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Accepted papers to appear in an upcoming issue

Optica Publishing Group posts prepublication articles as soon as they are accepted and cleared for production. See the FAQ for additional information.

NiO Thickness Measurement using Rectangular-Typed Sagnac Interferometer as The Material Transport Layer in a Perovskite Solar Cell

Abdullahi Usman, Apichai Bhatranand, Yuttapong Jiraraksopakun, Khalid Sabo Muhammad, and Prathan Buranasiri

DOI: 10.1364/AO.520596 Received 01 Feb 2024; Accepted 18 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: This work aims to utilize a phase-shifting technique in a rectangular-typed Sagnac interferometer (RTSI) tomeasure the thickness of a thin film of nickel (II) oxide (NiO) in an electron transport layer (ETL) in perovskite solarcell preparation. The NiO layer is deposited on a fluorine-doped tin oxide (FTO) glass substrate. In the RTSI setup, thesignal output from the interferometer is divided into the reference and testing arms using a nonpolarizing beamsplitter (NPBS). The balanced photodetectors then detect the signal, with the FTO/NiO layer placed in the testing armand pure FTO in the reference arm. By analyzing the signal intensities at polarization settings of 0° to 180°, the phaseshift and thickness of the NiO layer can be determined. The thickness values of FTO and NiO films obtained throughthree different phase-shifting algorithms of three-, four-, and five-steps are calculated. The obtained NiO thicknessvalues are validated against scanning electron microscopy (SEM). Finally, by considering the NiO thickness value thatexhibits the lowest percentage error compared to one from SEM, it is confirmed that the three-stepping algorithm isthe most suitable scheme for obtaining intensities at 0°, 45°, and 90°. Therefore, the proposed setup shows promiseas a replacement for SEM in thickness measurements.

A New Analytical Propagation Model for Underwater Free-Space Optical Communication Through Realistic Levels of Oceanic Absorption and Scattering

Anthony Davis, Lewis Roberts, Samuel Dolinar, and Michael Cheng

DOI: 10.1364/AO.511438 Received 10 Nov 2023; Accepted 17 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: Optical communications (OC) through water bodies is an attractive technology fora variety of applications. Thanks to current single-photon detection capabilities, OC receiversystems can reliably decode very weak transmitted signals. This is the regime where pulseposition modulation is an ideal scheme. However, there has to be at least one photon thatgoes through the pupil of the fore optics and lands in the assigned time-bin. We estimatethe detectable photon budget as function of range for propagation through ocean water, bothopen and coastal. We make realistic assumptions about the water’s inherent optical properties,specifically, absorption and scattering coefficients, as well as the strong directionality of thescattering phase function for typical hydrosol populations. We adopt an analyical (hence very fast)path-integral small-angle solution of the radiative transfer equation for multiple forward-peakedscattering across intermediate to large optical distances. Integrals are performed both along thedirectly-transmitted beam (whether or not it is still populated) and radially away from it. We usethis modeling framework to estimate transmission of a 1-Joule pulse of 532 nm light throughopen ocean and coastal waters. Thresholds for single-photon detection per time bin are a few kmand a few 100 m respectively. These are indicative estimates that will be reduced in practice dueto sensor noise, background light, turbulence, bubbles, and so on, to be included in future work.

Reducing noise in polarization-sensitive optical coherence tomography for high-quality local phase retardation imaging

Guoqiang Chen, wenai wang, and Yanqiu Li

DOI: 10.1364/AO.515942 Received 13 Dec 2023; Accepted 17 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: Local phase retardation (LPR) is increasingly recognized as a crucial biomarker forassessing disease progression. However, the presence of speckle noise significantly challengesits accuracy and polarization contrast. To address this challenge, we propose a signal14 processing strategy aimed at reducing the impact of noise on LPR measurements. In thisapproach, the LPR is reconstructed by polar decomposition after averaging multiple Muellermatrices from different overlapping sub-spectra. To optimize measurement accuracy, wesystematically combined and traversed different sub-spectral numbers and bandwidths. Byexamining the quarter-wave plate and glass slide, high-accuracy phase retardationmeasurements were successfully verified, and the maximum polarization contrast wasimproved by %. Moreover, experimental results from multi-tissue imaging vividly illustratethat the equivalent number of looks (ENL) and polarization contrast were improved by 18%and 19%, respectively. This outcome indicates that our proposed strategy can effectively reducethe noise spikes, enhancing tissue discrimination capabilities.

Calibration for endoscopic 3D shape measurement with cone beam projection

Yuta Takahashi, Takeshi Higashiguchi, and Toshitaka Wakayama

DOI: 10.1364/AO.516747 Received 21 Dec 2023; Accepted 17 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: We demonstrate a calibration method for endoscopic three-dimensional shape measurement with cone beam projection.In this method, changes in the shape of the optical sectioning profiles are quantified and fitted while scanning a calibrationboard in the depth direction, using a cubic function. In accuracy tests using a flat plate and a ring reference gauge, theproposed method obtains an accuracy of 0.02 mm for in the depth dimension and 0.09 mm in the radial dimension. Theseresults represent 88% and 55% improvements compared to previous analysis. For medical applications, an earexamination simulator was employed, and our measurement results were compared to ground truth data obtained bymicrofocus X-ray computed tomography. The surface deviation of our method relative to the ground truth data was ±0.36mm during manual operation. A comparison of the measurement results before and after calibration revealed animprovement in peak agreement with ground truth data, with the deviation shifting from 0.2 mm to −0.05 mm. Ourstrategy achieves digital transformation of 3D endoscopy, which would benefit a number of medical fields.

Optical system design of a femi-class ultra-high resolution spectrometer based on the quaternary dispersion of a middle echelon grating

Qingsheng Xue, Junhong Song, Fengqing Lu, Jun Ma, and Diansheng Cao

DOI: 10.1364/AO.521955 Received 19 Feb 2024; Accepted 17 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: In order to meet the needs of the femmi ultra-high spectral resolution test, an optical system of the Femmi ultra-highspectral resolution spectrometer in the spectral range of 190-800nm is designed based on the quaternary dispersion ofthe middle echelon grating under the condition that the volume and weight of the spectrometer do not increase sharply.After optimization design, the spectral resolution can reach 51.149fm in the full field of view of 0.5mm, at the wavelengthof 191nm, the spectral resolution in the whole spectral range is better than 150fm, and the maximum distortion of thesystem is 0.2288%, which can provide a feasible reference for the subsequent design of the spectrometer optical systemto realize the simultaneous detection of wide band and high spectral resolution.

Multifunctional 2-bit Coded Reconfigurable Metasurface Based on Graphene-Vanadium Dioxide

Binggang Xiao, Zhonghua Wei, Panpeng Ge, Xinyi Wang, Lihua Xiao, Jianyuan Qin, D. P. Zhang, Hongmei Mi, and Jiabin Yu

DOI: 10.1364/AO.515723 Received 11 Dec 2023; Accepted 17 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: In this paper, a graphene-vanadium dioxide-based reconfigurable metasurface unitstructure is proposed. Using the change in graphene Fermi energy level on the surfaceof the unit structure to satisfy the 2-bit coding condition, four reflection units with aphase difference of 90 can be discovered. The modulating impact of the multi-beamreflection wave with 1 bit coding is then confirmed. Then we study the control ofsingle beam reflected wave by metasurface combined with convolution theorem in 2bit coding mode. Finally, when vanadium dioxide is in an insulating condition, thestructure can also be transformed into a terahertz absorber. It is possible to switchbetween a reflection beam controller and a terahertz multifrequency absorber simplyby changing the temperature of the vanadium dioxide layer without retooling a newmetasurface. Moreover, compared with 1-bit coded metasurface, it increases theability of single beam regulation, which makes the device more powerful for beamregulation.

A simple DFS and AOA simultaneous measurement scheme without directional ambiguity with co-frequency self-interference signal cancellation

Yuan Fang and Jianxin Ma

DOI: 10.1364/AO.519921 Received 24 Jan 2024; Accepted 16 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: A photonic method based on a dual-polarization dual-parallel Mach–Zehndermodulator (DPol-DPMZM) for simultaneous measurement of the Doppler frequency shift(DFS) and angle of arrival (AOA) of microwave signals is proposed and demonstrated bysimulation. The upper arm of each sub-DPMZMs is driven by the echo and self-interferencesignals from the antenna while the lower arm is driven by the reference signal 1 and referencesignal 2. The phase and amplitude of the reference signal 1 are adjusted to match the interferencesignals for achieving the self-interference cancellation (SIC). At central office (CO), DFS andAOA can be acquired in real time without directional ambiguity by processing thedownconverted two low-frequency tones in the photocurrent. Simulation results show that thepresence of the SI signal will seriously interfere with the observation of the SOI signalfrequency and waveform, and the self-interference cancellation depth of about 42 dB can beobtained after the SIC. The measurement errors of the DFS without direction ambiguity arewithin 0.2 Hz. After Hilbert transforming the intermediate frequency (IF) signal, the AOA canbe measured from -87.31° to +87.31° with errors less than 3.9°. The system has a largebandwidth, excellent real-time performance, better invisibility, and is expected to be used inmodern electronic warfare systems.

All-optical instantaneous RF signal frequency measurement system based on a linear ACF with a steep slope

KUN ZHI, Chongjia Huang, Erwin Chan, peng hao, and Xudong Wang

DOI: 10.1364/AO.519808 Received 23 Jan 2024; Accepted 14 Mar 2024; Posted 15 Mar 2024  View: PDF

Abstract: A new microwave photonic structure for measuring the frequency of an RF signal is presented. The frequency of an unknown RF signal can be determined by simply measuring the system output optical powers. The proposed frequency measurement system can be designed so that the ratio of the two system output optical powers as a function of the RF signal frequency or the amplitude comparison function (ACF) has a steep linear slope over a wide frequency range. This enables the RF signal frequency to be measured in high resolution and high accuracy. The proposed frequency measurement system has a simple and compact structure, and is free of high-speed photodetectors as well as electrical components and instruments. It also has a fast response time compared to many reported photonics-based frequency measurement systems. A proof-of-concept experiment is carried out. Experimental results show a linear ACF with a slope of more than 4.4 dB/GHz over a frequency measurement range of 5 to 26 GHz and a frequency measurement accuracy of better than ±0.1 GHz.

Experimental Saturation Characteristics of PMT andAdaptive Compensation Algorithm for OWC

Weijie Liu, Weihao Yuan, Nuo Huang, and Zhengyuan Xu

DOI: 10.1364/AO.516465 Received 18 Dec 2023; Accepted 13 Mar 2024; Posted 13 Mar 2024  View: PDF

Abstract: Orthogonal frequency division multiplexing (OFDM) utilizes numerous sub-carriers to achieve hightransmission data rates. The frequency selectivity of the channel becomes a crucial factor influencingthe communication performance of OFDM-based systems. In optical wireless communication (OWC)systems, the photomultiplier tube (PMT) may experience saturation when the incident optical powerapproaches its saturation threshold. This paper, for the first time, characterizes the saturation levels of ahigh-speed PMT based on the measured amplitude in the time domain and the output response of the PMTin the frequency domain. Additionally, an adaptive optical saturation compensation algorithm, leveragingan electronically controlled variable optical attenuator, is proposed to realize a reliable OWC system.Experimental results demonstrate that the proposed saturation compensation method achieves a highertolerance to large dynamic signal and background radiation compared with that without compensation,while maintaining a satisfactory bit error rate.

4.4 kW Nd:YAG slab amplifier with high repetition frequencyand high beam quality

Chenggong Zhang, Dong Xueyan, xingwei yan, Zilei Liu, Quanhao Chen, xin li, Fengtu Yang, and Haozhu Wang

DOI: 10.1364/AO.520472 Received 31 Jan 2024; Accepted 13 Mar 2024; Posted 14 Mar 2024  View: PDF

Abstract: Using the self-developed fused indium wetting technology andplanar waveguide, the uniform heat dissipation of the slab crystaland the uniform pumping of the pump light were achieved,respectively. Based on the Master Oscillator Power Amplification(MOPA) scheme, the power was then amplified when the seedlight source passed through the Nd:YAG slab crystal three times.Additionally, the image transfer system we added to theamplified optical path achieved high beam quality. Finally, weobtained a rectangular pulsed laser with an output averagepower of 4461 W, a repetition frequency of 20 kHz, a pulse widthof 62 ns, an optical-to-optical conversion efficiency of 26.8% andbeam quality of βx = 7.0 and βy = 7.7.

Fabrication of high-quality, small blaze anglegrating for visible short-wave infraredhyperspectral cameras

TI YUNZAN, xiaotao mi, jingxuan zhou, sibo jiang, pengyuan chen, shuo li, zhiyi wang, and tingyu wang

DOI: 10.1364/AO.516926 Received 27 Dec 2023; Accepted 12 Mar 2024; Posted 13 Mar 2024  View: PDF

Abstract: This study outlines the development of a low line density, small blaze angle grating,optimized for a visible to short-wave infrared hyperspectral camera. An analysis of gratingspecifications was conducted to meet the precise requirements of this application, particularlyfocusing on the stringent tolerance limits for the blaze angle. A specialized ruling tooladjustment device was designed to adhere to these exacting blaze angle tolerances. The gratinggroove shape was examined using atomic force microscopy (AFM), and the theoreticaldiffraction efficiency of the grating was calculated based on these observations. Additionally,laser-based methods were employed to measure the actual diffraction efficiency of the grating,while interferometry was used to assess the grating's diffraction wavefront. The test resultsdemonstrate our capability to fabricate high-quality gratings with low line density and smallblaze angles, suitable for advanced hyperspectral imaging applications.

Algorithm and Software for Field Distortion Correction in a Commercial SD-OCT for Corneal Curvature Assessment

Maron Dolling, Lara Buhl, Gereon Hüttmann, Reginald Birngruber, and Hinnerk Schulz-Hildebrandt

DOI: 10.1364/AO.505107 Received 05 Sep 2023; Accepted 12 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: Accurate assessment of corneal curvatures using frequency domain optical coherencetomography (OCT) with galvanometer scanners remains challenging due to the well-known scanfield distortion. This paper presents an algorithm and software for correcting the distortion usingonly two simple measurements in which a readily available standard sphere is positioned in differentdepths in front of the OCT scanner. This offers a highly accessible and easily reproducible methodfor the field distortion correction (FDC). The correction was validated by measuring differentspherical phantoms and conducting corneal curvature measurements of ex vivo porcine corneasusing a commercial spectral-domain OCT system and a clinically approved swept-source OCT as a reference instrument. Thus, the error in radius measurements of spherical phantoms was reducedby >90% and astigmatism by >80% using FDC. In explanted porcine eyes, the error in astigmatism measurements with the Telesto was reduced by 75% for power and 70% for angle. The best fitting sphere radius was determined up to a deviation of 0.4% from the Anterion. This paper describes a new correction algorithm for OCT immanent distortion that is applicable to any scanning OCT setup and enables precise corneal curvature measurements. The MATLAB software for the FDC is publicly available on GitHub: https://github.com/MaDolling/OCT_Field_Distortion_Correction.

Effect of gamma-irradiation on thephotoluminescence of silver-doped lithiumtriborate glass

Volodymyr Adamiv, Yaroslav Burak, Natalia Volodko, Ulyana Dutchak, Taras Izo, Ihor Teslyuk, and Andriy Luchechko

DOI: 10.1364/AO.514966 Received 05 Dec 2023; Accepted 12 Mar 2024; Posted 15 Mar 2024  View: PDF

Abstract: The optical-luminescence properties, including absorption spectra,photoluminescence excitation spectra, and changes in photoluminescence spectra afterirradiation with γ-rays in the dose range of 1-3 Gy, were measured for novel LiB3O5:Agdosimeter glass. A clear dependence of the intensity of the photoluminescence (PL), with amaximum of approximately 300 nm, on the dose value when excited by light with λexc = 220nm was observed. The mechanism of PL in γ-irradiated LiB3O5:Ag glass is proposed to be aconsequence of radiation annihilation with the emission of relaxed exciton-like electronicexcitations with the participation of impurity defects (Ag0) in the glass structure. Consideringthat LiB3O5 has an effective atomic number of Zeff = 7.39, which is the closest to Zeff = 7.42 ofhuman body tissue, LiB3O5:Ag glass can be very promising for γ-dosimetry in medical practiceduring radiation therapy of patients with cancer. The presence of lithium Li(6) and boron B(10)isotopes increases the sensitivity of LiB3O5:Ag glass to neutrons, which is promising for themanufacture of individual neutron dosimeters for applications in a new field of radiationtherapy - neutron therapy.

Estimation of Modified Zernike Coefficients fromTurbulence-Degraded Multispectral Imagery using DeepLearning

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

DOI: 10.1364/AO.521072 Received 06 Feb 2024; Accepted 12 Mar 2024; Posted 15 Mar 2024  View: PDF

Abstract: We investigate how wavelength diversity affects the performance of a deep learning model that predictsthe modified Zernike coefficients of turbulence-induced wavefront error from multispectral images. Theability to perform accurate predictions of the coefficients from images collected in turbulent conditionshas potential applications in image restoration. The source images for this work were a point objectand extended objects taken from a character-based dataset and a wavelength-dependent simulation wasdeveloped that applies the effects of isoplanatic atmospheric turbulence to the images. The simulationutilizes a phase screen resampling technique to emulate the simultaneous collection of each band of amultispectral image through the same turbulence realization. Simulated image data was generated forthe point and extended objects at various turbulence levels and a deep neural network architecture basedon AlexNet was used to predict the modified Zernike coefficients. Mean squared error (MSE) resultsdemonstrate a significant improvement in predicting modified Zernike coefficients for both the pointobject and extended objects as the number of spectral bands is increased. However, the improvement withthe number of bands was limited when using extended objects with additive noise.

A free-space coupled, large-active-areasuperconducting microstrip single-photon detectorfor photon-counting time-of-flight imaging

Yuze Wang, Weijun Zhang, XINGYU ZHANG, Guangzhao Xu, Jiamin Xiong, CHEN ZHI-GANG, Yiyu Hong, Xiaoyu Liu, PUSHENG YUAN, Ling Wu, Zhen Wang, and Lixing YOU

DOI: 10.1364/AO.519475 Received 18 Jan 2024; Accepted 11 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: Numerous applications at the photon-starved regime require a free-space coupling single-photon detector with a largeactive area, low dark count rate (DCR), and superior time resolutions. Here, we developed a superconducting microstrip singlephoton detector (SMSPD), with a large active area of 260 μm in diameter, a DCR of ~5 kcps, and a low time jitter of ~171 ps,operated at near-infrared of 1550 nm and a temperature of ~2.0 K. As a demonstration, we applied the detector to a single-pixelgalvanometer scanning system and successfully reconstructed object information in depth and intensity using a time-correlatedphoton counting technology.

Light-field photography using differential high-speedaperture coding

Suyi Huang, Qin Yang, Zihao Deng, manhong yao, Zibang Zhang, Xiaoli Liu, Jianping Li, peng junzheng, and Jingang Zhong

DOI: 10.1364/AO.520338 Received 29 Jan 2024; Accepted 11 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: Programmable aperture light-field photography enables acquiring angular information without compromising spatial resolution.However, direct current (DC) background noise is unavoidable in images recorded by programmable aperture light-field photography,leading to reducing the contrast of reconstructed images. In addition, it requires sacrificing temporal resolution to obtain angular information,making it facing challenge to capture dynamic scenes. In this paper, we propose programmable aperture light-field photography usingdifferential high-speed aperture coding. This method effectively reduces DC noise and produces high-contrast refocused images.Furthermore, we build a light-field camera based on a 1250 Hz spatial light modulator and a 1250 fps high-speed camera, achieving dynamiclight-field photography at 1110(H)×800(V) resolution and 24 fps. Our results demonstrate significant improvements in image contrast andexhibit considerable promise for diverse applications.

Depth of focus and intensity distribution of lensacon illuminated by partially coherent Gaussian Schell vortex beam

Abdu Alkelly, Huda Alsarhi, Shukri Kaid, and Hassan Al-Ahsab

DOI: 10.1364/AO.520732 Received 01 Feb 2024; Accepted 11 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: Using the extended Huygens-Fresnel principle, a cross-spectral density formula wasdeveloped for a Gaussian Schell model vortex (PCGSMV) beam diffracted through Lensacon (lenswith axicon). The intensity and depth of focus (DOF) shaped by the lensacon were calculated.Our numerical results show the relationship between the intensity distribution and depth of focuswith the beam waist width as well as the spatial correlation of the coherence length. Furthermore,the relationship between the beam spot size and propagation distance was investigated. In thecase of the lensacon tandem, the maximum intensity was greater than that attained by the axiconalone for the same beam parameters, and the DOF was smaller than that of the axicon alone.The vortex structure canceled out the low value of the spatial degree of coherence length. Ournumerical model exhibited high-intensity values and high-quality Bessel rings along the DOFwhich are critical for various applications.

Generation of elliptic perfect optical vortex beams and their propagation in free-space

Bikash Kumar Das, Camilo Granados, and Marcelo Ciappina

DOI: 10.1364/AO.521826 Received 19 Feb 2024; Accepted 11 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: We investigate the generation of an elliptical perfect optical vortex (EPOV) beam through the optical Fourier transformation of an elliptical Bessel-Gauss beam and derive an analytical expression for its complex field amplitude. Our analysis includes the examination of the beam’s propagation in free-space and the influence of topological charge on its transmission. The EPOV beam’s propagation in free-space can be categorized into non-diffracting and diffracting stages. During the non-diffracting stage, the beam intensity and profile size are effectivelypreserved over a short transmission distance. Subsequently, in the diffracting stage, the beam profile initially contracts and then expands with a change in the beam intensity. Notably, the impact of topological charge on the beam intensity and profile size varies across different stagesof propagation. We compare our results with those of the circular perfect optical vortex beam propagating in free-space, observing that the EPOV beam exhibits non-diffracting behavior forshort propagation distances compared to its circular counterpart. The signature of the focusingbehavior of the EPOV beam is observed at a relatively small propagation distance compared tothe circular case.

Design of highly efficient far-field beamshapers with irregular maskless microlensarrays

Dmitrii Stefanidi, Leo Maximilian Wilhelm, Peter Schreiber, Robert Brüning, and Andreas Tünnermann

DOI: 10.1364/AO.516730 Received 21 Dec 2023; Accepted 10 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: Regular tandem microlens arrays are well described and widely used for beamshaping and homogenization. Applying absorbing slides between the entrance and exit lensletsand channel-wise variation of the slides’ shape and size allows flexible control of the beam’sintensity profile and silhouette. The downside of absorbing slides is a significant transmissionloss, limiting the achievable level of system efficiency. This work describes more efficientmethod for micro-optical beam shaping with maskless irregular microlens arrays (iMLA). TheiMLAs are completely absorption-free elements, enhancing the overall efficiency of the opticalsystem. We describe basic design rules for iMLAs, including stray-light suppression,tolerancing and modelling under consideration of manufacturing imperfections.

Underwater image restoration via adaptive color correction and dehazing

Jiening Zhang, Qing Yu, and Guojia Hou

DOI: 10.1364/AO.514749 Received 08 Dec 2023; Accepted 10 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: Due to the absorbing and scattering effects, underwater images are often degradedby low contrast, color cast and haze, which have limited their further applications to underwatervision task. To address this issue, we propose a hybrid framework by adaptive color correctionand dehazing for underwater image restoration. Specifically, according to the color attenuationprinciple, we first design an adaptive color compensation strategy to correct the color cast ofthe underwater image. In addition, based on underwater image formation model, we develop arobust dehazing algorithm, in which a new scoring formula across three indicators (i.e.,darkness, distance, and blurriness) and a Double-Max DCP method are proposed to estimatethe background light and transmission map, respectively. Experimental results validate that theproposed method is effective in color correction and dehazing. Both qualitative and quantitativecomparisons further demonstrate that the proposed method outperforms several state-of-the-artmethods.

Tuning the parameters of a Free-Space Optical (FSO) channel using Machine Learning.

Zélia Maria Lima, Rogean Silva, Yosdan Camejo, Aderaldo Irineu Araujo, and Glendo Guimarães

DOI: 10.1364/AO.522124 Received 21 Feb 2024; Accepted 10 Mar 2024; Posted 13 Mar 2024  View: PDF

Abstract: The present work use artificial intelligence (AI) methodology to simulate the data transmission processthrough free space optical (FSO) technology. With machine learning procedure, the data is obtainedby multiparametric simulation using the software Optisystem. For the first simulation set, the inputparameters were distance, attenuation, gain in the input signal amplifier and gain in the output signalamplifier. For the second set, the effects of beam divergence and receiver diameter were also evaluated.Additional sets were added to increase the data and characterize the underfitting and overfitting processes.With the data generated, artificial intelligence models were trained using the Decision Tree Regression(DTR), Random Forest Regression (RFR), Gradient Boosting Regressor (GBR), Histogram GradientBoosting Regression (HGBR) and AdaBoost + Decision Tree Regression (ADDTR). The results showedthat for the first scenario the models (DTR) and (RFR) showed an excellent estimate for the maximumquality factor (MaxQFactor), with a value of the coefficient of determination R2 above 95.00 %, and,for the second scenario, the algorithms (DTR) and (RFR) also have shown excellent results, with R2above 94.00%. The results obtained from the artificial intelligence procedures were compared graphicallywith the values obtained by multiparametric numerical simulation, confirming the effectiveness of themethodology used to predict the output values of the FSO channel.

Near-Infrared Reflecting/Transmitting Smart Windows Based on Chiral Nematic Liquid Crystals with Enhanced Electrically Switching Performance

MEHDI JAFARIMOGHADAM, Mohammad Mohammadimasoudi, MOHSEN ZALI, and KAVEH YAZDI

DOI: 10.1364/AO.516346 Received 18 Dec 2023; Accepted 09 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: This paper addresses global warming concerns stemming from energyconsumption, particularly in buildings, which contribute 40% to global energy use. Smartwindows that reflect near infrared (NIR) radiation have emerged as a solution to reduce indoortemperatures. Chiral nematic liquid crystals (CLCs) play a crucial role in this technology.Numerous approaches have been explored for regulating indoor temperatures using liquidcrystals (LCs). Despite achieving ideal transparency, rapid switching speeds, negligible powerconsumption, and user control over switching, reported samples often face challenges whenattempting to revert from either the focal conic state or the transmitting state back to the initialreflecting state. In this work, for the first time to our knowledge, CLC cells with electricallyreversibility are visually demonstrated rapidly switching between reflective and transmittingmodes. Cell thickness emerged as a pivotal factor in achieving smart window reversibility,with 3 micrometers identified as the optimal choice. samples exhibited effective IR reflection,high visible transparency, and complete reversibility, marking a significant step towardpractical smart windows to combat global warming.

Dynamic control of defocus, astigmatism and tilt aberrations with a large area foveal liquid crystal lens

Tigran Galstian and Louis Begel

DOI: 10.1364/AO.517797 Received 14 Jan 2024; Accepted 09 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: We have recently reported the dynamic adjustment of the focal length in an electrically tunable liquid crystal "foveal" lens the center of which can be shifted over a large working area. In the present work, we show that this design allows also independently generating astigmatism with arbitrary axis and tilt of light wavefront by simply changing the phase and the voltage differences between 4 control electrodes. Furthermore, we also demonstrate the capability of generating highly localised negative (defocusing) lenses with the same device by using a dual frequency liquid crystal.

Efficiency Enhancement of Ultrathin CIGS Solar Cells by Optimal Bandgap Grading. Part III: Piecewise-Homogeneous Grading

Faiz Ahmad, Peter Monk, and Akhlesh Lakhtakia

DOI: 10.1364/AO.516850 Received 21 Dec 2023; Accepted 09 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: In Parts I [Appl. Opt. 58, 6067 (2019)] and II [Appl. Opt. 61, 10049 (2022)], we used a coupled optoelectronicmodel to optimize a thin-film CIGS solar cell with a graded-bandgap photon-absorbing layer, periodicallycorrugated backreflector, and multilayered antireflection coatings. Bandgap grading of the CIGS photonabsorbing layer was continuous and either linear or nonlinear, in the thickness direction. Periodiccorrugation and multilayered antireflection coatings were found to engender slight improvements inthe efficiency. In contrast, bandgap grading of the CIGS photon-absorbing layer leads to significantenhancement of efficiency, especially when the grading is continuous and nonlinear. However, practicalimplementation of continuous nonlinear grading is challenging compared to piecewise-homogeneousgrading. Hence, for this study, we investigated piecewise-homogeneous approximations of the optimallinear and nonlinear grading profiles, and found that an equivalent efficiency is achieved using piecewisehomogeneous grading. An efficiency of 30.15% is predicted with a 3-layered piecewise-homogeneous CIGSphoton-absorbing layer. The results will help fellow experimentalists to implement optimal designs forhighly efficient CIGS thin-film solar cells.

3D nanoprinted catadioptric fiber sensor for dual-axis distance measurement during vitrectomy

Florian Lux, Aybuke Calikoglu, Carolin Klusmann, Matthias Hillenbrand, and Çağlar Ataman

DOI: 10.1364/AO.518950 Received 28 Jan 2024; Accepted 08 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: Retinal damage is a common intraoperative complication during vitrectomy, causedby a complex interplay between the suction of the vitrectome, the cut- and aspiration rate, and thedistance of the instrument to the retina. To control this last factor, we developed two miniaturizedfiber-optic distance sensors based on low-coherence interferometry for direct integration intothe vitrectome. Both sensors have a diameter of 250 µm, which makes them compatible with a25G vitrectome. The first sensor measures distance in the lateral direction. The second sensor iscapable of simultaneously measuring distance in both the lateral and the axial direction. Axialand lateral directions correspond to the direction of the cutter port of the vitrectome and thedirection along the vitrectome’s shaft, respectively. In both sensors, a free-form mirror deflectsand focuses the beam in the lateral direction. In the dual-axis distance sensor, an additionallens is integrated into the free-form mirror for distance measurement in the axial direction. Thebeam-shaping micro-optics at the tip of the sensor fibers were fabricated through two-photonpolymerization, and are selectively gold coated for increased reflectivity of the mirror. Distancemeasurements were successfully demonstrated in artificial samples and in ex-vivo pig eyes witha back-end that uses a current-tuned VCSEL as a swept-source. We experimentally demonstratethat the complete sensor system can attain a SNRmax of up to 80 dB. The small dimensions of thedeveloped sensors make them a potential solution for various other medical applications.

Polarization-sensitive optical coherencetomography for birefringence measurementof calcite nonlinear uniaxial crystal

Kosar Kasmaee, mohammad dehshiry, and Alireza Khorsandi

DOI: 10.1364/AO.518550 Received 12 Jan 2024; Accepted 08 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: We report on the new application of a time-domain optical coherence tomography(TD-OCT) device used to measure the ordinary no and extraordinary ne indices of calcitebirefringence crystal at room temperature. A 1.25±0.05-mm-thick slab of calcite crystal is cut,polished and used as a sample in the OCT arm. While the calcite slab is axially scanned, theraw carrier ordinary signals came from its front and rare facets are received and denoised witha set of digital filters. The extraordinary signals are generated by the change of beampolarization using a 90-rotating polarizer plate. It is found that the wavelet transform iscapable of reaching the highest signal-to-noise ratio (SNR) of about 24.50 and .91 fordenoising the ordinary and extraordinary signals, respectively. Quantitative measurement ofno and ne is carried out by extracting a desired envelope from the denoised signals usingstandard methods. An average values of 1.660 and 1.444 are obtained for no and ne,respectively, using the wavelet-denoised signals. The weight of the results are finally searchedwith ones obtained from two sets of dispersion equations. We found a very good agreementbetween the wavelet-denoised OCT- and dispersion equation-based values with a very lowrelative differences of 0.04% and 2.8% for no and ne, respectively, when Ghosh equation isused and averaged ones of 1.3% and 4.2% for no and ne, respectively, when Zhao et al equationis applied.

All-solid-state ultrafast 2 μm laser with 1.83 W outputpower

Jiajia Mao, ping hu, Xue Zhou, Baitao Zhang, Tao Li, Tianli Feng, Jingliang He, and Kejian Yang

DOI: 10.1364/AO.521887 Received 18 Feb 2024; Accepted 08 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: In this paper, a high-power all-solid-state ultrafast 2 μmmode-locked laser is investigated. The particularity of thislaser is the simultaneous utilization of two Tm:YAPcrystals in the same resonant cavity, independentlypumped by two laser diodes. Using a 20% output coupler,pulses with output power as high as 1.83 W are achievedat a wavelength of 1938 nm with a pulse duration of1.97 ps and a pulse repetition frequency of 100 MHz. Toour knowledge, this mode-locked laser achieves thehighest output power of any mode-locked Tm:YAPultrafast laser reported to date. In addition, this paperprovides a new approach to solve the problem of lowoutput power due to multi-mode low-brightness laserdiode pumping.

A method for optical proximity correctionbased on a vector imaging model

Ruixuan Wu, Lisong Dong, and Yayi Wei

DOI: 10.1364/AO.518578 Received 10 Jan 2024; Accepted 07 Mar 2024; Posted 07 Mar 2024  View: PDF

Abstract: Optical proximity correction (OPC) has become an indispensable step in integratedcircuit manufacturing. It requires a huge amount of calculation to obtain a sufficiently accurateOPC model and implement pattern correction. In this paper, the authors proposed an edgebased OPC (EBOPC) method based on a vector imaging model, where the analytical correlationbetween the cost function and movement of each edge segment is established by chain rule.Firstly, the mask pattern is segmented and down-sampled to get the mask image in order toreduce the total data. Secondly, the aerial image, various parameters on each evaluating pointsand the final cost value are obtained in proper sequence. In each part of the OPC process, theprocedures of solution and derivation are both recorded. After obtaining the cost value, thechain rule is applied, by which the differential relation between the cost value and movementof each segment is built. According to this differential relation, the next movement of eachsegment is decided under a quasi-Newton method. All results obtained by the proposed methodare compared with results from commercial software. The comparison shows that the proposedOPC method has well OPC accuracy in few iterations.

Scanning Defocussing Particle Tracking for the Experimental Characterization of Flows in Demanding Microfluidic Systems

Quentin Galand, David Blinder, Pierre Gelin, Dominique Maes, and Wim De Malsche

DOI: 10.1364/AO.515604 Received 13 Dec 2023; Accepted 07 Mar 2024; Posted 08 Mar 2024  View: PDF

Abstract: A novel scanning PIV technique is proposed to characterize flows in microfluidic applications. Three-dimensional information is acquired by oscillating the target sample over a fixed focal plane, allowing the reconstruction of particle trajectories with micron accuracy over an extended depth. This technology is suited for investigating acoustic flows with unprecedented precision in microfluidic applications. In this contribution, we describe the experimental setup and the data processing pipeline in detail, we study the technique's performance by reconstructing pressure-driven flow, and we report the three-dimensional trajectory of a 2 µm particle in an acoustic flow in a 525 µm x 375 µm microchannel with micrometric accuracy.

A Power-Modulated Integrated-Path DifferentialAbsorption Lidar for Probing Benzene Concentration

Jiahui Liu, Shichun Li, Yi Wang, Si Yu Guan, Dengxin Hua, Wenhui Xin, and Yuehui Song

DOI: 10.1364/AO.518447 Received 09 Jan 2024; Accepted 06 Mar 2024; Posted 07 Mar 2024  View: PDF

Abstract: Aimed at the regional open-path detection of benzene (C6H6) in the atmosphere, a power-modulated integrated-pathdifferential absorption (PM-IPDA) lidar is introduced and demonstrated. Two tunable interband cascade lasers(ICLs) with about 3.2 μm wavelength are utilized to generate the required PM optical signal. These two operationcentral wavelengths (CWs) of the PM-IPDA lidar are respectively 3 6.6 nm and 3187.1 nm, which can mitigate theinfluence of significant gases such as H2O, CH4, and HCl on the detection performance. In this work, the fast Fouriertransform (FFT) algorithm is used to retrieve the measured values with the time resolution of 0.1s corresponding to104 sampling bins at the sampling rate of 100 kSps/s. The modulated frequency of the PM-IPDA lidar is selected as10 kHz by laboratory experiments. The slow fluctuation characteristic of the benzene absorption spectrum withinthe vicinity region of 3.2 μm reduces the impact of small wavelength fluctuations on the performance of PM-IPDAlidar, although a scheme modulated only the driving current causes wavelength fluctuations of approximately ±0.2nm. These laboratory experiments also indicate the PM-IPDA lidar can reduce the error resulted from 1/f noise.Open-path observation experiments show that the detection limit is about 0.60 mg·m-3 and that the PM-IPDA lidarcan be used for the regional open-path real-time detection of benzene.

Fluorescence modulation of quantum dots in subsurface defects of optical elements by a linearly polarized light

Wen He, Yajie Wu, Yana Cui, Chunyang Wang, Xuelian Liu, and Bo Xiao

DOI: 10.1364/AO.510388 Received 31 Oct 2023; Accepted 06 Mar 2024; Posted 07 Mar 2024  View: PDF

Abstract: The limited excitation efficiency of quantum dots in the detection of subsurface defects in optical elements by quantum dot fluorescence gives rise to insufficient accuracy. To enhance the excitation efficiency of quantum dots, we studied the modulation of the polarization direction of linearly polarized incident light on quantum dot fluorescence. We first apply density matrix evolution theory to study the quantum dots interacting with linearly polarized incident light and emitting fluorescence. The fluorescence intensity exhibits cosine oscillations versus modulated laser polarization. It reaches a maximum value at the polarization angle zero, and then decreases as the angle becomes larger until $\pi/2$. The experimental results for quantum dot in both solutions and subsurface defect of optical elements confirmed these results. For optical elements tagged with CdSe/ZnS quantum dots, the fluorescence intensity increases by 61.7\%, and the area for the detected subsurface defects increases by 142.9\%. Similarly, for C and InP/ZnS quantum dots, there are also increases in both fluorescence intensity and the area of subsurface defects. Our study suggests that the subsurface defect detection in optical elements by the linearly polarized incident light could enhance the detection accuracy of subsurface defects in optical elements, and potentially achieve super-resolution imaging of subsurface defects.

Surface profile measurement and parametersanalysis of silicon wafer in the upright state

Ying Yang, Sen Han, Linghua Zhang, and Yuhang Shen

DOI: 10.1364/AO.515440 Received 07 Dec 2023; Accepted 06 Mar 2024; Posted 07 Mar 2024  View: PDF

Abstract: A novel approach is presented for assessing silicon wafer surface profiles using aninterferometer and vertically rotatable wafer holder. This approach significantly enhancesprecision and reduces costs, and outperforms traditional techniques in measurement consistencyand accuracy. It effectively reduces sample distortion and positional shifts owing to the removaland reinstallation of the wafers. Using this method, the global backsurface-referenced idealrange (GBIR) of 0.385μm, Warp of 0.193μm, and other parameters were obtained,demonstrating its practicality in efficiently capturing key surface profile metrics for siliconwafers. This innovation promises substantial improvements in high-volume wafer surfaceprofile testing, overcoming prevalent technological challenges in this industry.

Denoised single-pixel imaging in a Fourieracquisition mode

Pengcheng Xia, Le Wang, and Shengmei Zhao

DOI: 10.1364/AO.515719 Received 18 Dec 2023; Accepted 06 Mar 2024; Posted 07 Mar 2024  View: PDF

Abstract: The degradation of imaging quality caused by environmental noise during Fouriersingle pixel imaging (FSPI) is a big problem. In the paper, we propose simple and efficientdenoised single-pixel imaging schemes by using linear filters to reduce the impact. Two filters,such as average filter and Gaussian filter are employed, and their corresponding schemes arenamed SCH-A, SCH-G, respectively. The experimental and simulation results show that bothschemes can effectively reduce the impact of environmental noise, and have greater robustness incomparison with those using the deringing SPI and conventional SPI. Compared with SCH-A, thereconstructed image by SCH-G keeps more details and edges. With the increase of template size,SCH-A and SCH-G have better filtering effects with the same variance. Meanwhile, the largerthe template size is, the better filtering effect SCH-A has. This conclusion is also applicable toSCH-G under the same variance.

On-site Calibration Method for a Binocular Vision Sensor based on Phase-shiftAlgorithm

Mingwei Shao, Lingkun Xu, Hailong Ma, and Jianjun Chen

DOI: 10.1364/AO.506693 Received 25 Sep 2023; Accepted 05 Mar 2024; Posted 08 Mar 2024  View: PDF

Abstract: When a binocular vision sensor (BVS) is installed in a narrow space, traditionalcalibration methods are limited as all targets should be placed in more than three different positions.To solve this problem, an on-site calibration method based on the phase-shift algorithm is proposedin our paper. Intrinsic parameters of these two cameras should be first calibrated offline. Series ofphase-shift patterns are projected onto any one target with known three-dimensional information todetermine the relationship between two cameras. The target utilized in our proposed method can beselected arbitrarily, which is suitable to achieve the on-site calibration of BVS, especially inindustrial vibration environments. Experiments are conducted to validate the effectiveness androbustness of our proposed method.

(Zhejiang University) Quantitative diagnosis of out-of-plane displacementderivative with temporal-phase-shift shearography

Zhaorui Xu, jie li, Rongsheng Ba, Liqun Chai, Lan Wu, qiao xu, and Dong Liu

DOI: 10.1364/AO.521159 Received 07 Feb 2024; Accepted 05 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: Shearography, or digital speckle pattern shearing interferometry, is highly sensitive to material strainmeasurements. Among various shearography systems, the temporal-phase-shift Michelson Shearography system isextensively researched for its high-accuracy diagnosis of the out-of-plane displacement derivative. However, thesystem faces difficulties achieving quantitative measurement due to the need to calibrate parameters and controlrandom noise. Therefore, finding solutions to improve system design and performance is essential. This paperfocuses on obtaining more accurate and reliable results for out-of-plane displacement derivatives. Several methodsto calibrate the magnification, the equivalent pixel size and the shear amount are discussed. A temporal-phase-shiftshearography system with an online universal calibration module is designed and tested for performance. Therandom noise and stability are under 1με RMS, and the repeatability is better than 2.75%. Besides, the system'saccuracy is verified by digital speckle pattern interferometry with a 2.5% relative error.

High-speed full-color computer-generated holography using a digital micromirror device and fiber-coupled RGB laser diode

Shuhei Yoshida

DOI: 10.1364/AO.509556 Received 17 Oct 2023; Accepted 05 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: Computer-generated holography (CGH) can be used to display three-dimensional (3D) images and has a special feature that no other technology possesses: it can reconstruct arbitrary object wavefronts. In this study, we investigated a high-speed full-color reconstructionmethod for improving the realism of 3D images produced using CGH. The proposed method uses a digital micromirror device (DMD) with a high-speed switching capability as the hologram display device. It produces 3D video by time-division multiplexing using an optical system incorporating fiber-coupled laser diodes (LDs) operating in red, green, and blue wavelengths. The wavelength dispersion of the DMD is compensated for by multiplying plane waves on the hologram. Fourier transform optics are used to separate the object, conjugate, and zeroth-order light, thus eliminating the need for an extensive 4f system. The resources used in this research, such as the programs used for the hologram generation and the schematics of the LD driver, are available on GitHub.

Angular uniformity improvement of diffractivewaveguide display based on region geometryoptimization

Ziyin Li, Chen Gao, Haifeng Li, Rengmao Wu, and Xu Liu

DOI: 10.1364/AO.515428 Received 06 Dec 2023; Accepted 05 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: Augmented reality (AR) near-eye displays have significantly progressed due to advances in nanostructure fabrication.However, for diffractive waveguide AR displays requiring exit pupil expansion, the angular uniformity of each exit pupilposition still needs to improve. In this paper, an angular uniformity improvement method based on region geometryoptimization is proposed. This optimization method essentially introduces the interaction number of the light with thegrating as one of the variables to manipulate the energy distribution. This distribution is obtained by the rigorous coupledwave analysis (RCWA) method and ray tracing process and is further optimized by a multi-objective genetic algorithm. Amodel is built and the feasibility of the proposed method is verified. The diffractive waveguide system has a 10mm ×10mm exit pupil size at the eye relief of 25mm and a field of view (FOV) of 21°×12°. After optimization, the overall opticalefficiency of the central field and the angular uniformity at the center exit pupil position increased from 0.9% and 66% to3.1% and 80%.

Numerical Analysis and Early Detection ofPlasmodium Falciparum using highperformance plasmonic biosensor withexternal sensing approach

Tazin Fatema, Sumaya Arafin, Md Sohanur Rahman, and Md Walid Hassan

DOI: 10.1364/AO.521186 Received 11 Feb 2024; Accepted 05 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: The authors propose a surface plasmon resonance (SPR) sensor based onphotonic crystal fibers (PCF) using three hexagonal ring lattices. The sensor can detectbiomolecules with maximum wavelength and amplitude sensitivities of ,000 nm/RIUand 1310.93 RIU-1 respectively in the RI range of 1.32 to 1.42. It can detect infected redblood cells with plasmodium falciparum for RI 1.402, 1.373, 1.395 and 1.383 in variousmalaria-infected red blood cell stages, including ring phase, trophozoite phase, andSchizont phase. Furthermore, the sensor will be able to detect biomolecules such as viruses,proteins, DNA/RNA strands, acetone, ethanol, hexane, isopropanol, hexanol, formic acid,allyl cyanide, and others in its’ range. These impressive results and identification capacity,the proposed sensor would benefit the biomaterial field and appropriate for the earlyidentification of malaria disease.

Monolithic 24cm2 flexible triple-junction solarcell encapsulated module based on theipsilateral electrode welding technology

xiaolong luo, Junhua Long, qiangjian sun, zhitao chen, Xiaoxu Wu, Xuefei Li, Menglu Yu, Huyin Zhao, Xiaoming Shen, and Shulong Lu

DOI: 10.1364/AO.518102 Received 09 Jan 2024; Accepted 05 Mar 2024; Posted 07 Mar 2024  View: PDF

Abstract: The inverted metamorphic multi-junction solar cell is anticipated to be widelyapplied in stratospheric flight because of its exceptional properties of flexibility and lightweight.We propose an ipsilateral welding technology based on Ti/Au electrodes to simplify thefabrication process of GaInP/GaAs/InGaAs solar cells and encapsulate large-sized flexible solarcells. After annealing at 200 ℃ for 2 h, the Ti/Au electrode achieved a low specific contactresistivity of 2.9×10-7 Ω·cm2. The performance of ohmic contact remains stable after thethermal cycling tests. The Ti/Au electrode can require less heat input for welding to reduce therisk of microcrack formation of the solar cells. By employment of this electrode, a 24 cm2 solarcell achieved a conversion efficiency of 34.74 %. A flexible solar cell module with an efficiencyof 32.82 % under AM 1.5G illumination was obtained by the ipsilateral electrode weldingtechnology.

Regulating the crystal orientation of vapor-transport-depositedGeSe thin films by post annealing treatment

Shaoqiang Chen, Shiqi Zheng, Deyang Qin, Rui Wang, Yunxin Pan, Guo-En Weng, Xiaobo Hu, Junhao Chu, and Hidefumi Akiyama

DOI: 10.1364/AO.521605 Received 15 Feb 2024; Accepted 05 Mar 2024; Posted 07 Mar 2024  View: PDF

Abstract: Recently, GeSe has emerged as a highly promising photovoltaic absorber materialdue to its excellent optoelectronic properties, nontoxicity, and high stability. Althoughmany advantages make GeSe well suited for thin-film solar cells, the power conversionefficiency of GeSe thin-film solar cell is still much below the theoretical maximumefficiency. One of the challenges lies in controlling the crystal orientation of GeSe toenhance solar cell performance. The two-step preparation of GeSe thin films has notyet been reported to grow along the [111] orientation. In this work, we have studied theeffect of post annealing treatment on the GeSe thin films and the performance of thesolar cells. It was found amorphous GeSe films can be converted into polycrystallinefilms with different orientations by changing the post-annealing temperature. [111]-oriented and [100]-oriented GeSe thin films were successfully prepared on the samesubstrate by optimizing the annealing conditions. With the structure ofAu/GeSe/CdS/ITO cell devices, PCEs of 0.14% and 0.16% were ultimately achieved.

Tight focusing of vector optical field with polarization varying along complex curves of Poincaré sphere

Jiaqi lv, Jin-Song Li, Ji-Xiang Guo, Ling-Fei Zheng, and Shuo Liu

DOI: 10.1364/AO.519214 Received 16 Jan 2024; Accepted 04 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: Different from the scalar optical field with spatially uniform polarization, the vector optical field exhibits inhomogeneous distribution of polarization on the cross section. Manipulating the variation of polarization in single optical beam is important to acquire flexible and controllable focused optical field. Previous studies mainly focused on the vector optical field with its polarization varying along circular trajectory of the Poincaré sphere. Here, we demonstrate the tight focusing behaviors of the vector optical field with the polarization varying along complex curves of the Poincaré sphere, which is generated by the joint modulation of azimuthal phase and amplitude distributions of orthogonally polarized components. The longitudinal polarization component with multipolar pattern in rotation symmetry can be achieved with similar distribution of the total focused field. The transverse and longitudinal spin angular momentum distributions in focal space are discussed. Approximiately pure transverse spin angular momentum can be constructed and manipulated in the focal space, which provides the possibility to manipulate the 3D spin flux for the applications of nano and spin photonics.

Automatic detection of quartz glass subsurface defects by laser scattering method based on ellipsoidal mirror

Gengyang Chen, Tengda Zhang, Weixun Zhang, and Rongsheng Lu

DOI: 10.1364/AO.514950 Received 07 Dec 2023; Accepted 04 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: With the improvement of quality requirements of optical components, the detection of sub-surface defects of optical components has become a very key technology. The existing detection methods still have some limitations in detection depth and detection efficiency. In this paper, a defect scattering light collection method based on ellipsoidal mirror model is used to analyze the scattering light collection efficiency under different experimental conditions theoretically, and the favorable conditions for improving the scattering light collection are proposed. After simulation verification, the use of ellipsoidal reflectors to collect scattered light can effectively avoid the impact of surface defects compared to lenses. At the same time, an experimental system based on this method is set up to filter the stray light by mean filtering method. The system detected three scratches (2um in width and 252nm in depth) on the underside of a piece of quartz glass. The results show that the system can clearly detect the subsurface defects of optical components.

Design and Simulation of Light Field Patterns for Angular Color Variation Reduction in MicroLED and MiniLED RGB Arrrays

Shu-Mei Yang, Chia-Hsin Chao, Yu-Sheng Chen, Ming-Hsien Wu, YEN-HSIANG FANG, and Chien-Chung Lin

DOI: 10.1364/AO.516502 Received 28 Dec 2023; Accepted 04 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: We propose a methodology to mitigate angular color variation in full-color micron-scale LED arrays. By simulating light field distribution for red (AlGaAs) and green/blue (GaN) light across various RGB microLED sizes, we can select matching light field patterns for RGB chips, reducing angular color variation from 0.0201 to 0.0030. Applying this method to full-color miniLED assemblies achieves a reduction from 0.0128 to 0.0032 by matching light field patterns with varying substrate thicknesses. This straightforward approach aligns with current mass transfer processes, offering practical implementation.

RDDA method for characterization of photonic nanojets

Mohammad Ataii and Jafar Amjad

DOI: 10.1364/AO.520467 Received 30 Jan 2024; Accepted 04 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: This paper presents reduced discrete dipole approximation method (RDDA) as a flexible and powerful tool for computing field distribution of photonic nanojets (PNJs) based on the superposition of the induced dipole radiation fields at the surface of microparticles with low number of lattice points. PNJs constructed by microparticles of spherical and ellipsoidal shapes have been studied by RDDA method. The incident beam polarization and filling factor effects in the PNJs shape and confinement strength have been characterized by quality factor Q. The best confined PNJs have been found in microellipsoids with specific range of shape parameter and filling factor values.

The Rotation of Reference Frame Dependent Polarimetric Variables for Equidistant Fisheye Lens Projections

E. Blocker and Kenneth Voss

DOI: 10.1364/AO.519772 Received 25 Jan 2024; Accepted 03 Mar 2024; Posted 04 Mar 2024  View: PDF

Abstract: The effect of a linear polarizer is conventionally well defined when viewed along the axis normal to its face. However, even for ideal linear polarizers, the non-normal viewing geometries attainable with wide-angle lenses require further considerations. A method to describe the polarization state of light passed through a linear polarizer and observed with an equidistantfisheye lens is described.

Trapping of low and high refractive index nano-spherical particles by using highly focused Laguerre-Gaussian beam

Fatima Thabit and Mohamed Shukri

DOI: 10.1364/AO.519752 Received 22 Jan 2024; Accepted 03 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: We have investigated the trapping force on the low and high refractive index nanoparticles using a highly focused Laguerre-Gaussian beams. It was clearly found that the increase of the beam mode 𝑝 causes a substantial increase in the number of trapping zones which frequently increase the number of particles to be captured simultaneously. Also, the decrease of 𝑝 values would increase the trap area which consequently lead to simultaneous trapping of several particles in the same area. The change of 𝑝, 𝑙, Δ𝑛 and 𝑧1 values have a direct influence on the gradient forces. The smaller values of scattering and thermal forces relative to the gradient force values represent essential conditions for the stability of the trapping performance.trapping performance.

An Er3+/Yb3+/Ho3+ tri-doped no-core fibertemperature sensor based on FluorescenceIntensity Ratio Technology: Towards highstability and accurate measurements

huifang wang, Zhiyuan Yin, Dianchang Song, Wei Liu, Xue Zhou, xin yan, Xuenan Zhang, and Tonglei Cheng

DOI: 10.1364/AO.509856 Received 20 Oct 2023; Accepted 03 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: IIn this paper, the green up-conversion (UC) fluorescence emission fromEr3+/Yb3+/Ho3+ tri-doped tellurite glass was investigated for temperature sensing. The dopingof Ho3+ ions can not only enhance the chance of energy level transition, but also avoid theinfluence of thermal effect caused by the proximity of 2H11/2 and 4S3/2 energy levels. Theluminescence characteristics at different Yb3+ and Ho3+ ions concentration doping molar ratioswere investigated, and the strongest luminescence characteristics were exhibited when Yb3+ions concentration at 5 mol% and Ho3+ at 0.2 mol%. Based on it, a tri-doped TeO2-ZnO-Bi2O3(TZB) no-core fiber was fabricated and connected with multimode fibers (MMFs) to form atemperature sensor. The temperature sensing performance of the tri-doped TZB temperaturesensor was evaluated in detail over the temperature range of 255-365 K. The repeatability andstability of the temperature sensor was experimentally verified. The Er3+/Yb3+/Ho3+ tri-dopedsensor can be used for noninvasive optical temperature sensing in the fields of environmentalmonitoring, biological sensing, and industrial process temperature control, etc.

Design and testing of a multi-electrodeapodised acousto-optic filter for arbitrarypolarised light

Samuel Dupont, Jean-Claude Kastelik, and Jurgen Vanhamel

DOI: 10.1364/AO.514229 Received 24 Nov 2023; Accepted 03 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: We present an original acousto-optic tuneable filter able to filter visible light from400 nm to 650 nm. It is designed to interact simultaneously with two polarizations. It showsadjustable optical bandwidth and apodization capabilities. These features make it suitable forpractical spectroscopic applications. Experimental validation is presented.

High -precision binocular camera calibration method based on a3D calibration object

Xiaowen Zhang, Tiegang Lv, Wang Dan, and Zhang Minghao

DOI: 10.1364/AO.517411 Received 29 Dec 2023; Accepted 03 Mar 2024; Posted 07 Mar 2024  View: PDF

Abstract: A high-precision binocular camera calibration method is proposed to address theissues of poor calibration accuracy and large calibration errors in current practical applications.This method uses a triangular stereo sphere as the calibration object and employs steps, such asellipse fitting, Cholesky decomposition, homography matrix solution, and nonlinearoptimization, to compute the intrinsic and extrinsic parameters, distortion parameters, andrelative pose of the binocular camera. Moreover, this method simplifies the correspondencesbetween primitives, enabling simultaneous calibration of multiple viewpoint cameras. Thismethod is also suitable for both binocular cameras consisting of two different structuredmonocular cameras and those composed of two image sensors within the same structure.Experimental results showed that this method outperforms traditional algorithms in terms ofbinocular camera calibration accuracy, calibration errors between left and right cameras, androbustness, resulting in a significant improvement in overall algorithm performance.

Modelling the atmospheric refractive indexstructure parameter using macrometeorologicalobservations

Ravikiran Hegde, Anand Sarma, S Satheesh, and Krishnamoorthy K

DOI: 10.1364/AO.519025 Received 14 Jan 2024; Accepted 02 Mar 2024; Posted 04 Mar 2024  View: PDF

Abstract: The dynamic fluctuations in the atmospheric refractive index, commonly referred toas optical turbulence, causes phase distortions of the electromagnetic waves propagatingthrough the atmosphere. The consequent scintillations have large implications for free-spaceoptical communication, laser remote sensing, and directed energy applications. The refractiveindex structure parameter (𝐶2𝑛), quantifying the strength of these fluctuations, is usuallyestimated using high-frequency micrometeorological measurements, employing sonicanemometer-thermometers or scintillometers. Despite providing highly accurate information,these systems are immensely complex and costly, especially for frequent field applications andremote locations. In this study, we have developed an empirical multinomial model forestimating 𝐶2𝑛 using three-year macrometeorological data and validated it against collocatedand concurrent micrometeorological measurements, from a tropical semi-arid location. Thissimpler model would be handy for applications in remote locations having weather stationmeasurements alone.

Quantitative assessment whole-rock iron content in magnetite protolith based on terahertz time-domain spectroscopy

Mingrui Zhang, Zhiyuan Zheng, Tong Zhang, Siqi Zhang, Ren Huang, Xiaodi Zheng, Junfeng Shen, Haochong Huang, and Zili Zhang

DOI: 10.1364/AO.517400 Received 08 Jan 2024; Accepted 02 Mar 2024; Posted 04 Mar 2024  View: PDF

Abstract: The absorption coefficient and refractive index of terahertz spectra were used to establish a correlation with the whole-rock iron (TFe) content in different depth of the Bayan Obo protolith. Base on the elemental contents, three machine learning algorithms, namely partial least squares regression (PLSR), random forest (RF) and Multi-Layer Perceptron Regressor (MLP), were used to develop a quantitative analytical model for TFe content of the protolith minerals. Results show that superior to the refractive index, the absorption coefficient can be used for rapid and effective investigation of TFe content in minerals. Among the three algorithms, MLP has higher detection accuracy, with a model coefficient of determination R^2 reaching 0.944. These findings demonstrate that terahertz time-domain spectroscopy can be used to rapidly and nondestructively quantify the TFe elemental content of protolith.

Multi-mode heralded single photons based on the DLCZ

Wang Shengzhi and Mingjie Wang

DOI: 10.1364/AO.517993 Received 09 Jan 2024; Accepted 01 Mar 2024; Posted 04 Mar 2024  View: PDF

Abstract: High-quality single-photon sources are crucial for the development of simple quantum devices. Quantum communication stands at the forefront of cutting-edge technologies, promising unprecedented levels of security and efficiency. A cornerstone of this revolutionary field is the development of high-speed single photon sources, which play a pivotal role in quantum key distribution (QKD) and other quantum communication protocols. In this context, the concept of space multimode emerges as a promising avenue to propel the capabilities of single photon sources to new heights. We have spatial multiplexing technology to develop single photon sources that deliver high-speed heralded single photons in DLCZ scheme. We propose a spatial multiplexing single photon source scheme based on the DLCZ. In our experimental system, the single-photon generation rate increases sixfold by expanding from one to six spatial modes, and the second order correlation function of single photons is less than 0.5. We show that expanding the spatial degrees of freedom of the quantum storage scheme based on DLCZ does not affect the single-photon properties. The generation rate of the single photon can be significantly increased through spatial multiplexing with a feedback circuit. Our approach offers a promising path to creating a high speed photon source based on spatial multi-mode scheme.

Half-spectrum OFDM to quadruple the spectralefficiency of underwater wireless opticalcommunication with digital power divisionmultiplexing

xiaojian hong, Yuan Wang, Tianyi Zhang, Chao Fei, Jiahan Tian, Junwei Zhang, Yitong Xie, Fei Zhang, JI DU, Guowu Zhang, Gaoxuan Wang, and Sailing He

DOI: 10.1364/AO.514639 Received 30 Nov 2023; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: Improving spectrum efficiency (SE) is an effective method to further enhance the data rate of bandwidth-limited underwaterwireless optical communication (UWOC) systems. Non-orthogonal frequency division multiplexing (NOFDM) with a compressionfactor of 0.5 can save half of the bandwidth without introducing any inter-carrier-interference (ICI) only if the total number ofsubcarriers is large enough, and we termed it as half-spectrum OFDM (HS-OFDM). To the best of our knowledge, this is the firstreported work on a closed-form HS-OFDM signal in the discrete domain from the perspective of correlation matrix. Due to thespecial mathematical property, no extra complex decoding algorithm is required at the HS-OFDM receiver, making it as simple asthe conventional OFDM receiver. Compared with traditional OFDM, HS-OFDM can realize the same data rate but with a largersignal-to-noise ratio (SNR) margin. To fully use the SNR resource of the communication system, we further propose a digital powerdivision multiplexed HS-OFDM (DPDM-HS-OFDM) scheme to quadruple the SE of the conventional OFDM for the bandwidthstarved UWOCs. The experimental results show that HS-OFDM can improve the receiver sensitivity by around 4 dB as opposed tothe conventional 4QAM-OFDM with the same data rate and SE. With the help of the DPDM-HS-OFDM scheme, the data rate of themulti-user UWOC can reach up to 4.5 Gbps under the hard-decision forward error correction (HD-FEC) limit of bit error rate (BER)of 3.8 × 10-3. Although there is some performance degradation in comparison with the single-user HS-OFDM, the BER performanceof the multi-user DPDM-HS-OFDM is still superior to that of the conventional single-user 4QAM-OFDM. Both the single-user HSOFDM and multi-user DPDM-HS-OFDM successfully achieve 2-Gbps/75-m data transmission, indicating that the DPDM-HS-OFDMscheme is of great importance in bandwidth-limited UWOC systems and also has guiding significance to underwater wirelessoptical multiple access.

Down-Conversion and Frequency SpectrumConvolution of the Broadband Signal Basedon the Active Mode Locking Technology

Feng Xiong, Yibei Wang, Yalan Wang, Dangwei Wang, Runhua Liu, Pengfei Du, Jin Zhang, Anle Wang, Haixia Cong, and Xiaoniu Peng

DOI: 10.1364/AO.517148 Received 27 Dec 2023; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: A multi-function processor for broadband signal based on the active mode lockingoptoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The centralfrequency down-conversion and frequency spectrum convolution of the target broadband signal(TBS) are realized by just tunning the wavelength of the optical carrier or by time domainproduct, respectively. To achieve the central frequency down-conversion of the TBS, an opticaltunable delay line (OTDL) is adopted to match the delay time of the OEO loop with therepetition period of the TBS. Then the spectrum convolution of the TBS is produced by justinjecting a lower frequency signal consistent with the free spectral range (FSR) of the OEOloop. Moreover, the frequency convolution repetition is also multiply increased by harmonicmode-locking injection. The equivalent bandwidth of the TBS is enlarged by ~ 50 timesbenefits from the frequency convolution. The central frequency conversion flexibility and thebandwidth compatibility are also discussed in detail. This work provides a multi-functionprocessor system and may have potential usage in multifunctional integrated radar systems.

Magneto-optical fiber-based orbital angular momentum mode converters

Geng Chen, Bao-Jian Wu, Qi-Quan Wang, Feng Wen, and Kun Qiu

DOI: 10.1364/AO.510563 Received 27 Oct 2023; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: Orbital angular momentum (OAM) mode division multiplexing (MDM) systems can support large-capacity and high-speed rate information transmission, in which the OAM mode conversion devices play an important role. In this paper, the mode conversion principle of magneto-optical fiber-based long period grating (MOF-LPG) is analyzed for further developing new magneto-optical (MO) OAM mode converters, including three types of CP01 to OAM±1,1, OAM±1,1 to OAM±2,1 and OAM±1,1 to CP02. It is shown that, the magnetic tunability of the mode converters through the propagation constants of the eigenmodes is useful for compensating for process errors and increasing the operating wavelength range.

An ultra-compact SOI based higher-ordermode pass wavelength demultiplexer

Jinhua Chen, WeiWeii Pan, and CHEN JI

DOI: 10.1364/AO.517258 Received 27 Dec 2023; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: We proposed an ultra-compact mode filtering wavelength demultiplexer design witha footprint of 3 × 3 𝜇𝑚2. Our device can route input 𝑇𝐸1 mode signals at 1310 nm and 1550nm to different output ports, while simultaneously blocking fundamental transverseelectric(𝑇𝐸0) mode input. Our device is designed based on the topology optimization algorithm,which results in an ultra-compact footprint combining wavelength routing and mode filteringfunctions for the first time. Our final optimized devices demonstrated insertion losses of 1.26dB and 1.47 dB for the C and O band output ports, respectively, with inter-port crosstalk as lowas -21.25 dB and -30.99 dB. The extinction ratio between 𝑇𝐸1 mode and 𝑇𝐸0 mode are 24.02dB and 30.12 dB at the 1310 nm and 1550 nm output ports. The combination of small footprint,broad transmission bandwidth, 𝑇𝐸1 to 𝑇𝐸0 mode selectively filtering, C and O band 𝑇𝐸1 modedemultiplexing functions make this a uniquely versatile device that can play an important rolein future high density mode-wavelength multiplexing systems.

Highly Enhancement Photoluminescence of Monolayer MoS2 in Hybrid Plasmonic Systems

Yan Huang, Ruoying Kanyang, cizhe fang, Xiaoxi Li, Xiangyu Zeng, yibo wang, Liu Yan, Yue Hao, and Genquan Han

DOI: 10.1364/AO.517514 Received 02 Jan 2024; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: Monolayer molybdenum disulfide (MoS2) has weak light-matter interaction due to ultrathin thickness, which limits its potential application in lasing action. In this study, we propose a hybrid structure consisting of nanocavity and Au nanoparticles to enhance the photon emission efficiency of monolayer MoS2. Numerical simulations show that photoluminescence (PL) emission is significantly enhanced by introducing localized surface plasmon resonance (LSPR) to the proposed structure. Furthermore, an exciton energy band system is proposed to elucidate the physical mechanism of the PL process. By optimizing the spacer thickness, a high Purcell enhancement factor of 95 can be achieved. The results provided by this work pave the way to improve the PL efficiency of two-dimensional (2D) material, which constitutes a significant step towards the development of nanodevices such as nanolasers and sensors.

Description and reconstruction of typical structured light beams with vector spherical wave functions

Yiyu Shi, zhiwei cui, Zhanfei Liu, Shuaishuai Ren, and Fuping Wu

DOI: 10.1364/AO.518878 Received 12 Jan 2024; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: It is well known that the generalized Lorenz-Mie theory (GLMT) is a rigorous analytical method for dealing with the interaction between light beams and spherical particles, which involves the description and reconstruction of the light beams with vector spherical wave functions (VSWFs). In this paper, a detailed study on the description and reconstruction of the typical structured light beams with VSWFs is reported. We first systematically derive the so-called beam shape coefficients (BSCs) of typical structured light beams, including the fundamental Gaussian beams, Hermite-Gaussian beams, Laguerre-Gaussian beams, Bessel beams, and Airy beams, with the aid of angular spectrum decomposition method (ASDM). Then based on the derived BSCs, we reconstruct these structured light beams using VSWFs and compare the results of reconstructed beams with that of original beams. Our results will be useful to the study of typical structured light beams interaction with spherical particles in the framework of GLMT.

Transport of Intensity and Phase: Applications to Digital Holography

Nouf Alanazi, Austin Scott, hammid al-ghezi, Muhammad Faryad, Akhlesh Lakhtakia, and Partha Banerjee

DOI: 10.1364/AO.519170 Received 15 Jan 2024; Accepted 29 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: We first review transport of intensity and phase and show their use as aconvenient tool to directly determine the unwrapped phase of an imaged object, eitherthrough conventional imaging or using digital holography. For both cases, either thetraditional transport of intensity and phase, or with a modification, viz., electricallycontrollable transport of intensity and phase, can be used. The use of digital holographywith transport of intensity for 3D topographic mapping of fingermarks coated withcolumnar thin films is shown as an illustrative application of this versatile technique.

Microwave Photonics Frequency MeasurementWith Improved Accuracy Based on ArtificialNeural Network

Xin An, Zhangyi Yang, Zuoheng Liu, Youdi Zhang, and Wei Dong

DOI: 10.1364/AO.519402 Received 18 Jan 2024; Accepted 28 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: Photonics-assisted techniques for microwave frequency measurement (MFM) showgreat potential for overcoming electronic bottlenecks, with wild applications in radar andcommunication. The MFM system based on stimulated Brillouin scattering (SBS) effect canmeasure the frequency of multiple high-frequency and wide-band signals. However, theaccuracy of the MFM system in multi-tone frequency measurement is constrained by the SBSbandwidth and the nonlinearity of the system. To resolve this problem, a method based onartificial neural network (ANN) is raised, which can establish a nonlinear mapping between themeasured two-tone signal spectra and the theoretical frequencies. Through simulationverification, the ANN optimized frequencies within the range of (0.5, 27) GHz of the MFMsystem show 79%, 76%, 70%, 44% reduction in errors separately under 20 dB, 15 dB, 10 dB,0 dB four spectral signal-to-noise ratios (SNR) conditions, and the frequency resolution isimproved from 30 MHz to 10 MHz.

Impact of different receiver geometry on reconfigurable intelligent surface assisted multi-cell VLC system in presence of light path blockage

Deblina Sabui, Sourish Chatterjee, and Gufran Khan

DOI: 10.1364/AO.516475 Received 21 Dec 2023; Accepted 28 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: We report the effect of integrating metasurface-aided reconfigurable intelligent surfaces (RIS) on the signal-to-interference-plus-noise ratio (SINR) and data rate of a multi-cell visible light communication (VLC) system. RIS has been deployed in the channel between transmitter and receiver to redirect the reflected light in the desired directions, even in the absence of line-of-sight (LoS) links. Results show that the introduction of RIS has improved average SINR but reduced average illumination level compared to no-RIS system. As the quantity of RIS increases, a discernible improvement in the maximum SINR value is observed. Here, three different receiver geometries, namely photodiode (PD), freeform diversity receiver (FDR), and modified FDR (MFDR), have been adopted. The impact of individual receivers has been reported in the presence of light path blockage. MFDR geometry is found to be most suitable with more coverage probability compared to the other two receivers. With (40cm x 24cm) RIS area, during blockage, MFDR maintains an average SINR 21.95 dB, which is 97.29% and 14.24 % greater than PD and FDR, respectively.

Surface hardness determination of Laser Cladding using laser-induced Breakdown Spectroscopy and Machine Learning(PLSR, CNN, ResNet and DRSN)

Jiacheng Yang, Linghua Kong, and Ye Hongji

DOI: 10.1364/AO.516603 Received 21 Dec 2023; Accepted 28 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: In this study, we employed laser-induced breakdown spectroscopy (LIBS) along with machinelearning algorithms, which encompass partial least squares regression(PLSR), deep convolutionalneural network(CNN), deep residual neural network(ResNet), and deep residual shrinkage neuralnetwork(DRSN), to estimate the surface hardness of laser cladding layers( The layers wereproduced using Fe316L, FeCrNiCu, Ni25, FeCrNiB, and Fe313 powders, with 45 steel and Q 5serving as substrates ). The research findings indicate that both linear and nonlinear modelscan effectively fit the relationship between LIBS spectra and surface hardness. Particularly, themodel derived from the ResNet exhibits superior performance with an 𝑅2value as high as 0.9967.We hypothesize that the inclusion of numerous noises in the LIBS spectra contributes to the enhanced predictive capability for surface hardness, thereby leading to the superior performance of the ResNet compared to the DRSN.

Phase screen prediction using deep phase network for FSO links

Ming Li, Zhigeng Wu, Tianyi Wang, Pengxin Zhang, and Milorad Cvijetic

DOI: 10.1364/AO.517151 Received 28 Dec 2023; Accepted 28 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: Due to the presence of air turbulence in free-space optical (FSO) links, randomfluctuations in both wavefront phase and amplitude of the optical signal are reduced afterit propagates through the air channel, which degrades the performance of free-space opticalcommunication (FSOC) systems. Phase screen reflects the phase distortions resulting from airturbulence. Accordingly, accurate prediction with respect to phase screen is of significance forthe FSOC. In this paper we propose a phase screen prediction method based on the deep phasenetwork (DPN). The advantages of the proposed method include strong robustness against airturbulence, low model depth, few number of parameters as well as low complexity. The resultsreveal that our DPN enables desired inference accuracy and faster inference speed comparedto the existed models by combining the mean square deviation loss function with the pixelpenalty terms. More concretely, the accuracy of phase screen prediction can reach up to 95%,the average time consumed to predict the phase screen is the order of millisecond only undervarious turbulence conditions. Also, our DPN outperforms the traditional Gerchberg-Saxtonalgorithm in convergence speed.

Optical design of visible/short-wave infraredcommon-aperture optical system with longfocal length and wide field of view

Aqi Yan, Weining Chen, Qianxi Li, min guo, and HAO WANG

DOI: 10.1364/AO.517643 Received 03 Jan 2024; Accepted 27 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: Addressing the urgent need for long-distance dim target detection with a wide fieldof view and high sensitivity, this paper proposes a visible and short-infrared dual-bandcommon-aperture optical system characterized by a broad field and extended focal length. Toachieve system miniaturization and high sensitivity target detection, the visible and infraredoptical systems share a Ritchey-Chretien primary and secondary mirror. The primary opticalpath is segmented into visible light (0.45μm to 0.75μm) and short-wave infrared (SWIR) (2μmto 3μm) bands by a dichroic spectral splitter prism. The SWIR optical system utilizes fourshort-wave cooled infrared detectors, and wide-field stitching is achieved using a field-ofview divider. While ensuring high cold shield efficiency of cooled infrared detectors, thiscommon-aperture optical system delivers visible and SWIR dual-band images with expansivefields, elongated focal lengths and sizable apertures. The visible-light optical system has afocal length of 277mm, a field of view of 2.3°×2.3°, and an entrance pupil diameter of130mm. Meanwhile, the SWIR optical system features a focal length of 480mm, a field ofview of 2.26°×1.8°, and an entrance pupil diameter of 160mm. Design outcomes suggest thatthe imaging quality of the optical system approaches the diffraction limit. This visible/SWIRcommon-aperture optical system exhibits high sensitivity, a large field of view, compactstructure, and excellent imaging quality, thereby meeting the requirements for long-distancedim target detection and imaging.

Uncertainty of digital image correlation undervideo compression and DSP optimization

Xiaowen Li, Xinqiao Tang, Yinuo Xu, Wei Sun, and Zhenning Chen

DOI: 10.1364/AO.511691 Received 09 Nov 2023; Accepted 26 Feb 2024; Posted 01 Mar 2024  View: PDF

Abstract: The storage and transmission of videos at high spatial resolution remains a greatchallenge in image-based optical techniques. The uncertainty of digital image correlation (DIC)was assessed following speckle video compression under High Efficiency Video Coding(HEVC/H.265). First, the evaluation criterion for the DIC accuracy affected by compressionwas provided. The stability of H.265 video compression in DIC was studied consideringdifferent compressed frames under different target quantization parameters (QPs) andcompression ratios (CRs). The deformation uncertainty of the DIC itself as affected by H.265video compression was further investigated through uniform translation and non-uniformsinusoidal deformation performance. Moreover, the optimized digital speckle pattern (DSP)was re-evaluated considering video compression-induced uncertainty. DSPs with parametersof different diameters and randomness were compressed using various QPs and CRs. Inaddition, DSP evaluation was performed under both translation and non-homogeneousdeformation conditions. The feasibility of the re-optimized DSP under H.265 videocompression was validated using a defective bending beam, and DSP videos with a speckle sizeof 8 pixels reached a high CR within an acceptable margin of error.

Pixel-wise calibration method for telecentric structured light system

Yuchen Yang and Song Zhang

DOI: 10.1364/AO.509535 Received 19 Oct 2023; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: This paper introduces a pixel-wise calibration method designed for a structured light system utilizing a camera attached with a telecentric lens. In the calibration process, a white flat surface and another flat surface with circle dots serve as the calibration targets. After deriving the properties of the pinhole projector through conventional camera calibration method using circle dots and determining the camera's attributes via 3D feature points estimation through iterative optimizations, the white surface calibration target was positioned at various poses and reconstructed with initial camera and projector calibration data. Each 3D reconstructions was fitted with an ideal virtual ideal plane that was further used to create the pixel-wise phase-to-coordinate mapping. To optimize the calibration accuracy, various angled poses of the calibration target are employed to refine the initial results. Experimental findings validate that the proposed approach offers high calibration accuracy for a structured light system using a telecentric lens.

Phase retrieval algorithm applied to high-energyultrafast lasers

Jikai Wang, Abdolnaser Ghazagh, Sonam Ravi, Stefan Baumbach, Benjamin Dannecker, Michael Scharun, Dominik Bauer, Stefan Nolte, and Daniel Flamm

DOI: 10.1364/AO.516820 Received 21 Dec 2023; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: A standardized phase retrieval algorithm is presentedand applied to an industry-grade high-energy ultrashortpulsed laser to uncover its spatial phase distribution.We describe in detail how to modify the well-knownalgorithm in order to characterize particularly stronglight sources from intensity measurements only. Withcomplete information about the optical field of the unknown light source at hand, virtual back propagationcan reveal weak points in the light path such as apertures or damaged components.

Temperature dependent index of refraction Sellmeier modelfor crystalline and polycrystalline materials

Michael Thomas

DOI: 10.1364/AO.511188 Received 05 Nov 2023; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: The temperature dependence of optical window materials remains an important issue for a variety ofapplications from spacecraft, laser components to high speed aircraft. Concerning the refractive index in regions oftransparency, and current models are empirically based polynomial fits for the Sellmeier model strength and modelocation parameters. These polynomial fit functions limit the ability to accurately extrapolate beyond theexperimental range used to develop the fit functions. Thus, the development of a physics based model as a functionof temperature is an important goal for these critical materials. Such a model will allow extrapolation to higher andlower temperatures as long as the physical mechanisms don’t change. For vibrational modes, a thermal average ofthe anharmonically shifted energy levels is investigated and compared to experimental data. The first anharmonicterm can be estimated using the Morse potential based on a multiphonon absorption model. Experimentally, thesemodes red shift, and this is consistent with the developed temperature dependent index of refraction Sellmeiermodel. This red-shifting phenomena can also be applied to electronic transition shifts. In addition, the temperaturedependent oscillator number density can be obtained from known expansion coefficient models and experimentaldata. Other model parameters, in particular the electronic and vibrational mode polarizability, still needexperimental grounding for a given material. The method is incorporated into a modified Sellmeier model format.

Multiple HAPS-based Space-Air-Ground Network withFSO Communication: A Performance Analysis

Deepshikha Singh, Swaminathan R, and Anh Pham

DOI: 10.1364/AO.515707 Received 11 Dec 2023; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: Due to the fact that the existing generation of wireless communication cannot possibly keep up with thecurrent traffic explosion and emerging applications, research and development on next-generation (i.e. sixthgeneration 6G) wireless technologies is being carriedout worldwide. In this regard, it is anticipated that thespace-air-ground (SAG) network with free space optics(FSO) communication can provide terabits per secondthroughput necessary to sustain various potential 6Gapplications. However, FSO communications are susceptible to atmospheric turbulence, pointing errors, andbeam scintillation effects. To remedy the severe atmospheric effects, we propose a multiple high-altitude platform station (HAPS)-based SAG network with HAPSselection scheme. For the proposed system, we have derived the closed-form expressions for outage probability,average symbol error rate (SER), ergodic capacity, andoutage capacity over Málaga distribution with pointingerrors. Further, the asymptotic expressions for outageprobability, average SER and outage capacity were derived to enhance the comprehension of the system froma practical standpoint. It is observed from the numerical results that the multiple HAPS-based FSO systemperforms better than the existing HAPS-based FSO systems.

A scheme for WDM-PON with colorless opticalnetwork units based on flat optical frequency comb

Yian Cao and Shibao Wu

DOI: 10.1364/AO.516098 Received 17 Dec 2023; Accepted 26 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: A new scheme for wavelength division multiplexing passive optical network (WDM-PON) based on flat optical frequencycomb (OFC) is proposed. Using OFC as the optical source of the system can realize the colorlessness of optical networkunits (ONUs), and the direct detection of the downlink data further simplifies the ONU structure. The coherentdemodulation of the uplink data improves the system performance due to the coherence of the comb lines. In thisresearch, the proposed system is studied for its performance and power budgeting. The results show the flexibility,effectiveness and practicability of the proposed scheme, which can be applied to future high-capacity optical accessnetworks.

Highly Sensitive Optical Fiber Pressure SensorBased on FPI and Vernier Effect via FemtosecondLaser Plane-by-Plane Writing Technology

Hu Xixi, Dan Su, and Xueguang Qiao

DOI: 10.1364/AO.516751 Received 21 Dec 2023; Accepted 21 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: In this paper, a highly sensitive pressure sensor based on fiber-optic Fabry-Pérotinterferometers (FPI) and the Vernier effect (VE) is proposed and experimentally demonstrated.For the sensing cavity, we employ a closed capillary-based FPI𝑠, and for the reference cavity, we usean FPI𝑟 created through femtosecond laser refractive index modulation, which remains imperviousto pressure changes. Connecting these two FPIs in series produces a VE-based cascaded sensorwith a clear spectral envelope. The femtosecond laser micromachining technique provides precisecontrol over the length of FPI𝑟 and facilitates adjustments to the VE’s amplification degree.Experimental results reveal significant pressure sensitivities of -795.96 pm/MPa and -3219.91pm/MPa, respectively, representing a 20-fold and 80-fold improvement compared to FPI𝑠 (-39.80pm/MPa). This type of sensor has good sensitivity amplification and, due to its all-silica structure,can be a promising candidate for high-temperature and high-pressure sensing, especially in harsh environments.

Analysis and implementation of volumereflection gratings in photorefractive lithiumniobate for edge enhancement

Austin Scott and Partha Banerjee

DOI: 10.1364/AO.512442 Received 15 Nov 2023; Accepted 21 Feb 2024; Posted 27 Feb 2024  View: PDF

Abstract: Diffraction from volume reflection gratings written in bulk photorefractive lithiumniobate is modeled for the case of longitudinally varying index modulation depths. Numericalsolutions to the Helmholtz equation are found, in the spatial frequency domain, leading totransfer functions for the volume reflection grating. These transfer functions are then used toshow the spatial frequency filtering effect of the volume reflection grating on input light fieldscontaining 2D spatial information. It is shown, first through simulations and then byexperiment, that the 0th order transmitted beam undergoes 2D edge enhancement.

Optical experimental results using Singer product apertures

Paul Shutler and Kevin Byard

DOI: 10.1364/AO.514108 Received 29 Nov 2023; Accepted 01 Feb 2024; Posted 22 Feb 2024  View: PDF

Abstract: We present the first optical experimental results obtained using the recently developed Singerproduct apertures. We also show that Fenimore and Cannon’s fine sampling and delta decoding techniquescan be combined with the fast direct vector decoding algorithm for Singer product apertures. We demonstrateresolutions and decoding speeds comparable to, or better than, those currently reported in the opticalliterature. Taken together these make possible coded aperture video in the optical domain.