Accurate measurement of the frequency offset of the laser based on electromagnetically induced transparency

Shuang Ren, Yu Tang, Chuang Yang, Siyuan Wang, and Hanbin Zhou

DOI: 10.1364/AO.523527 Received 11 Mar 2024; Accepted 01 May 2024; Posted 01 May 2024  View: PDF

Abstract: We propose a method using electromagnetically induced transparency (EIT) to measure the frequency offset of the laserrelative to a cavity's resonance frequency, thereby reducing the laser detuning when preparing Rydberg atoms. Laserreflection by the vapor cell enables observation of two EIT peaks corresponding to the co-propagating and counterpropagating beams, and the peaks' position is related to laser detuning, allowing us to estimate the frequency offset of theprobe and coupling laser. The method reduces the measurement uncertainty compared to directly observing saturatedabsorption spectroscopy (SAS) and EIT, making it suitable for applications that require strict control over laser detuning.

Neural radiance fields-based holography [Invited]

Kang Minsung, Fan Wang, KAI KUMANO, Tomoyoshi Ito, and Tomoyoshi Shimobaba

DOI: 10.1364/AO.523562 Received 11 Mar 2024; Accepted 01 May 2024; Posted 01 May 2024  View: PDF

Abstract: This study presents a novel approach for generating holograms based on the neural radiance fields (NeRF)technique. Generating real-world three-dimensional (3D) data is difficult in hologram computation. NeRFis a state-of-the-art technique for 3D light-field reconstruction from 2D images based on volume rendering.The NeRF can rapidly predict new-view images that are not included in a training dataset. In this study,we constructed a rendering pipeline directly from a radiance field generated from 2D images by NeRFfor hologram generation using deep neural networks within a reasonable time. The pipeline comprisesthree main components: the NeRF, a depth predictor, and a hologram generator, all constructed using deepneural networks. The pipeline does not include any physical calculations. The predicted holograms ofa 3D scene viewed from any direction were computed using the proposed pipeline. The simulation andexperimental results are presented.

Optimizing LED Array Irradiance Uniformity with a Particle Swarm Optimization-Based Scheme: Application in a Parallel Photoreacto

Junnan Mei and Jun Zou

DOI: 10.1364/AO.524246 Received 20 Mar 2024; Accepted 01 May 2024; Posted 01 May 2024  View: PDF

Abstract: This study proposes a particle swarm optimization (PSO)-based method for enhancing the irradiance uniformity of LEDarrays in parallel photoreactors. We construct a spatial irradiance model, validating its accuracy with near-field opticaltest data and optical simulation results. An evaluation function for irradiance uniformity guides the iterative optimizationof the LED arrangement, taking into consideration practical constraints such as chip size, heat dissipation, and circuitboard wiring. The optimized LED array demonstrated a significant improvement in irradiance uniformity, increasingfrom 75.27% to 95.61%. This showcases the feasibility and effectiveness of our PSO-based method for practicalapplications in optimizing LED arrays.

Experimental optical encryption with full complex modulation

Juan González Moncada, Alejandro Velez-Zea, and John Barrera Ramírez

DOI: 10.1364/AO.518364 Received 09 Jan 2024; Accepted 30 Apr 2024; Posted 30 Apr 2024  View: PDF

Abstract: We present a novel method to achieve experimental encryption using double random phase encoding with full complexmodulation and a single phase-only spatial light modulator. Our approach uses double phase encoding to generate phaseonly holograms containing complex-valued input planes for a joint transform correlator (JTC) cryptosystem. Thisapproach enables users to independently manipulate both the phase and amplitude of the cryptographic keys and objects,thereby significantly enhancing the versatility of the optical cryptosystem. We validate the capabilities of our proposedscheme by generating optimized random phase masks and using them to experimentally encrypt various grayscale andbinary objects. The experimental complex modulation obtained with the system detailed in this work, in conjunction withoptimized random phase masks results in an enhancement in the quality of the decrypted objects during reconstruction.Both numerical simulations and experimental findings corroborate the effectiveness of our proposal.

High-speed eye-tracking based on a synchronized imaging mechanism by dual-ring infrared lighting source

Xiao Zhang, Lihui Wang, Yuan He, Zhiwei Mou, and Yiqi Cao

DOI: 10.1364/AO.521840 Received 01 Mar 2024; Accepted 30 Apr 2024; Posted 30 Apr 2024  View: PDF

Abstract: : It is a challenge for conventional monocular-camera single-light source eye trackingmethods to achieve high-speed eye tracking. In this work, a dual-ring infrared lighting sourcewas designed to achieve bright and dark pupils in high-speed. The eye tracking method used adual-ring infrared lighting source and synchronized triggers for the even and odd camera framesto capture bright and dark pupils. A pupillary corneal reflex was calculated by the centercoordinates of the Purkinje spot and the pupil. A map function was established to map therelationship between pupillary corneal reflex and gaze spots. The gaze coordinate wascalculated based on the mapping function. The average detection time of each gaze spot was3.76 milliseconds.

Digitizing translucent object appearance by validating computed optical properties

Duc Minh Tran, Mark Bo Jensen, Pablo Santafé, Stefan Källberg, Alejandro Ferrero, Morten Hannemose, and Jeppe Frisvad

DOI: 10.1364/AO.521974 Received 21 Feb 2024; Accepted 30 Apr 2024; Posted 30 Apr 2024  View: PDF

Abstract: The optical properties available for an object are most often fragmented and insufficient for photorealistic rendering of the object. We propose a procedure for digitizing a translucent object with sufficient information for predictive rendering of its appearance. Based on object material descriptions, we compute optical properties and validate or adjust this object appearance model based on comparison of simulation with spectrophotometric measurements of the bidirectional scattering-surface reflectance distribution function (BSSRDF). To ease this type of comparison, we provide an efficient simulation tool that computes the BSSRDF for a particular light-view configuration. Even with just a few configurations, the localized lighting in BSSRDF measurements is useful for assessing the appropriateness of computed or otherwise acquired optical properties. To validate an object appearance model in a more common lighting environment, we render the appearance of the obtained digital twin and assess the photorealism of our renderings through pixel-by-pixel comparison with photographs of the physical object.

Speckle visual cryptography for credentials authentication

Yishi Shi, anqing jiao, Zhu Yupeng, Zhaoke Mi, Long Zhou, Qiaofen Zhu, and changjun ke

DOI: 10.1364/AO.522918 Received 04 Mar 2024; Accepted 30 Apr 2024; Posted 30 Apr 2024  View: PDF

Abstract: Based on the high random distribution characteristic of natural speckle image, a newmethod of speckle visual cryptography is designed by combining the natural speckle imagewith the secret key in visual cryptography. And for the first time, we design a authenticationsystem for user credentials by combining speckle visual cryptography and QR code. By usingspeckle visual cryptography method, the image of QR code carrying user authenticationinformation is hidden in the speckle image, and the speckle image is printed on the papercredentials. Through simulation and analog experiments, we verify the possibility of applyingspeckle visual cryptography to user credentials authentication system, and compare theimproved grayscale reordering algorithm and grayscale reordering algorithm, and prove thatthe improved grayscale reordering algorithm has more advantages in this system by comparingPSNR and SSIM. Finally, the anti-interference ability and the uniqueness of the virtual secretkey in the system are analyzed to prove that the secret key has high anti-interference abilityand security.

Measurement method of virtual image distance for head-mounted display based on a variable-focus liquid lens

Shuangjiang Huang, Lihui Wang, Yutao huang, Yuan He, and Shi Bai

DOI: 10.1364/AO.524353 Received 27 Mar 2024; Accepted 30 Apr 2024; Posted 30 Apr 2024  View: PDF

Abstract: The distance from the virtual image to the human eye is an important factor inmeasuring the comfort of a head-mounted display (HMD). However, accurately measuring theirdistance is challenging due to the dynamic changes in virtual presence and distance. In thispaper, we proposed a virtual image distance measurement prototype based on a variable-focusliquid lens and derived a virtual image distance calculation model. We built a variable-focusliquid lens experimental platform to verify the method’s correctness. In addition, we proposed animproved optimization algorithm that can efficiently and accurately search for the optimal focallength corresponding to the maximum sharpness moment of the virtual image within the focallength value space. Verified in an experimental scene of 0.5 m to 3.5 m, we observed that theerror between the object image distance and the virtual image distance at the same focal length isabout 5 cm. The proposed virtual image distance measurement method can accurately measurethe distance value of the virtual image in the HMD. This method can be widely used in virtualand augmented reality, especially in the task of constructing realistic scenes.

Hybrid mode for absorption enhancement in the Ga2O3 nanocavity photodetector with grating electrodes

Jiayong Yang, cizhe fang, Tong-Zhou Li, yibo wang, Xiaoxi Li, Xiangyu Zeng, Liu Yan, Yue Hao, and Genquan Han

DOI: 10.1364/AO.524563 Received 22 Mar 2024; Accepted 30 Apr 2024; Posted 30 Apr 2024  View: PDF

Abstract: Gallium oxide (Ga2O3) photodetectors have drawnincreased interest for their widespread applicationsranging from military to civil. Due to the inherent oxygenvacancy defects, they seriously suffer from trade-offs thatmake them incompetent for high-responsivity, quickresponse detection. Herein, a Ga2O3 nanocavityphotodetector assisted with grating electrodes isdesigned to break the constraint. The proposed structuresupports both the plasmonic mode and the Fabry-Pérot(F-P) mode. Numerical calculations show that theabsorption of 99.8% is realized for ultra-thin Ga2O3 (30nm), corresponding to a responsivity of 12.35 A/W.Benefiting from optical mechanisms, the externalquantum efficiency (EQE) reaches 6040%, which is 466times higher than that of bare Ga2O3 film. Furthermore,the proposed photodetector achieves a polarizationdependent dichroism ratio of 9.1, enabling polarizationphotodetection. The grating electrodes also effectivelyreduce the transit time of the photo-generated carriers.Our work provides a sophisticated platform fordeveloping high-performance Ga2O3 photodetectors withthe advantages of simplified fabrication processes andmultidimensional detection.

Linear systems characterization of the topographical spatial resolution of optical instruments

Peter de Groot, Zoulaiha Daouda, Leslie Deck, and Xavier Colonna de Lega

DOI: 10.1364/AO.521868 Received 26 Feb 2024; Accepted 29 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: Lateral resolving power is a key performance attribute for Fizeau interferometers,confocal microscopes, interference microscopes, and other instruments that measure surfaceform and texture. Within a well-defined scope of applicability, limited by surface slope, texture,and continuity, a linear response model provides a starting point for characterizing spatialresolution under idealized conditions. Presently, the instrument transfer function (ITF) is astandardized way to quantify linear response to surface height variations as a function of spatialfrequency. Here we build upon the ITF idea and introduce terms, mathematical definitions, andappropriate physical units for applying a linear systems model to surface topographymeasurement. These new terms include topographical equivalents of the point spread, linespread, and edge spread functions, as well as a complex-valued transfer function that extendsthe ITF concept to systems with spatial-frequency dependent topography distortions. As anexample, we consider the experimental determination of lateral resolving power of a coherencescanning interference microscope using a step-height surface feature to directly measure theITF. The experiment illustrates the proposed mathematical definitions and provides a directcomparison to theoretical calculations performed using a scalar diffraction model.

A Comprehensive Review on the Directed Design of Optomechanical Crystal Cavities Using Intelligent Algorithms

Zhe Yu, Jian Bai, and Qianbo Lu

DOI: 10.1364/AO.522776 Received 01 Mar 2024; Accepted 29 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: Optomechanical crystal cavities are devices based on optomechanical interactionsto manipulate photons and phonons on periodic subwavelength structures, enabling precisemeasurement of force and displacement. The performance of the target structures varieswhen applied to different applications. Optomechanical crystal cavities now rely on empiricalforward design, which is inefficient. Therefore, a desired shift is toward directed design witha "problem-oriented" strategy. The directed optimization problem’s non-convex nature andextensive parameter space necessitate substantial computational resources, driving the need forintelligent algorithms in sub-wavelength structure design. Intelligent algorithms can surpass theconstraints of traditional methods and discover novel structures that are effective in differentmaterials, topologies, modes, and wavelengths.This paper provides an extensive overview tothe intelligent algorithms for guiding the directed design of optomechanical crystal cavities.It presents a systematic classification of fifteen algorithmics, including but not limited totopology algorithms, particle swarm optimisation algorithms, convolutional neural networks,and generative adversarial networks. The article provides a comprehensive review and thorough28 analysis of the principle, current application state, as well as the advantages and disadvantages of29 each intelligent algorithm. By using these intelligent algorithms, researchers can enhance the30 efficiency and accuracy of optimising optomechanical crystal cavities in a broader design space.

Experimental demonstration of scene-based cophasing in optical synthetic aperture imaging using the SPGD algorithm

Makoto Hirose and Norihide Miyamura

DOI: 10.1364/AO.522829 Received 06 Mar 2024; Accepted 29 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: Large-aperture telescopes based on optical synthetic aperture imaging are investigated for recent highresolution spaceborne observations. An enabling technique of aperture synthesis is a cophasing methodto suppress a piston-tip-tilt error between sub-apertures. This paper proposes a scene-based cophasingtechnique using the stochastic parallel gradient descent (SPGD) algorithm, assuming application to highresolution Earth observation. A significant advantage of the SPGD algorithm is a model-less cophasingcapability based on extended scenes, but the simultaneous scene-based piston-tip-tilt correction betweenmultiple apertures has not been demonstrated. In this paper, we developed a tabletop synthetic apertureimaging system with 37 sub-apertures and demonstrated extended-scene-based piston-tip-tilt control byoptimizing applied voltages to 111 actuators simultaneously. The demonstration experiments used notonly static scenes but also a time-varying dynamic scene for observation targets. In every measurement, theproposed scene-based approach reduced the initially defined piston-tip-tilt errors, and the image sharpnesssignificantly improved, although the correction rate in the dynamic scene observation was slower. Finally,this paper discusses the influence of scene dynamics on image-based cophasing.

High beam quality continuous-wave 320 nm UV laser from walk-off compensated intracavity frequencydoubled Pr:YLF laser

Yue Zuo, Bo Wu, and Yonghang Shen

DOI: 10.1364/AO.522760 Received 03 Mar 2024; Accepted 29 Apr 2024; Posted 30 Apr 2024  View: PDF

Abstract: We report a high beam quality continuous-wave (CW) 320 nm ultra-violet (UV) laser. End-pumped praseodymium-dopedyttrium lithium fluoride (Pr:YLF) laser is constructed in a typical V-shaped cavity structure while the UV output isobtained through intra-cavity frequency doubling using LiB3O5 (LBO). We investigate the dependence of UV output power,as well as the spot profile on the LBO length, and find the ‘walk-off’ within LBO affects both the frequency doublingefficiency and the beam quality of the UV output severely. Rotated twin LBO crystals are then applied to substitute forsingle long LBO crystal to compensate the walk-off effect, resulting in high power 320 nm output up to 410 mW underabsorbed pump power of 4.0 W at 444 nm, with high beam quality of 𝑴𝟐𝒙 = 𝟏.𝟎𝟐 and 𝑴𝟐𝒚 = 𝟏.𝟎𝟒.

An optimized optical system with 1.65m wide-field corrector for the 6.5m high-performance MUltiplexed Survey Telescope

Yifan Zhang, Qi Bian, Liquan Guo, Stephen Schectman, Zhongtian Tian, Jinlong Huang, Chen Xu, Zheng Cai, Song Huang, Lu Lu, Yamin Zheng, Shude Mao, and Lei Huang

DOI: 10.1364/AO.521907 Received 20 Feb 2024; Accepted 28 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: The development of modern large-scale spectroscopicsurvey telescopes responds to the urgent demand forspectral information in astronomical research. TsinghuaUniversity has previously proposed a 6.5m MUltiplexedSurvey Telescope composed of a Ritchey-Chretien configuration and a 1.8m multi-element wide-field corrector, achieving excellent performance and world-leadingsurvey efficiency. However, an optimized 1.65m multielement corrector with five lenses is proposed to overcome the constraints on glass uniformity and verificationin fabrication of the previous corrector design. It maintains outstanding image quality with the 80% enclosedenergy diameter not more than 0.559 arcsec within 3°FoV over at up to 55° zenith angle. The optimized optical system does not revise the working mode of theADC and the curvature of the primary mirror, while ensuring the reasonability and accuracy of manufacturingof large corrector elements. It provides a more feasible reference optical design for the MUltiplexed SurveyTelescope in subsequent iterations and communicationswith manufacturers.

Coded aperture compressive temporal imaging via unsupervised lightweight local-global networks with geometric characteristics

Youran Ge, Gangrong Qu, Yuhao Huang, and Duo Liu

DOI: 10.1364/AO.510414 Received 26 Oct 2023; Accepted 28 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: Coded aperture compressive temporal imaging (CACTI)utilizes compressive sensing (CS) theory to compressthree dimensional (3D) signals into 2D measurementsfor sampling in a single snapshot measurement, whichin turn acquires high-dimensional (HD) visual signals.To solve the problems of low quality and slow runtimeoften encountered in reconstruction, deep learning hasbecome the mainstream for signal reconstruction andhas shown superior performance. However, currentlyimpressive networks are typically supervised networkswith large size models and require vast training setsthat can be difficult to obtain or expensive. This limitstheir application in real optical imaging systems. In thispaper, we propose a lightweight reconstruction networkthat recovers HD signals only from compressed measurements with noise, and design a block consisting ofconvolution to extract and fuse local and global features,stacking multiple features to form a lightweight architecture. In addition, we also obtain unsupervised lossfunctions based on the geometric characteristics of thesignal to guarantee the powerful generalization capability of the network in order to approximate the reconstruction process of real optical systems. Experimentalresults show that our proposed network significantly reduces the model size and not only has high performancein recovering dynamic scenes, but also the unsupervisedvideo reconstruction network can approximate its supervised version in terms of reconstruction performance.

Measurements of seeing-induced crosstalk in tip-tilt corrected solar polarimetry

Harsh Mathur, Nagaraju Krishnappa, Hemanth Pruthvi, and Kolandaiswamy Sagayanathan

DOI: 10.1364/AO.519245 Received 17 Jan 2024; Accepted 28 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: We present measurements of seeing-induced cross-talk using spectropolarimetric observations of sunspots recorded simultaneously in the H𝛼 and Ca ii 8662 Å lines with the Kodaikanal Tower Tunnel (KTT) telescope. The Kodaikanal Tower Tunnel telescope is integrated and installed with an image stabilization system consisting of a tip-tilt and an autoguider system. Additionally, the spectropolarimeter at KTT is upgraded to allow for the simultaneous recording of spectropolarimetric observations in three spectral lines. The tip-tilt system is shown to have a cut-off frequency of 80 Hz, effectively reducing the seeing induced cross-talk in the measured Stokes parameters at least by a factor of 2.

Deterministic direct design method for a spherical-aberration-free singlet lens with reduced coma

lin jiandong, Qi An, and Naibo Zhang

DOI: 10.1364/AO.520247 Received 01 Feb 2024; Accepted 27 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: It is still challenging to find a spherical-aberration-free singlet lens with well corrected coma due to an undesired and complicated residual high order coma. In this paper, we present a spherical-aberration-free singlet lens with reduced coma containing high order coma contribution. This design algorithm is to deduce the front aspherical surface parameters from the back spherical surface using meridional ray tracing to find the proper values of the back focal length and the back spherical radius to reduce the coma. The exemplary lens demonstrates an excellent well-balanced and diffraction-limited performance at the field angle ranging from 0.0° to 2.5° with working F# equal to 1.65.

Compact, low-loss and high polarized-extinction ratio and terahertz TM-pass polarizer based on a hybrid plasmonic waveguide with Graphene ridge

Ji Xu, han yao, Tiantian Chi, huichao cheng, Wenjie Yue, Biying Liu, Xinrong Zhang, sheng li, Baifu Zhang, Yuqing Lu, and Ning Liu

DOI: 10.1364/AO.520254 Received 28 Jan 2024; Accepted 26 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: A compact, low-loss and high polarized extinction ratio TM-pass polarizer based on a graphene hybrid plasmonic waveguide(GHPW) hasbeen demonstrated for terahertz band. A ridge coated by graphene layer and the hollow HPW with an semiround arch (SRA) Si core isintroduced to improve structural compactness and suppress the loss. Based on this, a TM-pass polarizer has been designed, which caneffectively cut off the unwanted TE mode, and the TM mode passes with negligible loss. By optimizing the angle of ridge, the height ofridge, air gap height and the length of the tapered mode converter, an optimum performance with a high polarization extinction ratio of30.28 dB and a low insert loss of 0.4 dB is achieved in the 3 THz band. This work provides a scheme for the design and optimization ofpolarizers in THz band, which has potential application value in integrated terahertz systems.

Two dimensional Keitz method for the radiant power measurement of planar ultraviolet excilamps

qianwen zhu, FUSHENG LI, Qiuyi Han, and Shanduan Zhang

DOI: 10.1364/AO.520397 Received 29 Jan 2024; Accepted 26 Apr 2024; Posted 26 Apr 2024  View: PDF

Abstract: The planar XeBr* and XeCl* excilamps emitting the non-coherent narrow-band UVB light (280-315 nm), are now widely used to cure the psoriasis and vitiligo as well as to synthetic Vitamin D. The two-dimensional integral formula has been deducted in this study, which is a good method and has great practical significance to calculate the total radiant power and assess the energy efficiency of planar UV lamp. The measured radiant power of planar white LED lamps through a two-dimensional Keitz formula has been compared with that of gonio-photometer, verifying the applicability of the formula. The optimum measurement distance is dependent on the lamp length, as 1.5L≤D≤3.5L, for which the derivation from the two methods can be controlled within 10%. The planar XeBr* excilamps have been measured and compared to the coaxial excilamps, which show the similar patterns of change for the radiant characteristics. Since planar radiant power formula only needs to measure the normal illuminance at a certain distance from the symmetric center of the lamp, it is more convenient to use and low-cost method to promote the development of large-sized planar ultraviolet lamps.

THz-TDS with gigahertz Yb-based dual-comb lasers: noise analysis and mitigation strategies

Benjamin Willenberg, Christopher Phillips, Justinas Pupeikis, Sandro Camenzind, Lars Liebermeister, Robert Kohlhaas, Björn Globisch, and Ursula Keller

DOI: 10.1364/AO.522802 Received 04 Mar 2024; Accepted 26 Apr 2024; Posted 26 Apr 2024  View: PDF

Abstract: We investigate terahertz time-domain spectroscopy using a low-noise dual frequency comb laser based ona single spatially-multiplexed laser cavity. The laser cavity includes a reflective biprism, which enablesgeneration of a pair of modelocked output pulse trains with slightly different repetition rate and highlycorrelated noise characteristics. These two pulse trains are used, respectively, to generate the THz wavesand detect them by equivalent time sampling. The laser is based on Yb:CALGO, operates at a nominalrepetition rate of 1.18 GHz, and produces 110 mW per comb with 77 fs pulses around 1057 nm. We performTHz measurements with Fe-doped photoconductive antennas, operating these devices with gigahertz 1-µmlasers for the first time and obtain THz signal currents approximately as strong as those from referencemeasurements at 1.55 µm and 80 MHz. We investigate the influence of the laser’s timing noise properties onTHz measurements, showing that the laser’s timing jitter is quantitatively explained by power-dependentshifts in center wavelength. We demonstrate reduction in noise by simple stabilization of the pump power,and show up to 20-dB suppression in noise by the combination of shared pumping and shared cavityarchitecture. The laser’s ultra-low noise properties enable averaging of the THz waveform for repetitionrate differences from 1 kHz to 22 kHz, resulting in a dynamic range of 55-dB when operating at 1 kHz andaveraging for two seconds. We show that the obtained dynamic range is competitive and can be wellexplained by accounting for the measured optical delay range, integration time, as well as the measurementbandwidth dependence of the noise from transimpedance amplification. These results will help enable anew approach to high-resolution THz-TDS enabled by low-noise gigahertz dual-comb lasers.

Renormalizing the high-harmonics interference via double-slit and grating

Yukiaki Ishida and Makoto Kuwata-Gonokami

DOI: 10.1364/AO.523629 Received 12 Mar 2024; Accepted 26 Apr 2024; Posted 26 Apr 2024  View: PDF

Abstract: A method to utilize high-harmonics interference in Young's double-slit followed by a grating holds promise for precisely measuring the refractive-index spectrum in the extreme ultraviolet (EUV) region. The measurement is currently bottlenecked by the time it takes to fit the Fresnel interference patterns. The conventional fitting procedure, involving multiple numerical integrals for evaluating the EUV propagation through the slit and grating, takes ≥30min on a standard laptop computer, while the data acquisition time is ~200s. Here, we apply an analytic Gaussian integral and bypass one of the numerical integrals associated with the grating. The analysis reduces to evaluating a renormalized EUV propagation in a simple double-slit interferometer, and the estimated fitting time on a laptop becomes as short as 112s. Our study enables real-time analysis during measurements, facilitating mass optical-data collection of EUV lithography materials.

Demonstration of Photonics-based D-band Integrated Localization and Communication

Qigejian Wang, Yirui Deng, Deepak Mishra, Yixuan Xie, Elias Aboutanios, and Shaghik Atakaramians

DOI: 10.1364/AO.516852 Received 22 Dec 2023; Accepted 26 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: The Terahertz spectrum has the ability to provide high-speed communication and millimeter-level resolution. As a result, terahertz-integrated sensing and communication (ISAC) has been identified as a keyenabler for 6G wireless networks. This work discusses a photonics-based D-band communication systemfor integrated high-resolution localization and high-speed wireless communication. Our empirical resultsshow that a communication rate of 5 Gbps over a distance of 1.5 meters and location identification of thetarget with millimeter-level (< 4 mm) range resolution can be conducted simutaneously using the samesignal. We also show that the error due to the thickness of the beam splitter can be eliminated, while thequantization error and the random drift errors are the limiting factors of the resolution achieved. Thisexperimental demonstration using D-band communication indicates that terahertz ISAC can be realizedfor 6G networks while considering the underlying system restrictions, e.g. bandwidth limit and lensdiameter

Nearly Polarization-Insensitive Angular Filters Enabled by Metal-Dielectric Photonic Crystal in the Visible Region

Yihong Chen, Jianjun Lai, Tianyu Xu, and Bingyu Lu

DOI: 10.1364/AO.517695 Received 08 Jan 2024; Accepted 26 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: We report on the design and fabrication of nearlypolarization-insensitive angular filters, which have beendeveloped through the optimization of one-dimensionalAg/MgF2 photonic crystals (PCs). We evaluate differentinitial systems for optimization and compare their resultsin terms of both wavelength and angular selectivity. Ourfindings reveal that relaxing the strict periodic conditionof initial photonic crystals with a small number of latticeshas enabled improvement in angular selectivity viaFabry–Pérot resonances in dielectric layers, achievingtransmission as high as 81% at normal incidence byoptimizing dielectric layers’ thickness. Simulation resultsdemonstrate that the transmitted beam through theangular filtering sample at 633 nm has allowable angleswithin 29° and 33° for TE and TM polarizationrespectively, with a transmission over 80% at normalincidence. This proposed and demonstrated angular filterrepresents a novel way to utilize 1D metal-dielectric PCsas polarization-insensitive angular filters, overcoming themain drawback of low transmission. This angular filterwill have significant applications in lighting, beammanipulation, optical coupling, and optical detectors.

Polarization-insensitive graphene-based bandnotched frequency selective absorber at terahertz

Asal Malekara, Changiz Ghobadi, and Javad Nourinia

DOI: 10.1364/AO.518847 Received 12 Jan 2024; Accepted 26 Apr 2024; Posted 29 Apr 2024  View: PDF

Abstract: This paper introduces a new polarization-insensitive graphene-based frequencyselective absorber (FSA) with a reflective notch designed for terahertz applications. Theproposed structure features two absorption bands on either side of a central reflection band. Thedesign comprises a lossy frequency selective surface (FSS), a bandstop FSS with a metalbacking, and an air spacer in between. A wideband absorber structure is developed in the firststep, leveraging graphene as an absorbent material in the lossy layer to achieve widebandabsorptive characteristics. Subsequently, a reflection band is introduced by integrating abandstop, lossless FSS layer into the absorber structure. The overall structure demonstrates twodistinct absorption bands, characterized by absorptivity exceeding 80%, within the frequencyranges of 0.30 to 0.57 THz and 0.67 to 0.90 THz. Simultaneously, a reflection notch is achievedat 0.60 THz. Extensive simulations assessed the performance of the designed FSA. Theproposed structure exhibits stability under oblique incidence up to 40 degrees and allowstunable absorption specifications by adjusting the chemical potential of graphene. It isnoteworthy that the FSA reflector offers advantages such as eliminating the need forcomplicated, high-cost 3-D structures and welding of the lumped resistors.

Autofocusing of laser lithography through crosshair projection method

Wei Wei, Jingsong Wei, Tianyu Gao, and Xiaozhong Xu

DOI: 10.1364/AO.523160 Received 05 Mar 2024; Accepted 25 Apr 2024; Posted 25 Apr 2024  View: PDF

Abstract: In laser direct writing lithography, there is not any image information from samplesurface, which makes it difficult to find the position of the focal plane. To overcome theproblem, an autofocusing through crosshair projection method is proposed in this work. Thecrosshair on the reticle is inserted into the lighting path and imaged onto the sample surface.The addition of the crosshair projection increases the image information from the samplesurface, meeting the requirement for the image information in focusing and improving thefocusing environment. Furthermore, this work presents a new division of the focusing curvebased on the range of the perpendicular feature extracted from the crosshair projection duringthe focusing process. The perpendicular feature can be extracted from the crosshair projectionin the focusing zone but not in the flat zone. Compared with the traditional division, this newdivision enables the use of the perpendicular feature to directly determine the zone of thecurrent sample position and find the focusing zone during the focusing process. This cancompletely filter out the interference of local fluctuations in the flat zone, greatly facilitatingthe sample focusing. The autofocusing process was designed based on this division, andexperiments were carried out accordingly. The focusing accuracy is about 0.15 μm, which is inthe range of depth of focus of the optical system. The results show that the proposed methodprovides a good solution to achieve accurate focusing based on the crosshair projection imagefrom the sample surface in laser lithography

Imaging through thick scattering media based on envelope-informed learning with simulated training dataset

bin wang, Yaoyao Shi, Wei Sheng, Meiling Zhang, and Youwen Liu

DOI: 10.1364/AO.521140 Received 05 Mar 2024; Accepted 24 Apr 2024; Posted 25 Apr 2024  View: PDF

Abstract: Computational imaging faces significant challenges in dealing with multiplescattering through thick complex media. While deep learning has addressed some ill-posedproblems in scattering imaging, its practical application is limited by the acquisition of trainingdataset. In this study, the Gaussian-distributed envelope of the speckle image is employed tosimulate the point spread function (PSF) and the training dataset is obtained by the convolutionof the handwritten digits with the PSF. This approach reduces the requirement of time andconditions for constructing the training dataset and enables a neural network trained on thisdataset to reconstruct objects obscured by an unknown scattering medium in real experiments.The quality of reconstructed objects is negatively correlated with the thickness of the scatteringmedium. Our proposed method provides a new way to apply deep learning in scattering imagingby reducing the time needed for constructing training dataset.

Intercomparison of flux, gradient, and variance-based optical turbulence (Cn²) parameterizations

Maximilian Pierzyna, Oscar Hartogensis, Sukanta Basu, and Rudolf Saathof

DOI: 10.1364/AO.519942 Received 05 Feb 2024; Accepted 24 Apr 2024; Posted 24 Apr 2024  View: PDF

Abstract: For free-space optical communication (FSOC) or ground-based optical astronomy, abundant data of optical turbulence strength ($C_n^2$) is imperative but typically scarce. Turbulence conditions are strongly site-dependent, so their accurate quantification requires in-situ measurements or numerical weather simulations. If $C_n^2$ is not measured directly, e.g., with a scintillometer, $C_n^2$ parameterizations must be utilized to estimate $C_n^2$ from meteorological observations or model output. Even though various such parameterizations exist in the literature, their relative performance is unknown. We fill this knowledge gap by performing a systematic three-way comparison of a flux-based, a gradient-based, and a variance-based parameterization. Each parameterization is applied to both observed and simulated meteorological variables, and the resulting $C_n^2$ estimates are compared against observed $C_n^2$ from two scintillometers. The variance-based parameterization yields the overall best performance, and unlike other approaches, its application is not limited to the lowest part of the atmospheric boundary layer, i.e.\ the surface layer. We also show that $C_n^2$ estimated from the output of the Weather Research and Forecasting (WRF) model aligns well with observations, highlighting the value of mesoscale models for optical turbulence modeling.

Hybrid meta/refractive lens design with inverse design using physical optics

Ko-Han Shih and Christopher Renshaw

DOI: 10.1364/AO.516890 Received 21 Jan 2024; Accepted 23 Apr 2024; Posted 24 Apr 2024  View: PDF

Abstract: Hybrid lenses are created by combining metasurface optics with refractive optics, where refractive elements contribute optical power, while metasurfaces correct optical aberrations. We present an algorithm for optimizing metasurface nanostructures within a hybrid lens, allowing flexible interleaving of metasurface and refractive optics in the optical train. To efficiently optimize metasurface nanostructures, we develop a scalar field, ray-wave hybrid propagation method. This method facilitates the propagation of incident and derived adjoint fields through optical elements, enabling effective metasurface optimization within the framework of adjoint gradient optimization. Numerical examples of various lens configurations are presented to illustrate the versatility of the algorithm and showcase the benefits offered by the proposed approach, allowing metasurfaces to be positioned beyond the image space of a lens. Taking a F/2, 40° field-of-view, midwave infrared lens as an example, the lens exhibits an average focusing efficiency of 38% before the integration of metasurfaces. Utilizing the new algorithm to design two metasurfaces—one in the object space and one in the image space—results in significant enhancement of the average focusing efficiency to over 90%. In contrast, a counterpart design with both metasurfaces limited to the image space yields a lower average focusing efficiency of 73%.

Detection method of micro and macro scale defects in silicon carbide castings

Jinyang Wang, Jingjing Chen, and Haiping Shen

DOI: 10.1364/AO.517305 Received 28 Dec 2023; Accepted 23 Apr 2024; Posted 23 Apr 2024  View: PDF

Abstract: Aiming at challenges such as low efficiency, high missing rate, difficulty inidentifying contour defects , and difficulty in extracting tiny defects, a defect detection methodfor extracting micro and macro scale defects was proposed in this paper. After preprocessingthe image, contour detection is performed to identify the contours. Subsequently, a contourcomplementation algorithm is employed to complement the unclosed contours. Finally, thedetection of micro scale defects is conducted based on the grayscale variation of the center of themicro scale efects. The experimental results show that compared with the traditional method, theproposed algorithm can accurately detect the bubble defects of different scales in silicon carbidecastings, and can identify the complex defects better.

Measuring the radius of curvature of spherical surfaces with actively tunable Fizeau and Twyman-Green interferometers

Minjae Kim, Arjent Imeri, and Syed Reza

DOI: 10.1364/AO.519083 Received 21 Jan 2024; Accepted 22 Apr 2024; Posted 24 Apr 2024  View: PDF

Abstract: Accurate and repeatable measurements of radius of curvature (RoC) of sphericalsample surfaces is of great importance in optics. This importance lies in the ubiquitous useof spherical optical elements such as curved mirrors and lenses. Due to a high measurementsensitivity, interferometric techniques are often deployed for accurate characterization of thesample surface RoC. One method by which a typical Commercial Fizeau or Twyman-Green(TG) interferometer measures surface RoC is via scanning between two principal retroreflectiveoptical configurations - namely the confocal and catseye configurations. Switching betweenthese two configurations is typically achieved by moving an optical head along the axis of thepropagating laser beam and the RoC is estimated by measuring the magnitude of mechanicalmotion to switch between the two principal configurations. In this paper, we propose a motion-freecatseye/confocal imaging-based sample RoC measurement system. The necessity of bulk motionto switch between the two configurations is circumvented via the use of an actively-controlledvarifocal lens. We demonstrate the usefulness of the proposed innovation in RoC measurementswith either the TG of the Fizeau interferometers. Furthermore, we converted a commercialmotion-based Zygo ⃝c RoC measurement system into a motion-free one by introducing a tunablelens inside the apparatus and using it to accurately characterize the RoC of different test samples.We also compute the wavefront aberrations for all spherical samples surfaces from the recordedmeasurement data.

Efficient fiber coupling system for simultaneous beam shaping and wavelength combining based on misaligned stepped prism multiplexing

Chang Liu, Lirui Xue, Ziyi Yan, Qian Qinyu, Haitao Chen, and Liwen Cheng

DOI: 10.1364/AO.524357 Received 21 Mar 2024; Accepted 22 Apr 2024; Posted 22 Apr 2024  View: PDF

Abstract: Due to the substantial contrast in output beam quality between the two directions of the semiconductor laser,efficiently coupling the beam directly into the fiber is difficult. In this study, an efficient shaping method based on stepmultiplexing of misaligned step prisms is proposed and investigated systematically. Firstly, multiple laser stacks eliminate darkareas to improve beam quality through the full utilization of the transmission and reflection surfaces of the misaligned steppedprisms, which also improves the efficiency of the stepped prisms. Simultaneously realizes laser wavelength combining basedon reflective surface multiplexing, without affecting the quality of the beam on the premise of improving the output power.Finally, the whole beam shaping system is completed without using cut-transform-rearranging prisms. The method couplesfour groups of laser stacks into a single fiber with a fiber diameter of 400 µm and a numerical aperture of 0.22. It can be seenfrom the computer-simulated model outputs that the final fiber output power can reach 2255.4 W and that the system as a wholehas a light-to-light translation rate of 88.1%.

DESIGNING BLACK MIRRORS

Ronald Willey

DOI: 10.1364/AO.523277 Received 06 Mar 2024; Accepted 22 Apr 2024; Posted 23 Apr 2024  View: PDF

Abstract: Black mirrors or total absorbers are related to neutral density (ND) filters which areprimarily concerned with transmittance whereas a black mirror has no transmittance andendeavors to reduce the reflectance to zero in the wavelength band of interest by absorbing allthe light. Metal layers must be used for the absorptance capabilities and dielectric layers tocreate the interference phases which maximize the electric field at the absorbing layers. Aliterature search for designing ND filters came up with eight [1-8] references which supplieduseful data on the indices of refraction of metals. There is a description of black mirror designprinciples found in Ref 9. Very thin layers of metal are required whose indices of refractionvary with thickness and require special software handling in the design process. The proceduresand additional viewpoint on designing black mirrors is reported here.

Generation of switchable and tunable rectangular pulse and multi-pulse with different peak power at L band

lei huang, Boxin Li, Dongfang Jia, and Chunfeng Ge

DOI: 10.1364/AO.520957 Received 04 Feb 2024; Accepted 22 Apr 2024; Posted 24 Apr 2024  View: PDF

Abstract: A novel L-band erbium-doped fiber laser, utilizing a nonlinear optical loop mirror (NOLM) as mode-locker, is presentedin this study. Through precise adjustments of the polarization controllers (PCs), the laser achieves the generation ofrectangular pulses with distinct single wavelengths, λ1=1593 nm and λ2=1571 nm, as well as dual-wavelength operation.The laser's operational mode can extend further to include harmonic mode-locking (HML). Furthermore, theinvestigation reveals the emergence of trapezoidal pulses and low-peak-power rectangular pulses within proximity of theconventional rectangular pulses. Notably, the evolutions of these low peak power pulses with the pump power also adhereto the peak power clamping (PPC) effect. Remarkably, the relative positioning of these pulses remains consistent acrossvarying pump power levels or harmonic orders. Intriguingly, the evolution of the trapezoidal pulse with respect to pumppower stands in stark contrast to that of the h-shaped pulse.

Research on a text information security protection method based on computer-generated holograms

Qiuyang Wang, Ge Aiming, Chen Xindi, Jiangbo Wu, Shuo Liu, and Zhu Dongsheng

DOI: 10.1364/AO.523616 Received 11 Mar 2024; Accepted 22 Apr 2024; Posted 24 Apr 2024  View: PDF

Abstract: Currently, computer-generated holograms (CGH) basedon ray tracing technology are generated faster and faster,and the reconstructed scenes are getting bigger and bigger and contain more and more information. Based onthis situation there are also more applications of usingCGH to hide information, but there is a lack of researchon the ability to hide information. To address this issue, this paper proposes a point-sampling CGH methodbased on ray tracing. Our method utilizes ray tracingtechniques to rapidly sample text information at different depths in the scene and hides the depth-encoded textinformation in the carrier image using discrete cosinetransform (DCT). The reconstructed image after embedding shows good results, with a peak signal-to-noiseratio (PSNR) of 29.56 dB between the hidden images before and after embedding. The PSNR value between theembedded carrier image and the original carrier image is51.66 dB, making it difficult for the human eye to distinguish, thereby effectively protecting the generated CGH.We also analyzed the maximum information density andobserved that computational holograms obtain the maximum information density at 200×200 resolution.

3DOF Displacement Sensor Based on Self-imaging Effect of Optical Micro-gratings

hao yang, Mengdi Zhang, Lijun Guo, Zhiyong Yang, Mengwei Li, and Chenguang Xin

DOI: 10.1364/AO.523811 Received 14 Mar 2024; Accepted 21 Apr 2024; Posted 22 Apr 2024  View: PDF

Abstract: In recent years, there is an increasing demand for multiple degrees of freedom (DOF) measurement system with high performance and high integration. Here, we report a 3DOF displacement sensor based on Self-imaging effect of optical micro-gratings. Optical field distribution behind a micro-grating with a period of 3μm is analyzed theoretically. Transmission properties of a double-grating structure are investigated in theory. In the experiment, 3DOF displacement measurement within a range of 1mm is demonstrated. Using an interpolation circuit with subdividing factor of 1000, displacement measurement with theoretical resolution of 3nm is realized. The experimental resolution is ~8nm. An error within 2μm is obtained experimentally within a range of 1mm for 3DOF measurement. With a few optical components such as beam splitter prism and beam expanders, the sensor shows potential in developing ultra-compact multi-DOF displacement measuring systems. Together with a nanometric resolution, the 3DOF displacement sensor has shown great potential in applications such as high-precision mechanical engineering and semiconductor processing.

Graphene-integrated microring cavity for electronically controlled molecular fingerprinting

Dazhi Du, qing zhang, and Zhi-Hong Zhang

DOI: 10.1364/AO.519693 Received 22 Jan 2024; Accepted 19 Apr 2024; Posted 22 Apr 2024  View: PDF

Abstract: Microring cavities supporting whispering-gallery modes(WGMs) have an exceptionally high-quality factor (Q) anda small mode volume, greatly improving the interactionbetween light and matter, which has attracted greatattention in various micro/nanoscale photonic devicesand potential applications. Recently, two-dimensional vander Waals (vdW) materials like graphene have emergedas a potential platform for next-generation biosensing byenabling the confinement of light fields at the nanoscale.Here, we propose a novel approach to achieve molecularfingerprint retrieval by integrating graphene into amicroring cavity, and conducting numerical simulationsusing Finite-Difference Time-Domain (FDTD) method.The hybrid cavity exhibits high-quality WGMs with a highQ factor of up to 800. Moreover, the resonant wavelengthcan be electronically controlled through modulation ofgraphene's Fermi level, enabling coverage of the entirefree spectral range at infrared frequencies. By depositinga thin layer of bio-molecular material (e.g., CBP) onto thesurface of our hybrid cavity, we are able to accurately readout the absorption spectrum at multiple spectral points,thereby achieving broadband fingerprint retrieval for thetargeted bio-molecule. Our results pave the way for highlysensitive, chip integrated, miniaturized, and electricallymodulated infrared spectroscopy biosensing.

Dwell time for optical fabrication using modified discrete convolution matrix method

Xi ming Liu, Longxiang Li, Xingchang Li, Feng Zhang, and Xuejun Zhang

DOI: 10.1364/AO.523682 Received 12 Mar 2024; Accepted 19 Apr 2024; Posted 22 Apr 2024  View: PDF

Abstract: More accurate dwell time calculation methods are necessary to achieve superior error convergence in producing optically critical components. Although the discrete convolution matrix method finds widespread application, which still have approximate errors in the non-uniform discrete form of tool paths. To address this issue, this paper introduced a modified matrix elements method and presented the general Voronoi polygon area weight calculation forms under different tool path discretization forms. The mechanism of the modified model is explained through analysis. The validity of the modified model is verified by numerical simulation. The modified approximation significantly improved uniformity distribution of surface residuals. This improvement holds promise as a guiding principle for the fabrication of ultra-precision optical components.

Estimation of non-uniform motion blur using a patch-based regression convolutional neural network (CNN)

Luis Varela, Laura E Boucheron, Steven Sandoval, David Voelz, and Abu Bucker Siddik

DOI: 10.1364/AO.521076 Received 06 Feb 2024; Accepted 18 Apr 2024; Posted 19 Apr 2024  View: PDF

Abstract: The non-uniform blur of atmospheric turbulence can be modeled as a superpositionof linear motion blur kernels at a patch level. We propose a regression convolutional neuralnetwork (CNN) to predict angle and length of a linear motion blur kernel for varying sizedpatches. We analyze the robustness of the network for different patch sizes and the performanceof the network in regions where the characteristics of the blur are transitioning. Alternating patchsizes per epoch in training, we find coefficient of determination scores across a range of patchsizes of 𝑅2 > 0.78 for length and 𝑅2> 0.94 for angle prediction. We find that blur predictions inregions overlapping two blur characteristics transition between the two characteristics as overlapchanges. These results validate the use of such a network for prediction of non-uniform blurcharacteristics at a patch level.

Study on VLC channel Model in Underground Mining Scenarios with Extinction Effect and Shadowing Effect

Chao Li, xing wang, Zhenliang Dong, Fengyuan Shi, Ting Yang, and Ping Wang

DOI: 10.1364/AO.520073 Received 25 Jan 2024; Accepted 18 Apr 2024; Posted 18 Apr 2024  View: PDF

Abstract: In this work, a novel visible light communication (VLC) channel model is proposed for underground mining scenarios taking into account the impact of coal dust particles and obstacles. Specifically, the extinctioneffect of the coal dust particles is analyzed on the basis of the Mie theory, and the quantitative formula ofthe influence on channel direct current (DC) gain is derived. Meanwhile, the effect of random shadowingphenomenon is investigated and quantified with the geometric and statistical model considering theposition, size, and shape of the obstacles. The channel impulse response, path loss, root mean square delayspread, and bit error rate (BER) are further investigated in two different underground mining scenarios,namely, mining roadway and coal mine working face. Simulation results show that the shadowing effectplays a major role in the influence of DC gain attenuation. Furthermore, the BER performance would benoticeably degraded due to the presence of coal dust particles and obstacles, especially when the receiveris located far from the transmitter. This work will benefit the design of the VLC systems in undergroundmines.

Stably polarized 795 nm Vertical-Cavity Surface-Emitting Lasers with Anti-phase SiNx Surface Gratings

qiuxue Fu, Yurun Sun, suzhen Yu, Bocang Qiu, Jianrong Dong, and Yongmin Zhao

DOI: 10.1364/AO.524882 Received 27 Mar 2024; Accepted 17 Apr 2024; Posted 18 Apr 2024  View: PDF

Abstract: 795 nm vertical-cavity surface-emitting lasers (VCSELs) with dielectric surface gratings to control the output polarizationare designed and fabricated. The calculated results demonstrate that a well-designed SiNx surface grating positioned onthe surface of an anti-phase VCSEL structure enhances the reflectivity difference between the two polarization modescompared to a conventional GaAs surface grating, consequently resulting in a larger gain anisotropy in VCSELs and a highorthogonal polarization suppression ratio (OPSR). Characterization shows that a peak-to-peak OPSR of 30.3 dB isachieved at 85 °C for 795 nm VCSELs with a SiNx surface grating of 5 μm in diameter and an oxide aperture of ~4 μm,demonstrating the effectiveness of the SiNx surface grating in polarization control for 795 nm VCSELs.

E-Beam Synthesized Fast Switching TiO2/SnO2 Type-II Heterostructure Photodetector

Rajib Nanda and Mitra Sarkar

DOI: 10.1364/AO.522709 Received 29 Feb 2024; Accepted 17 Apr 2024; Posted 18 Apr 2024  View: PDF

Abstract: A fast-switching TiO2/SnO2 heterostructure thin film (TF) photodetector synthesized by electron beam evaporation technique, isanalysed in this study. The substrate utilized is n-type silicon (Si), while Gold (Au) is employed as the top electrode. To assesssample morphology and confirm elemental composition, field emission scanning electron microscopy (FESEM), energy dispersivex-ray spectroscopy (EDS), and chemical mapping were conducted. Structural characteristics were determined using X-raydiffraction (XRD) analysis. The XRD analysis confirmed the presence of various phases of TiO2 (anatase and rutile) and SnO2(rutile). UV-Vis spectroscopy revealed multiple absorption peaks, at 447 nm, 495 nm, 560 nm, and 673 nm within the visiblespectrum. The device demonstrates high detectivity (D*) of 1.737×109 Jones and a low noise equivalent power (NEP) of 0.765×10-10 W. Evaluation of the device's switching response through current-time characteristic (I-T) analysis indicates rapid switching witha rise time and fall time of 0.33s and 0.36s, respectively.

Radial basis point interpolation for strain field calculation in digital image correlation

Jiayi Du, ZHAO JIAN, Jiahui Liu, and Dong Zhao

DOI: 10.1364/AO.520232 Received 29 Jan 2024; Accepted 17 Apr 2024; Posted 17 Apr 2024  View: PDF

Abstract: In order to extract smooth and accurate strain fields from the noisy displacement fields obtained by digital image correlation (DIC), a point interpolation meshless (PIM) method with radial basis function (RBF) is introduced for full-field strain calculation, which overcomes the problems of slow calculation speed and unstable matrix inverse calculation of the element-free Galerkin method (EFG). The radial basis point interpolation method (RPIM) with three different radial basis functions and the moving least squares (MLS) and pointwise least squares (PLS) methods are compared by analyzing and validating the strain fields with high strain gradients in simulation experiments. The results indicate that RPIM is nearly 80% more computationally efficient than the MLS method when a larger support domain is used, and the efficiency of RPIM is nearly 26% higher than that of the MLS method when a smaller support domain is used; the strain calculation accuracy is slightly lower than that of the MLS method by 0.3-0.5%, but the stability of the calculation is significantly improved. Different from the PLS method, which is easily affected by the noise and the size of the strain calculation window, RPIM is insensitive to the displacement noise and the size of the support domain and can obtain similar calculation accuracy. RPIM with Multiquadric (MQ) radial basis functions perform well in balancing computational accuracy and efficiency and is insensitive to shape parameters. Application cases show that the method can effectively compute the strain field at the crack tip, validating its applicability to the study of the plastic region at the crack tip. In conclusion, the proposed RPIM-based method provides an accurate, practical, and robust approach for full-field strain measurements.

Theoretical Analysis of Buried Heterostructure Laser for Stable Dual Wavelength Generation

Soumi Pal, Arpit Khandelwal, and Nitin Bhatia

DOI: 10.1364/AO.522669 Received 29 Feb 2024; Accepted 17 Apr 2024; Posted 17 Apr 2024  View: PDF

Abstract: Stable dual wavelength emission from a laser is desirable for microwave signal generation using the opticalheterodyning method. As both optical wavelengths are generated from the same cavity, the phase noise ofthe generated microwave signal is minimized. In this work, we exploit the inherent birefringence in theburied heterostructure (BH) semiconductor laser to generate dual polarized modes. We carefully analyzethe mode competition between various modes in the cavity and propose the desirable gain modificationconditions for stable dual mode oscillations when the laser is operating near the threshold. We show thatthe required asymmetry in the gain for two stable modes can be obtained from the mode confinementfactors and facet losses. We also show the applicability of our results to a homogeneously broadenedmultimode laser.

Leveraging statistical – spectral correlations of random metasurfaces for steganography and multi-wavelength cryptography

Romil Audhkhasi, Maksym Zhelyeznyakov, Steven Brunton, and Arka Majumdar

DOI: 10.1364/AO.523914 Received 14 Mar 2024; Accepted 16 Apr 2024; Posted 17 Apr 2024  View: PDF

Abstract: The ability to tailor the spectral response of photonic devices is paramount to theadvancement of a broad range of applications. The vast design space offered by disorderedoptical media provides enhanced functionality for spectral tailoring, while also making itchallenging to map the spectral properties of such complex systems to their structural attributes.In this work, we investigate correlations between the configuration statistics of randommetasurfaces and their spectral transmissivity in the visible, and leverage those to develop areduced phase space. In the latter part of the manuscript, we use this reduced phase space todesign a pixelated color filter that hides visual data within a preselected cover image forsteganography. Furthermore, we design a pair of color filters that can collectively encrypt agiven grayscale image in their spectral transmissivities. We envision such devices to createopportunities for the development of compact, next-generation cryptographic systems. Morebroadly, the results presented in this manuscript provide new avenues for optimizing large-scalerandom metasurfaces to achieve enhanced optical functionalities for a wide variety ofapplications.

Structured polarized laser beams for controlled spiral-shaped mass transfer in azopolymer thin films

Alexey Porfirev, Svetlana Khonina, Denis Porfiriev, and Nikolay Ivliev

DOI: 10.1364/AO.521196 Received 07 Feb 2024; Accepted 16 Apr 2024; Posted 16 Apr 2024  View: PDF

Abstract: We present an approach for the realization of controlled spiral-shaped mass transferin azopolymer thin films and the fabrication of spiral microreliefs. For such laser processing,we propose to use light fields with structured polarization distributions generated by atransmissive spatial light modulator. The projection lithography approach is utilized,transferring the pattern directly to the surface of azopolymer thin films. The shaped polarizationdistributions with different dependencies of the polarization vector orientation on the azimuthalangle allow us to drive surface waves on the sample along a spiral trajectory. Additionally, theability to control the concavity of the formed microreliefs is demonstrated. This approach canbe effectively modified for the direct laser fabrication of more complex nano-/micro-elementsas well as their arrays.

Optimizing Thin-Film Silicon Solar Cells with Nanostructured TiO2 and Silver Back Reflector for Enhanced Energy Conversion Efficiency

Basma Abu-elmaaty, Tawfik Ismail, Ala H. Sabeeh, and Khawaji Ibrahim

DOI: 10.1364/AO.521845 Received 18 Feb 2024; Accepted 16 Apr 2024; Posted 16 Apr 2024  View: PDF

Abstract: This paper investigates the improvement of energy conversion efficiency in thin-film silicon solar cells by employing periodic nanostructures of TiO2 on the silicon active layer and aback reflector featuring periodic nanostructures of silver. The objective is to increase the opticalpath length, enhance absorption probability for longer wavelengths, and subsequently improvesolar cell performance. Three silicon-based solar cell configurations are proposed and simulatedusing the finite difference time domain (FDTD) method to assess their performance. Electricalcharacteristics are obtained through the drift-diffusion method. The resulting short-circuit currentdensity increased from 40.93 mA/cm2to 65.28 mA/cm2to 95.373 mA/cm2for the three cells,leading to significant improvements in conversion efficiency with observed values of 20.39%,33.26%, and 47.28%, respectively, in the optimized structures. Furthermore, we compare thesimulation results of the three structures with those of a reference structure and several structurespreviously proposed in the literature.

A fiber-optic gyroscope for rotational seismic ground motion monitoring of the Campi Flegrei volcanic area

Marialuisa Capezzuto, guido gaudiosi, Lucia Nardone, ezio dalema, Davide DAmbrosio, roberto manzo, Antonio Giorgini, Pietro Malara, Paolo De Natale, Gianluca Gagliardi, Luigi Santamaria Amato, Danilo Galluzzo, and Saverio Avino

DOI: 10.1364/AO.518354 Received 17 Jan 2024; Accepted 16 Apr 2024; Posted 17 Apr 2024  View: PDF

Abstract: The real-time monitoring of densely populated areas with high seismic and volcanicrisk is of crucial importance for the safety of people and infrastructures. When an earthquakeoccurs, the Earth surface experiences both translational and rotational motions. The latter areusually not monitored, but their measurement and characterization is essential for a fulldescription of the ground motion. Here we present preliminary observational data of a highsensitivity rotational sensor based on a 2-km long fiber-optic Sagnac gyroscope, presentlyunder construction in the middle of the Campi Flegrei Volcanic Area (Pozzuoli, Italy). We haveevaluated its performances by analyzing data continuously recorded during an acquisitioncampaign of five months. The experimental setup was composed by a digital nine-componentseismic station equipped with both the rotational sensor and conventional seismic sensors(seismometers, accelerometers and tiltmeters). During this experiment we detected seismicnoise and ground rotations wavefield induced by small – medium local earthquakes (MD < 3).The prototype gyroscope shows a very promising sensitivity in the range of 5 × 10―7 ―8 ×10―9𝑟𝑎𝑑 𝑠 𝐻𝑧over the frequency bandwidth 5 mHz – 50 Hz. Future upgrades andperspectives are discussed.

Radiant fluence from ray tracing in optical multipass systems

Miroslaw Marszalek, Lukas Affolter, OGUZHAN KARA, Klaus Kirch, Karsten Schuhmann, Manuel Zeyen, and Aldo Antognini

DOI: 10.1364/AO.520267 Received 28 Jan 2024; Accepted 15 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: Optical multipass cells are used in photochemical reactors and laser excitation of weak transitions. In theseapplications, estimation of the radiation dose in a volume of interest allows to assess the performance andoptimize the design of the cell. We adopt radiant fluence as the figure of merit and employ the radiativetransfer equation to derive analytical expressions for average radiant fluence in a given volume of interest.These expressions involve quantities that are either obtained with Monte Carlo ray tracing methods orapproximated on the grounds of geometry arguments. Furthermore, we compute the spatial distribution offluence by applying either of the two methods to individual volume elements in a rectangular voxel grid.Ray tracing is performed with Zemax OpticsStudio 18.9.

High-accuracy centroid location of CCD imaging retroreflective targets in close-range photogrammetry

Kaifeng Ma, Guiping Huang, and Junzhen Meng

DOI: 10.1364/AO.518097 Received 08 Jan 2024; Accepted 15 Apr 2024; Posted 16 Apr 2024  View: PDF

Abstract: Depending on the size, shape, and gray intensity distribution of charge-coupled device (CCD) imaging retro-reflectivetargets (RRTs) in close-range photogrammetry, and based on conventional grayscale centroiding, this paper proposesgrayscale threshold variable-index weighted centroiding (GTVIWC). The centroid location accuracy of CCD imaging RRTswas analyzed and compared using simulated and measured target images, respectively. The experimental resultsdemonstrated that the centroid location accuracy of the algorithms used in the experiment was relatively high, reachingthe subpixel level. Among them, GTVIWC has the highest location accuracy. The root mean square error (RMSE) of thecentroid location for the simulated and measured CCD imaging RRTs reaches 0.0011 pixels and 0.0122 pixels,respectively. The correctness, reliability, and high accuracy of the proposed algorithm are verified.

Fractional Fourier-transform off-axis digital holographic imaging

junjie zhang, YiWei Liu, Wanying Cui, and Zhuqing Jiang

DOI: 10.1364/AO.520086 Received 25 Jan 2024; Accepted 15 Apr 2024; Posted 16 Apr 2024  View: PDF

Abstract: A fractional Fourier-transform digital holographic imaging method with resolution enhancement features ispresented. In optical configuration, an extended fractional Fourier-transform optical setup is set in the objectarm of an off-axis digital holographic recording system,to record a fractional Fourier transform hologram via theoptical interference of the fractional Fourier transformwavefront of an object wave with a reference wave. Forreconstruction imaging, the reconstruction approach forfractional Fourier transform holograms is given. In experiment, the fractional Fourier-transform digital holograms are recorded under the different recording parameters, and their amplitude images are effectivelyreconstructed. The imaging results demonstrate that thereconstruction-imaging resolution of fractional-orderFourier-transform holograms is obviously enhancedcompared to that of conventional image-plane holograms. The presented fractional Fourier-transform digital holographic imaging with resolution enhancementand optical configuration flexibility provides a novelway for off-axis digital holographic imaging.

Active visual continuous seam tracking based on adaptive feature detection and particle filter tracking

Rong Fan, Peng Zhang, Fengyun Guo, Jie Rong, and Xupeng Lian

DOI: 10.1364/AO.520506 Received 30 Jan 2024; Accepted 15 Apr 2024; Posted 16 Apr 2024  View: PDF

Abstract: Welding seam tracking based on online programming is the future trend of intelligent production. However, most of the existing image processing methods have certain limitations in the adaptability, accuracy and robustness of weld feature point detection. The on-line welding method of gas metal arc welding (GMAW) based on active vision sensing is studied in this paper. Steger sub-pixel detection method is used to guarantee the accuracy of feature point extraction, and self-adaptive search window and self-adaptive slope extraction are proposed on this basis, which has certain robustness and universality for continuous weld detection. When arc light and other serious interference makes it difficult to obtain weld information, particle filter is used to make the best prediction of weld position. Finally, the welding robot platform based on laser vision sensing was built to test various continuous welds of butt weld, fillet weld and lap weld. Through the detection test of weld point on laser stripe image and the tracking performance test during welding tracking, experimental results show that the detection speed is 27ms, the accuracy of detection and tracking can respectively reach 0.80pixel and 0.78mm, which meet the requirements of weld detection and tracking.

An Irradiance Uniformity Reshaping Method for Solar Simulators Based on Computer Generated Spatial Filter Films

ZeSheng Qin, Jia-Yong Song, Changwen Xue, Haojie Li, Chang Ma, Lifeng Bian, and Chen Yang

DOI: 10.1364/AO.516380 Received 18 Dec 2023; Accepted 15 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: The irradiance uniformity is critical to the accuracy of photovoltaic device testresults. Therefore, to post-correct the irradiance uniformity inherent in artificial lightingsystems, a spatial irradiance filter scheme for film patterns is proposed based on the physicalphenomenon of a positively related relationship between inkjet concentration and thetransparency of the flexible film. The scheme first establishes the characteristic equationbetween the irradiance absorption and pattern grayscale values and then generates the spatialfiltering pattern by utilizing the light intensity distribution to be calibrated, matrix operations,and bilinear interpolation. To evaluate its performance, an STM32 microprocessor-basedirradiance distribution measurement system was developed and used to test and verify singlelamp, planar array, and curved surface array light sources. The results reveal that the correctedirradiance uniformity improves by 15.5%, 24. 01 %, and 13.11%, all of which achieve the ClassA irradiance uniformity of the IEC 60904-9 standard.

Simulation Method for Multichromatic Light Spots in Mosaic Aperture Telescopes with Large Image Planes during Deployment

Tang Hongyang, Zhiyuan Liao, jisong jiang, Mo Chen, Hao Xian, Hua Li, and Sheng Liao

DOI: 10.1364/AO.519473 Received 18 Jan 2024; Accepted 14 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: In optical systems, diffraction limits significantly impact spot simulations. Thisstudy addresses this problem by applying the Fourier transform to calculate spots in imagingsystems. Typically, a 1 mm image plane suffices; however, mosaic aperture telescopes withnotable wavefront discontinuities require an approximately 10 mm simulation image plane.This necessitates high sampling rates for pupils, posing challenges for conventional methods.Our model overcomes this challenge by leveraging an interpolation technique to align multiwavelength spots on a uniform image plane grid, thus effectively analyzing spot translation andspreading in imaging systems with diffraction limits.

Characterization of perfect sinusoidal grating profile using artificial neural network for plasmonic based sensors

Moustapha Godi Tchéré, Stephane Robert, Bernard Bayard, Julie Dutems, Hugo Bruhier, Yves JOURLIN, and damien jamon

DOI: 10.1364/AO.520109 Received 25 Jan 2024; Accepted 14 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: In this paper, we present a system intended to detect a targeted perfect sinusoidal profile of a diffraction gratingduring its manufactured process. Indeed, the sinusoidal nature of the periodic structure is essential to ensureoptimal efficiency of specific applications as plasmonic sensors (Surface Plasmon Resonance SPR based sensors).A neural network is implemented to characterize the geometrical shape of the structure under testing at the endof the Laser Interference Lithography (LIL) process. This decision tool operates in classifier mode prior to furtherprocessing. Then, the geometrical parameters of the structure can be reliably determined if necessary. Twosolutions can be considered: the detection of a fixed geometrical shape operating on a binary mode and theidentification of a geometrical shape from a limited number of profiles. These methods are validated in the contextof plasmonic sensors on experimental sinusoidal grating structures with a grating period of 627 nm.

Laser cleaning of dirty grease on steel sluice cables

Guisheng Fang, Jianjun Pang, Daming Wu, Yehang Pan, and Wei Zhao

DOI: 10.1364/AO.522387 Received 28 Feb 2024; Accepted 14 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: Steel cables used to raise sluices require a layer of corrosion-resistant grease, which must be periodically replaced. This time-consuming and laborious, and conventional manual cleaning, mechanical cleaning and chemical cleaning methods have many drawbacks. In this paper, a nanosecond pulsed fiber laser is used to clean hardened surface grease from such cables. An experimental system was designed to study the effects of parameters such as laser power, scanning speed, cleaning frequency, and defocusing amount. Macroscopic and microstructural observations were conducted on the surfaces of steel cables before and after cleaning using cameras, optical microscopy, scanning electron microscopy, and energy dispersive spectrometry. With the optimal parameters, laser cleaning can effectively remove hardened grease from steel cable surfaces without damaging the galvanized layer and the steel wire matrix. Ablation, gasification, and evaporation are the main mechanisms by which grease and dirt are removed. This study lays a foundation for optimizing the laser cleaning of steel sluice cables at work sites.

Compact Auto-aligning Interferometers with Picometer Precision

Xiang Lin, Peng Qiu, Xiaofang Ren, Yurong Liang, and Hao Yan

DOI: 10.1364/AO.522813 Received 01 Mar 2024; Accepted 14 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: This research introduces a compact, auto-aligning interferometer engineered formeasuring translations with a wide angular working range and picometer precision above 1 Hz.It presents a design ensuring automatic beam alignment during movement through secondaryreflection from a corner reflector. The sensor head, a 20×10×10 mm3all-glass, quasi-monolithicstructure, exhibits a displacement sensitivity below 1 pm/Hz1/2above 1 Hz and a wide angularworking range of ±200 mrad. This versatile optical design holds promise to improve thesensitivity in applications such as laser ranging, optical seismometers, precision manufacturingand metrology.

Using broadband long-wavelength channel to increase the capture range of segment piston phase retrieval for segmented-aperture systems

Joseph Tang and James Fienup

DOI: 10.1364/AO.518565 Received 12 Jan 2024; Accepted 13 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: Segmented-aperture systems, such as the James Webb Space Telescope (JWST),requires fine piston alignment between primary mirror segments. Computer simulation experiments show that using a broadband long-wavelength channel, illustrated with the Mid InfraredInstrument (MIRI) onboard the JWST, can extend the capture range of segment piston phaseretrieval significantly (in the case of JWST with MIRI, up to hundreds of microns), greatlyreducing the requirements on coarse phasing.

A model for partitioning the nonphytoplankton absorption coefficient of seawater in the ultraviolet and visible spectral range into the contributions of non-algal particulate and dissolved organic matter

Matthew Kehrli, Dariusz Stramski, Rick Reynolds, and Ishan Joshi

DOI: 10.1364/AO.517706 Received 25 Jan 2024; Accepted 12 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: Non-algal particles and chromophoric dissolved organic matter (CDOM) are twomajor classes of seawater constituents which contribute substantially to light absorption in theocean within the ultraviolet (UV) and visible (VIS) spectral region. The similarities in thespectral shape of these two constituent absorption coefficients, ad(λ) and ag(λ) respectively,have led to their common estimation as a single combined non-phytoplankton absorptioncoefficient, adg(λ), in optical remote sensing applications. Given the different biogeochemicaland ecological roles of non-algal particles and CDOM in the ocean, it is important to determineand characterize the absorption coefficient of each of these constituents separately. We describean ADG model that partitions adg(λ) into ad(λ) and ag(λ). This model improves upon a recentlypublished model [Appl. Opt. 58, 3790 (2019)] through implementation of a newly assembleddataset of hyperspectral measurements of ad(λ) and ag(λ) from diverse oceanic environments tocreate the spectral shape function libraries of these coefficients, a better characterization ofvariability in spectral shape of ad(λ) and ag(λ), and a spectral extension of model output toinclude the near-UV (350–400 nm) in addition to the VIS (400–700 nm) part of the spectrum.We developed and tested two variants of the ADG model; the ADG_UV-VIS model whichdetermines solutions over the spectral range from 350 to 700 nm, and the ADG_VIS modelwhich determines solutions in the VIS but can also be coupled with an independentextrapolation model to extend output to the near-UV. This specific model variant is referred toas ADG_VIS-UVExt. Evaluation of the model with development and independent datasetsdemonstrate good performance of both ADG_UV-VIS and ADG_VIS-UVExt. Comparativeanalysis of model-derived and measured values of ad(λ) and ag(λ) indicates negligible or smallmedian bias, generally within ±5% over the majority of the 350–700 nm spectral range butextending to or above 10% near the ends of the spectrum, and the median percent differencegenerally below 20% with a maximum reaching about 30%. The presented ADG models aresuitable for implementation as a component of algorithms in support of satellite ocean colormissions, especially the NASA PACE mission.

Stretchable metal-dielectric-metal metasurface for dynamic radiation management

Sijie Pian, Chengtao Lu, Zhuning Wang, and Yaoguang Ma

DOI: 10.1364/AO.522582 Received 28 Feb 2024; Accepted 12 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: Radiative cooling devices offer passive characteristics and hold significant potential for reducing energy consumption.However, to address changing climate needs, dynamic devices with tunable radiation properties are crucial. Here, wepropose a novel design for tunable radiative thermal management utilizing a reconfigurable flexible metasurface emitter.By applying biaxial stretching to alter the metasurface's periodicity, its optical response can be continuously modulated.At ambient temperature, with a stretch ratio of 1.6, the device is predicted to achieve thermal management powermodulations of 173 W/m2 and 42 W/m2 during daytime and nighttime, respectively. This approach holds promise forimproving the efficiency of thermal management systems in applications such as buildings and smart windows.

Quantification of surface layer turbulence using Sensible Heat values from Energy Balance versus Aerodynamic methods

Steven Fiorino, Yogendra Raut, Jaclyn Schmidt, Laura Slabaugh, Blaine Fourman, Jack McCrae, Benjamin Wilson, and Santasri Bose-Pillai

DOI: 10.1364/AO.521086 Received 06 Feb 2024; Accepted 12 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: Surface layer optical turbulence values in the form of refractive index structurefunction, Cn2, are often calculated from surface layer temperature, moisture, and windcharacteristics and compared to measurements from sonic anemometers, differentialtemperature sensors, and imaging systems. A key derived component needed in the surfacelayer turbulence calculations is the “Sensible Heat” value. Typically, the sensible heat iscalculated using the “Bulk Aerodynamic Method” that assumes a certain surface roughness anda “friction velocity” that approximates the turbulence drag on temperature and moisture mixingfrom the change in the average surface layer vertical wind velocity. Theseassumptions/approximations generally only apply in free convection conditions. A more robustmethod, that applies when free convection conditions are not occurring, to obtain the sensibleheat is via the Energy Balance or Bowen Ratio method. The use of the Bowen Ratio – the ratioof sensible heat flux to latent heat flux – allows a more direct assessment of the opticalturbulence-driving surface layer sensible heat flux than do more traditional assessments ofsurface layer sensible heat flux. This study compares surface layer Cn2 values using sensibleheat values from the bulk aerodynamic and energy balance methods to quantifications fromsonic anemometers posted at different heights on a sensor tower. The research shows thatsensible heat obtained via the Bowen Ratio method provides a simpler, more reliable, and moreaccurate way to calculate surface layer Cn2 values than what is required to make suchcalculations from bulk aerodynamic method-obtained sensible heat.

Measurement of light absorption by chromophoric dissolved organic matter (CDOM) using a type-II liquid capillary waveguide: assessment of an achievable accuracy.

Rüdiger Röttgers, Michael Novak, and Mathias Belz

DOI: 10.1364/AO.516580 Received 21 Dec 2023; Accepted 12 Apr 2024; Posted 12 Apr 2024  View: PDF

Abstract: Light absorption by chromophoric dissolved organic matter (CDOM) in the ocean isoften measured using liquid waveguide capillary cells coupled to spectral array detectors. Thistype of optical setup is affected by several sources of uncertainties related to the waveguide andthe detector. Uncertainties from the waveguide arise from error in the effective path length andthe effects of water salinity, while errors related to the detector are due to non-linearity in theresponse, internal stray light, and wavelength accuracy. Here, uncertainties in themeasurements of the spectral absorption coefficient of CDOM due to the optical setup itselfwere investigated in detail. The related systematic errors were very often significant (2 - 15%)and larger than expected from simple measurement uncertainty (1%). However, they can becorrected by characterizing the detector's response for non-linearity and stray light, regularlyperforming calibrations for the detector's wavelength response, and routinely measuring thewaveguide's effective path length. Including such corrections and timely calibrations reducesthe uncertainties related to the spectrophotometric measurements to about 2%. Uncertaintiesrelated to the necessary handling of samples are not included here.

Step-adaptive accelerated demodulation algorithm for LFM pulse-based distributed acoustic sensing

Zhongquan Chen, Junfeng Jiang, Kun Liu, Mingjiang Zhang, Shuang Wang, Zhenyang Ding, and T. Liu

DOI: 10.1364/AO.522323 Received 25 Feb 2024; Accepted 12 Apr 2024; Posted 16 Apr 2024  View: PDF

Abstract: We propose a novel step-adaptive cross-correlation algorithm tailored for distributed acoustic sensing systems based on linear frequency modulation pulses, aiming for rapiddemodulation. This algorithm adjusts its step length through an adaptive "successive refinement"search strategy, which greatly improves computational efficiency by reducing the number ofcross-correlation computations. Experimental results have shown that the demodulation time canbe reduced by approximately 15 times compared to the conventional method, while maintainingthe same demodulation result.

Comparative analysis of 𝐂 𝟐 𝐧 estimation methods for sonic anemometer data

Melissa Beason, Guy Potvin, Detlev Sprung, Jack McCrae, and Szymon Gladysz

DOI: 10.1364/AO.520976 Received 04 Feb 2024; Accepted 12 Apr 2024; Posted 19 Apr 2024  View: PDF

Abstract: Wind speed and sonic temperature measured with ultrasonic anemometers are oftenutilized to estimate the refractive index structure parameter 𝐶2𝑛, a vital parameter for opticalpropagation. In this work, we compare four methods to estimate 𝐶2𝑛 from 𝐶2𝑇 using the sametemporal sonic temperature data streams for two separated sonic anemometers on ahomogenous path. Values of 𝐶2𝑛 obtained with these four methods using field trial data arecompared to those from a commercial scintillometer and from the differential image motionmethod using a grid of light sources positioned at the end of a common path. In addition to thecomparison between the methods, we also consider appropriate error bars for 𝐶2𝑛 based on sonictemperature considering only the errors from having a finite number of turbulent samples. TheBayesian and power spectral methods were found to give adequate estimates for strongturbulence levels, but consistently overestimated the 𝐶2𝑛 for weak turbulence. The nearestneighbors and structure function methods performed well under all turbulence strengths tested.

Simulation of the reflection of a high energy laser beam at the sea surface for hazard and risk analyses

Frederic Schwenger, Adrian Azarian, and Michael Henrichsen

DOI: 10.1364/AO.516715 Received 26 Dec 2023; Accepted 11 Apr 2024; Posted 12 Apr 2024  View: PDF

Abstract: The application of a high energy laser beam in a maritime scenario necessitates alaser safety concept to prevent injury to personnel or uninvolved third parties from uncontrolledreflections of laser light from the sea surface. Therefore, it is crucial to have knowledge of theamount and direction of reflected laser energy, which varies statistically and depends largelyon the dynamics of the wavy sea surface. These dynamics are primarily influenced by windspeed, wind direction, and fetch. An analytical model is presented for calculating the time averaged spatial intensity distribution of the laser beam reflected at the dynamic sea surface.The model also identifies the hazard areas inside which laser intensities exceed a fixed exposure limit. Furthermore, as far as we know, our model is unique in its ability to calculate the probabilities of eye-damaging glints for arbitrary observer positions, taking into account theslope statistics of gravity waves. This is a critical first step towards an extensive risk analysis.The simulation results are presented on a hemisphere of observer positions with fixed radii fromthe laser spot center. The advantage of the analytical model over our numeric (dynamic) modelis its fast computation time. A comparison of the results of our new analytical model with thoseof the previous numerical model is presented.

A dynamic accuracy measurement method for star trackers using a time synchronized high accuracy turntable

Rui Lu, Jianfu Zhang, Xing Han, Yanpeng Wu, and Lin Li

DOI: 10.1364/AO.507209 Received 11 Jan 2024; Accepted 11 Apr 2024; Posted 12 Apr 2024  View: PDF

Abstract: Star trackers are typically used in a spacecraft to provide absolute attitude information to the on-board attitude controlsystem for its high accuracy. The performance of the star tracker is rather important. Attitude incorrectness provided bystar trackers may lead to bad navigation with big deviations, even failure of satellites. Therefore, how to realize andverify the accuracy is crucial. As a matter of fact, it is difficult to validate accuracy of star trackers on ground, especiallyfor star trackers under high dynamic conditions. In this paper, an accuracy measurement method for star trackers under dynamic conditions is proposed, utilizinghigh accuracy swing table to provide reference to compare. To this end, swing table, star tracker, and the test equipmentare synchronized, in order to reduce systematic errors. As the motion trajectory of the swing table can be setbeforehand, initial attitude of the star tracker can be predicted through a set of coordinate transformations. As a result,the star tracker is able to keep tracking, regardless of the angular velocity of the swing table. This makes the statisticalsample points more sufficient and the results more reliable. Moreover it can evaluate the angular velocity of startrackers up to 20°/s. In comparison with the conventional method with simulated stars, this method utilizes realnavigation stars as observation targets making the measurement results much closer to the on-orbit performance. Lastbut much more important, it can also verify the performances of a star tracker in one experiment, such as sensitivity,static performance, capture probability and so on. Experimental results demonstrate that the proposed method iseffective, especially for high dynamic star trackers. Such a measurement environment is close to the in-orbit conditionsand it can satisfy the stringent requirement for star trackers under high dynamics

Cost-effective 3D-printed rotatable reflectors for two-dimensional beam steering

Ching-Kai Shen, Wei-An Tsui, Pin-Hung Yeh, Cheng-Lin Tsai, Yi-Wen Cheng, and Jui-che Tsai

DOI: 10.1364/AO.522895 Received 05 Mar 2024; Accepted 10 Apr 2024; Posted 11 Apr 2024  View: PDF

Abstract: In this paper, we have developed a 2D optical scanning module comprising cascaded3D-printed one-axis rotating mirrors with large areas (30 × 30 mm2 for the X-direction scanand 60 × 25 mm2 for the Y-direction scan). Each mirror device comprises a square orrectangular silicon substrate coated with aluminum, serving as the mirror. A 3D-printedstructure, including the mirror frame (with four embedded mini permanent magnets on thebackside), torsion springs, and base, is combined with the mirror; two electromagnets aresituated under the mirror as the actuation mechanism. We apply DC voltage to theelectromagnets to create magnetic force. The electromagnets can interact with the permanentmagnets to make the mirror rotate. The X-scan of the 2D scanning module can achieve a staticoptical scan angle of ~11.8 degrees at the -X corners, and the corresponding Y-scan angle is~4.5 degrees, both with 12 VDC. Moreover, we have observed a fan-shaped distortion, aphenomenon not thoroughly studied previously for combining two single-axis scan mirrors.Therefore, we also perform simulation to establish and demonstrate a correlation between thesimulation prediction and experimental results. The 2D scanning module can be a low-costalternative to the expensive conventional galvanometer scanners, and it can be used to upgradea rangefinder to a simplified LiDAR.

Intensity Correlation Imaging Design for Geostationary Satellite Inspection

David Hyland

DOI: 10.1364/AO.520963 Received 05 Feb 2024; Accepted 08 Apr 2024; Posted 09 Apr 2024  View: PDF

Abstract: Recent advances in the reduction of the integration time required of IntensityCorrelation Imaging have opened the possibility of significant improvements in astronomicalimaging. This paper discusses the application of ICI to the fine resolution imaging ofgeostationary satellites conducted by ground-based observatories.

Multilayer lensless camera for improving the condition number

Tomoya Nakamura, Reina Kato, Kazuya Iwata, Yasushi Makihara, and Yasushi Yagi

DOI: 10.1364/AO.521126 Received 12 Feb 2024; Accepted 05 Apr 2024; Posted 05 Apr 2024  View: PDF

Abstract: Computational lensless imaging is a technology that realizes the thinning of the optical system by replacing optical imaging with image reconstruction processing.While the conventional optical design used a single coded mask and an image sensor, recent studies have proposed optical designs that use multiple stacked coded apertures for multidimensional imaging and wide-field imaging.In this study, we investigate the effect of the multi-layering of the coded aperture on the performance of two-dimensional spatial imaging.We demonstrate through simulations and optical experiments that the multi-layering of the coded aperture improves the condition number of the transmission matrix of the optical system, and as a result, improves the accuracy of image reconstruction in lensless imaging.

Thermal active optical technology to achieve in-orbit wavefront aberration correction for optical remote sensing satellites

xiaoyi zheng, Shikai Zan, Xueying Lv, fan zhang, and Liu Zhang

DOI: 10.1364/AO.517834 Received 05 Jan 2024; Accepted 02 Apr 2024; Posted 02 Apr 2024  View: PDF

Abstract: Image quality and resolution are important factors affecting the application value ofremote sensing images. Although increasing the optical aperture of space optical remote sensors(SORSs) improves image resolution, it exacerbates the effects of the space environment onimaging quality. Thus, this study proposes thermal active optical technology (TAO) to enhanceimage quality while increasing the optical aperture of SORSs by actively correcting in-orbitwavefront aberrations. Replacing traditional wavefront detection and reconstruction withnumerical calculation and simulation analysis, more realistic in-orbit SORS wavefrontaberrations are obtained. Numerical and finite element analyses demonstrate that nonlinearitiesin TAO control lead to the failure of traditional wavefront correction algorithms. To addressthis, we use a neural network algorithm combining CNN and ResNet. Simulation results showthat the residual of the systematic wavefront RMS error for SORS reduces to 1/100 λ. The staticand dynamic modular transfer functions are improved, and the structural similarity index isrecovered by over %, highlighting the effectiveness of TAO in image quality enhancement.The static and thermal vacuum experiments demonstrate the wide applicability and engineeringprospects of TAO.

Modified Failproof Physics-Informed Neural Network Framework for Fast and Accurate Optical Fiber Transmission Link Modeling

Joshua Uduagbomen, Mark Leeson, Zheng Liu, Subhash Lakshminarayana, and Tianhua Xu

DOI: 10.1364/AO.524426 Received 25 Mar 2024; Accepted 31 Mar 2024; Posted 08 Apr 2024  View: PDF

Abstract: Physics-informed neural networks (PINNs) have recently emerged as an important and ground-breakingtechnique in scientific machine learning for numerous applications including in optical fiber communications. However, the vanilla/baseline version of PINNs is prone to fail under certain conditions because ofthe nature of the physics-based regularization term in its loss function. The use of this unique regularization technique results in a highly complex non-convex loss landscape when visualized. This leads tofailure modes in PINN-based modeling. The baseline PINN works very well as an optical fiber modelwith relatively simple fiber parameters and for uncomplicated transmission tasks, but struggles, whenthe modeling task becomes relatively complex, reaching very high error, for example, numerous modeling tasks/scenarios in soliton communication and soliton pulse development in special fibers such asErbium-doped dispersion compensating fibers. We implement two methods to circumvent the limitationscaused by the physics-based regularization term to solve this problem, namely, the so-called Scaffoldingtechnique for PINN modeling and the Progressive Block-Learning PINN modeling strategy to solve thenonlinear Schrödinger equation (NLSE), which models pulse propagation in an optical fiber. This helpsPINN learn more accurately the dynamics of pulse evolution and increases accuracy by two to three ordersof magnitude. We show in addition that this error is not due to the depth or architecture of the neuralnetwork but a fundamental issue inherent to PINN by design. The results achieved indicate a considerablereduction in PINN error for complex modelling problems, with accuracy increasing by up to two orders ofmagnitude.

Versatile Luminescence Thermometry via Intense Green Defect Emission from an Infrared-Pumped Fluorosilicate Optical Fiber

Alexander Pietros, Kacper Rebeszko, Jacob Rosenbaum, Miranda Stone, Thomas Hawkins, Maxime Cavillon, John Ballato, and Peter Dragic

DOI: 10.1364/AO.514785 Received 26 Dec 2023; Accepted 24 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: An all-glass optical fiber capable of two distinct methods of optical thermometry isdescribed. Specifically, a silica-clad, barium fluorosilicate glass core fiber, when pumped inthe infrared, exhibits visibly intense green defect luminescence whose intensity and upper-statelifetime are strong functions of temperature. Intensity-based optical thermometry over the rangefrom 25°C to 130°C is demonstrated, while lifetime-based temperature sensitivity is shownfrom 25°C to 100°C. Time-domain measurements yielded a relative sensitivity of 2.85% K-1 at373 K (100C). A proof-of-concept distributed sensor system using a commercial digital single lens reflex camera is presented, resulting in a measured maximum relative sensitivity of 1.13%K-1 at 368 K (95C). The sensing system described herein stands as a new blueprint for defect based luminescence thermometry that takes advantage of pre-existing, and relativelyinexpensive, optical components and allows for the use of standard cameras or simply directhuman observation.