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A modified homodyne laser interferometer based onphase modulation for simultaneous measuringdisplacement and angle

Xianfan Wang, jing su, Jianhua Yang, lijun miao, and Tengchao Huang

DOI: 10.1364/AO.425294 Received 16 Mar 2021; Accepted 06 May 2021; Posted 07 May 2021  View: PDF

Abstract: Many researchers from scientific and industrial fields have devoted their efforts into the laser interferometer, aiming toimprove the measurement accuracy and extend the practical applications. Here, we present a modified homodyne laserinterferometer based on phase modulation for simultaneous measuring displacement and angle. The active sawtoothwave phase modulation enhances immunity of this interferometer to the environmental fluctuations and laser powerdrift. Based on polarized optic theory and sinusoidal measurement retro-reflector, a modified Michelson-typeinterferometer configuration is designed to simultaneously measure displacement and angle. Phase difference betweenthe reference and measurement interference signals can be obtained using the sawtooth wave phase modulation and zerocrossing detection technique, where the real-time displacement and angle values can be derived directly. Experimentalresults demonstrate our proposed interferometer has good static and dynamic performance.

2D Zonal Integration with Unordered Data

Greg Smith

DOI: 10.1364/AO.426162 Received 25 Mar 2021; Accepted 06 May 2021; Posted 07 May 2021  View: PDF

Abstract: Numerical integration of two dimensional gradient data is an important step for many slope measuringoptical instruments. However existing methods are limited by low accuracy or data location restrictions.The zonal integration algorithm in this paper is a generalized process that works with unordered data viaTaylor series approximations of finite difference calculations. This method does not require iteration, andall significant steps rely on matrix calculations for a least-squares solution. Simultaneous integration andinterpolation is achieved with high accuracy and arbitrary data locations.

Experimental and theoretical analysis of Dy3+-dopedfiber lasers for efficient yellow emission

Md Amin, Stuart Jackson, and Matthew Majewski

DOI: 10.1364/AO.426360 Received 01 Apr 2021; Accepted 06 May 2021; Posted 07 May 2021  View: PDF

Abstract: We report a detailed experimental and theoretical analysis of the 4F9/2 to 6H13/2 lasing transition of adysprosium (Dy3+)-doped ZBLAN fiber, a strong candidate for future compact and highly efficient yellow laser emission. Experimentally, we used a gallium nitride (GaN) laser diode emitting at 447 nm as apump source and measured yellow laser output that was generated with a maximum slope efficiency of33%, which is less than a half of the Stokes limit (of ∼78%). This result commensurate with two otherreports of yellow emission from Dy3+. As a result, we developed a numerical model to understand andanalyse the improvement potential of this fiber laser system. For reliable spectroscopic data input to numerical model, we measured the absorption and emission cross sections from Dy3+-doped ZBLAN glass.We investigated the potential causes of the low experimental slope efficiency and found contributionsfrom the background loss of the fiber and excited-state absorption (ESA) of the intracavity yellow light.We estimated the signal re-absorption cross section using the emission cross section and the McCumberrelation that was subsequently used in our numerical model to compare successfully with our experimental results. We show that the ESA can be reduced for future Dy3+-doped yellow laser systems by cascadelasing or co-doping with a suitable rare earth ion desensitiser.

Rugged diode-pumped Alexandrite laser as anemitter in a compact mobile lidar system foratmospheric measurements

Alexander Munk, Michael Strotkamp, Bernd Jungbluth, Jan Froh, Thorben Mense, Alsu Mauer, and Josef Höffner

DOI: 10.1364/AO.422634 Received 23 Feb 2021; Accepted 06 May 2021; Posted 07 May 2021  View: PDF

Abstract: We present design and performance data of two diode-pumped Alexandrite lasersdeveloped explicitly as laser emitters in mobile potassium resonance lidar systems. The lasersyield an output power of up to 1.75 mJ at a repetition rate of 500 Hz with a beam quality ofM² < 1.1 in both spatial directions. Reliable single longitudinal mode (SLM) operation with aunrivalled narrow linewidth of 3.3 MHz at a potassium resonance line at 769.898 nm isachieved. The wavelength can be switched from pulse to pulse in a range of several GHz so thepotassium line can be scanned. The lasers are finally integrated in highly efficient lidar systemswith a power consumption of 500 W for the whole lidar system. The extremely high spectralrequirements are investigated and the performance for different working points regardingrepetition rates and pump durations is investigated. Several weeks of remotely controlledoperation of the prototype in a field campaign were conducted without changes of the outputparameters. Approximately 1,000 hours of reliable single longitudinal mode operation wasachieved during the campaign and measurements of Doppler-Mie wind observations in thestratosphere and of the potassium layer in the mesopause were conducted simultaneously evenat daytime.

Doppler-shift Compensated Spatial HeterodyneSpectroscopy for Rapidly Moving Sources

Marcus Burke, Raymond Fonck, John Harlander, and George Mckee

DOI: 10.1364/AO.426159 Received 26 Mar 2021; Accepted 06 May 2021; Posted 07 May 2021  View: PDF

Abstract: High resolution-luminosity product measurements of neutral beam emission inmagnetized plasmas are severely limited by the artificial Doppler broadening inherent to the useof large diameter collection optics. In this paper, a broadening compensation method is developedfor the spatial heterodyne spectroscopy interferometric technique. The compensation techniquegreatly reduces the artificial broadening, thereby enabling high resolution measurements at asignificantly higher photon-flux than previously available. Compensated and uncompensatedmeasurements of emission generated by impact excitation of 61 keV deuterium neutrals in atokamak plasma at the DIII-D National Fusion Facility are presented. The spectral width of thecompensated measurement is ∼0.13 nm, which is comparable to the instrument resolution. Thiswidth is ∼ 4× smaller than the uncompensated width, which for the 20 cm diameter collectionlens system utilized in this study is ∼0.5 nm.

Temporally-averaged speckle noise in wavefrontsensors for beam projection in weak turbulence

Gregory Allan, Ryan Allured, Jonathan Ashcom, Lulu Liu, and Kerri Cahoy

DOI: 10.1364/AO.424637 Received 10 Mar 2021; Accepted 06 May 2021; Posted 07 May 2021  View: PDF

Abstract: Adaptive optics (AO) compensation for imaging or coherent illumination of a remoteobject relies on accurate sensing of atmospheric aberrations. When a coherent beacon is projectedonto the object to enable wavefront sensing, the reflected reference wave will exhibit randomvariation in phase and amplitude characteristic of laser speckle. In a Shack-Hartmann WavefrontSensor (SHWFS) measurement, speckle effects cause fluctuations in the intensity of focal spotsand errors in the position of their centroids relative to those expected from purely atmosphericphase aberrations. The resulting error in wavefront measurements negatively impacts the qualityof atmospheric phase conjugation.This paper characterizes the effect of reflected laser speckle on the accuracy of SHWFSmeasurements for ground-to-space beam projection systems in weak turbulence conditions. Weshow via simulation that the speckle-induced error in centroiding depends the ratio betweenbeacon diameter and the diffraction limited resolution of the lenslet, and confirm these resultswith experimental data. We provide experimental validation that averaging of SHWFS lensletspot intensities over speckle realizations converges to the incoherent intensity as expected. Wefurther show that the effects of shot noise and speckle noise add in quadrature, simplifyingnoise analysis. Finally, we characterize the effect of temporal averaging under typical conditionsof target motion and integration time. This work provides a straightforward set of relationswhich can help investigators more accurately estimate the required integration time for wavefrontsensing in the presence of laser speckle.

Tapered Mach-Zehnder interferometer based on PbSquantum dots modified by polymers for copper ionsensing

Yuyang Yan, Meiduan Fan, Shuang Zhou, Xiaolan Sun, Longfei Ma, Ruoyu li, and Alan Kost

DOI: 10.1364/AO.425453 Received 25 Mar 2021; Accepted 06 May 2021; Posted 07 May 2021  View: PDF

Abstract: An optical fiber interferometer coated with PbS quantum dots (QDs) was developed for copper ion (Cu2+) detection.The QDs were modified by a multifunctional copolymer that enabled QD surface ligation, dispersion andcoordination with Cu2+. Cu2+ coordination with the polymer induced changes of the surrounding refractive index(SRI) of the interferometer. The sensor was highly selective for Cu2+ and showed a linear detection range of 0-1000μM, with a limit of detection (LOD) of 2.20 μM in both aqueous and biological solutions.

Accurate and practical feature extraction from noisyholograms

SIDDHARTH RAWAT and Anna Wang

DOI: 10.1364/AO.422479 Received 16 Feb 2021; Accepted 04 May 2021; Posted 04 May 2021  View: PDF

Abstract: Quantitative phase imaging using holographic microscopy is powerful and non-invasive imagingmethod, ideal for studying cells and quantifying theirfeatures such as size, thickness, and dry mass. However, biological materials scatter little light, and theresulting low signal to noise ratio in holograms complicates any downstream feature extraction and henceapplications. More specifically, unwrapping phasemaps from noisy holograms often fails or requiresextensive computational resources. We present a strategy for overcoming the noise limitation: rather thana traditional phase-unwrapping method, we extractthe continuous phase values from holograms by usinga phase-generation technique based on conditionalgenerative adversarial networks employing a Pix2Pixarchitecture. We demonstrate that a network that istrained on random surfaces can accurately generatephase maps for test objects such as dumbbells, spheres,and biconcave discoids. Furthermore, we show thateven a rapidly-trained network can generate faithfulphase maps when trained on related objects. We areable to accurately extract both morphological andquantitative features from the noisy phase maps ofhuman leukemia (HL-60) cells, where traditional phaseunwrapping algorithms fail. We conclude that deeplearning can decouple noise from signal, expandingpotential applications to real-world systems that maybe noisy.

Construction of Integrated Multi-layer Textile for Solar-drivenSteam Generation

Mengyu Xie, Jiaqi Qian, Yao Li, Hanmei Yang, Jiangang QU, Xiaolin Hu, and Qinghui Mao

DOI: 10.1364/AO.422841 Received 19 Feb 2021; Accepted 04 May 2021; Posted 04 May 2021  View: PDF

Abstract: Solar steam generation has widespread application in wastewater treatment,seawater desalination, liquid-liquid separation and other fields, providing potentialopportunities for producing fresh water. Up until now, most researchers in this fieldfocused on enhancing the evaporation rate of solar steam generation device. However,problems in terms of its portability and flexibility still exist when it comes to realapplication scenario. Herein, we propose a novel integrated multi-layer textilecomposed of reduced graphene oxide/cotton (RGO/cotton) fabric, cotton yarn andpolypropylene (PP) fabric for solar-driven steam generation. The evaporation rateobtained by the integrated multi-layer textile as prepared is 0.83 kg·m−2·h−1 under onesun solar radiation, which is 3.16 times higher than that of blank experiment and issuperior to many previously reported works. Its remarkable evaporation performanceis mainly attributed to the inherent multi-layer structures, where porous RGO/cottonfabric exhibits ultra-water vapor permeability, hydrophilic cotton yarn supplies watercontinuously, and low-density hydrophobic PP fabric hinders heat sustainably. Basedon the results of application performance evaluation, the integrated multi-layer textilewith scalable manufacturability, portability, durability, and flexibility is expected toboost the development of solar-driven steam generation.

High-harmonic diffractive lens colorcompensation

Zichan Wang, Youngsik Kim, and Tom Milster

DOI: 10.1364/AO.421032 Received 16 Feb 2021; Accepted 04 May 2021; Posted 07 May 2021  View: PDF

Abstract: Large diameter, high-harmonic diffractive lenses could find applications in futurespace telescopes. Residual chromatic aberrations from these lenses can cause significantblurring. Solutions to reduce chromatic dispersion and other aberrations to diffraction-limitedperformance are discussed. A design example based on a 240 mm diameter, 1 m focal lengthmulti-order diffractive engineered (MODE) lens operating over the astronomical R-Band (589nm to 727 nm) is presented. The design example uses a relay subsystem with 4 times smallerdiameter than the primary. This color corrector includes both refractive and diffractive opticalelements and reduces the longitudinal chromatic aberrations more than a factor of 30 comparedto the primary lens alone, while maintaining effective focal length and numerical aperture ofthe system.

Digital holographic microscopy for real-timeinvestigation of 3D microstructural dynamics ofstarch-kefiran based nanocomposite

vahideh farm rad, Amin Babaei-Ghazvini, Ramin Jamali, Iman Shahabi-Ghahfarrokhi, and Ali-Reza Moradi

DOI: 10.1364/AO.423075 Received 18 Feb 2021; Accepted 04 May 2021; Posted 07 May 2021  View: PDF

Abstract: Investigating the real-time phenomena in bio-polymers have received large attention because of their increasing demands as polymer substitution. 3D morphometry of the polymer surfaces may be very impactful in such studies. Here, digital holographic microscopy (DHM) is applied for quantitative measurementof the rare morphological changes of UV-A and UV-C exposed nanocomposites during their incubationwith excess water. By reconstruction of the recorded successive digital holograms the time evolution ofthe swelled regions of the samples is derived. Our results clearly show that the higher water-swelling ofUV-A irradiated starch/kefiran/ZnO may be attributed to its higher hydrophilicity.

Novel approach for the design of uniform-fieldelectrodes in transversely excited CO2 lasers

Alireza pezh

DOI: 10.1364/AO.425882 Received 25 Mar 2021; Accepted 04 May 2021; Posted 07 May 2021  View: PDF

Abstract: Uniform-field electrodes, which are commonly used in transversely excited CO2 lasers, are essential toproduce a very uniform pulsed glow discharge. In this paper, a simple and novel approach associated withpractical parameters is developed for the design of Chang and Ernst profiles. This approach not only providesa maximally flat distribution of field on the electrode surface but also optimizes the geometric parameterk0.The design procedure is investigated using this method based on both approximate and exact solutions andfor simplicity an iterative method will also be suggested. Furthermore, a monolithic and composite profile ispresented to calculate the 3D shape of realistic electrodes that brings desired values of field uniformity andaspect ratios. The obtained distribution of electric field is discussed to explore more details about theproposed approach. The approximate and exact methods are in good agreement and the boundaries ofdischarge region are determined by the field distribution across the discharge midplane. The results showthat a square discharge also requires an optimum applied voltage.

Optimized plasmonic reversible logic gate for low losscommunication

Kuldeep Choudhary and Santosh Kumar

DOI: 10.1364/AO.428158 Received 19 Apr 2021; Accepted 02 May 2021; Posted 07 May 2021  View: PDF

Abstract: With the development of plasmonic optical waveguide, numerous nanostructures based on differentmaterials can be fabricated in a control way. While doing reversible computing, reversible logic gates arethe necessary requirement to reduce the loss of information with less power consumption. The proposeddesign of Feynman logic gate is simulated by cascading metal-insulator-metal optical waveguide based onMach-Zehnder interferometers. The footprint of proposed Feynman logic gate is 62µm*9µm, extinctionratio is 10.57 dB and insertion loss of -0.969 dB and -1.191dB which is much better as compared to electrooptic based exiting Feynman logic gate. The results are obtained by simulating the proposed structure usingfinite difference time domain (FDTD) method and verified by using mathematical computation inMATLAB.

EFFECT OF INPUT PULSE DURATION ON LASER-INDUCED UNDERWATER ACOUSTIC SIGNALS

JANAPATI YELLAIAH and Prem Kiran Paturi

DOI: 10.1364/AO.422471 Received 10 Feb 2021; Accepted 02 May 2021; Posted 03 May 2021  View: PDF

Abstract: The characteristics of laser induced underwater acoustic signals (LIUAS) generated by focused 10 ns and 30 ps laserpulses of different energies under similar experimental conditions are compared. The time domain signals, and timefrequency analysis of the underwater acoustic signals were used to understand the role of input pulse duration and energyon the evolution of underwater acoustic signal (UAS). In the time domain, the peak-to-peak (Pk-Pk) overpressure of theUAS is decreasing, and the arrival time (At) is increasing as a function of propagation distance for both ns and ps laserinduced breakdown (LIB) of water. With increasing incident energy of both ns and ps laser pulses, the Pk - Pkoverpressures of acoustic signals are increasing almost linearly. In the time-frequency domain, the spectrogram obtainedvia STFT provides spectral information and At of both direct and reflected signals simultaneously. The spectrogramrevealed that the transient UAS has broad acoustic spectra spanning from 10 – 800 kHz, perpendicular to the laserpropagation direction. The initial acoustic impulse resulted in two major frequencies centred around 105 kHz and 690kHz with a standard error of 30 kHz. Upon reflection from the water-air interface, only the peak frequency correspondingto ~105 kHz was reflected while the longer frequency is observed to get dissipated. Our results demonstrate the ns-LIB ismore suitable for applications compared to the ps-LIB owing to stronger acoustic impulse of both direct and reflectedsignals.

Degree of polarization of photoluminescencefrom facets of InP as a function of strain: someexperimental evidence

Daniel T. Cassidy and Samuel Lam

DOI: 10.1364/AO.405394 Received 13 Aug 2020; Accepted 01 May 2021; Posted 03 May 2021  View: PDF

Abstract: Previous work demonstrated a good fit to degree of polarization (DOP) of luminescence measurements on {110} facets of InP using a simple dependence of DOP of luminescence on strain: −퐾푒 (푒1 − 푒3), where 퐾푒 is a positive calibration constant, and 푒1 and 푒3 are normalcomponents of strain in the plane of the facet and along 110¯and h001i directions [Appl. Opt. 43, 1811 (2004)]. Recent analytic modelling, which by necessity to be analytic must make simplifyingassumptions, has suggested that unless the measurements are along crystallographic axes, thedependence of the DOP of luminescence on strain is more complicated: −퐾푒 (1.315 푒1−0.799 푒3)for measurements from an InP facet, with a similar ‘excess’ 푒1 for GaAs [Appl. Opt. 59, 5506(2020)]. In this work we fit finite element simulations (FEM) to DOP measurements of thephotoluminescence from facets of InP bars with {111}퐵 v-grooves that have been placed in acylindrical bending moment. We find that the more complicated dependence of DOP on strain, asderived by the analytic model, fits the data better than the previously assumed simple dependence.This finding thus corroborates the analytical model and should have an impact on understandingthe strain-dependent operation of optoelectronic devices.

Detection of Small Targets in the Infrared: An Infrared Search and Track Tutorial

Ronald Driggers, Eytan Pollak, Robert Grimming, Ectis Velazquez, Robert Short, Gerald Holst, and Orges Furxhi

DOI: 10.1364/AO.424767 Received 12 Mar 2021; Accepted 01 May 2021; Posted 03 May 2021  View: PDF

Abstract: Airborne target detection in the infrared has been classically known as Infrared Search and Track orIRST. From a military point of view, it can be described as target detection at long ranges where thetarget image is sub-pixel in size. Here, the target is “unresolved.” It can also describe the detection ofaircraft near the observer using distributed apertures in a spherical detection field. From a commercialpoint of view, an important application is drone detection near live airport operations.As drones become more common, the dual-use functionality of IRST systems is expanding. Technologyimprovements for IRST systems include the wide proliferation of infrared staring focal planes. New readout integrated circuits (ROICs) allow for time-delay-integration (TDI) of large format detectors. Starestep sensors in the future appear to be as common as gimbal-scanned thermal imagers. Detectionprobability analysis and IRST sensor design is different than targeting system design. We provide atutorial here on IRST system calculations as well as discussions on broadband versus spectral calculationsand new technology considerations.

A double-path parallel convolutional neural networkfor removing speckle-noise in different types of OCTimages

Zhengjie Shen, Manhui Xi, Chen Tang, Min Xu, and Zhenkun Lei

DOI: 10.1364/AO.419871 Received 14 Jan 2021; Accepted 30 Apr 2021; Posted 03 May 2021  View: PDF

Abstract: Speckle noises widely exist in optical coherence tomography (OCT) images. In this paper, we propose an improveddouble-path parallel convolutional neural network (called DPNet) to reduce speckles. We increase the networkwidth to replace the network depth to extract deeper information from the original OCT images. In addition, we usethe dilated convolution and residual learning to increase the learning ability of our DPNet. We use 100 pairs ofhuman retinal OCT images as the training dataset. Then we test the DPNet model for denoising speckles on fourdifferent types of OCT images, mainly including human retinal OCT images, skin OCT images, colon crypt OCT images,and quail embryo OCT images. We compare the DPNet model with the adaptive complex diffusion (ACD) method, thecurvelet shrinkage (Curvelet) method, the shearlet-based total variation (TV-SH) method, and the OCTNet method.We qualitatively and quantitatively evaluate these methods in terms of image smoothness, structural informationprotection, and edge clarity. Our experimental results prove the performance of the DPNet model and it allows us tobatch and quickly process different types of poor-quality OCT images without any parameter fine-tuning under atime-constrained situation.

Algorithm optimization of Cross-Interfaces ComputedTomography into Full field

Chen LIng, Haiyan Chen, Yu Gao, and Yue Wu

DOI: 10.1364/AO.425829 Received 23 Mar 2021; Accepted 30 Apr 2021; Posted 03 May 2021  View: PDF

Abstract: Tomographic approaches in confined space requires advanced imaging algorithm which can properly consider therefractive distortion as the imaging rays pass through the optical wall. Our previous work established an algorithm(Cross-Interfaces Computed Tomography, CICT) and practically solved tomographic problems in confined space.However, critical restriction was found in CICT: images simulated at small azimuth angles are contaminated withnoticeable signal loss and become unusable. Based on this recognition, this work has developed an improvedtomography approach, namely Full-field Cross-Interfaces Computed Tomography (FCICT), to extend the availableview angles to all perspectives. The key of this approach involves the 3D domain discretization using voxelparallelepipeds instead of traditional voxel layers to establish the ray-tracing relationship between imaging planesand the measurement domain. The imaging process of FCICT is first validated by quantitatively comparing the gridimaging locations in measured and simulated projections of a calibration plate. By evenly distributing the viewangles in whole azimuth angle range, the FCICT reconstruction is then numerically validated by reconstructing asimulated double-cone flame phantom, and the reconstruction presents a high correlative coefficient of ~98% withthe original phantom. Finally, the FCICT is employed to reconstruct an ethylene-air premixed flame. Comparisonsshow the re-projections generated by the FCICT reconstruction are in highly accordance with measured flameimages, with the mean correlative coefficients of more than 95%.

The dielectric function of vanadium oxide thin films by thermal annealing

Adolfo Canillas, Frank Güell, Oriol Arteaga, PAULINA R. MARTÍNEZ-ALANIS, Michel Vergnat, Hervé RINNERT, and Blas Garrido

DOI: 10.1364/AO.420476 Received 29 Jan 2021; Accepted 30 Apr 2021; Posted 30 Apr 2021  View: PDF

Abstract: The dielectric function of VOx and V2O5 thin films is determined with the use of a spectroscopic Mueller matrix ellipsometer from 1.5 to 5.0 eV. The complex dielectric function of the films is calculated using the measured Mueller matrices filtered with the Cloude decomposition. VOx shows high absorption in the UV region, a Tauc-Lorentz gap around 2.4 eV and non-vanishing absorption in the visible. V2O5 shows a high absorption band centered at 2.87 eV, an indirect optical band gap at 1.95 eV and a direct optical band gap at 2.33 eV. The ellipsometric characterization is supported by Raman, X-ray photoelectron and photoluminescence spectroscopy.

3-Component Flow Velocity Measurementswith Stereoscopic Picosecond LaserElectronic Excitation Tagging (S-PLEET)

Colter Russell, Naibo Jiang, Paul Danehy, Zhili Zhang, and Sukesh Roy

DOI: 10.1364/AO.420067 Received 21 Jan 2021; Accepted 29 Apr 2021; Posted 30 Apr 2021  View: PDF

Abstract: Nonintrusive 3-component (3C) velocity measurements of free jet flows wereconducted by S-PLEET (Stereoscopic Picosecond Laser Electronic Excitation Tagging) at100 kHz. The fundamental frequency of the burst-mode laser at 1064 nm was focused togenerate the PLEET signal in a free jet flow. A stereoscopic imaging system was used tocapture the PLEET signals. The 3C centroids of the PLEET signal were determined byutilizing simultaneous images from two cameras placed at an angle. The temporal evolutionsof the centroids were obtained and used to determine the instantaneous, time resolved 3Cvelocities of the flows. The free jets with various inlet pressures of 10–40 bars exhaustinginto atmospheric pressure air (i.e., the underexpanded free jet with large pressure ratios,Reynolds numbers from the jet ranged from 39,000 to 145,000) were measured by S-PLEET.Key 3C turbulent properties of the free jets, including instantaneous and mean velocities wereobtained with an instantaneous measurement uncertainty of about ±10 m/s which is about 2%of the highest velocities measured. Computation of higher-order statistics includingcovariances related to turbulent kinetic energy and Reynolds stress component weredemonstrated. The new 3C nonintrusive and unseeded velocimetry technique could provide anew tool for the flow property measurements in ground test facilities; the measured highfrequency turbulence properties of free jet flows could be useful for turbulence modeling andvalidations.

Optimization of multiscale digital specklepatterns for multiscale deformationmeasurement using stereo-digital imagecorrelation

Zhenning Chen, Xinxing Shao, Wei Sun, Xiaoyuan He, and Jie Zhao

DOI: 10.1364/AO.423350 Received 23 Feb 2021; Accepted 29 Apr 2021; Posted 30 Apr 2021  View: PDF

Abstract: Simultaneous monitoring of multiple fields of views (FOVs) by multiscale stereodigital image correlation (stereo-DIC) can quantify the deformation of a material whenlocalized phenomena occur within a larger FOV or moving object. In multiscale deformationmeasurement via stereo-DIC, optimization of the digital speckle patterns (DSPs) is essentialto achieve high accuracy and efficiency. This work optimizes and fabricates multispectralDSPs used for multiple scales. First, an optimization of the DSP for two FOVs is achievedusing both spatial modulation and specified spectra. A spatially modulated DSP is comparedwith two spectral DSPs achieved by visible and ultraviolet-excited blue light. Then, aspatially modulated visible DSP fabricated by an ultraviolet printer overlaid with anultraviolet-excited blue DSP fabricated by a photosensitive seal is designed for multiscalestereo-DIC measurements of three FOVs. Experiments were performed to illustrate thefunctionality and utility of this multiscale DSP. Such experimental analyses can supplyadequate full-field data to validate localized or kinetic mechanical behavior.

On axis sample viewer with flexible working distancefor an X-ray spectroscopy beamline

Roelof van Silfhout

DOI: 10.1364/AO.423932 Received 02 Mar 2021; Accepted 29 Apr 2021; Posted 30 Apr 2021  View: PDF

Abstract: Conducting research using micrometer-sized X-ray beams with small samples is common at modern synchrotron X-raysources. Often, the relative alignment between X-ray beam and sample is time consuming. An on-axis or coaxial camerasystem with a view of the sample in a direction along the path of the X-ray beam with its field of depth set to coincide withthe location of the focal spot of the X-ray beam is preferred. Besides the use of a drilled mirror, we propose the use ofPellicle mirror to create an on-axis viewer allowing various sample environments and X-ray beam sizes.

The possibility of intracavity terahertzdifference frequency generation in a twofrequency GaAsP/AlGaAs/GaAs quantum welllaser

Alexander Dubinov, Vladimir Utochkin, and Anna Razova

DOI: 10.1364/AO.425366 Received 18 Mar 2021; Accepted 29 Apr 2021; Posted 30 Apr 2021  View: PDF

Abstract: We consider the GaAsP/AlGaAs/GaAs laser design with two different quantumwells for simultaneous generation of TE0 and TM0 modes having different frequencies innear-IR range. We theoretically investigate the possibility of effective difference frequencygeneration in the 7.5 – 8 and 10.5 – 11 THz regions in the laser design proposed. Resonantincrease of second-order susceptibility in AlGaAs in these ranges provides sufficientgeneration efficiency. We demonstrate an output power-conversion factor for the differencefrequencies in the 7.5 – 8 THz range to be up to 4 – 8 MW-1 at room temperature in such alaser.

Observations Regarding Emissions in ShortGated ns/ps/fs-LIBS for Fuel-to-Air RatioMeasurements in Methane-Air Flames

Mark Gragston, Paul Hsu, Naibo Jiang, Sukesh Roy, and Zhili Zhang

DOI: 10.1364/AO.418453 Received 24 Dec 2020; Accepted 29 Apr 2021; Posted 30 Apr 2021  View: PDF

Abstract: A study of short-gated 10 nanosecond (ns), 100 picosecond (ps), and 100 femtosecond (fs)-laser induced breakdown spectroscopy (LIBS) was conducted for fuel-to-air ratio (FAR)measurements in an atmospheric Hencken flame. The intent of the work is to understand whichemission lines are available near the optical range in each pulse width regime and whichemission ratios may be favorable for generating equivalence ratio calibration curves. Theemission spectra in the range of 550 - 800 nm for ns-LIBS and ps-LIBS are mostly similar withslightly elevated atomic oxygen lines by ps-LIBS. Spectra from fs-LIBS show the lowestcontinuum background and prominent individual atomic lines, though has significantly weakerionic emission from nitrogen. A qualitative explanation based on assumed local thermodynamicequilibrium and electron temperatures calculated by the NII(565nm) and NII(594nm) emissionsis presented. In studying line emission ratios for FAR calculation, it is found that H(656nm)/NII(568 nm) is best for FAR measurements with ns-LIBS and remains viable for ps-LIBS,while H(656 nm)/OI(777 nm) is optimal for the ps-LIBS and fs-LIBS cases. Due to lowcontinuum background and short time delay for spectra collection, fs-LIBS is very promisingfor high-speed FAR measurements using short-gated LIBS.

Strong extrinsic chirality in biaxial hyperbolicmaterial 𝜶- MoO3 with in-plane anisotropy

Biyuan Wu, Mingjun Wang, WU FENG, and Xiaohu Wu

DOI: 10.1364/AO.426098 Received 30 Mar 2021; Accepted 29 Apr 2021; Posted 03 May 2021  View: PDF

Abstract: Chirality has always been a hot research topic since it possesses potentialapplications in polarization optics, chemical and bio-sensing. In the previous works, intrinsicchirality has been extensively explored, but its development is limited due to the complexity infabrication of chiral metamaterials. Therefore, there is an urgent need to simplify fabricationand design of compact devices with chiral response. Extrinsic chirality has shown greatpotential since it can be realized in nonchiral anisotropic planar structure with low-costfabrication techniques. In this paper, the extrinsic chirality of biaxial hyperbolic material 𝛼-MoO3 with in-plane anisotropy has been investigated. By analyzing the effect of thickness of𝛼-MoO3 film, the angle of incidence, azimuth angle and wavelength of incidence on the circulardichroism (CD), the maximum CD can reach 0.77. This strong extrinsic chirality of the 𝛼-MoO3 film results from the mutual orientation of the 𝛼-MoO3 film and the incident light. Inaddition, 𝛼-MoO3 film can still maintain strong extrinsic chirality when the azimuthal angle isapproximately from 20° to 57° and the angle of incidence is from 55° to 80°.

High-speed computational ghost imaging based onauto-encoder network under low sampling rate

Wei Feng, Xingyu Sun, Xiuhua Li, Junhui Gao, Xiaodong Zhao, and Daxing Zhao

DOI: 10.1364/AO.422641 Received 12 Feb 2021; Accepted 29 Apr 2021; Posted 30 Apr 2021  View: PDF

Abstract: Computational ghost imaging is difficult to apply under low sampling rate. In this paper, we propose a highspeed computational ghost imaging based on auto-encoder network to reconstruct images with high quality under lowsampling rate. The auto-encoder convolutional neural network is designed and the object images can be reconstructedaccurately without labeled images. Experimental results show that our method can greatly improve the peak signal-tonoise ratio and structural similarity of test samples, which are up to 18 and 0.7 respectively under low sampling rate. Ourmethod only needs 1/10 of traditional deep learning samples to achieve fast and high-quality image reconstruction, andthe network also has a certain generalization to the grayscale images.

Photonic Approach for SimultaneousMeasurement of Microwave DFS and AOA

jianing Zhao, Zhenzhou Tang, and Shilong Pan

DOI: 10.1364/AO.422946 Received 17 Feb 2021; Accepted 28 Apr 2021; Posted 30 Apr 2021  View: PDF

Abstract: A photonic scheme that can simultaneously estimate the microwave Dopplerfrequency-shift (DFS) and angle-of-arrive (AOA) is demonstrated. In the proposed system,the transmitted signal is independently mixed with two echo signals by a dual-channelmicrowave photonic mixer. By measuring the frequency of the intermediate frequency (IF)signals output from the two channels and the phase difference between them, both the DFS(with direction identification) and AOA parameters can be obtained. In a proof-of-conceptexperiment, the errors are less than ± 0.08 Hz for the DFS measurement within a range of±100 kHz and less than ±1.3° for the AOA measurement ranging from 0° to 90°, respectively.

Analytical determination of the complexrefractive index and the incident angle of anoptically isotropic substrate by ellipsometricparameters and reflectance

Chia-Wei Chen, Matthias Hartrumpf, Thomas Längle, and Jürgen Beyerer

DOI: 10.1364/AO.423793 Received 02 Mar 2021; Accepted 28 Apr 2021; Posted 30 Apr 2021  View: PDF

Abstract: An analytical solution for the determination of both angle of incidence (AOI) andthe complex refractive index from combined ellipsometric and reflectometric measurementsat optically isotropic substrates is presented. Conventional ellipsometers usually measure flatsurfaces because the curvatures of the surface alter the reflected or transmitted light whichcauses experimental errors due to the deviation of the incident angle. However, in real industrialapplications, the shapes of samples are usually curved or even free-form. In this case, theknowledge of the AOI is essential. The proposed method provides a simple way to measurethe AOI and the complex refractive index of nonplanar samples without extra or complicatedhardware.

Optimal Optical Path Difference of Asymmetric Common-pathCoherent-dispersion Spectrometer

Shasha CHEN, Ruyi Wei, xie zhengmao, Yinhua Wu, Lamei Di, wang feicheng, and zhai yang

DOI: 10.1364/AO.425491 Received 19 Mar 2021; Accepted 28 Apr 2021; Posted 30 Apr 2021  View: PDF

Abstract: Optical path difference (OPD) is a very significant parameter in asymmetric common-path coherentdispersion spectrometer (CODES), which directly determines the performance of the CODES. In order toimprove the performance of the instrument as much as possible, a temperature-compensated optimal opticalpath difference (TOOPD) method is proposed. The method does not only consider the influence oftemperature change on the OPD, but also effectively solves the problem that the optimal OPD cannot beobtained simultaneously at different wavelengths. Taking the spectral line with a Gaussian-type powerspectral density distribution as a representative, the relational expression between the OPD and the visibilityof interference fringes formed by the CODES is derived for the stellar absorption/emission line. Further, theoptimal OPD is deduced according to the efficiency function, and the relationship between the optimal OPDand wavelength is analyzed. Then based on the materials' dispersion characteristics, different opticalmaterials are combined and added to the interferometer's reflected and transmitted optical path to implementthe optimal OPD at different wavelengths, thereby improving the detection precision. Meanwhile, thematerials whose refractive index negatively changes with temperature are selected to reduce or even offset thetemperature impact on OPD, hence the system's stability is improved and further improve the detectionprecision. Under certain input conditions, the material combination which approximate the optimal OPD isperformed within the range of 0.66-0.9 μm. The simulation results show that, the maximal difference betweenthe optimal OPD obtained by the efficiency function and the OPD produced by the material combination is0.733 mm for absorption line and 1.122 mm for emission line respectively, which is reduced by one timecompared with only one material; while the influence of temperature on the OPD can be reduced by 2-3orders of magnitude by material combination, which greatly ameliorates the stability of the wholespectrometer. Hence, TOOPDA method provides a new idea for further improving the high-precision radialvelocity (RV) detection of the asymmetric common-path CODES.

Optical design for laser tweezers Raman spectroscopysetups for increased sensitivity and flexible spatialdetection

Tobias Dahlberg and Magnus Andersson

DOI: 10.1364/AO.424595 Received 12 Mar 2021; Accepted 28 Apr 2021; Posted 30 Apr 2021  View: PDF

Abstract: We demonstrate a method to double the collection efficiency in Laser Tweezers Raman Spectroscopy(LTRS) by collecting both the forward and back-scattered light in a single-shot multitrack measurement.Our method can collect signals at different sample volumes, granting both the pinpoint spatial selectivityof confocal Raman and the bulk sensitivity of non-confocal Raman simultaneously. Further, we displaythat our approach allows for reduced detector integration time and laser power. To show this, we measurethe Raman spectra of both polystyrene beads and bacterial spores. For spores, we can trap them at 2.5mW laser power and acquire a high signal-to-noise ratio Power spectrum of the CaDPA peaks using anintegration time of 2 x 30 seconds. Thus, our method will enable the monitoring of biological samplessensitive to high intensities for longer times. Additionally, we demonstrate that by a simple modification,we can add polarization sensitivity and retrieve extra biochemical information.

Research on Indoor Location Technology Based on LED Visible Light and QR Code

Qian Wu and Yan He

DOI: 10.1364/AO.426233 Received 09 Apr 2021; Accepted 27 Apr 2021; Posted 30 Apr 2021  View: PDF

Abstract: For the current problems of extremely complex visible light indoor positioning algorithms and devices and low positioning accuracy, this paper proposes an improved indoor positioning method combining LED visible light and QR codes. Firstly, initial positioning is achieved by loading a QR code image containing LED position information on the LED light, which is recognized and decoded by the receiver. Then the position of the receiver is precisely located by proposing an improved indoor positioning method. The experiment shows that the smallest average error of 4.0326cm in positioning, which greatly improves the indoor positioning accuracy.

The effect of surface modification on wettability andtribology by laser-textured in Al2O3

xiubing Jing, yani xia, shuxian zheng, Chengjuan Yang, huan qi, and Syed Husain Imran Jaffery

DOI: 10.1364/AO.423564 Received 26 Feb 2021; Accepted 27 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: The life of ceramic tools restricts the development of the manufacturing industry,and can be increased through the enhancement of surface performance. Laser surfacetexturing is a feasible technology to improve ceramic tool life based on therelationship between surface properties and the laser-texturing process. In this study,Al2O3 substrates have been textured by an Ytterbium fiber laser system with awavelength of 1064 nm and a pulse duration of 50 ns. First, the damage threshold ofAl2O3 was measured to provide a basis for selecting laser-texturing parameters. Thesurface morphology was characterized using a white confocal scanning microscopeand a scanning electron microscope (SEM) to investigate the characteristics of laserprocessing. Water contact angles were measured to investigate the relationshipbetween laser parameters and changes in wettability. The surface energy of the superhydrophobic ceramic was calculated based on the contact angle. Combined XPSmeasurement was used to explore the mechanism of wettability changes from thechemical component and microstructure perspectives. The friction coefficient ofAl2O3 was determined by a ball-on-disc wear test. The results showed that lasertexturing can significantly improve the surface hydrophobicity and friction stability.

Design method for off-axis reflective anamorphicoptical system with aberration balance and constraintcontrol

Yue Wu, Liping Wang, Xu Zhang, Jie Yu, BO YU, and Chun-shui Jin

DOI: 10.1364/AO.427713 Received 14 Apr 2021; Accepted 27 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: This paper proposes a design method for off-axis reflective anamorphic optical system (ORAOS). This method first applies vector aberration theory to establish a mathematical model to balance the aberration of an ORAOS. It then builds the error function of structural parameters and constraints through spatial ray tracing and grouping design. Next, it introduces automatic adjustment of weight factors for dynamic balance of aberrations and constraints. A particle swarm simulated annealing algorithm is used to iteratively calculate the initial structure of the ORAOS. Finally, we use an extreme ultraviolet (EUV) lithographic projection objective with off-axis six-reflective anamorphic mirrors ( βx=1/4,βy=1/8) as an example to verify the effectiveness of this method. We obtain an EUV lithographic anamorphic objective with a numerical aperture of 0.55 and a root mean square wavefront error better than1/30λ ( λ=13.5 nm).

Measurement of radius of curvature directly ininterferometer confocal position

Pavel Psota, Marek Stasik, Jan Kredba, Vit Ledl, and Jakub Necasek

DOI: 10.1364/AO.421188 Received 29 Jan 2021; Accepted 27 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: This paper presents a new method for radius of curvature measurement by interferometers. The radius measurement iscarried out directly in the interferometer confocal position without the need for a specific hardware and thus allows tomeasure a much more diverse range of optical surfaces than standard methods. The method is based on measuring anumber of phase maps and displacements at several steps through the confocal null position. Radius of curvature is thencomputed as the tangent slope of the measured defocus – displacement pair values in the confocal position. A relativeaccuracy of the method is approximately 0.05% which makes the method suitable for a vast number of applications.Results of the method are verified using standard confocal-cat’s eye technique.

Numerical investigation of high birefringence andnonlinearity tellurite glass photonic crystal fiber withmicro-structured core

Jiangyun Wang

DOI: 10.1364/AO.423029 Received 22 Feb 2021; Accepted 26 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: In this paper, A novel photonic crystal fiber(PCF) based on tellurite glass with high birefringence and high nonlinearityis designed. Six small air holes arranged nearly rectangular are added in the fiber core to form the near-elliptic core. Byusing the finite element method(FEM) with the help of COMSOL Multiphysics software, we investigate and simulate thebirefringence, effective mode area, nonlinear coefficient and the dispersion characteristics. Simulation results show thatby optimizing the structure parameters of the core, at the wavelength of 1.55μm, the birefringence is up to 5.05×10−2;nonlinear coefficient can reach a maximum of 1896 W-1km-1, moreover, the zero dispersion points can be obtainedaround the wavelength 1.55μm. The proposed structure is easy for fabrication. The advantage of the proposed PCF haspotential applications in polarization control, communication systems and super-continuum generation.

Nanosecond pulsed single-frequency two-stageholmium-doped fiber MOPA at 2054 nm and2090 nm

Dominik Lorenz, Clement Romano, Marc Eichhorn, and Christelle Kieleck

DOI: 10.1364/AO.424061 Received 02 Mar 2021; Accepted 26 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: A single-frequency polarization-maintaining holmium-doped fiber master oscillatorpower amplifier operating at signal wavelengths of 2054 nm and 2090 nm is presented. Thetwo-stage setup delivers up to 240 W peak power and 6.7 µJ pulse energy for a pulse widthof 30.2 ns at a repetition rate of 100 kHz. The first amplifier stage is designed by simulation,tailored for high gain at the signal wavelength range, favoring amplification at 2090 nm. Thedesign is discussed and the measured values are compared with the simulation. The secondstage is investigated regarding the efficiency for co- and counter-pumping. Stimulated Brillouinscattering was found to be the limiting factor for pulse peak power scaling in the second stage.The measured output pulse shapes are discussed and compared to pulse shapes derived with theFrantz-Nodvik model.

Real-time quantitative phase imaging by single-shotdual-wavelength off-axis digital holographic microscopy

Huang Meng, Hongpeng Qin, and Zhuqing Jiang

DOI: 10.1364/AO.424666 Received 12 Mar 2021; Accepted 26 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: A single-shot dual-wavelength digital holographic microscopy with adjustable off-axis configuration is presented, bywhich a real-time quantitative phase imaging for living cells is realized. With this configuration, two sets ofinterference fringes corresponding to their wavelengths can be flexibly recorded onto one hologram just at one shot.The universal expression on the dual-wavelength hologram recorded under any wave-vector orientation angles ofreference beams is given. In order to refrain from the effect of zero-order spectrum as possible, we can flexibly selecttheir carry frequencies for the two wavelengths via this adjustable off-axis configuration, according to thedistribution feature of object’s spatial-frequency spectrum. This merit is verified by the experiment of quantitativephase imaging for the micro-channel of a microfluidic chip. The reconstructed phase maps of living onion epidermalcells exhibit the cellular internal life activities, vividly displaying the progress of nucleus, cell wall, cytoskeleton, andthe substance transport in microtubules inside living cells, for the first time to our knowledge. These imaging resultsdemonstrate the presented method in availability and reliability for real-time quantitative phase imaging.

Underwater image sharpening based on structurerestoration and texture enhancement

Lin Sen, kaichen chi, Tong Wei, and Zhiyong Tao

DOI: 10.1364/AO.420962 Received 29 Jan 2021; Accepted 26 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: Light can be absorbed and scattered when traveling through water, which results in underwater opticalimages suffering from blurring and color distortion. To improve the visual quality of underwater opticalimages, we propose a novel image sharpening method. We utilize the relative total variation model todecompose images into structure and texture layers in a novel manner. On those two layers, the Red-BlueDark Channel Prior (RBDCP) and detail lifting algorithms are proposed, respectively. The RBDCP modelcalculates background light based on brightness, gradient discrimination, and hue judgment which thengenerates transmission maps using red-blue channel attenuation characteristics. The linear combinationof the Gaussian kernel and binary mask is employed in the proposed detail lifting algorithm. Furthermore, we combine the layers of restoration structure and enhancement texture for image sharpening, inspired by the concept of fusion. Our methodology is both rich texture information and effective in colorcorrection and atomization removal through RBDCP. Extensive experimental results indicate that the proposed method effectively balances image hue, saturation, and clarity.

Numerical and experimental study on theimpact of chromatic dispersion on O-banddirect-detection transmission

Yang Hong, Kyle Bottrill, Natsupa Taengnoi, Naresh Kumar Thipparapu, Yu Wang, Jayanta Sahu, David Richardson, and Periklis Petropoulos

DOI: 10.1364/AO.424962 Received 15 Mar 2021; Accepted 26 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: The recent emergence of efficient O-band amplification technologies has enabledthe consideration of O-band transmission beyond short-reach. Despite the O-band being a lowchromatic dispersion (CD) window, the impact of CD will become increasingly significantwhen extending the reach of direct-detection (DD) systems. In this work, we first numericallyinvestigate the 3-dB bandwidth of single-mode fibers (SMF) and the CD-restricted transmissionreach in intensity-modulation DD systems, confirming the significant difference between lowand high-dispersion O-band wavelengths. We then carry out experimental transmission studiesover SMF for distances of up to 70 km at two different wavelengths, the low-dispersion1320 nm and the more dispersive 1360 nm, enabled by the use of an O-band bismuth-dopedfiber amplifier as a pre-amplifier at the receiver. We compare three 50-Gb/s optical DD formats,namely Nyquist on-off keying (OOK), Nyquist 4-ary pulse amplitude modulation (PAM4) andKramers-Kronig detection-assisted single-sideband quadrature phase shift keying (KK-QPSK)half-cycle subcarrier modulation. Our results show that at both wavelengths, OOK and QPSKexhibit better bit error rate performance than PAM4. When transmitting over 70-km of SMF atthe less dispersive wavelength of 1320 nm, 50-Gb/s OOK modulation offers more than 1.5-dBoptical power sensitivity improvement at the photodiode (PD) compared to 50-Gb/s QPSK.Conversely, at 1360 nm, the required optical power to the PD can be reduced by more than 3dB by using QPSK instead of OOK.

Fast processing of underwater polarization imagingbased on optical correlation

Huajun Zhang, Mingyuan Ren, Hantao Wang, Jinren Yao, and Yu Zhang

DOI: 10.1364/AO.423184 Received 26 Feb 2021; Accepted 26 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: Underwater polarization differential imaging requires the estimation of different parameters, and theparameters can be accurately obtained by using optical correlation. However, optical correlation as a criterion function to estimate parameters takes a lot of time. To expedite the parameters estimation process,we propose two operations to process underwater polarization images. One operation is to update the analyzer angle range to reduce the number of processed images. The other is image downsampling, whichreduces the amount of calculation for the corresponding images. In experiments, we confirmed the feasibility of our method. We have obtained an average of 42 times the calculation speed increase under theconditions of updating the analyzer angle range three times and reducing the image scale by 16 times.The results of our method are consistent with those of traditional methods. This established method isconducive to the practical application of underwater polarization differential imaging.

Compact computational spectrometer usingsolid wedged low finesse etalon

Shawn Redmond, Salvatore Di Cecca, and William Herzog

DOI: 10.1364/AO.423440 Received 24 Feb 2021; Accepted 26 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: A novel two layer hybrid solid wedged etalon was fabricated and combined with atraditional imager to make a compact computational spectrometer. The hybrid wedge,comprised of Nb2O5 and Infrasil 302, was designed to operate from 0.4-2.4 µm. Initialdemonstrations, however, used a CMOS imager and demonstrated operation from 0.4-0.9 µmwith spectral resolutions <30 cm-1 from single snapshots. The computational spectrometeritself operates similarly to a spatial Fourier Transform spectrometer (FTIR), but rather thanuse conventional Fourier-based methods or assumptions, the spectral reconstruction used anon-negative least squares fitting algorithm based on analytically computed wavelengthresponse vectors determined from extracted physical thicknesses across the entire twodimensional wedge. This new computational technique resulted in performance and spectralresolutions exceeding those that could be achieved from Fourier processing techniquesapplied to this wedge etalon. With an additional imaging lens and translational scanning, thesystem can be converted into a hyperspectral imager.

Rapid Detection of Cellulose and HemicelluloseContents of Corn Stover Based on Near-infraredSpectroscopy Combined with Chemometrics

Na Wang, Longwei Li, Jinming Liu, Jianfei Shi, Yang Lu, Bo Zhang, Yong Sun, and Wenzhe Li

DOI: 10.1364/AO.418226 Received 28 Dec 2020; Accepted 26 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: The feasibility of near-infrared spectroscopy (NIRS) combined with chemometrics for the rapid detection of thecellulose and hemicellulose contents in corn stover was discussed. Competitive adaptive reweighted sampling(CARS) and genetic simulated annealing algorithm (GSA) were combined (CARS-GSA) to select the characteristicwavelengths of cellulose and hemicellulose and reduce the dimensionality and multicollinearity of the NIRS data.The whole spectra contained 1845 wavelength variables. After CARS-GSA optimization, the number ofcharacteristic wavelengths of cellulose (hemicellulose) was reduced to 152 (260), accounting for 8.24% (14.09%)of all wavelengths. The coefficients of determination of the regression models for predicting the cellulose andhemicellulose contents were 0.968 and 0.996, the root mean square errors of prediction (RMSEP) were 0.683 and0.648, and the residual predictive deviations (RPD) were 5.213 and 16.499, respectively. The RMSEP of thecellulose and hemicellulose regression models was 0.152 and 0.190 lower for CARS-GSA than for the full-spectrum,and the RPD was increased by 0.949 and 3.47, respectively. The results showed that the CARS-GSA modelsubstantially reduced the number of characteristic wavelengths and significantly improved the predictive ability ofthe regression model.

Infrared Image Impulse Noise Suppression UsingTensor Robust Principal Component Analysis andTruncated Total Variation

Yan Zhang, Shao Yuyi, Shen Jinyue, Lu Yao, Zhouzhou Zheng, Yaya Sidib, and Bin Yu

DOI: 10.1364/AO.421081 Received 01 Feb 2021; Accepted 25 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: Infrared image denoising is an essential inverse problem that has been widely applied in many fields. However, whensuppressing impulse noise, existing methods lead to blurred object details and loss of image information. Moreover,computational efficiency is another challenge for the existing methods when processing infrared images with large resolution.In this paper, an infrared image impulse noise suppression method is introduced based on the tensor robust principalcomponent analysis. Specifically, we propose a randomized tensor singular value thresholding algorithm to solve the tensorkernel norm based on matrix stochastic singular value decomposition and tensor singular value threshold. Combined withthe image blocking, it can not only ensure the denoising performance, but also greatly improve the efficiency of the algorithm.Finally, truncated total variation is applied to improve the smoothness of the denoised image. Experimental results indicatethat the proposed algorithm outperforms the state-of-the-art methods in computational efficiency, denoising effect, and detailfeature preservation.

Influence of multiple reflections on the transmissioncoefficients of uniaxial plane-parallel plates

German Caro, Veiras F. E., Eduardo Acosta, and Liliana Perez

DOI: 10.1364/AO.421824 Received 10 Feb 2021; Accepted 25 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: A study on the influence of multiple reflections on the transmission coefficients of uniaxial plane-parallelplates is presented. Two representative models are analyzed: one that considers only the first transmissionand a rigorous one, taking into account the multiple reflections within the plate. Modules, phases andthe interference between p and s transmitted fields are evaluated in a wide range of angles of incidenceby means of three emblematic examples that illustrate the effects of thickness, birefringence, and opticalaxis orientation. For simplicity, whereas the optical axis can form an arbitrary angle with the interface, itis restricted to the plane of incidence. A complete theoretical framework is provided along with generalreference guidelines derived from numerical examples.

Design method of wide field-of-view imagingsystems using Gaussian radial basisfunctions freeform surfaces

Junhao Ni, Tong Yang, Dewen Cheng, and Yongtian Wang

DOI: 10.1364/AO.423022 Received 19 Feb 2021; Accepted 25 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: High optical performance systems with wide field-of-view (FOV) have importantapplications in remote sensing. The radial basis functions, which have a prominent localcharacteristic in surface description, have attracted many attentions in recent years. In thispaper, an effective design method for the wide FOV imaging system using Gaussian radialbasis functions freeform surfaces is proposed. The FOV of optical system is extended from arelatively small value to a larger one, and the Gaussian radial basis functions surfaces areextended stepwise based on certain criterions. A high image quality and small distortion offaxis freeform three-mirror system with a wide FOV (60°×0.6°) is designed as an example.Tolerance analysis considering both surface figure error and assembly error is performed. Thedesign results demonstrate the effectiveness of the proposed method.

Spectrally Resolved, 1D, Mid-Infrared Imaging of Temperature, CO₂, and HCl in Propellant Flames

Austin McDonald, Ryan Tancin, and Christopher Goldenstein

DOI: 10.1364/AO.422654 Received 24 Feb 2021; Accepted 24 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: This work presents a high-speed, spectrally resolved, mid-infrared imaging diagnostic for providing 1-dimensional measurements of gas temperature and relative mole fraction of CO2 and HCl in flames. Animaging spectrometer and a high-speed mid-infrared camera were used to provide 1D measurements ofCO2 and HCl emission spectra from 86 to 2402 cm-1 with a spectral resolution of 0.46 cm-1, and simulated emission spectra were least-squares fit to the data to determine the aforementioned gas properties.Measurements were acquired in HMX and AP-HTPB flames burning in air at 1 atm. This diagnosticwas applied to characterize how the path-integrated gas temperature of HMX flames varies in time andwith distance above the burning surface. Additionally, Abel inversion with Tikhonov regularization wasapplied to determine the radial distribution of temperature and relative concentration of CO2 and HClwithin the core of AP-HTPB flames. The results demonstrate that this diagnostic has potential to furtherour understanding of propellant combustion physics by quantifying thermochemical flame structure atrates up to 2 kHz.

Error Analysis of Channeled Stokes Polarimeters

Luis Gonzalez-Siu and Neil Bruce

DOI: 10.1364/AO.423739 Received 25 Feb 2021; Accepted 24 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: In this work, an analysis of passive polarimeters with spectral channeling, referred to as Stokes Channeled Spectropolarimeters, is presented. The SCS setup is composed of two thick birefringent retardersfollowed by a horizontal linear polarizer. The simulation of these polarimeters and two extraction methods for the incident Stokes vector are also presented. The effects of different retarders thickness ratio, theglobal retardance factor, retardance errors, axes alignment error, and Gaussian noise on RMS errors of therecovered Stokes parameters are presented. Two different, previously published data extraction methodsare presented and compared. We find the best polarimeter configurations from the cases studied, and ourresults suggest a mixed extraction process, using different extraction methods for different Stokes parameters, could give better results reducing the RMS errors by about a factor of five. It is worth mentioningthat, even though a calibration procedure is needed to account for the effect of errors, this is out of thescope of this work.

Bandwidth and stability of the StochasticParallel Gradient Descent algorithm for phasecontrol in coherent beam combination.

Matts Björck, Markus Henriksson, and Lars Sjökvist

DOI: 10.1364/AO.422630 Received 12 Feb 2021; Accepted 23 Apr 2021; Posted 23 Apr 2021  View: PDF

Abstract: The bandwidth and stability limits of the Stochastic Parallel Gradient Descent (SPGD)algorithm used for coherent beam combination is investigated by deriving an analytical modelfor the phase control loop. The analytical model is compared to experiments and numericalsimulations using a laboratory tiled coherent beam combination setup. The setup consisted offour sub-beams from fiber-optic collimators and used a back reflected signal as feedback. Arotating phase plate was used to induce phase disturbances into the system. The analyticalmodel compared favorably to numerical simulations and experiments as well as to otherstudies found in the literature. The results can be used to provide an estimate of theachievable phase control bandwidth of coherent beam combination systems using SPGD as acontrol algorithm.

Automated defect identification from carrier fringepatterns using Wigner-Ville distribution and machinelearning based method

Ankur Vishnoi, Aditya Madipadaga, Sreeprasad A., and G Rajshekhar

DOI: 10.1364/AO.424967 Received 12 Mar 2021; Accepted 23 Apr 2021; Posted 23 Apr 2021  View: PDF

Abstract: The paper presents a method for automated defect identification from fringe patterns. The method relieson computing the fringe signal’s Wigner-Ville distribution followed by a supervised machine learningalgorithm. Our machine learning approach enables robust detection of fringe pattern defects of variedshapes and alleviates the limitations associated with thresholding based techniques which require carefulcontrol of the threshold parameter. The potential of the proposed method is demonstrated via numericalsimulations for identifying different types of defect patterns at various noise levels. In addition, thepractical applicability of the method is validated by experimental results.

Research of Wavelength Calibration Methods in Laser Wavelength Measurement

Liang Yuan and Lirong Qiu

DOI: 10.1364/AO.417682 Received 28 Dec 2020; Accepted 23 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: At present, the accurate wavelength calibration plays an important role in laser spectrum measurements. Although the wavelength calibration methods have been investigated for long time, however, there is no techniques which are particularly designed for laser spectral calibration to the best of our knowledge. In this paper, a mathematical model for calibrating pulse laser wavelength is first established. According to the analysis formula of dispersion aberration, a flat-field concave grating in the near-infrared band is designed. Then, a wavelength calibration model based on concave grating spectroscopy is proposed. Through adjusting the spectra of each pixel, we design a calibration algorithm based on the cubic spline interpolation and kernel regression methods. By compensating and interpolating spectral data, accurate wavelengths are obtained. Finally, some experiments verify the calibration performance of the proposed method. Meanwhile, the uncertainty of measurement is also analysed.

Weak value amplification for angular velocity measurements

Senzhi Fang, Huatang Tan, Gaoxiang Li, and Qinglin Wu

DOI: 10.1364/AO.420231 Received 20 Jan 2021; Accepted 23 Apr 2021; Posted 28 Apr 2021  View: PDF

Abstract: The weak-value-amplification technique has shown great importance in the measurement of tinyphysical effects. Here we introduce a polarization-dependent angular velocity measurement systemconsisting of two Glan prisms and a true zero-order half wave plate, where a non-Fourier limitedGaussian pulse acts as the meter. The angular velocities measurements results agree well with theoretical predictions and its uncertainties are bounded by the Cram´er-Rao bound. We also investigateuncertainties of angular velocities for different numbers of detected photons and the smallest reliablepost-selection probability which can reach 3.42 ∗ 10−6.

Adaptation of the polarimetric multi-spectralAerosol Limb Imager for high altitude aircraftand satellite observations

Matt Kozun, Adam Bourassa, Doug Degenstein, Craig Haley, and Sheng-Hai Zheng

DOI: 10.1364/AO.419249 Received 13 Jan 2021; Accepted 22 Apr 2021; Posted 22 Apr 2021  View: PDF

Abstract: An elegant breadboard prototype of the Aerosol Limb Imager (ALI) has beendeveloped to meet key performance parameters that will meet requirements for the retrievalof aerosol from the upper troposphere and stratosphere from limb scattered sunlight radiancemeasurements. Similar in concept to previous high altitude balloon based generations, thisinstrument pairs a liquid crystal polarization rotator with an acousto-optic tunable filter to capturepolarimetric multi-spectral images of the atmospheric limb. This design improves the verticalresolution, signal to noise ratio and athermalization, all of which will facilitate observation froma moving high altitude aircraft platform, which provides an analogous platform to the spatiallyvarying measurements that would be made from a satellite. Finally, a preliminary design ispresented for a satellite-based generation of ALI.

Pupil-modulation ghost phase imaging

Takanori Nomura, Takafumi Ito, and Koshi Komuro

DOI: 10.1364/AO.425319 Received 18 Mar 2021; Accepted 22 Apr 2021; Posted 23 Apr 2021  View: PDF

Abstract: Computational ghost imaging (CGI) allows to reconstruct images under a low signal-to-noise-ratio(SNR)condition. However, CGI cannot retrieve phase information; it is unsuitable for observation of transparent objects such as living cells. In this paper, a phase imaging method with the architecture of CGI isproposed. The proposed method realizes phase imaging with a simple optical setup by introducing pupilmodulation differential phase contrast (PMDPC) to CGI. In PMDPC, phase information can be obtainedfrom intensity distributions which have phase gradient information and its optical setup is similar to thatof CGI. Therefore, the two methods are highly compatible, and the introduction of PMDPC to CGI canbe easily achieved. Numerical simulation and an optical experiment demonstrated the feasibility of theproposed method.

Demonstration of exact reconstruction condition forangle-limited computed tomography ofchemiluminescence

Jia Wang, Mingzhe Li, Junxia Cheng, Zhenyan Guo, Dangjuan Li, and Shenjiang Wu

DOI: 10.1364/AO.420223 Received 22 Jan 2021; Accepted 21 Apr 2021; Posted 21 Apr 2021  View: PDF

Abstract: Computed tomography of chemiluminescence (CTC) is an effective technique for three-dimensional (3D) combustiondiagnostics. It reconstructs the 3D concentrations of intermediate species or 3D images of flame topology by multiplechemiluminescence projections captured from different perspectives. In the previous studies of CTC systems, it wasassumed that projections from arbitrary perspectives are available. However, for some practical applications, the rangeof view angles and the number of projections might be restricted due to the optical access limitation, greatly affectingthe reconstruction quality. In this paper, the exact reconstruction condition for angle-limited computed tomography ofchemiluminescence was studied based on Mojette transform theories, and it was demonstrated by numericalsimulations and experiments. The studies indicate that the object tested within limited angles can be well reconstructedwhen the number of grids, the number of projections and the sampling rate of projections satisfy the exactreconstruction condition. By increasing the sampling rate of projections, high-quality tomographic reconstruction canbe achieved by a few projections in a small angle range. Although this technique is discussed under combustiondiagnostics, it can also be used and adapted for other tomography methods.

Turbulence scale effects and resolutionrequirements in aero-optics

Edwin Mathews, Kan Wang, Meng Wang, and Eric Jumper

DOI: 10.1364/AO.421304 Received 01 Feb 2021; Accepted 21 Apr 2021; Posted 21 Apr 2021  View: PDF

Abstract: While optical aberrations caused by atmospheric turbulence have been extensivelyinvestigated and well characterized, recent research has identified structural differences in opticalphase distortions caused by aircraft-induced, compressible turbulence. These so-called aerooptical distortions can be a critical obstacle in the development of airborne optical systems andreduce the fidelity and on-target intensity of optical beams. Using a model index-of-refractionspectrum that accounts for changes in density due to both temperature and pressure fluctuationsin aero-optically active flow fields, expressions for the two-dimensional phase distortion overan aperture are developed. From these results, relations among OPDrms, turbulent flow scales,and aperture size are examined while accounting for the effects of piston and tip/tilt corrections.Additionally, using the model spectrum, resolution requirements for wavefront sensors andnumerical simulations of aero-optical flows are examined.

Investigation of the temperature dependent S-branchRaman linewidths of oxygen and carbon dioxide in anoxyfuel relevant mixture

Henry Misoi, Jonas Hölzer, and Thomas Seeger

DOI: 10.1364/AO.424232 Received 08 Mar 2021; Accepted 21 Apr 2021; Posted 21 Apr 2021  View: PDF

Abstract: The temperature dependence of the O2 and CO2 S-branch linewidths in a 30% / 70% O2-CO2 mixture between 295 Kand 1900 K has been studied by a picosecond time-resolved pure rotational coherent anti-Stokes Raman scattering(RCARS) approach. The S-branch Raman linewidths are required for diagnostics of thermodynamic properties inoxyfuel combustion processes by RCARS, where this mixture is of special interest, because it is regularly used to replaceair when transiting from air-fed to oxyfuel combustion. The obtained linewidths for oxygen and carbon dioxide show astrong deviation from pure self-broadened linewidths and previously used Q-branch linewidths respectively. Adiscussion on the expected impact on RCARS thermometry and concentration evaluations as well as a description ofspecific properties of oxygen and carbon dioxide and their effect on the dephasing behavior of the Raman coherencesand thereby the Raman linewidths is included, along tabulated linewidths data of both molecules.

Mid-infrared optical parametric oscillator pumped by ahigh-pulse-energy, Q-switched Ho3+:YAG laser

Michael Griesbeck, Hendrik Büker, Madeleine Eitner, Katharina Goth, Peter Braesicke, Marc Eichhorn, and Christelle Kieleck

DOI: 10.1364/AO.424039 Received 02 Mar 2021; Accepted 21 Apr 2021; Posted 21 Apr 2021  View: PDF

Abstract: A high-pulse-energy mid-infrared light source is presented, based on a ZGP OPO pumped by an activelyQ-switched high-pulse-energy Ho3+:YAG laser. The Ho3+:YAG pump laser source is capable of generating a pulse energy of 15 mJ from a single Ho3+:YAG rod at room temperature at a pulse repetitionfrequency (PRF) of 700 Hz. A maximum power of 20.1 W at a central wavelength of 2090 nm could be obtained in continuous operation, with a slope efficiency of 45.1%. A good beam quality with an M2 betterthan 1.3 was achieved in Q-switched operation. The presented laser architecture was used as a suitablepump source for a ZGP-based OPO. Operated at a PRF of 2 kHz and pumped with a pulse energy of 8 mJ,a low conversion threshold of 1.5 W and a maximum total output power of 6.3 W could be obtained in alinear ZGP-based OPO. At maximum power, the peak power of the generated mid-infrared radiation exceeded 120 kW, while the beam quality was affected by the strong gain lens building inside the nonlinearmaterial as a consequence of the high-energy pump pulses.

Simple reformulation of the coordinate transformationmethod for gratings with vertical facet or overhangingprofile

Xianshun Ming and Liqun Sun

DOI: 10.1364/AO.423209 Received 19 Feb 2021; Accepted 21 Apr 2021; Posted 22 Apr 2021  View: PDF

Abstract: We reformulate the coordinate transformation method (C method) for gratings with vertical facet or overhanging profile (overhanging gratings), in which no tensor concept is involved, only the knowledge ofelementary mathematics and Maxwell0s equations in the rectangular coordinate system is used, and provide detailed recipe for programming. This formulation is easy to understand and implement. It adoptsthe strategy of rotating coordinate system from Plumey et al. [J. Opt. Soc. Am. A 14,610(1997)], and expresses it with the method of changing variables from Li et al. [Appl. Opt. 38, 304(1999)]. We investigateseveral typical overhanging gratings by the reformulated C method, and validate and compare the resultswith the Fourier Modal Method (FMM), which shows that it is superior especially for metal deep smoothgratings. This reformulation can facilitate the research in light couplers and so on for optical engineers.

Sub-kilohertz linewidths fiber laser byusing Bragg grating filters

Qiao Wen, Zhihao Sun, and Ji LI

DOI: 10.1364/AO.421214 Received 04 Feb 2021; Accepted 20 Apr 2021; Posted 21 Apr 2021  View: PDF

Abstract: A single-frequency (SF) fiber laser is widely applied to many fields. A superior ultra-narrowbandwidth fiber filter, which consists of an in-series commercial common fiber Bragg grating (FBG) filterswith a partial overlap in their reflection spectrum, is presented in this paper. The bandwidth of the filter witha center reflection wavelength of 1549.83nm reaches 0.024 nm at 3 dB reflection, a remarkable reductionof approximately 300% by combination usage narrower than a single FBG used bandwidth. We alsodemonstrate a method to further narrowing the filter bandwidth by increasing the usage times of filters, andthe bandwidth can reach 0.0025 nm at 3 dB reflection. A long linear cavity single-frequency fiber laser withan entire cavity of 30 cm is implemented with the linewidth of 941 Hz at 3dB, which is about 210% narrowerthan the previous record of SF fiber laser based on FBG filters in a linear cavity. This research may open anew door towards ultra-narrow-bandwidth filters and SF fiber lasers.

Efficiency Analysis of Cellular/LiFi Traffic Offloading

Haitham khallaf, ABD EL-RAHMAN EL-FIKKY, Mohamed Elwekeil, Abdulaziz El-Fiqi, Ehab Mohamed, and Hossam Shalaby

DOI: 10.1364/AO.419593 Received 14 Jan 2021; Accepted 20 Apr 2021; Posted 21 Apr 2021  View: PDF

Abstract: Data offloading is a promising low-cost and power-efficient solution for the expected high demands forhigh-speed connectivity in the near future. In this paper, we investigate offloading efficiency in a cellular/light fidelity network. This offloading efficiency is a measure of the ratio of traffic carried by lightfidelity (LiFi) network to the total traffic carried by both LiFi and cellular networks. We consider thetwo scenarios of opportunistic and delayed offloading. Effects of user density, user mobility, LiFi-signalblocking, and channel characteristics are investigated. We use Zemax to simulate LiFi channels in theproposed model. Based on our results, delayed offloading can achieve up to 60% offloading efficiency.While, opportunistic offloading achieves up to 18% offloading efficiency.

Vector-apodizing phase plate coronagraph: design, current performance, and future development [Invited]

David Doelman, Frans Snik, Emiel Por, Steven Bos, Gilles Otten, Matthew Kenworthy, Sebastiaan Haffert, Michael Wilby, Alexander Bohn, Ben Sutlieff, Kelsey Miller, Mireille Ouellet, Jos de Boer, Christoph Keller, Michael Escuti, Shuojia Shi, Nathaniel Warriner, Kathryn Hornburg, Jayne Birkby, Jared Males, Katie Morzinski, Laird Close, Johanan Codona, Joseph Long, Lauren Schatz, Jennifer Lumbres, Alexander Rodack, Alexander Hedglen, Kyle Van Gorkom, Olivier Guyon, Julien Lozi, Tyler Groff, Jeffrey Chilcote, Nemanja Jovanovic, Simon Thibault, Chris de Jonge, Guillaume Allain, Cédric Vallée, Deven Patel, Olivier Cote, Christian Marois, Phil Hinz, Jordan Stone, Andy Skemer, Zackery Briesemeister, Anna Boehle, Adrian Glauser, William Taylor, Pierre Baudoz, Elsa Huby, Olivier Absil, Brunella Carlomagno, and Christian Delacroix

DOI: 10.1364/AO.422155 Received 11 Feb 2021; Accepted 19 Apr 2021; Posted 21 Apr 2021  View: PDF

Abstract: Over the last decade, the vector-apodizing phase plate (vAPP) coronagraph has been developed from concept to on-sky application in many high-contrast imaging systems on 8-m class telescopes. The vAPP is an geometric-phase patterned coronagraph that is inherently broadband, and its manufacturing is enabled only by direct-write technology for liquid-crystal patterns. The vAPP generates two coronagraphic PSFs that cancel starlight on opposite sides of the point spread function (PSF) and have opposite circular polarization states. The efficiency, that is the amount of light in these PSFs, depends on the retardance offset from half-wave of the liquid-crystal retarder.Using different liquid-crystal recipes to tune the retardance, different vAPPs operate with high efficiencies (>96%) in the visible and thermal infrared (0.55 μm to 5 μm). Since 2015, seven vAPPs have been installed in a total of six different instruments, including Magellan/MagAO, Magellan/MagAO-X, Subaru/SCExAO, and LBT/LMIRcam.Using two integral field spectrographs installed on the latter two instruments, these vAPPs can provide low-resolution spectra (R~30) between 1 μm and 5 μm. We review the design process, development, commissioning, on-sky performance, and first scientific results of all commissioned vAPPs.We report on the lessons learned and conclude with perspectives for future developments and applications.

Ultra-Wideband bandstop filter based on Fanoresonance and rectangular resonators

Siavash Pooretemad, Ali Reza Maleki Javan, and mehdi aslinezhad

DOI: 10.1364/AO.422475 Received 16 Feb 2021; Accepted 14 Apr 2021; Posted 14 Apr 2021  View: PDF

Abstract: In this study, the effect of length of the stub on the formation of the Fano resonancein structures which possess MIM waveguide coupled to rectangular cavities by the stub isinvestigated theoretically and numerically. The resulting Fano resonance is used to design anultra-wideband band-stop filter which can filter all wavelengths between twotelecommunication windows of λ = 850nm and λ =1310nm .The structure is based on tworectangular cavities coupled to the MIM waveguide by stub that are located at an adjusteddistance from each other and the interference superposition of reflected and transmitted wavesfrom each other will make this filtering phenomenon. The center wavelength of the band-stopof the structure is highly adjustable by changing the dimensions of the structure. Thetheoretical and the numerical results are respectively based on transmission-line model (TLM)and the finite difference time domain (FDTD) method. The theoretical results comply wellwith the numerical ones. To analyze the Fano resonance, Temporal coupled mode theory(CMT) is also exploited. The proposed structure has significant applications in highlyintegrated optical circuits.

A Design Framework for MetasurfaceOptics-based Convolutional Neural Networks

Nickolas Vamivakas, Carlos Mauricio Villegas Burgos, Yuhao Zhu, and Tianyi Yang

DOI: 10.1364/AO.421844 Received 17 Feb 2021; Accepted 14 Apr 2021; Posted 15 Apr 2021  View: PDF

Abstract: Deep learning using convolutional neural networks (CNNs) has been shown tosignificantly out-perform many conventional vision algorithms. Despite efforts to increase theCNN efficiency both algorithmically and with specialized hardware, deep learning remainsdifficult to deploy in resource-constrained environments. In this paper, we propose an end-to-endframework to explore how to optically compute the CNNs in free-space, much like a computationalcamera. Compared to existing free-space optics-based approaches which are limited to processingsingle-channel (i.e., grayscale) inputs, we propose the first general approach, based on nanoscalemetasurface optics, that can process RGB input data. Our system achieves up to an order ofmagnitude energy saving, simplifies the sensor design, all the while sacrificing little networkaccuracy.

Optical Configuration of 푁 : 2푁 ReversibleDecoder using LiNbO3 based Mach-ZehnderInterferometer

Shashank Awasthi, Barnali Chowdhury, ZUHAIB Haider, Jalil Ali, Preecha Yupapin, Sanjeev Metya, and Alak Majumder

DOI: 10.1364/AO.422790 Received 16 Feb 2021; Accepted 08 Apr 2021; Posted 09 Apr 2021  View: PDF

Abstract: In the days when integrated circuit (IC) designers are facing an uphill task in limitingthe energy / heat dissipation, reversible computing emerges as a potential candidate with vastapplication in the fields like nanotechnology, quantum dot cellular automata and low power IC.Optical reversible logics have turned up to offer high speed and low energy computations withalmost no loss of input information when a certain (arithmetic or logical) operation is performed.This article explores an optical implementation of Optimised Fredkin gate which is employed todesign 푁 : 2푁 reversible decoder. The optical designs have been carried out using electro-opticeffect of Lithium Niobate (LiNbO3) based Mach-Zehnder Interferometer under Beam PropagationMethod using Optiwave’s OptiBPM tool. The mathematical model of output power of thesedesigns is also performed along with its validation in MATLAB.

Pressure measurement in gas flows using laser-induced grating lifetime

Christopher Willman, Laurent Le Page, Paul Ewart, and Benjamin Williams

DOI: 10.1364/AO.419973 Received 18 Jan 2021; Accepted 07 Apr 2021; Posted 07 Apr 2021  View: PDF

Abstract: Optical diagnostics of gas-phase pressure are relatively unusual. In this work we demonstrate a novel,rapid and robust method to use laser-induced grating scattering (LIGS) to derive this property in realtime. Previous pressure measurements with LIGS have employed a signal fitting methodology but this isrelatively time consuming and requires specialist understanding. In this paper, we directly measure a decay lifetime from a LIGS signal and then employ a calibration surface constructed using a physics-basedmodel to convert this value to pressure. This methodology was applied to an optically-accessible single cylinder internal combustion engine, yielding an accuracy of better than 10 % at all tested conditionsabove atmospheric pressure. This new approach complements the existing strength of LIGS in preciselyand accurately deriving temperature with a simple analysis method, by adding pressure information witha similarly simple method.

(Fraunhofer) Classification of airborne 3D point clouds regarding separation of vegetation in complex environments

Dimitri Bulatov, Dominik Stütz, Jorg Hacker, and Martin Weinmann

DOI: 10.1364/AO.422973 Received 19 Feb 2021; Accepted 31 Mar 2021; Posted 12 Apr 2021  View: PDF

Abstract: Classification of outdoor point clouds is an intensely studied topic, in particular, with respect to the separation of vegetation from the terrain and man-made structures. In presence of many overhanging and vertical structures, the (relative) height is no longer a reliable criterion for such a separation. An alternative would be to apply supervised classification; however, thousands of examples are typically required for an appropriate training. In this paper, an unsupervised and rotation-invariant method is presented and evaluated for three datasets with very different properties. The method allows to detect planar patches by filtering and clustering so-called superpoints, whereby the well-known but suitably modified Random Sampling and Consensus (RANSAC) approach plays a key role for plane estimation in outlier-rich data.The performance of our method is compared to that produced by supervised classifiers with parameters common for remote sensing: Random Forest as learner and features sets for point cloud processing, like covariance-based features or point descriptors. It is shown that for point clouds resulting from airborne laser scans, the detection accuracy of the proposed method is over 96\% and as such, higher than that of standard supervised classification approaches. Because of artifacts caused by interpolation during 3D stereo matching, the overall accuracy was lower for photogrammetric point clouds (74 to 77\%). However, using additional salient features, such as NGRDI index, the results became more accurate and less dependent on the data source.

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