fields propagating in GRIN media is then of highly importance for optical simulation and design. Though ray tracing can be used to evaluate some basic effects in GRIN media, the field properties are not considered and evaluated. The general physical optics techniques, like FEM or FDTD, can be used to calculate field in GRIN media, but they need high numerical effort or may even be impractical for large-scale components. Therefore, there still exists a demand for a fast physical optics model of field propagation through GRIN media on a large scale, which will be explored in this paper.
Iterative optimization in tomographic deconvolution phase microscopy
Yijun Bao and Thomas Gaylord
Doc ID: 319530 Received 12 Jan 2018; Accepted 23 Feb 2018; Posted 26 Feb 2018 View: PDF
Abstract: Tomographic deconvolution phase microscopy (TDPM) is a three-dimensional (3D) quantitative phase imaging (QPI) method using partially coherent light that can be implemented on a commercial microscope platform. However, the measurement procedure is relatively time-consuming because it requires many illumination angles. In the present work, an edge-preserving iterative optimization algorithm is presented and applied to TDPM, so that the required number of illumination angles is reduced from 15 to 3, while the measurement accuracy remains high. In addition, the iterative algorithm does not require matrix representation of operators, so the memory requirement is reduced.
Four-Mirror Flat-Field Anastigmat Aplanat Telescopeswith Simple Closed-Form Solutions
Doc ID: 314158 Received 22 Nov 2017; Accepted 23 Feb 2018; Posted 21 Mar 2018 View: PDF
Abstract: A four-mirror anastigmat aplanat telescope can be constructed using a Gregorian-form Mersenne afocaltelescope as a feeder for a concentric spherical Schwarzschild objective. A flat image field can be obtainedby adjusting the ratio of the radii of the mirrors. With the tertiary at the image of the aperture stop (theprimary) the primary’s conic K1 can be varied, using the tertiary’s conic K3 to cancel the primary’s sphericalaberration without affecting the system’s coma or astigmatism. For a spherical primary, the tertiary is anoblate ellipsoid (K3 > 0). The minimum obscuration ratio is 44.7% (20% by area).
Matched coordinates in the framework of polynomial modal methods for complex metasurfaces modeling
Kofi EDEE, jean-pierre Plumey, Antoine Moreau, and Brahim Guizal
Doc ID: 317977 Received 18 Dec 2017; Accepted 22 Feb 2018; Posted 22 Feb 2018 View: PDF
Abstract: The Polynomial Modal Method is one of the most pow- erful methods for modeling diffraction from lamellar gratings. In the present work, we show that applying it to the so-called matched coordinates leads to impres- sive results. After giving the new formulation of the PMM under matched coordinates in the general frame- work of bi-periodic structures, we provide numerical examples to demonstrate the effectiveness of the pro- posed approach.
Joint Camera-Blur and Pose Estimation from Aliased Data
Joel LeBlanc, Brian Thelen, and Alfred Hero
Doc ID: 309908 Received 24 Oct 2017; Accepted 21 Feb 2018; Posted 22 Feb 2018 View: PDF
Abstract: A joint-estimation algorithm is presented that enables simultaneous camera-blur and pose estimation from a known calibration target in the presence of aliasing. Specifically, a parametric Maximum Likelihood (ML) PSF estimate is derived for characterizing a camera's optical imperfections through the use of a calibration target in an otherwise loosely controlled environment. The imaging perspective, ambient light-levels, target-reflectance, detector gain and offset, quantum-efficiency, and read-noise levels are all treated as nuisance parameters. The Cramer-Rao Bound (CRB) is derived, and simulations demonstrate that the proposed estimator achieves near optimal MSE performance. The proposed method is applied to experimental data to validate both the fidelity of the forward-models, as well to establish the utility of the resulting ML estimates for both system identification and subsequent image restoration.
Which tone-mapping operator is the best? Acomparative study of perceptual quality
Xim Cerda-Company, C. Parraga, and Xavier Otazu
Doc ID: 314149 Received 24 Nov 2017; Accepted 20 Feb 2018; Posted 21 Feb 2018 View: PDF
Abstract: Tone-mapping operators (TMO) are designed to generate perceptually similar low-dynamic range imagesfrom high-dynamic range ones. We studied the performance of fifteen TMOs in two psychophysicalexperiments where observers compared the digitally-generated tone-mapped images to their correspondingphysical scenes. All experiments were performed in a controlled environment and the setups weredesigned to emphasize different image properties: in the first experiment we evaluated the local relationshipsamong intensity-levels, and in the second one we evaluated global visual appearance amongphysical scenes and tone-mapped images, which were presented side by side. We ranked the TMOs accordingto how well they reproduced the results obtained in the physical scene. Our results show thatranking position clearly depends on the adopted evaluation criteria, which implies that, in general, thesetone-mapping algorithms consider either local or global image attributes but rarely both. Regarding thequestion of which TMO is the best, KimKautz  and Krawczyk  obtained the better results across thedifferent experiments. We conclude that a more thorough and standardized evaluation criteria is neededto study all the characteristics of TMOs, as there is ample room for improvement in future developments.
Efficient, Nonlinear Phase Estimation with the Non-Modulated Pyramid Wavefront Sensor
Doc ID: 319913 Received 17 Jan 2018; Accepted 17 Feb 2018; Posted 20 Feb 2018 View: PDF
Abstract: The sensitivity of the the pyramid wavefront sensor (PyWFS) has made it a popular choice for astronomical adaptive optics (AAO) systems.The PyWFS is at its most sensitive when it is used without modulation of the input beam.In non-modulated mode, the device is highly nonlinear. Hence, all PyWFS implementations on current AAO systems employ modulation to make the device more linear.The upcoming era of 30-m class telescopes and the demand for ultra-precise wavefront control stemming from science objectives that include direct imaging of exoplanets make using the PyWFS without modulation desirable. This article argues that nonlinear estimation based on Newton's method for nonlinear optimization can be useful for mitigating the effects of nonlinearity in the non-modulated PyWFS.The proposed approach requires all optical modeling to be pre-computed, which has the advantage of avoiding real-time simulations of beam propagation. Further, the required real-time calculations are amenable to massively parallel computation.Numerical experiments simulate a PyWFS with faces sloped 3.7$^\circ$\ to the horizontal, operating at a wavelength of 0.85 $\mu$m, and with an index of refraction of 1.45.A singular value analysis shows that the common practice of calculating two "slope" images from the four PyWFS pupil images discards critical information and is unsuitable for non-modulated PyWFS.Instead, this article advocates estimators that use the raw pixel values not only from the four geometrical images of the pupil, but from surrounding pixels as well.The simulations indicate that nonlinear estimation can be effective when the Strehl ratio of the input beam is greater than 0.3, and the improvement relative to linear estimation tends to increase at larger Strehl ratios.At Strehl ratios less than about 0.5, the performances of both the nonlinear and linear estimators are relatively insensitive to noise, since they are dominated by nonlinearity error.
Fractal Spiral Zone Plates
Ziwen Ji, Huaping Zang, FAN chunzhen, Junqiao Wang, Chenglong Zheng, lai Wei, Chuanke Wang, and leifeng cao
Doc ID: 308128 Received 10 Oct 2017; Accepted 01 Feb 2018; Posted 06 Mar 2018 View: PDF
Abstract: We present diffractive optical elements with an extended depth of focus, fractal spiral zone plates (FSZPs), which combinea fractal structure and spiral zone plates (SZPs) to generate a sequence of coaxial vortices in the focal volume along thepropagation direction. The axial irradiance of the FSZPs is examined and compared with that of SZPs and that of fractalzone plates (FZPs), both experimentally and in a simulation. The focusing properties of the FSZPs with differentparameters are investigated and a potential application to edge-enhancement images is also shown.
Are hue and saturation carried in different neural channels?
Sarah Regan, Robert Lee, Donald MacLeod, and Hannah Smithson
Doc ID: 312522 Received 02 Nov 2017; Accepted 30 Jan 2018; Posted 30 Jan 2018 View: PDF
Abstract: Chromatic discrimination data show that a smaller physical stimulus change is required to detect a change in hue than to detect a change in saturation (D. B. Judd, Palette 30, 1968; M. V Danilova and J. D. Mollon, Proc. R. Soc. London. Ser. B 283, 2016) and, on this basis, it has been suggested that hue and saturation are carried in different neural channels (R. G. Kuehni, Color Space and Its Divisions, 331, 2003). We used an adaptation paradigm to test explicitly for separate mechanisms, measuring hue and saturation detection thresholds before and after adaptation to hue and saturation stimuli. Within-condition adaptation did not elevate detection thresholds significantly more than between-condition adaptation. We therefore did not find psychophysical evidence for a neural channel that that extracts hue thresholds more effectively than the neural channel or channels that determine saturation thresholds.
Improved photon-pair generation from transitionmetaldichalcogenide monolayers embedded in onedimensionalphotonic crystals
Doc ID: 312477 Received 03 Nov 2017; Accepted 18 Jan 2018; Posted 19 Jan 2018 View: PDF
Abstract: We have theoretically investigated photon-pair generation from transition-metal dichalcogenide (TMDC)monolayers embedded in photonic crystals (PCs) by a rigorous quantum model of spontaneous parametric downconversion. The mean number of output photon-pairs and the signal-field energy spectrum have been elaborated.Two different structures are studied, one is a monolayer embedded in the defective PC and the other is a stackof layers in the dielectric multilayer structure. It is clearly shown that an obvious improvement of thegeneration rate of the correlated photon-pairs from the monolayer can be realized in these two structures andthe enhancement can reach as much as 14 orders of magnitude. Such an effective improvement utilizes theimproved field localization at the defect state or the band-edge state, the propagation of photons in the PCs andcoherent superposition, the similar phenomena can occur for other TMDC monolayers. These phenomena are verybeneficial for the design of optical devices.