High energy laser propagation through natural convection of air: A benchmark for validation of numerical simulation

Joseph Fiordilino, Shannon Sweitzer-Siojo, Tam Vo, Joseph Mays, and Daniel King

DOI: 10.1364/JOSAA.520664 Received 07 Feb 2024; Accepted 19 Apr 2024; Posted 19 Apr 2024  View: PDF

Abstract: The study of propagation medium effects on lasers continues to be an active areaof research. High energy laser (HEL) propagation through planetary atmosphere is particularlynuanced as the beam generates its own flow field and suffers from additional degrading effects.Herein, we construct experimental setups conducive to probing the physics of the laser-atmosphereinteraction and generating validation datasets for high fidelity predictive software. Measured andderived parameters are presented and predictive models are generated utilizing random forestregression.

Quantitative evaluation of the impact of variation of optical parameters on the estimation of blood hematocrit and oxygen saturation for dual-wavelength photoacoustics

Subhadip Paul, Harishankar Patel, and Ratan Saha

DOI: 10.1364/JOSAA.521238 Received 08 Feb 2024; Accepted 19 Apr 2024; Posted 22 Apr 2024  View: PDF

Abstract: Photoacoustic (PA) spectroscopy is considered to be one of the most effective ways to measure the levelsof hematocrit (H) and oxygenation saturation (SO2) of blood, which are essential for diagnosing bloodrelated illnesses. This simulation study aims to investigate the impact of individual optical parametersi.e., optical absorption coefficient (µa), scattering coefficient (µs) and anisotropy factor (g) on the accuracyof this technique in estimating the blood properties. We first performed the Monte Carlo simulations,using realistic optical parameters, to obtain the fluence maps for various samples. The wavelengths ofthe incident light were chosen to be 532, 700, 1000 and 1064 nm. Thereafter, the k-Wave simulations wereexecuted incorporating those fluence maps to generate the PA signals. The blood properties were obtainedusing the PA signals. We introduced variations in µa, µs and g ranging from -10% to +10%, -10% to +10%and -5% to +1%, respectively at 700 and 1000 nm wavelengths. One parameter, at both wavelengths, waschanged at a time keeping others fixed. Subsequently, examined how accurately the blood parameterscould be determined at physiological hematocrit levels. A 10% variation in µa induces a 10% changein H estimation but no change in SO2 determination. Almost no change has been seen for µs variation.However, a 5% (-5% to 0%) variation in g factor resulted in approximately 160% and 115% changes in thePA signal amplitudes at 700 and 1000 nm, respectively leading to ≈ 125% error in hematocrit estimationand ≈ 14% deviation in SO2 assessment when nominal SO2 = 70% . It is clear from this study that thescattering anisotropy factor is a very sensitive parameter and its small change can result in large errors inthe PA estimation of blood properties. In future, in vitro experiments with pathological blood (inducingvariation in g parameter) will be performed and accordingly, evaluate the accuracy of the PA technique inquantifying blood H and SO2.

Light consistency correction for liquid crystal tunable filter hyper-spectral imaging system

jianxin zhang, Yupeng Zhang, Miao Qian, and Xinen Zhang

DOI: 10.1364/JOSAA.515706 Received 12 Dec 2023; Accepted 18 Apr 2024; Posted 19 Apr 2024  View: PDF

Abstract: In hyperspectral images, every pixel encompasses continuous spectral information.Compared with traditional colorimeters, using hyperspectral imaging systems (HIS) for fabriccolor measurement can obtain richer color information. However, measuring fabric colorswith Liquid Crystal Tunable Filter (LCTF) HIS encounters challenges related to light consistency. In this paper, we adopted an innovative approach, integrating Gradient BoostedDecision Trees (GBDT) with a sliding window algorithm to develop a uniformity calibrationmodel addressing the illumination uniformity issue. To address the consistency issues acrossvarious light sources, we further adopted a deep neural network (DNN) model to correct thereflectance measurements under different light sources. Subsequently, this model was mergedwith the uniformity calibration model to form a light consistency correction model. Throughcalibration, we successfully reduced the color difference of the corrected samples from 3.636to 0.854, an enhancement of 76.51%. This means that after calibration we can achieveconsistency in fabric color measurements under non-uniform lighting and different lightsources.

Non-invasive accelerated imaging through scattering medium via multi-stage complexity guidance

Qinlei Xiang, Guangmang Cui, Fu Liao, Shigong Shi, Weize Cui, and Jufeng Zhao

DOI: 10.1364/JOSAA.517626 Received 03 Jan 2024; Accepted 18 Apr 2024; Posted 19 Apr 2024  View: PDF

Abstract: The research of scattering imaging is of great significance to the development of various fields, but the existing scattering imaging methods are difficult to combine the advantages of non-invasive, real-time and high quality. In this paper, a new scattering imaging technique is proposed which optimize the traditional autocorrelation imaging technique by multi-stage complexity guidance and the initial acceleration module. We introduce the complexity difference index into the phase iterative recovery step for effective complexity guidance, and add the initial module based on ER iteration to realize fast startup. A series of experiments are carried out to test the performance of the new technique. The results show that the proposed technique significantly improves the scattering reconstruction speed. Meanwhile, the accuracy and clarity of the reconstructed image are significantly higher than the traditional method in terms of fast imaging. Moreover, this technique has better robustness to noise compared to the traditional autocorrelation imaging technique.

Optical skyrmions in Bessel profile

Srinivasa Rao Allam

DOI: 10.1364/JOSAA.522001 Received 20 Feb 2024; Accepted 17 Apr 2024; Posted 18 Apr 2024  View: PDF

Abstract: Optical skyrmions formed in terms of polarization are topological quasi-particles and have garnered much interest in the optical community owing to their unique inhomogeneous polarization structure and simplicity in their experimental realization. These structures belong to the Poincaré beams satisfying the stable topology. We theoretically investigated the non-diffracting and self-healing Poincaré beams based on the superposition of two orthogonal Bessel modes by mode matching technique. These Poincaré beams are topologically protected, and we suggest them as optical skyrmions. These optical skyrmions are quasi-skyrmions and their range of propagation depends on the range of superposed Bessel modes. The polarization structure of these optical skyrmions has no change upon the propagation. A necessary experimental configuration is suggested to realize variable order skyrmions in Bessel modes experimentally. This work can provide a new direction for the generation of skyrmions with completely new textures and features with reference to existing skyrmions originating from Laguerre-Gaussian modes.

Information propagation of focus wave mode localized waves in anisotropic turbulent seawater

guoqing zhao, Yixin Zhang, Qingze Yan, Lin Yu, Yun Zhu, and lifa hu

DOI: 10.1364/JOSAA.516209 Received 18 Dec 2023; Accepted 16 Apr 2024; Posted 16 Apr 2024  View: PDF

Abstract: The evolution of the information transfer capability of a new optical system forunderwater focused wave mode localized wave (FWMLW) in anisotropic weakly turbulentabsorbing seawater is studied. By developing the probability distribution function as well asthe detection probability of the vortex modes carried by the FWMLW and the average bit errorrate of the FWMLW underwater system, the information capacity of the FWMLW system withpointing error is modeled. Through numerical analysis of the effects of turbulent seawater andoptical system parameters on the built light intensity, the detection probability and theinformation capacity models, we find that the FWMLW system has an optimal delay timedetermined by the spectrum bandwidth when the spectrum bandwidth is greater than 1. Theinformation capacity of the FWMLW system is higher than that of the X localized wave systemunder the same turbulent seawater channel condition, and FWMLW is a better optical signalsource for vortex mode division multiplexing underwater systems than Bessel Gaussian beam.

Performance analysis of FSO communication system with random jamming over composite Málaga turbulence fading channel

wang jingyu, Dingshan Gao, Li Juan, Chen Dongliang, Deqiang Ding, and Dong Yuzhao

DOI: 10.1364/JOSAA.514356 Received 27 Nov 2023; Accepted 16 Apr 2024; Posted 17 Apr 2024  View: PDF

Abstract: The performance analysis of a free space optical (FSO) communicationsystem in the presence of random jamming is presented over a Málaga () distributedchannel model with pointing errors and atmospheric attenuation. Firstly, the probabilitydensity function expressions of the transmission channel, signal-to-jamming ratio, andsignal-to-noise ratio are derived respectively. Then, considering the probability of thejammer and Gaussian white noise, the closed-form expressions for the ergodic channelcapacity, outage probability, and average bit error rate are derived, respectively. Moreover,asymptotic expressions for the aforementioned performance metrics are also derived toascertain the diversity gain of the system. Extensive Monte Carlo simulations areperformed to demonstrate the credibility of this theoretical analysis. Results indicate thatthe adverse impact of random jamming is higher than that of Gaussian noise for the FSOcommunication system. Besides, this observation highlights the pulsating nature of thejamming effect, showcasing that within high signal-to-jamming ratio regions, a lowprobability jammer exerts the most significant impact on the FSO system.

Fractioned-pattern radiation mapping I: Modeling

Juan Valencia-Estrada, Jorge Garcia-Marquez, and Romain ETIENNE

DOI: 10.1364/JOSAA.518371 Received 09 Jan 2024; Accepted 16 Apr 2024; Posted 17 Apr 2024  View: PDF

Abstract: Here, we present a technique that predicts the radiation's distribution inany optical system. It is based on decomposing the emitting source power byassigning a fraction of the total power to each emitted ray. All kinds of power lossesin the rays' optical paths are considered. Fractioned radiation patterns are createdin the last optical system surface, each associated with a single ray. We refer tofractioned patterns as those that conform to a whole radiating pattern. Thus, theirradiance of the completely illuminated surface is calculated by adding the opticalsystem's fractioned radiation maps. This method is non-zero étendue. The resultpresented here allows for predicting the radiation patterns accurately with ahandful of equations and can help design any image and non-image-forming opticalsystems.

Fractioned-patterned radiation mapping II: Assessment

Juan Valencia-Estrada, Jorge Garcia-Marquez, alejandro espinoza-garcia, Clement Lartige, Romain ETIENNE, and Philippe Dupont

DOI: 10.1364/JOSAA.518373 Received 12 Jan 2024; Accepted 16 Apr 2024; Posted 17 Apr 2024  View: PDF

Abstract: In part one, the authors proposed a theoretical background for predicting the radiation's distribution in any optical system based on decomposing the emitting source power. Here, we describe the validity of this decomposition through a practical example that uses a radiating source and a single surface optical system. This source is calibrated in a metrology testbed that guarantees its traceability to the candela (cd), the International System (SI) base unit for luminous intensity Iv. A second example, this time numerical, shows the method’s performance in a multi-surface optical system.

Optical Phase and Amplitude Measurements of Underwater Turbulence via Self-Heterodyne Detection

Nathaniel Ferlic, Alan Laux, and Linda Mullen

DOI: 10.1364/JOSAA.520917 Received 07 Feb 2024; Accepted 16 Apr 2024; Posted 17 Apr 2024  View: PDF

Abstract: The creation of underwater optical turbulence is drivenby density variations that lead to small changes inthe water’s refractive index which induce optical pathlength differences that affect light propagation. Measuring a laser beam’s optical phase after traversing theseturbulent variations can provide insight into how thewater’s turbulence behaves. The sensing technique tomeasure turbulent fluctuations is a self-heterodyne beatnote enhanced by light’s orbital angular momentum(OAM) to obtain simultaneous optical phase and amplitude information. Experimental results of this methodare obtained in a water tank that creates a thermallydriven flow called Rayleigh-Bénard (RB) convection.The results show time-varying statistics of the beatnotethat depend on the incident OAM mode order and thestrength of the temperature gradient. Beatnote amplitude and phase power spectral densities are comparedto analytic theory to obtain estimates of the turbulentlength scales using the Taylor hypothesis that includemean flow speed, turbulent strength and length scales,and flow dynamics due to intermittency in the RB process

Field correlations of multimode optical beams in underwater turbulence

Yahya Baykal, Muhsin Gökçe, Yalçin Ata, and Hamza Gerçekcioglu

DOI: 10.1364/JOSAA.522599 Received 28 Feb 2024; Accepted 15 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: For the multimode optical beams, field correlations at the receiver plane are found inunderwater turbulence. Field correlations in underwater of single high order beams arespecial cases of our formulation. Variations of field correlations against the underwaterturbulence parameters and the diagonal length from various receiver points, are examinedfor different multimode and single high order beams. Stronger underwater turbulence isfound to reduce the field correlations of the multimode and single high order opticalbeams. The results will be of help in heterodyne detection analysis and fiber coupling2efficiency in an underwater medium experiencing turbulence.

Estimating the time-evolving refractivity of a turbulent medium using optical beam measurements: a data assimilation approach

Anjali Nair, Qin Li, and Samuel Stechmann

DOI: 10.1364/JOSAA.518013 Received 11 Jan 2024; Accepted 14 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: In applications such as free-space optical communication, a signal is often recoveredafter propagation through a turbulent medium. In this setting, it is common to assume that limitedinformation is known about the turbulent medium, such as a space- and time-averaged statistic(e.g., root-mean-square), but without information about the state of the spatial variations. It couldbe helpful to gain more information if the state of the turbulent medium can be characterizedwith the spatial variations and evolution in time described. Here, we propose to investigate theuse of data assimilation techniques for this purpose. A computational setting is used with theparaxial wave equation, and the extended Kalman filter is used to conduct data assimilationusing intensity measurements. To reduce computational cost, the evolution of the turbulentmedium is modeled as a stochastic process. Following some past studies, the process has only asmall number of Fourier wavelengths for spatial variations. The results show that the spatialand temporal variations of the medium are recovered accurately in many cases. In some timewindows in some cases, the error is large for the recovery. Finally, we discuss the potential use ofthe spatial variation information for aiding the recovery of the transmitted signal or beam source.

Correlation and polarization singularities of radially polarized Gaussian Schell-model vortex beam propagating in oceanic turbulence

Qian Xu, Yonggen Xu, Wenli Liu, and Liang Zhao

DOI: 10.1364/JOSAA.520531 Received 01 Feb 2024; Accepted 13 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: The correlation and polarization singularities as the important parameters of a radiallypolarized Gaussian Schell-model vortex (PRGSMV) beam propagating in oceanic turbulence have beeninvestigated in detail. On the one hand, the correlation singularity of the beam will first split, and thengenerate new correlation singularity, finally vanish in pairs. The longer propagating distance, the largerrate of dissipation of mean-square temperature, and the lower initial correlation lengths reduce thestability of correlation singularities. On the other hand, polarization singularities also split duringtransmission. The different initial correlation lengths cause the uneven distribution of polarizationsingularities, and the high order topological charge leads to the generation of new polarizationsingularities at short distances. Our numerical findings may be of great significance for detection andimaging of the oceanic optical telecommunication links.

Long-term Measurement and Characterization of Boundary Layer Optical Turbulence

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

DOI: 10.1364/JOSAA.520980 Received 07 Feb 2024; Accepted 13 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: The United States Naval Academy long-term scintillation measurement campaign was a multi-year effort to characterize optical turbulence in the near-maritime boundary layer. At its core, the field experiment consists of in-situ measurements of bulk atmospheric and oceanographic parameters, as well as path-averaged measurements of the refractive index structure parameter, $C_n^2$, collected using a large-aperture scintillometer. The field experiment ran from 1 January 2020 through 26 September 20 , representing the most comprehensive collection of optical turbulence measurements in the near-maritime atmospheric boundary layer to date. These measurements are publicly available as a unified dataset at https://github.com/cdjellen/otbench.

A HDR Vision Sensor with Neuro-Memristive Skin Detection for Edge Computing

Francesco Paissan, Michela Lecca, Roberto Passerone, Elisabetta Farella, and Massimo Gottardi

DOI: 10.1364/JOSAA.516912 Received 27 Dec 2023; Accepted 12 Apr 2024; Posted 12 Apr 2024  View: PDF

Abstract: Human skin classification is an essential task for several machine vision applications such as human-machine interfaces, people/object tracking, and classification. In this paper, we describe a hybrid CMOS/memristor vision sensor architecture embedding skin detection over a wide dynamic range. In-sensor RGB to rg-chromaticity color space conversion is executed on the fly through a pixel-level automatic exposure time control. Each pixel of the array delivers two pre-filtered analog signals, the r and g values, suitable for being efficiently classified as skin or non-skin through an analog memristive Neural Network (NN), without the need for any further signal processing. Moreover, we study the NN performance and theorize how it should be added in the hardware. The skin classifier is organized in an array of column- level memristor-based NN to exploit the nano-scale device characteristics and non-volatile analog memory capabilities, making the proposed sensor architecture highly flexible, customizable for various use-case scenarios, and low-power. The output is a skin bitmap that is robust against variations of the illuminant color and intensity.

Autofocusing in digital holography based on adaptive genetic algorithm

Zhongyang Wang, Hong-wei Ma, Yuan Chen, and Dengxue Liu

DOI: 10.1364/JOSAA.518105 Received 23 Jan 2024; Accepted 11 Apr 2024; Posted 11 Apr 2024  View: PDF

Abstract: In digital holography (DH), determining the reconstruction distance is critical to thequality of the reconstructed image. However, traditional focal plane detection methods requireconsiderable time investment to reconstruct and evaluate holograms at multiple distances. Toaddress this inefficiency, this paper proposes a fast and accurate autofocusing method based onan adaptive genetic algorithm. This method only needs to find several reconstruction distancesin the search area as an initial population, and then adaptively optimize the reconstructiondistance through iteration to determine the optimal focal plane in the search area. In addition,an off-axis digital holographic optical system was used to capture the holograms of the USAFresolution test target and the coin. The simulation and experimental results indicated that,compared with the traditional autofocusing, the proposed method can reduce the computationtime by about 70% and improve the focal plane accuracy by up to 0.5mm.

Polarization structure of optical vortices in inclined Laguerre-Gaussian beams passed through a uniaxial crystal

Yuriy Egorov and Aleksandr Rubass

DOI: 10.1364/JOSAA.523057 Received 05 Mar 2024; Accepted 11 Apr 2024; Posted 11 Apr 2024  View: PDF

Abstract: The work shows that in linearly polarized Laguerre-Gaussian beams passing through an anisotropic medium at an angleto the optical axis of the crystal, the distribution of optical vortices is devoid of axial symmetry. It is shown that thetrajectories of movement of polarization singularities in the plane of the Laguerre-Gauss beam are different for differentcases of input linear polarization at angles 45o g = ± and there is an exchange of optical vortices, provided that the signof the topological charge is preserved. It is shown that when the axis of an anisotropic medium is tilted, the movement ofoptical vortices occurs, accompanied by topological reactions of creation, destruction, or displacement of opticalvortices to the periphery of the beam. It is characteristic that at angles of inclination by linear polarization 45og= +,topological reactions of creation and annihilation occur, and at angles 45o g = - , topological reactions of displacement ofoptical vortices to the periphery of the beam occur.

High-Efficiency Scattering Field Modeling in Metallic Components: A Machine Learning-Inspired Approach

Po-Jui Chiang, CHIH-LUNG TSENG, and Chien-Kun Wang

DOI: 10.1364/JOSAA.507016 Received 27 Sep 2023; Accepted 09 Apr 2024; Posted 11 Apr 2024  View: PDF

Abstract: We present a highly efficient method for characterizing the scattering field distributionof surface plasmon polaritons in metallic components by combining the eXtended PseudospectralFrequency-Domain (XPSFD) method with an iterative, machine learning-inspired procedure.Shifting away from traditional matrix operations, we utilize the ’Adam’ optimizer—an effectiveand swift machine learning algorithm—to solve the scattering field distribution. Our methodencompasses the derivation of the associated cost function and gradient differentiation of the field,leveraging spectral accuracy at Legendre collocation points in the Helmholtz equation. We refinethe total-field/scattered-field (TF/SF) formulation within the XPSFD framework for optimizedincident field management, and employ Chebyshev-Lagrange interpolation polynomials for rapid,accurate computation of broad-band results. To ensure global accuracy, we introduce uniquephysical boundary conditions at subdomain interfaces. Demonstrating our method’s robustnessand computational efficiency, we model perfect electric conductors (PEC) and silver nanocylinders,and apply our approach to analyze the excited electric field on subtly distorted metallic surfaces,particularly plasmonic structures, thereby validating its wide-ranging effectiveness.

Time characteristics of aero-optical imaging degeneration of ellipsoidal dome under variable operating conditions

Jiaqian Yu, Chonghui Zhu, and song long

DOI: 10.1364/JOSAA.515243 Received 13 Dec 2023; Accepted 09 Apr 2024; Posted 10 Apr 2024  View: PDF

Abstract: As the missile flies at high speed, the index and surface shape of the conformal dome will vary under the influence of theaero-optical effect, which will degenerate the performance of the seeker’s imaging detection system. However, manyprevious studies on aero-optical imaging deterioration of optical domes are usually carried out under fixed operatingconditions, which are not in line with the real flight scene of the missile. Besides, the aero-optical imaging degenerationof the dome is diverse as the flight time increases. Therefore, it is of great significance to study the time characteristics ofaero-optical imaging degradation of optical domes under variable work conditions. In this study, taking an air-to-airmissile as an example, the Zernike polynomials, wavefront aberration, Strehl ratio, and image simulation are applied toevaluate the aero-optical imaging deterioration of an ellipsoidal dome in the flight time range of 0-10 s under variableworking conditions. The simulation results show that, as the flight time increases, (1) the dynamic range of tilt, defocus,astigmatism, coma, and wavefront aberration increase; (2) the Strehl ratio and the peak signal-to-noise ratio (PSNR) ofthe simulated images decrease. Therefore, the influence of flight time on the aero-optical image degradation of theellipsoidal dome is gradually serious.

Saturation of anisoplanatic error in Kolmogorov and non-Kolmogorov turbulence

Jeremy Bos

DOI: 10.1364/JOSAA.520981 Received 07 Feb 2024; Accepted 05 Apr 2024; Posted 08 Apr 2024  View: PDF

Abstract: This work explores the conditions resulting in saturation of angular anisoplanatic error. When turbulence ismodeled with a von Kármán outer scale or when piston and aperture tilt are compensated the anisoplanaticerror can saturate to less than one squared radian. In Kolmogorov turbulence anisoplanatic error is limitedto values smaller than one when the ratio of the Fried parameter to the outer scale is 0.349. To understandthe effect of compensation on saturation both a first-order asymptotic approach and numerical integrationare considered for both plane and spherical wave sources and in non-Kolmogorov turbulence. Asymptoticexpressions are found to agree with the numerical results as long the ratio of outer scale to aperture size isless than five. For a plane wave propagating in Kolmogorov turbulence, the compensated anisoplanaticerror is found to saturate when D/r0 = 3.9 when the outer scale is equal to the aperture size. When aspherical wave source is considered D/r0 increases to 5.8; as expected these values are related by a factor of1.8. This work also formulates the anisoplanatic error in terms of an integrated strength parameter and themean turbulence height allowing extension to arbitrary path geometries and power law exponents. Usingthis approach I find smaller power law exponents increase the mean turbulence height thereby decreasingthe isoplanatic angle; the opposite applies as the power law exponent is increased relative to Kolmogorovturbulence.

Performance Evaluation and Comparative Research of Underwater Wireless Optical Communication System by Using Different Structured Beams

Shuang He, Peng Zhang, Hao Yu, Dongsheng Tian, Hang Chen, Hui Dai, Pengfei Ye, Dashuai Wang, and Shoufeng Tong

DOI: 10.1364/JOSAA.517901 Received 09 Jan 2024; Accepted 05 Apr 2024; Posted 05 Apr 2024  View: PDF

Abstract: Structured beams have attractedincreasing interest in free-space and fiberbased optical communications.Underwater wireless opticalcommunication (UWOC) is becoming aprospective technique in marineexploration. We investigated UWOCperformance using different representativestructured beams. The transmissionperformances of the Gaussian, BesselGaussian (BG), Ince-Gaussian (IG), andradially polarized Gaussian (RPG) beamswere experimentally demonstrated andevaluated in underwater channelssubjected to thermal gradient. Theexperimental results show that the BG, IG,and RPG perform better against thethermal gradient. Compared with theGaussian beams, the beam wanders of BG,IG, and RPG beams under the thermalgradient have been reduced by 56.9%,8.2% and 59%, the scintillation indiceshave been decreased by 12.8%, 17.3% and28.9%, and the BER performance of theBG, IG and RPG beams have beenimproved by ∼5.5 dB, ∼3.7 dB, and∼ 5.2dB at the forward error correctionthreshold (FEC threshold). Based on theabove results, the RPG beam is a morepromising light source for UWOC. Theexperimental results provide a promisingbeam choice for UWOC.

Randomness of Optical Turbulence Generated by Rayleigh-Bénard Convection using Intensity Statistics

Nathaniel Ferlic, Svetlana Avramov-Zamurovic, Owen O'Malley, Thomas Kelly, and Kyle Judd

DOI: 10.1364/JOSAA.520909 Received 07 Feb 2024; Accepted 03 Apr 2024; Posted 05 Apr 2024  View: PDF

Abstract: The experimental study of optical turbulence provesdifficult due to challenges in generating controllableconditions in a laboratory environment. Confined water tanks that produce Rayleigh-Bénard (RB) convectionare one method to generate optical turbulence using acontrollable temperature gradient. It is of utmost concern to quantify the properties of the optical turbulencegenerated for characterization of other optical applications such as imaging, sensing, or communications. Inthis experimental study a Gaussian beam is propagatedthrough a RB water tank where two intensity measurements are made at the receivers pupil and focal plane.The pupil and focal plane results include quantificationof the intensity fluctuation distribution, scintillationdistribution, and refractive index structure constant atvarious values of the temperature gradient. The angleof arrival fluctuations are also calculated at the focalplane to obtain a second estimate of C2n. Comparisonof the pupil plane estimate for C2n using scintillationindex and focal plane angle of arrival fluctuations arecompared to preliminary predictions of C2n as a functionof RB temperature gradient showing C2n ∼ ∆T4/3. Theoutcomes of the study confirm that the RB process produces intensity fluctuations that follow gamma-gammaand log-normal probability density functions. Estimatesof the refractive index structure constant C2n produce thesame trends with different magnitudes when measuredfrom the pupil and focal plane.

Strong multiperiod forerunning light jet within a double prism gap: A counterintuitive manifestation of causality

Alexander Turchin and Mikhail Vasnetsov

DOI: 10.1364/JOSAA.516563 Received 27 Dec 2023; Accepted 02 Apr 2024; Posted 02 Apr 2024  View: PDF

Abstract: We report on the effect of a strong and protractedadvanced response in pulse transmission and reflectionin a double-prism scheme. In distinction to the wellknown activity on superluminal-like tunneling of anelectromagnetic pulse through a gap of a double prism,we consider an optical pulse refracting to a gap andsliding therein. The formation of multiperiod light jetrunning within the gap well before the incident pulse isshown with account of normal material dispersion andexcitation of leaky modes in the gap. Conditions for theparadoxical appearance of the forerunning jet arerevealed to have a geometrical nature as a specificrelation between phase and group velocities involved.This deduction assigns the stated effect to thecounterintuitive manifestation of causality with noreference to superluminal propagation.

Propagation of auto-focusing Hypergeometric Gaussian beams along a slant Path in oceanic turbulence

Wenhai Wang, Zhou Yu, Chengzhao Liu, Xu Zhou, Zhengda Hu, and Yun Zhu

DOI: 10.1364/JOSAA.519982 Received 25 Jan 2024; Accepted 02 Apr 2024; Posted 02 Apr 2024  View: PDF

Abstract: Compared to horizontal transmission, the oceanic dissipation rate and temperature-salinity distribution ratio are no longer constant but vary with depth, imposing greater complexity on oceanic turbulence when beams propagate through a slant path and resulting in more limitations on the performance of underwater wireless optical communication (UWOC) links. This study focuses on investigating the performance, especially the auto-focusing characteristic, of auto-focusing Hypergeometric Gaussian (AHGG) beams propagating along slant paths in oceanic turbulence. We theoretically derive the spatial coherence radius and the relative probability of the orbital angular momentum (OAM) mode for AHGG beams passing through such links. Numerical simulations reveal that AHGG beams exhibit superior propagation performance compared to Hypergeometric Gaussian beams. Lower beam orders and OAM numbers contribute to improved performance, while careful selection of autofocusing length can tangibly enhance detection performance as well. Additionally, tidal velocities and wind speeds have non-negligible effects on OAM signal probability. Our results further demonstrate that surface buoyancy flux, temperature gradients, and waterside friction velocity significantly affect beam transmission under varying wind conditions. These findings, particularly controlling the autofocusing length of AHGG beams to match the transmission distance, provide valuable insights for enhancing the quality of UWOC links.

Projected algebraic reconstruction techniquenetwork for high- fidelity diffuse fluorescence tomography reconstruction

Wenjing Sun, Limin Zhang, Lingxiu Xing, Zhuanxia He, Yanqi Zhang, and Feng Gao

DOI: 10.1364/JOSAA.517742 Received 04 Jan 2024; Accepted 01 Apr 2024; Posted 02 Apr 2024  View: PDF

Abstract: We propose a model-driven projected algebraic reconstruction technique (PART)-network (PART-Net) that leverages the advantages of the traditional model-based method andthe neural network to improve imaging quality of diffuse fluorescence tomography (DFT). Inthis algorithm, non-negative prior information is incorporated into the ART iteration processto better guide the optimization process, and thereby improve imaging quality. On this basis,PART in conjunction with residual convolutional neural network is further proposed to obtainhigh fidelity image reconstruction. The numerical simulation results demonstrate that thePART-Net algorithm effectively improves noise robustness and reconstruction accuracy by atleast 1-2 times, and exhibits superiority in spatial resolution and quantification, especially forsmall-sized target (r mm = 2 ), compared with the traditional ART algorithm. Furthermore,the phantom and in vivo experiments verify the effectiveness of the PART-Net, suggestingstrong generalization capability and a great potential for practical applications.

Synthesizing Polarization Singularity Lattices Using Phase Ramps

KAPIL GANGWAR, Sarvesh Bansal, and Paramasivam Senthilkumaran

DOI: 10.1364/JOSAA.521031 Received 07 Feb 2024; Accepted 27 Mar 2024; Posted 29 Mar 2024  View: PDF

Abstract: In this paper, a novel methodology for generating polarization singularity lattices using ramp phase structures in a polarization interferometer is presented. By applying differential tilts to distinct regions within the wavefront using a spatial light modulator, a phase-discontinuity line separating the two regions is formed. During propagation along this line, phase vortices are formed at discrete points about which the phase difference on either side of the ramp is π. This wavefront with phase vortices is superimposed with a plane wave in orthogonal polarization in a polarization interferometer, giving rise to polarization singularities. A common-path polarization interferometer is constructed using a spatial light modulator to reduce errors and complexity. Polarization fringes instead of intensity fringes obtained in this interferometer host polarization singularities. Lattices made up of a linear chain of polarization singularities—unusually of the same index polarity—are found here. Experimental results corroborate the theoretical predictions. This study shows that singularities can be produced with non-spiral phase plates by using linear phase ramps. The method discussed in this manuscript may find potential applications in optical trapping and particle steering.

Tutorial: Nonlinear Ray Tracing in Focused Fields. Part 3: Monochromatic Wavefront Aberration

qin yu and Bryan Hennelly

DOI: 10.1364/JOSAA.503924 Received 29 Aug 2023; Accepted 22 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: Using the flux tracing algorithm developed in the previous two parts, we examine the nonlinear rays that pass through the focus of a lens containing monochromatic aberrations. Lens aberration is modeled differently in the numerical propagation algorithms relating to the thin lens and the ideal lens case. For the former, an additive phase term is applied to the transmission function of the thin lens, which describes a distortion in the thickness function of the lens, and for the latter an additive phase term is added to the pupil function of the lens (the Fourier transform of the image plane). In both cases, the Zernike polynomials are applied to model various aberrations including spherical, defocus, comatic, astigmatism, trefoil, and quadrafoil. Despite the different methods of modelling aberration for the two types of lens, remarkably similar results are obtained for both cases. A discussion is also provided on the relation ship between classical wavefront aberration theory and nonlinear tracing. This paper demonstrates the extraordinary potential of nonlinear ray tracing to gain insights into complex optical phenomena.

Tutorial: Nonlinear Ray Tracing in Focused Fields. Part 2: Tracing the Flux.

Bryan Hennelly and qin yu

DOI: 10.1364/JOSAA.503925 Received 29 Aug 2023; Accepted 22 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: In this three part paper, we develop a method to trace the lines of flux through a three-dimensional wavefield by following a direction that is governed by the derivative of the phase at each point, a process that is best described as flux tracing but which we interchangeably name as 'nonlinear ray tracing'. In the first part we focused on the high-speed calculation of focused three-dimensional complex wavefields in the paraxial approximation for TEM00 and TEM01 laser modes. The algorithms developed in the first paper are first used to generate the three-dimensional grid of samples of the complex wavefield in the focal region. In this second part, we focus on tracing a flux through this three-dimensional point cloud. For a given 'ray' at an arbitrary position in the 3D volume, interpolation of the three-dimensional samples is applied to determine the derivative of the phase (normal to the direction of propagation) at the ray position, which is then used to direct the ray as it 'propagates' forward in a straight line over a short distance to a subsequent plane; the process is repeated between consecutive planes. The initial origin of the ray can be chosen arbitrarily at any point and the ray can be then be traced through the volume with appropriate interpolation. Results are demonstrated for focused wavefields in the absence of aberrations, corresponding to the cases highlighted in the first paper. Some of the most interesting results relate to focused Laguerre-Gaussian beams, for which the rays are found to spiral at different rates of curvature. In the third paper we extend the application of this algorithm to the investigation of lens aberration.

Tutorial: Nonlinear Ray Tracing in Focused Fields. Part 1: Calculating 3D Complex Wavefields.

qin yu and Bryan Hennelly

DOI: 10.1364/JOSAA.503926 Received 30 Aug 2023; Accepted 22 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: In this three part paper, we develop a method to trace the lines of flux through a three-dimensional wavefield by following a direction that is governed by the derivative of the phase at each point, a process that is best described as flux tracing but which we interchangeably name as ’nonlinear ray tracing’. In this first part, we focus on the high-speed calculation of three-dimensional complex wavefields, which is a necessary precursor to flux tracing. The basis of this calculation is the Angular Spectrum method, a well known numerical algorithm that can be used to efficiently and accurately calculate diffracted fields for numerical apertures <0.7. It is known that this approach yields identical predictions to the first Rayleigh-Sommerfeld solution. We employ the Angular Spectrum method to develop two algorithms that generate the 3D complex wavefield in the region of focus of a lens. The first algorithm is based on the thin lens approximation and the second is based on the concept of an ideal lens, which can be modelled using an optical Fourier transform. Both algorithms are investigated to calculate focused laser beams with TEM00 and TEM01 laser profiles. The three-dimensional sampling requirements of the focused field are investigated in addition to the computational and memory efficiency of the two algorithms. These two algorithms generate the 3D scaffold for the flux tracing method developed in the second paper and in the third paper we highlight the application of the method to understanding monochromatic lens aberration. Disregarding the second and third papers, the two algorithms developed in this paper are interesting for anyone seeking to compute focused fields in three-dimensions.

Virtually Measuring Layered Material Appearance

Kewei XU, Arthur cavalier, Benjamin Bringier, Mickaël Ribardière, and Daniel Meneveaux

DOI: 10.1364/JOSAA.514604 Received 30 Nov 2023; Accepted 17 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: This paper describes the design and the implementation of a virtual gonioradiometer dedicated to theanalysis of layered materials BSDF. For a given material sample, interfaces between layers are representedby geometric meshes, associated with elementary reflectances. Light scattering is performed using pathtracing. Our system is composed of 5 hemispherical sensors, which cells have uniform solid angles, anda close-to-uniform geometry. The upper hemisphere captures the reflected radiance distribution, whilethe other 4 collect the light energy lost by the sample sides. Sensor resolutions can be set to gather veryfine details of the BSDF. With the proposed system, any type of virtual surface reflection and transmissioncan be simulated, with several controllable surface layers, and with any type of reflection configuration,including direct reflections, two bounces of reflection, or all contributions. A series of results are providedwith several types of layered materials, as well as discussion and analysis concerning the assumptionsmade with analytical layered BSDF models. We also propose an in-depth study of the side effects thatinevitably appear when measuring such (real) material configurations. Finally, our system will be freelyavailable to the community (open source dissemination). ©