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

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

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

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

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

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

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

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

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

Rajib Nanda and Mitra Sarkar

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

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

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

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

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

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

Theoretical Analysis of Buried Heterostructure Laser for Stable Dual Wavelength Generation

Soumi Pal, Arpit Khandelwal, and Nitin Bhatia

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Improving the reliability of deep learning computational ghost imaging with prediction uncertainty based on neighborhood feature maps

Shoma Kataoka, Yasuhiro Mizutani, Tsutomu Uenohara, Erick Ipus, Kouichi Nitta, Osamu Matoba, Yasuhiro Takaya, and Enrique Tajahuerce

DOI: 10.1364/AO.511817 Received 22 Nov 2023; Accepted 15 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: Defect inspection is required in various fields. Many researchers have attempteddeep-learning algorithms for inspections. Deep-learning algorithms have advantages in termsof accuracy and measurement time. However, the reliability of deep learning outputs isproblematic in precision measurements. This study demonstrates that iterative estimation usingneighboring feature maps can evaluate the uncertainty of the outputs. The unconfident errorpredictions have higher uncertainties. In ghost imaging using deep learning, the experimentalresults show that removing outputs with higher uncertainties improves the accuracy byapproximately 15.7%.

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

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

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

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

Radiant fluence from ray tracing in optical multipass systems

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

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

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

A three-dimensional measurement method based on reusing equally spaced binary stripes

Fei Yan, Pei yue Wu, SunCheng SunCheng, yinping liu, and Jia liu

DOI: 10.1364/AO.516338 Received 18 Dec 2023; Accepted 15 Apr 2024; Posted 16 Apr 2024  View: PDF

Abstract: To eliminate the effect of nonlinear errors on measurement results, this paperpresents a new method to overcome the nonlinear response of commercial projectors andcameras by using binary stripes for coding. The method shifts the generated equally spacedbinary stripes by a fixed number of pixel points to obtain different stripe maps, followed bysequential projection of these binary stripes with a digital projector. The acquired binary stripesare reused in the 3D reconstruction combined with the phase-shift method and can be reducedto sinusoidal stripes with different phase shifts by a specific superposition method. In this paper,this method is combined with the traditional four-step phase-shift method for experiments. Theresults show that the accuracy of the wrapped phase obtained by the method proposed in thispaper is 13.88% higher than that obtained by the traditional sixteen-step phase-shift method.Similarly, the accuracy of the standard ball measurement is increased by 21.05%. Additionally,the point cloud on the surface of the complex object obtained by the proposed method issmoother and more delicate than that obtained by the traditional sixteen-step phase-shiftmethod.

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

Kaifeng Ma, Guiping Huang, and Junzhen Meng

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

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

Fractional Fourier-transform off-axis digital holographic imaging

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

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

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

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

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

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

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

Laser cleaning of dirty grease on steel sluice cables

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

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

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

Compact Auto-aligning Interferometers with Picometer Precision

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

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

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

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

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

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

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

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

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

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

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

Impact of the preservation media on ex vivo bone samples for full field mechanical testing

BRENDA GUZMAN VALDIVIA, Manuel De la Torre I., and Claudio Frausto-Reyes

DOI: 10.1364/AO.512047 Received 12 Nov 2023; Accepted 13 Apr 2024; Posted 15 Apr 2024  View: PDF

Abstract: The preservation method to store bone tissue for posterior analysis is a widespread practice. However, the methods’potential influence on the material’s mechanical properties is often overlooked during single-point experimentation.Saline and formaldehyde solutions are the most common among the employed preservation media. A full field analysis ofthe mice femoral bone deformation using non-destructive optical techniques is conducted to assess the influence of thestorage media on the viscoelastic properties of the tissue. Three different groups are subjected to a standard three-pointbending test. The first group is the control, with fresh post-mortem samples. The second and third groups used saline andformaldehyde solutions, respectively. During the mechanical test, the bone’s surface and internal deformation aremonitored simultaneously using Digital holographic interferometry and Fourier-domain optical coherence tomography.A mechanical comparison among the three groups is presented. The results show that after 48 hours of immersion insaline solution, the mice bones keep their viscoelastic behavior similar to fresh bones. Meanwhile, 48 hours informaldehyde modifies the response and affects the marrow structure. The high sensitivity of the optical phase alsomakes it possible to observe changes in the anisotropy of the samples. As a comparison, Raman spectroscopy analyses thethree bone groups to prove that the preservation media does not affect a single-point inspection.

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

Joseph Tang and James Fienup

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

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

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

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

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

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

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

Rüdiger Röttgers, Michael Novak, and Mathias Belz

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

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

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

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

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

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

Stretchable metal-dielectric-metal metasurface for dynamic radiation management

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

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

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

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

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

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

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

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

Frederic Schwenger, Adrian Azarian, and Michael Henrichsen

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

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

Focal Cone High Harmonic Generation Driven by a 400 TW Laser

John Gjevre, Robert Fedosejevs, José Antonio Pérez Hernández, Roberto Lera, Carlos Salgado, Luis Roso, Michael Ehret, and Ghassan Zeraouli

DOI: 10.1364/AO.516732 Received 22 Jan 2024; Accepted 11 Apr 2024; Posted 11 Apr 2024  View: PDF

Abstract: The generation of self focusing beams of extreme ultraviolet (XUV) radiation generated with a Focal Cone High Harmonic Generation (FCHHG) technique is examined for high energy lasers. The FCHHG geometry is created by passing a focusing laser beam through agas sheet prior to reaching focus and thus creating a converging beam of high harmonic radiation. This leads to a larger interaction area which increases the total area of XUV emission while not exceeding the saturation intensity of the target atoms or increasing the density of the atoms. Such a method allows for scaling of HHG to any incident laser power. An experiment was conducted demonstrating such scaling to incident 400TW pulses demonstrating both the expected spectralsignature of HHG and the converging cone of XUV radiation. It was found that this technique is very sensitive to spatial non-uniformity in the driving laser which becomes more prevalent in high energy laser systems.

Graphene-Enhanced Decagonal Patch Antenna for Terahertz Frequency Operation in Breast Cancer Detection

Ashok G, Veerendra Ganimidi, and Balachakravarthy Neelapu

DOI: 10.1364/AO.518504 Received 09 Jan 2024; Accepted 11 Apr 2024; Posted 11 Apr 2024  View: PDF

Abstract: This paper presents a decagonal patch antenna loaded with graphene designed for terahertz (THz) frequency applications, with a specific emphasis on its potential for early breast cancer detection. Theproposed antenna features a hybrid structure, integrating both copper and graphene materials. A decagonal graphene strip is intricately incorporated into the copper patch, yielding significant improvementsin reflection coefficient, bandwidth, and gain. The antenna, with dimensions of 155 × 130 × 13 µm3, isdesigned on a polyimide substrate, characterized by a dielectric constant of 3.5 and a loss tangent of 0.0027.To ensure relevance in medical contexts, the design is optimized to operate within the frequency range of2.1 to 5.7 THz, a critical spectrum for medical applications. Simulation results validate the effectiveness ofthe proposed antenna, demonstrating S11 < −10 dB within the frequency band of 2.1 to 5.7 THz (92.3%fractional bandwidth). The antenna exhibits an impressive bandwidth (BW) of 3.6 THz and a gain of 7.87dBi at 4 THz. These findings establish the graphene-loaded decagonal patch antenna as a highly promising solution for breast cancer detection applications, showcasing its potential in the realm of medicaldiagnostics.

Spatial and spectral beam characteristics in a terahertz broadband sub-wavelength imaging system using a solid immersion lens

Da-Hye Choi

DOI: 10.1364/AO.519288 Received 16 Jan 2024; Accepted 11 Apr 2024; Posted 11 Apr 2024  View: PDF

Abstract: This study focuses on the spatial and spectral beamcharacteristics in a terahertz (THz) broadband subwavelength imaging system using a solid immersion lens(SIL). Previously, we demonstrated a broadband subwavelength THz imaging system by integrating a SIL witha THz time-domain spectrometer (TDS). Key parametersthat influence beam characteristics and, consequently,imaging performance, such as SIL misalignmenttolerances and beam propagation from the SIL, constitutethe primary focus of this investigation. Numericalsimulations demonstrate that the system can toleratemillimeter-level transverse and longitudinal SIL positiondisplacements, underscoring its robustness for subwavelength imaging in a wide frequency range.Additionally, numerical simulations of beam propagationcharacteristics reveal that the system achieves subwavelength imaging resolution up to 1 mm from the SIL at0.5 THz, highlighting its potential for non-destructivetesting of subsurface structures. These findings gainexperimental validation through imaging stacked utilityknife blades with sub-wavelength structures ranging from0.2 to 2 THz

Tunable multifunctional terahertz metasurface based on indium antimonide medium

Ming-yang Jiang, Jiusheng Li, and R. Xiong

DOI: 10.1364/AO.519334 Received 17 Jan 2024; Accepted 11 Apr 2024; Posted 11 Apr 2024  View: PDF

Abstract: Active adjustable terahertz multifunctional devices are crucial forthe application of terahertz technology. In this article, we propose a compositemetasurface structure based on indium antimonide metal octagonal pattern,which achieves different functional switching by controlling the phase stateof indium antimonide material under different ambient temperature. Whenindium antimonide exhibits in the dielectric state, by stacking and encodingthe unit cell, the designed metasurface has the functions of two beam splittingbeams superposition, vortex beam and quarter beam superposition, and dualvortex beam superposition for circularly polarized and linearly polarizedwave incidence. When indium antimonide appears in the metallic state, theencoding metasurface alters the modulation function of incident circularlypolarized and linearly polarized terahertz waves. This terahertz metasurfaceprovides a new approach for the design of multifunctional devices that canflexibly regulate terahertz wave metasurfaces.

Assessment of the effects of 2,4-Ddichlorophenoxyacetic acid based herbicide exposure in eggs and embryos of golden apple snail (Pomacea canaliculata) using Optical Coherence Tomography (OCT)

Chutima Thanomsit, Chaiprasarn Khumsopha, JADSADA SAETIEW, Panomsak Meemon, Phochit Nanthanawat, and Samnao Saowakoon

DOI: 10.1364/AO.503470 Received 02 Oct 2023; Accepted 11 Apr 2024; Posted 12 Apr 2024  View: PDF

Abstract: This study aimed to evaluate the effects of herbicide 2, 4-D-dichlorophenoxy aceticacid on golden apple snail eggs and embryos. Additionally, the study assessed the applicabilityof Optical Coherence Tomography (OCT), a non-invasive depth-cross-sectional microscopicimaging technique, as a novel method for studying morphological changes in golden apple snaileggs and embryos, in comparison to the conventional approach of using white light microscopy.The study revealed that the herbicide 2,4-D-dichlorophenoxy acetic acid affected the hatcheryrate and morphological changes of the eggs and embryos. The lethal concentration (LC50),representing the concentration of a substance that is expected to cause death in half of thepopulation being studied, of the golden apple eggs and embryos increased with longer exposuretime and higher concentrations. The estimated median effective concentration (EC50), whichdenotes the concentration producing the desired effect in 50% of the exposed golden appleembryos, exhibited a similar trend of change as the LC50. When compared to the microscopicstudy, it was observed that OCT could be employed to investigate morphological changes ofgolden apple snail eggs and embryos, enabling evaluation of alterations in both 2D and 3Dstructures.

Experimental Investigations on FSO-Fiber Converged Communication System under Fog and Turbulence

Champalal Lalani, Aashish Mathur, and Nitin Bhatia

DOI: 10.1364/AO.519567 Received 22 Jan 2024; Accepted 11 Apr 2024; Posted 12 Apr 2024  View: PDF

Abstract: This paper presents experimental results for a free-space optical-fiber converged (FSO-FC) communicationsystem under varying turbulence and foggy conditions. Based on the experimental measurements, westatistically characterize the FSO channel under different levels of turbulence and fog in terms of theirrespective probability density functions (PDFs). Our experimental PDFs fit well with the theoretical PDFsproposed in the literature. We experimentally evaluate the average bit error rate (ABER) of the consideredFSO-FC communication system under different levels of turbulence and fog. Further, to compensate forthe effects of turbulence and fog, we use multi-mode fiber (MMF) as the receiver and compare the resultswith single-mode fiber (SMF) as the receiver. Finally, we show the improvement in ABER under varyingturbulence and fog by adding cyclic redundancy check (CRC) bits to data bits.

Thermo-optic reconfigurable three-mode (de)multiplexer based on asymmetrical horizontal directional coupler

Areez Khalil Memon, Hao Zhang, and Kaixin Chen

DOI: 10.1364/AO.519732 Received 22 Jan 2024; Accepted 11 Apr 2024; Posted 11 Apr 2024  View: PDF

Abstract: We propose a thermo-optic reconfigurable three-mode (de)multiplexer based onasymmetrical horizontal three-waveguide directional coupler formed with two identical singlemode waveguides and a three-mode waveguide. Over the whole wavelength range of 1540-1560 nm and for the TE (TM) polarization, our typical fabricated device with polymer materialshows coupling efficiencies as high as 94% (93%) and 93% (92%) for the mode conversionsof LP01-LP11a and LP01-LP11b, with the heating powers of 53.57 mW and 71.19 mW,respectively. Our proposed device could find applications in the fields of reconfigurable modedivision multiplexing systems.

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

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

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

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

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

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

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

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

High precision 3×3 coupler demodulation algorithm with an additional phase shift judgment module

Shumin Pan, Qiang Ge, Gang Zhang, Jian Xiao, Dingli Xu, and Benli Yu

DOI: 10.1364/AO.521162 Received 20 Feb 2024; Accepted 10 Apr 2024; Posted 12 Apr 2024  View: PDF

Abstract: Ellipse fitting algorithms (EFAs) have been widely used in 3×3 coupler demodulation systems to reduce the requirementfor symmetry of 3×3 couplers. Based on the relative stability of the splitting ratio and phase difference after theestablishment of the 3×3 coupler demodulation system, we solve the problem that EFA fails to work when thestimulating signal is small. Depending on the existence of a symmetry point about the origin, an additional phase shiftjudgment module is used to determine whether the Lissajous figure is larger than π rad. If the elliptical arc exceeds πrad, the EFA is executed. Otherwise, the previous parameters are used to ellipse correct. Parameters are updated in realtime to ensure high precision. The experimental results show that the total harmonic distortion (THD) of theameliorated algorithm is improved by 1.28% compared with the EFA without the judgment module with a stimulusamplitude of 30 mV. The proposed scheme can effectively improve the dynamic range of the 3×3 coupler demodulationto reach 125.64 dB @1 kHz &1% THD. The algorithm ensures the effective operation of the EFA under small phase shiftconditions and improves the accuracy of phase demodulation.

Adaptive decision threshold algorithm based on sliding window to reduce BER of Freespace optical communication systems

ruilei ying, Yunqiang Zheng, sentao wei, Yuanchen He, Xie Zhuang, mingze he, and Wei Wang

DOI: 10.1364/AO.519321 Received 22 Jan 2024; Accepted 10 Apr 2024; Posted 11 Apr 2024  View: PDF

Abstract: Free-space optical communication (FSOC) systems face susceptibility to severalfactors, such as transmission distance, atmospheric turbulence, and alignment errors. Theseelements contribute to fluctuations in the signal strength reaching the receiver. The resultantsignal fluctuations can result in misjudgments and an elevated Bit Error Rate (BER). This paperproposes an Adaptive Decision Threshold algorithm based on a Sliding Window (ADTSW).By estimating received signal parameters and delimiting the amplitude interval, the algorithmensures that the decision threshold tracks signal fluctuations, thereby reducing signalmisjudgment. The effectiveness of the algorithm is validated through simulations andexperimentation. When signal peak-to-peak fluctuation, simulation results demonstrate that theproposed algorithm achieves a one-order-of-magnitude reduction in BER compared totraditional Fixed Decision Threshold (FDT) method. Under the influence of weak atmosphericturbulence with different scintillation variance, both simulation and experimentation indicate aone-order-of-magnitude reduction in BER compared to FDT method. The ADTSW algorithmproves its capability in minimizing misjudgments, thereby effectively reducing BER andimproving communication quality.

Loss analysis of grating coupler for single mode fiber coupling into SOI waveguide

Ziyu Cai

DOI: 10.1364/AO.517141 Received 28 Dec 2023; Accepted 10 Apr 2024; Posted 11 Apr 2024  View: PDF

Abstract: SOI (Silicon On Insulator) technology is widely used in silicon photonic integrated circuits, how to improve the coupling efficiency of light coupling in free space and optical fibers into waveguides on SOI must be discussed. Grating coupling is a commonly used and highly efficient coupling method. This article discusses the causes of loss in grating couplers from three aspects: transmission, reflection, and mode mismatch, and proposes corresponding loss reduction solutions. The coupling efficiency of Si and SiN grating couplers optimized according to the loss reduction scheme has been improved by 25% and 45% respectively.

Research and application of wavelet high-frequency coefficient direct mapping algorithm in micro-defect target detection system of infrared objective lens

Youtang Gao, Hua Wen, Mei Ying, and Jiaxian Zhang

DOI: 10.1364/AO.517217 Received 26 Dec 2023; Accepted 09 Apr 2024; Posted 09 Apr 2024  View: PDF

Abstract: Various lens detection techniques for military sighting telescope have been thefocus of military production. However, most of the micro-defect detection of lenses aredetected manually, which cannot guarantee the measurement precision and quality. In order tosolve these problems, the wavelet high-frequency coefficients direct mapping algorithm isapplied to the automatic detection system of micro-defect target in infrared objective lens,thus obtaining a new method of detection. The detection method utilizes the correlationfeature between wavelet coefficient scales to process the acquired images to suppress thebackground signals. Then the noise is further filtered by pipelined filtering method to finallyrealize the detection of micro-targets. The experimental results show that a relatively highsignal-to-noise ratio can be obtained, the method can detect the small and dim target whosesignal-to-noise ratio is more than 2.5 times. The measurement range is ≥3 pixels and the errorprecision is 4±1 pixels.

Highly precise thickness measurement of multilayer films based on the crosscorrelation algorithm using a widely tunable MG-Y laser

Yang Cheung, Zhenguo Jing, Ang Li, Pengdong Cao, Da-Peng Zhou, and Wei Peng

DOI: 10.1364/AO.522283 Received 26 Feb 2024; Accepted 09 Apr 2024; Posted 09 Apr 2024  View: PDF

Abstract: Inspired by the demodulation algorithm of Fabry-Perot composite sensors in the fieldof fiber-optic sensing, this paper proposed a method based on a widely tunable modulatedgrating Y-branch (MG-Y) laser combined with the cross-correlation algorithm to achievehighly precise measurement of the optical thickness of each layer of a multilayer optical sample.A sample consisting of a double glass stack was selected, and the interference spectrum of thestacked sample was acquired using a widely tunable MG-Y laser. A fast Fourier transform(FFT) algorithm combined with a finite impulse response (FIR) bandpass filter was utilized toseparate the different frequency components of the multilayer optical sample. The normalizedspectra of each layer were reconstructed using the Hilbert transform. Subsequently, a crosscorrelation algorithm was employed to process the normalized spectrum and determine theoptical thickness of each layer with high precision. The samples were measured atpredetermined locations, with 150 consecutive measurements performed to assess the repetitionof the thickness. The standard deviation of these measurements was found to be lower than 1.5nm. The results show that the cross-correlation algorithm is advantageous in the opticalthickness measurement of multilayer films.

Design of optical passive semi-athermalization zoom lens

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

DOI: 10.1364/AO.517025 Received 26 Dec 2023; Accepted 09 Apr 2024; Posted 10 Apr 2024  View: PDF

Abstract: Traditional zoom lenses cannot clearly image during the entire zoom process whenthe ambient temperature changes and need to focus frequently at middle focal length positions.An innovative design method called optical passive semi-athermalization (OPSA) design forzoom optical systems is proposed, which based on the difference in focusing sensitivity offocusing group at short and long focal length positions, seeks out sensitive groups that have agreater impact on imaging quality at the short focal position. By changing the temperaturecharacteristics of the temperature-sensitive lenses in these groups, an OPSA zoom opticalsystem can be realized, which exhibits a compact structure and excellent imaging quality.Under ambient temperature of −40℃ to +60℃, the OPSA zoom lens needs to refocus onlyonce at the long focal length position, which can ensure image clearly during the entire zoomprocess. Remarkably, this innovative method not only mitigates the frequent focusingchallenges in traditional zoom lenses but also contributes to the diminutive size.

Automatic generation method for long-focallength unobscured freeform optical systems with small volume

Yiwei Sun, Yangjie Wei, Xinyu Di, and Ji Zhao

DOI: 10.1364/AO.524442 Received 20 Mar 2024; Accepted 09 Apr 2024; Posted 12 Apr 2024  View: PDF

Abstract: The existing design methods for long-focal-length unobscured freeform systemsrarely consider the imaging quality requirements and volume constraints simultaneously,causing that most of the final designs do not fulfill the requirement of light weight. This studyproposes a method to automatically design a long-focal-length unobscured reflective systemthat satisfies volume constraints while maintaining high imaging quality. First, a method toadaptively set the structural parameter range is proposed, and multiple parameters for differentsystemic specifications can be effectively calculated within it. Subsequently, the systemicvolume and area functions are constructed using the ray tracing method, where the tilt angles,distances between mirrors, and radii of curvature of the mirrors are chosen as the optimizationparameters. Third, a comprehensive objective function is jointly established combining rayobscuration and convergence as performance evaluation factors. Then, the structuralparameters of a long-focal-length unobscured system with small volume are easily obtained viathe simulated annealing method. Finally, the improved W-W method is used to further enhancethe imaging quality of the system, and an unobscured freeform reflective optical system withthree mirrors is automatically generated. Experimental results demonstrate that our method canautomatically calculate the parameter ranges to facilitate the search for structural parameters,and effectively design the long-focal-length unobscured freeform systems with small volumeand high-imaging-quality.

The optimal vector phase matching conditions in biaxial crystalline materials determined by extreme surfaces method: the case of orthorhombic crystals

Oleh Buryy, Dmytro Shulha, NAZARIY ANDRUSHCHAK, Andriy Danylov, Bouchta Sahraoui, and ANATOLIY ANDRUSHCHAK

DOI: 10.1364/AO.518690 Received 12 Jan 2024; Accepted 08 Apr 2024; Posted 09 Apr 2024  View: PDF

Abstract: The optimal geometries of vector phase matching are determined for the cases ofsecond harmonic, sum and difference frequency generation in a number of orthorhombicnonlinear optical crystals – KTP, KTA, KB5, KNbO3, LBO, CBO, LRB4. Extreme surfacemethod was used to define wave vectors directions of highest possible generation efficiency.As it is shown, in a significant number of cases vector phase matching ensures higherefficiencies than the scalar one.

Investigating turbulence distribution in the lower atmosphere using time-lapse imagery from a camera bank

Benjamin Wilson, Santasri Bose-Pillai, Jack McCrae, Steven Fiorino, Robert Freeman, and Laura Slabaugh

DOI: 10.1364/AO.521082 Received 06 Feb 2024; Accepted 08 Apr 2024; Posted 09 Apr 2024  View: PDF

Abstract: The atmosphere’s surface layer (first 50 -100 m above the ground) is extremelydynamic and is influenced by surface radiative properties, roughness, and atmospheric stability.Understanding the distribution of turbulence in the surface layer is critical to many applications,such as directed energy and free space optical communications. Several measurement campaignsin the past have relied on weather balloons or Sonic Detection and Ranging (SODARs) tomeasure turbulence up to the atmospheric boundary layer. However, these campaigns had limitedmeasurements near the surface. We have developed a time-lapse imaging technique to profileatmospheric turbulence from turbulence-induced differential motion or tilts between featureson a distant target, sensed between pairs of cameras in a camera bank. This is a low-cost andportable approach to remotely sense turbulence from a single site without deployment of sensorsat the target location. It is thus an excellent approach to study the distribution of turbulence inlow altitudes with sufficiently high resolution. In the present work, the potential of this techniquewas demonstrated. We tested the method over a path with constant turbulence. We explored theturbulence distribution with height in the first 20 m above the ground by imaging a 30 m watertower over a flat terrain on three clear days in summer. In addition, we analyzed time-lapse datafrom a second water tower over a sloped terrain. In most of the turbulence profiles extractedfrom these images, the drop in turbulence with altitude in the first 15 m or so above the groundshowed a ℎ𝑚 dependence, where the exponent m varied from -0.3 to -1.0, quite contrary to thewidely used value of -4/3.

Intensity Correlation Imaging Design for Geostationary Satellite Inspection

David Hyland

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

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

Cryogenic Focus Measurement System for a Wide-Field Infrared Space Telescope

Samuel Condon, Stephen Padin, James Bock, Howard Hui, Phillip Korngut, Chi Nguyen, and Jordan Otsby

DOI: 10.1364/AO.514784 Received 30 Nov 2023; Accepted 07 Apr 2024; Posted 08 Apr 2024  View: PDF

Abstract: We describe a technique for measuring focus errors in a cryogenic, wide-field, near-infrared space telescope.The measurements are made with a collimator looking through a large vacuum window, with a reflectivecold filter to reduce background thermal infrared loading on the detectors and optics. The vacuum windowand cold filter introduce wavefront error which we characterize using an autocollimating microscope. Forthe 300 mm diameter aperture f /3 space telescope, SPHEREx, we achieve a focus position measurementwith ∼ 15 µm systematic and ∼ 5 µm statistical error.

Analysis of the Hybrid Light Field Reconstruction and Comparison With Richardson-Lucy Light Field Deconvolution

Viktor Eckstein, Tobias Schmid-Schirling, Daniel Carl, and Ulrike Wallrabe

DOI: 10.1364/AO.516573 Received 20 Dec 2023; Accepted 07 Apr 2024; Posted 08 Apr 2024  View: PDF

Abstract: Conventional microscopes have a high spatial resolution and a low depth-of-field. Light field microscopes have a high depth-of-field but low spatial resolution. A new hybrid approach uses information from both systems to reconstruct a high-resolution light field [Applied Optics 58, A142 (2019)]. The resolution of the generated light field is said to be limited only by diffraction and the size of the pixels. In this paper, we evaluate this method. Using simulation data we compare the output of the hybrid reconstruction algorithm with its simulated ground truth. Our analyses reveal that the observed improvement in light field quality is not a consequence of data fusion or incorporation of information from a conventional camera. Rather, the enhancement originates from an intermediate interpolation step within the light field itself. This suggests that the required information is already inherent in the light field. By employing the Richardson-Lucy Light Field Deconvolution Algorithm, we demonstrate that existing algorithms have already been utilizing this information.

Research on measurement technology based on Stokes parametric polarization system

Zhenmin Zhu, Hongwei Qiu, Wenqing Long, Lifa He, Dongdong Xie, and Yinsong Ye

DOI: 10.1364/AO.517878 Received 05 Jan 2024; Accepted 07 Apr 2024; Posted 08 Apr 2024  View: PDF

Abstract: Structured light measurement systems often use polarization filters to reduce imageinterference from highly reflective areas. This method can be effective, but it may also reduce thebrightness of specific areas, particularly overly dark portions, which can affect the accuracy of themeasurement results. This paper proposes a measurement method for a polarization system based onStokes parameters to solve the problem. After adjusting the polarization filter to angles of 0°, 45°, and90°, the camera captures an image of the object and calculates the corresponding Stokes parameters togenerate the expected polarization angle histogram. Then, based on the detailed information on the angledistribution, the accurate mathematical model is used to screen the interval, and the optimal polarizationangle is determined by orthogonal processing while ensuring signal-to-noise ratio and image quality.Finally, image fusion technology synthesizes a set of fringe projection images with the preferredpolarization angles. Experiments have shown that this new method effectively addresses the issue ofinterference in the highlight region when using conventional polarization filters. Additionally, itsignificantly improves the quality of the fringe pattern. The polarization angle selection in theexperimental process is made more rapid and accurate through the quantitative mathematical modelcalculation of the polarization angle, significantly improving the system's measurement efficiency.

Parameters selection on multi-exposure fusion method for measuring surfaces with varying reflectivity in microscopic fringe projection profilometry

Junlin Du, Fan Yang, Hong Guo, Jiangping Zhu, and Pei Zhou

DOI: 10.1364/AO.517940 Received 08 Jan 2024; Accepted 07 Apr 2024; Posted 08 Apr 2024  View: PDF

Abstract: As industrial and scientific advancements continue, the demand for precise measurement of threedimensional (3D) shapes and surfaces is steadily increasing. However, accurate 3D measurement ofcertain surfaces, especially those with varying reflectivities, has always been a challenging issue. Multiexposure fusion methods have shown stable, high-quality measurement results, but the selection ofparameters for these methods has largely been based on experience. To address this issue, this paper hasimproved the multi-exposure fusion method and introduced a guided approach for parameter selection,significantly enhancing the completeness of measurement results. Additionally, a comparative model isdeveloped to experimentally validate the specific impacts of Gaussian window variance, Optimal GrayscaleRange, and attenuation factor variance on the integrity of 3D reconstruction. The experimental resultsdemonstrate that under the guidance of the parameter adjustment method proposed in this paper, themulti-exposure fusion for measuring the 3D topography of high dynamic surfaces improves the restorationcoverage from the original 86% (bright areas) and 50% (dark areas) to over 99%. This provides a selectionstrategy for parameter adjustment guidance in precise measurements based on the multi-exposure method.

Improving image quality of phase-added stereogram using deep learning

MAHIRO BABA, David Blinder, Fan Wang, Tobias Birnbaum, Peter Schelkens, Tomoyoshi Ito, and Tomoyoshi Shimobaba

DOI: 10.1364/AO.521087 Received 07 Feb 2024; Accepted 06 Apr 2024; Posted 08 Apr 2024  View: PDF

Abstract: The disadvantage of computer-generated holograms (CGHs) using the direct integral method are the highcomputational requirements with increased object points and hologram size. This can be addressed byphase-added stereogram (PAS), a fast calculation method for CGHs. PAS divides the hologram into smallblocks and calculates the point-spread functions (PSFs) of the object points in the Fourier domain of eachblock. The PSF can be approximated using sparse spectra, which accelerate calculations. However, thisapproximation degrades the image quality. In this study, we improved the image quality of the PAS usingdeep learning while maintaining high computational speed.

Initial design with confocal conic mirrors for freeform optical imaging systems

Linyue Fang, Hongzuo Chen, Ting Chen, and Rengmao Wu

DOI: 10.1364/AO.522603 Received 28 Feb 2024; Accepted 06 Apr 2024; Posted 08 Apr 2024  View: PDF

Abstract: Freeform surfaces which can offer high degrees of freedom for aberration correctionhave been widely used in various imaging applications. Initial design is particularly critical inthe optical design of freeform imaging systems due to the significantly expanded solution space.Here, we present a method to find an initial point for the optical design of four-mirror freeformimaging systems. The method is computationally simple, easily accessible and theoreticallysupported. The effectiveness of the method is demonstrated by designing a four-mirror confocalsystem without field-constant aberration and linear astigmatism. We also generalize theproposed method to four-mirror confocal systems with “double-pass surface”. The proposedmethod can yield a promising starting point for the design of freeform off-axis imaging systems.

Comparison of Two-Wavelength Pyrometer System and Spectral Pyrometry for High-Temperature Measurements

Sama Aljohani, IBRAHIM ALSHUNAIFI, Naif Alqahtani, and Bader Alfarraj

DOI: 10.1364/AO.522898 Received 07 Mar 2024; Accepted 06 Apr 2024; Posted 08 Apr 2024  View: PDF

Abstract: A pyrometer system is an optically passive, non-intrusive method that uses thermal radiation law to determine temperature. Itcombines electronic and optical instruments to detect low-level signals of radiation measurements. Surface high-temperaturemeasurements are successfully obtained using a two-wavelength pyrometer system. This study used a pyrometer system toachieve high stability, minimize errors due to changing emissivity, and remove background noise from the radiationmeasurement for surface high-temperature measurements. Temperature measurements were also obtained from Planck’smodel and compared the results with logarithmic assumption. The precision of these measurements is improved throughvariable optimization of the instruments, validation of the data, and calibration of the pyrometer system. The sixteentemperature measurements were obtained (800-1600°C temperature measurement range) with a correlation coefficient above97%. The response time between temperature readings is within 785 µs. Furthermore, the high-temperature measurementswere obtained with higher stability (±2.99°C at 1600°C) and less error (less than 2.29% for Si sensor). In addition, the error ofthe temperature measurement was reduced from 5.33% to 0.86% at 850°C by using Planck’s model compared with usinglogarithmic assumption. A cooling system temperature is also optimized to reduce the error temperature reading. It was foundto be at 10°C that the uncertainty was reduced from 2.29% at ambient temperature to 1.53% at 1600°C. The spectral pyrometrysystem was also used in comparison with two-wavelength pyrometer system to confirm that the calibration curves of the spectralpyrometry can be used to determine temperature measurements.

A Broadband Photoelectric Fusion Transceiver-multiplexed Conversion System with High Spurious Rejection

hanyu wang, Zeping Zhao, Zijian Hu, chaoquan wang, Zhang Weijie, and Jianguo Liu

DOI: 10.1364/AO.520602 Received 31 Jan 2024; Accepted 05 Apr 2024; Posted 08 Apr 2024  View: PDF

Abstract: In this paper, a broadband photoelectric fusion transceiver-multiplexed system isproposed to realize a frequency converter. The system achieves a high spurious suppressionratio through two frequency conversions, which utilizes the advantages of microwave andphotonics technology simultaneously to reduce the complexity of the system and improve theeffective spectrum utilization. In addition, the core components such as Mach-Zehndermodulator (MZM) are multiplexed in the up and down frequency conversion link. Highfrequency local oscillator (LO) signals are used to keep image frequency signals and variouskinds of spurious signals obtained by beating frequency outside the system bandwidth.Experimental results demonstrated that the operating frequency range from 2GHz to 18GHzwith high performance for both transmitter and receiver. The image rejection is 57.35dB forup-conversion and 46.56dB for down-conversion, and the in-band spurious suppressionachieves at least 55.02dB. At the same time, the spurious-free dynamic range (SFDR) can reachat least 89.11dB·Hz2/3.

Multi-layer lensless camera for improving the condition number

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

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

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

A method for the production of a compact source of atomic line spectra in the vacuum ultraviolet

Jennifer Hare, David Wilmouth, Jessica Smith, J. Klobas, Darin Toohey, Reem Hannun, Jasna Pittman, and James Anderson

DOI: 10.1364/AO.520905 Received 07 Feb 2024; Accepted 05 Apr 2024; Posted 05 Apr 2024  View: PDF

Abstract: Atomic emission spectra provide a means to identify and to gain insight into the electronic structure of emitting or absorbing matter. Detailed procedures are provided for the construction of the low-pressure electrodeless discharge lamps that yield targeted emission in the vacuum ultraviolet for the spectroscopic study of water vapor and halogen species aboard an array of airborne observation platforms in the upper atmosphere, as well as in laboratory environments. While specific to the production of Lyman-alpha, atomic chlorine, and atomic bromine emissions in this study, the configuration of the lamps and their interchangeability with respect to operation lend these procedures to constructing sources engaging a wide selection of atomic and molecular spectra with straightforward modifications. Features and limitations of each type of lamp are discussed, as well as methods to improve spectral purity and factors affecting operational lifetime.

Pixelated gradient thickness optical filter for visible light spectroscopy

Pratyasha Sahani, Shiori Nabana, Taiyu Okatani, Naoki Inomata, and Yoshiaki Kanamori

DOI: 10.1364/AO.519521 Received 22 Jan 2024; Accepted 05 Apr 2024; Posted 08 Apr 2024  View: PDF

Abstract: A miniature low-cost pixelated gradient thickness opticalfilter is proposed to achieve spectroscopy in the visiblewavelength range. The optical filter consists of a twodimensional array of metal-dielectric-metal thin filmsarranged in Fabry–Pérot filter configurations withdiscretely varying cavity thickness. The wavelengthselective characterization of each filter is performed bymeasuring the transmittance over the visible wavelengthrange. The pixelated gradient thickness filter is equippedwith a CMOS image sensor, and its performance as aspectroscopic module is evaluated by illuminatingdifferent monochromatic wavelengths on it. The targetspectra are successfully reconstructed from the outputsignals recorded in the sensor from the respectivepixelated gradient thickness filters. The technologicalcompetence of the proposed filter will enable its use inhandheld devices to widen its application range in day-today life.

Spectral imaging based on custom-made multi-strip filter arrays

Quan Guo, Dengshan Wu, and Weixing Yu

DOI: 10.1364/AO.522642 Received 28 Feb 2024; Accepted 05 Apr 2024; Posted 08 Apr 2024  View: PDF

Abstract: Spectral imaging technology based on on-chip spectroscopy can find applications in areas including aerospace, industrial,and consumer electronics and so on. Since each application normally requires a different set and number of spectralbands, the development of customized spectroscopy solutions with more compact size and lower cost becomes quiteimportant. In this paper, we demonstrate a compact, highly customizable imaging spectrometer scheme based on custommade multi-strip filter arrays, which maintains an average high transmission of ~85%, narrow bandwidth of ~30 nm, andhigh optical density of ~OD2 in the blocking regions across the visible to near-infrared waveband. Spectral imagingexperiments are conducted, and the accurate reconstruction of sparse spectral image data is demonstrated as well toprove the validity of the proposed scheme. As a result, the work reported in this paper allows researchers to develop acustomized spectral imaging equipment in a relatively easy way and also has a great potential to be engineered furtherfor scalable production with a quite low cost.

Wave optical propagation in realistic lens systems through multi-slice decomposition with phase compensation (MSDP)

YINGLI LIU, fengxu guo, Zijun Zhang, and Rengmao Wu

DOI: 10.1364/AO.522702 Received 01 Mar 2024; Accepted 04 Apr 2024; Posted 05 Apr 2024  View: PDF

Abstract: : Fast and accurate optical simulation is essential in the design and analysis of opticalsystems. We propose a wave propagation method through multi-slice decomposition with phasecompensation (MSDP) to calculate the wave propagation in realistic lens systems. Estimationof the phase compensation term for the refraction effect and diffraction effect is given and amodified phase compensation term is presented for calculation through optical systems with ahigh numerical aperture or incident optical field featuring a broad spectrum. The proposedmethod yields accurate and fast results for micro-optical lens systems with the radius ofcurvature comparable to illumination wavelength. In a macroscopic optical system with a highnumerical aperture, an object with abundant spatial frequencies is reconstructed from thediffraction patterns produced by a single aspherical lens, further demonstrating theeffectiveness and applicability of the method in realistic lens systems.

Analysis and experimental verification of the polarization characteristics of cube-corner reflectors

ning li, Fajia Zheng, Qibo Feng, Bin Zhang, and Fei Long

DOI: 10.1364/AO.516101 Received 14 Dec 2023; Accepted 04 Apr 2024; Posted 05 Apr 2024  View: PDF

Abstract: The polarisation effect of cube-corner reflectors (CCRs), which influences theperformance of optical systems, requires comprehensive analysis. This study developed amodel for the polarisation state of both uncoated solid and hollow CCRs using the Jonesmatrix derivation and Zemax software simulations. The accuracies of theoretical analyses andsimulations were verified using an experimental setup. Theoretical analysis, simulation, andexperimental results revealed that hollow CCRs are insensitive to the polarisation state of theincident light, exhibiting average variations of 0.8° and 0.7° in the polarisation direction andellipticity, respectively. Contrastingly, the high sensitivity of solid CCRs to the polarisationstate of the incident light varied across different incident regions. The propagation paths 2–1–3 and 3–1–2 with minor polarisation effects involved light that entered from one side of theCCR, traversed the bottom, and emitted from the other side. In these regions, the averagevariations in the polarisation direction and ellipticity were 10.7° and 6.6°, respectively,whereas more affected regions exhibited corresponding values of 44.8° and 20.0°. Thesefindings guide the enhancement and optimization of the performance of optical systems usingCCRs.

MPLC-based Low-modal-crosstalk Nine-mode-group Demultiplexer for DSP-Free IM/DD MDM Transmission over MMF

Jian Cui, Yu Deng, Liu Zhuo, Bin Hao, Chao Wu, Daiwei Tan, Ming Jiang, Leimin Zhang, Zhangyuan Chen, Yongqi He, Juhao Li, and Qi Wan

DOI: 10.1364/AO.516866 Received 25 Dec 2023; Accepted 04 Apr 2024; Posted 05 Apr 2024  View: PDF

Abstract: Weakly-coupled mode-division multiplexing (MDM) transmission technique over legacy laid multimode fiber (MMF) hasgreat economic efficiency and can enhance the capacity of short-reach optical interconnections enormously. In order tobe compatible with cost-efficient intensity-modulation/direct-detection (IM/DD) transceivers, weakly-coupled modegroup demultiplexers which can simultaneously receive each mode group of MMFs are highly desired. In this paper, wepropose a scalable low-modal-crosstalk mode-group demultiplexer over MMF based on multi-plane light conversion(MPLC), in which multiple input Hermite-Gaussian (HG) modes of MMF are firstly converted to bridging modes thatcomposed of HG00 modes distributed as a right-angled triangle in Cartesian coordinates, and then each HG00 modebelonging to a degenerate mode group are respectively mapped to different overlapped HGn0 modes with verticalorientation for simultaneous detection. With the help of bridging modes, the MPLC-based mode-group demultiplexer canefficiently demultiplex all mode groups in standard MMFs with less than 20 phase masks. A 9-mode-group demultiplexeris further designed for demonstration, and simulation results show that the MPLC-based demultiplexer achieves lowmodal-crosstalk of lower than -22.3 dB at 1550 nm and lower than -17.9 dB over the C-band for all the 9 mode groups withonly 16 phase masks.

Eliminating sticking effects among fine strokes in micro-patterning complex Chinese characters

xiangbin kong, Jingsong Wei, Zhihong Huang, and Tianyu Gao

DOI: 10.1364/AO.519026 Received 14 Jan 2024; Accepted 04 Apr 2024; Posted 05 Apr 2024  View: PDF

Abstract: In this work, the complex Chinese micro-patterns are experimentally inscribed inAgInSbTe (AIST) thin films through laser writing technique. The overlapping inscriptionstrategy is introduced to reduce the size of complex Chinese micro-patterns, and the erosionalgorithm is used to eliminate sticking effects among fine strokes in the complex Chinesemicro-patterns. The results show that the size of complex Chinese micro-patterns can bereduced to around 15.41×15.41 μm. The width of the finest stroke is only 0.45 μm. This methodcan achieve a fidelity of 95%. This work helps with the long-term preservation of massiveChinese character archives.

The Spatially Synchronous Fringe Detection: a method to facilitate the alignment of echelle grating mosaic by separating the compensative angular errors

jian Han, KUI DING, Lizon JEAN Louis, XIN QIQIGE, RUI WANG, Kai Zhang, and Dong Xiao

DOI: 10.1364/AO.519106 Received 17 Jan 2024; Accepted 04 Apr 2024; Posted 05 Apr 2024  View: PDF

Abstract: Alignment of mosaic grating is traditionally supported by two interferometricverifications: on the zero order to verify the grating surfaces, on the blaze to verify the grovesdirection. In the case of low frequency echelle grating an interferometric measurement on thezero order is hardly feasible due to extremely low contrast of the fringes. The completealignment has then to be carried out on high order (close to the blaze) where the twomisalignment errors (the tip and rotation) show the same effect on the interferogram. Acquiringa low and a high diffraction order image simultaneously, referred to as the SpatiallySynchronous Fringe Detection method (SSFD), is used to analyze the misalignment. Iterativeadjustment with the autocollimation configuration at the low and high order is used to separatethe compensative errors of ∆θx and ∆θz. A prototype mosaic with two 110mm×220mmsegments has been aligned with the support of this method. A numerical simulation of thealignment procedure as well as the error orientation analysis of this mosaic grating arepresented. The mosaic grating with an accuracy of ∆θx<0.64μrad, ∆θy<1.13μrad, ∆θz<0.65μrad,and a wavefront RMS error of 0.149λ has been completed. This method can greatly facilitatethe alignment of echelle mosaic for astronomical spectrograph.

Accurate Internal Cavities and Kissing bonds Sizing in Metal Plates by using the Time-of-Flight of LaserInduced Ultrasound Waves

Melissa Rojas-Romero, Orlando Medina, Francisco J. García-Rodríguez, Arturo González-Vega, Geminiano Martinez-Ponce, and Gerardo Gutiérrez-Juárez

DOI: 10.1364/AO.519588 Received 25 Jan 2024; Accepted 04 Apr 2024; Posted 05 Apr 2024  View: PDF

Abstract: This paper presents a nondestructive method for accurately identifying internal flaws in metal plates, which is crucial forensuring structural integrity in safety-critical applications. The technique relies on analyzing Laser-Induced Ultrasound(LIU) longitudinal wave time-of-flight, as demonstrated through a theoretical five-layer model. Experimental validationwas conducted using a piezo sensor in contact with a slab containing millimetric artificial cavities immersed in air,resulting in a discrepancy of 5.05%. In contrast, experiments performed in a water medium exhibited a lower discrepancyof 2.5% (discrepancy refers to differences between measurements obtained through experimental time-of-flight analysisand caliper measurements). The results obtained in water-based experiments affirm the accuracy of the proposed model.B-scan measurements and the five-layer model were utilized to generate 2D reconstructed images, enabling preciselocalization and sizing of cavities and kissing bonds between plates, finding an average size of a kissing bond of 30µm. Inconclusion, the proposed five-layer model, based on longitudinal wave time-of-flight analysis, provides a straightforwardframework for an easy cavity and kissing bonds measurements in metal plates.

Optical cavity reaching 10e-16 frequency stability with compact optical circulator based in-coupling optics

Timm Wegehaupt, Josep Sanjuan, Martin Gohlke, Pascal Grafe, Lee Kumanchik, Markus Oswald, Thilo Schuldt, and Claus Braxmaier

DOI: 10.1364/AO.522293 Received 23 Feb 2024; Accepted 03 Apr 2024; Posted 03 Apr 2024  View: PDF

Abstract: Future space missions will benefit from highly stable and compact optical frequency references. While many promising technologies are currently under investigation, optical cavities are a well-suited technique for applications, in which relative references are required. To improve the frequency stability of optical cavities, a key step in combining high performance with compactness and robustness is the further development of the in-coupling optics. Here, we present our work of using a fiber-coupled circulator based in-coupling for a high-finesse optical cavity. Implementing the new in-coupling board to an extensively characterized crossed cavity set-up allows us to identify possible differences to the commonly used free-beam technique. With a frequency stability of 5.5e-16 1/sqrt(Hz) at 1 Hz and with only a slight degradation in frequency stability below the mHz range, no circulator caused instabilities were observed.

Unsupervised speckle denoising in digital holographic interferometry based on 4-f optical simulation integrated CycleGAN

HongBo Yu, Fang Qiang, qinghe song, Silvio Montresor, Pascal Picart, and Hai-Ting Xia

DOI: 10.1364/AO.521701 Received 16 Feb 2024; Accepted 03 Apr 2024; Posted 05 Apr 2024  View: PDF

Abstract: The speckle noise generated during digital holographic interferometry (DHI) isunavoidable and difficult to eliminate, thus reducing its accuracy. We propose a self-superviseddeep learning speckle denoising method using a cycle-consistent generative adversarialnetwork to mitigate the effect of speckle noise. The proposed method integrates a 4-f opticalspeckle noise simulation module with a parameter generator. In addition, it uses an unpaireddataset for training to overcome the difficulty in obtaining noise-free images and paired datafrom experiments. The proposed method was tested on both simulated and experimental data,with results showing a 6.9% performance improvement compared with a conventional methodand a 2.6% performance improvement compared with unsupervised deep learning in terms ofthe peak signal-to-noise ratio. Thus, the proposed method exhibits superior denoisingperformance and potential for DHI, being particularly suitable for processing large datasets.

Synchronized measurement method of burning rate and combustion temperature of solid propellant specimen

Hu Ni, Rong Fan, Haihang Hu, Bin Yang, Zhinxin Wang, Di Cao, Yang Yang, and Zhiquan Shi

DOI: 10.1364/AO.520172 Received 29 Jan 2024; Accepted 03 Apr 2024; Posted 05 Apr 2024  View: PDF

Abstract: To characterize the combustion properties of solid propellants, this paper proposes a hybrid method that combines shadow imaging and radiative imaging. Using spectroscopy and filtering, shadow and radiative images are obtained, and the burning rate and temperature of solid propellant samples are measured simultaneously. Measurement accuracies of burning rate and combustion temperature of solid propellant specimen are validated by other independent measurement methods. Compared with target-line measurement method, the burning rate measurement relative deviation is less than 3.1%. Compared with thermocouple measurement, the temperature measurement relative deviation is less than 1.7%. On this basis, the synchronized measurements of burning rate and combustion temperature of different formulations of solid propellant specimens under different working conditions are carried out. The results show that burning rate and combustion temperature are important characterization parameters of solid propellant combustion. For a certain formula solid propellant specimen, the burning rate can be greatly increased by increasing the pressure, but the combustion temperature is not much increased. Therefore, the synchronized measurement of burning rate and combustion temperature can provide more direct data support for the evaluation of solid propellant combustion performance.

Accuracy enhancement and joint calibration method of multi-station triangulation network based on absolute ranging

Shuo Yang, Yang Linghui, Tengfei Wu, Shendong Shi, Luyao Ma, and jigui zhu

DOI: 10.1364/AO.520571 Received 31 Jan 2024; Accepted 03 Apr 2024; Posted 05 Apr 2024  View: PDF

Abstract: Precision measurement methods and technologies for large-scale three-dimensional coordinates are urgently demandedin advanced equipment manufacturing. The multi-station triangulation network represented by the rotary-laser scanningmeasurement system has the advantages of high precision, multitask parallel measurement capability and high degree ofautomation. It is widely used in the docking of large components, quality control of key points, and collaborativepositioning of production equipment. Nevertheless, due to the limitation in measurement principle, the positioningaccuracy along the depth direction is notably lower compared to other directions, and this difference becomes morepronounced with increasing distance. This paper proposes a method to address this issue by integrating a distancemeasurement station into the network. A novel cooperative target, coupled with a high-dynamic beam guidancemechanism, are designed to achieve fast absolute distance measurement to the target. The weighted fusion of distanceand angle observations effectively enhances measurement accuracy while preserving the advantages of highly automatedmeasurement. Additionally, a joint calibration method for extrinsic parameters of multi-type stations is introduced. Highprecision absolute distances are utilized to establish optical scale bars, complemented by the incorporation of physicalscale bars, thereby obviating the necessity for using external reference instruments such as laser trackers. Finally, a seriesof experimental verifications demonstrate the effectiveness of calibration and measurement methods. The RMSE of allmeasured points drop to 42.6% of that the triangulation method measures.

Optimized dielectric mirror coating designs for quasi-harmonic cavity resonance

Li-Wei Wei, Harold Hollis, Benno Willke, Aaron Spector, and Guido Mueller

DOI: 10.1364/AO.519987 Received 24 Jan 2024; Accepted 02 Apr 2024; Posted 03 Apr 2024  View: PDF

Abstract: High-finesse optical cavities have a wide range of applications, some of which are bichromatic. Thesuccessful operation of high-finesse bichromatic cavities can demand careful control on the temperaturedependence of the wavelength-dependent reflection phase from the dielectric mirror coatings that constitutethe optical cavity. We present dielectric coating designs that are optimized for minimal differential changein reflection phase between a quasi-second-harmonic field and its fundamental field under temperaturechanges. These designs guarantee cavity resonance at a wavelength of interest via the control of itsquasi-harmonic field. The proposed coating designs are additionally examined for their sensitivity tomanufacture errors in coating layer thickness with promising results.

Temperature effects in narrow-linewidth optical cavity control with a surrogate quasi-second-harmonic field

Li-Wei Wei, Jan Poeld, Dennis Schmelzer, Kanioar Karan, and Benno Willke

DOI: 10.1364/AO.519988 Received 24 Jan 2024; Accepted 02 Apr 2024; Posted 03 Apr 2024  View: PDF

Abstract: Fabry-Perot cavities are widely used in precision interferometric applications. Various techniques havebeen developed to achieve the resonance condition via the direct interrogation of the cavity with the mainlaser field of interest. Some use cases, however, require a surrogate field for cavity control. In this study weconstruct a bichromatic cavity to study the surrogate control approach, where the main and the surrogatefields are related by the second-harmonic generation with nonlinear optics. We experimentally verify thetemperature dependence of the differential reflection phase of a dielectric coating design optimized forthe surrogate control approach of the optical cavities of the light-shining-through-a-wall experiment AnyLight Particle Search II, and develop a comprehensive cavity model for quasi-second-harmonic resonancesthat considers also other important factors, such as the Gouy phase shift, for a detailed analysis of thesurrogate control approach.

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

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

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

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

Fast and High-Accuracy Collinear Reflection Mueller Imaging Polarimeter Implemented With Compound Calibration Method

Xiang Ma, Jinxian Wu, Yujie Hu, and Yanqiu Li

DOI: 10.1364/AO.517955 Received 05 Jan 2024; Accepted 02 Apr 2024; Posted 02 Apr 2024  View: PDF

Abstract: The collinear reflection Mueller matrix imaging polarimeter is suitable for characterizing thick samples with high-scattering depolarization, such as biological tissues or in-situ living organs. Achieving fast detection and high measurement accuracy is vital to prevent artifacts and accurately assess polarization characteristics in these applications. This paper demonstrates a fast collinear reflection imaging polarimeter based on liquid crystal variable retarders (LCVRs-CRMMIP). We propose a novel compound calibration method (CCM), which enhances measurement accuracy through light intensity correction and an improved equivalent calibration sample model. This method surpasses the double-pass eigenvalue calibration method (dp-ECM), enhancing accuracy by over times. Performance evaluations with standard samples, including mirrors, linear polarizers, and wave plates, reveal that the LCVRs-CRMMIP achieves rapid measurements (about 3 seconds), high precision with an error of less than 0.0017.

Exploring atmospheric optical turbulence: Observations across zenith angles

Lily Beesley, James Osborn, Richard Wilson, Oliver Farley, Ryan Griffiths, and Gordon Love

DOI: 10.1364/AO.519063 Received 22 Jan 2024; Accepted 02 Apr 2024; Posted 10 Apr 2024  View: PDF

Abstract: We present measurements of the atmospheric optical turbulence as a function of zenith angle using two identicalinstruments: Shack-Hartmann Image Motion Monitors(SHIMMs), to measure atmospheric parameters concurrently. One instrument was pointed near zenith, whilethe other collected data by tracking a single star untilit set and thus sampling zenith angles continuously tothe horizon. By comparing these measurements, wecan attribute changes in the atmospheric parameters tothe changing zenith angle rather than variations in local turbulence conditions. The primary purpose of thisexperiment is to make comparisons between the measurements of scintillation index, σ2Iand Fried parameter,r0, with current theories. In this demonstration, we findthat there is a strong agreement between the modelsand the instrument up until zenith angles of 70◦, abovewhich model and measurements begin to deviate. Wediscuss various ways in which limitations in modelsand our instrument may cause these deviations.

Experimental study of thin thermally grown oxide layer in thermal barrier coatings based on SWT-BP algorithm and terahertz technology

Manting Luo, Shuncong Zhong, Yi Huang, zhenghao zhang, WALTER WALTER NSENGIYUMVA, and Wanli Tu

DOI: 10.1364/AO.520751 Received 02 Feb 2024; Accepted 31 Mar 2024; Posted 01 Apr 2024  View: PDF

Abstract: As a promising nondestructive testing (NDT) technique with a very adaptive physical modeling of wavetransmission process, Terahertz technology is used for the detection and characterization of non-polar materialsand the evaluation of layered and/or defective structures. THz-TDS can also be used to perform the spectroscopicanalysis and detect structural defects in thermal barrier coatings (TBCs) of aero-engines. Although it is generallydifficult to measure the structure of the thin oxide layer of the thermal barrier coatings whose thickness isgenerally lower than 30 μm (the current axial resolution of the THz-TDS cannot exceed 30 μm). We were able tocomplete the detection of oxide layer within 1-29 μm through simulation by using SWT-BP algorithm. In this study,the analysis was performed on real-world samples, the fitting degree of the SWT-BP algorithm reached 0.77, andthe minimum prediction error was less than 0.1 μm. The paper also put forward some improvement measuresabout the experimental results.

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

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

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

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

Convolutional Neural Network for Improved Event-Based Shack-Hartmann Wavefront Reconstruction

Mitchell Grose, Jason Schmidt, and Keigo Hirakawa

DOI: 10.1364/AO.520652 Received 04 Feb 2024; Accepted 30 Mar 2024; Posted 01 Apr 2024  View: PDF

Abstract: Shack-Hartmann wavefront sensing is a technique for measuring wavefront aberrations, whoseuse in adaptive optics relies on fast position tracking of an array of spots. These sensorsconventionally use frame-based cameras operating at a fixed sampling rate to report pixelintensities, even though only a fraction of the pixels have signal. Prior in-lab experiments haveshown feasibility of event-based cameras for Shack-Hartmann wavefront sensing (SHWFS),asynchronously reporting the spot locations as log intensity changes at microsecond time scale.In our work, we propose a novel convolutional neural network (CNN) called Event-BasedWavefront Network (EBWFNet) that achieves highly accurate estimation of spot centroid positionin real-time. We developed a custom Shack-Hartmann wavefront sensing hardware with commonaperture for the synchronized frame- and event-based cameras so that spot centroid locationscomputed from the frame-based camera may be used to train/test the event-CNN-based centroidposition estimation method in an unsupervised manner. Field testing with this hardware allows usto conclude that the proposed EBWFNet achieves sub-pixel accuracy in real-world scenarios withsubstantial improvement over the state-of-the-art event-based SHWFS. An ablation study revealsthe impact of data processing, CNN components, and training cost function; and an unoptimizedMatlab implementation is shown to run faster than 800 Hz on a single GPU.

Optical MTF and aberrations predictions with the Helmholtz-Lagrange invariant

Paolo Sandri

DOI: 10.1364/AO.520107 Received 25 Jan 2024; Accepted 30 Mar 2024; Posted 02 Apr 2024  View: PDF

Abstract: The paper presents a method for a rough estimation of the aberrations and of the MTF of an opticalsystem by means of the Helmholtz-Lagrange invariant. The method allows for a rough understanding ofan optical design and can find application in those cases where approximated performance predictionsare needed, for example before running complex and time-consuming analyses. The simple approachpresented here is a rough approximation and should always be followed by the usual tolerance analyses.

Impact of thermal lensing on the TMI threshold powers in rod-type Yb doped fiber amplifiers

Marc Mermelstein

DOI: 10.1364/AO.517483 Received 02 Jan 2024; Accepted 29 Mar 2024; Posted 02 Apr 2024  View: PDF

Abstract: The impact of thermal lensing on the transverse mode instability (TMI) threshold power in rod type fiber amplifiers is investigated. Simulations are conducted with a three dimensional coupled mode analysis on a set of five scaled large pitch fiber (LPF) amplifiers. The LPF fibers are represented by surrogate step index fibers (SIF) with scaled cladding diameters, core diameters and numerical apertures for a fixed normalized frequency V equal to 3.0 and scaled modal field overlap integrals with the core and cladding. It is found that thermal lensing decreases the TMI threshold powers due to increases in the TMI non-linear gain. This gain increase is attributed to an increase in the non-linear gain overlap integrals that occurs with the reduction in the fundamental mode effective area. This result may offer opportunities for fiber designs that increase the TMI threshold powers.

Reflective Microring Resonator Based Microwave Photonic Sensor Incorporating Self-Attention Assisted Convolutional Neural Network

Yeming Chen, Xiaoyi Tian, Joel Sved, Liwei Li, Luping Zhou, Linh Nguyen, and Xiaoke Yi

DOI: 10.1364/AO.516204 Received 18 Dec 2023; Accepted 26 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: In this paper, a reflective microring resonator (MRR) based microwave photonic(MWP) sensor incorporating a self-attention convolutional neural network (CNN) is presented.An MRR cascaded with an inverse-designed optical reflector is adopted as the sensor probe toallow for utilizing the responses generated from both the clockwise and counterclockwiseresonant modes. Through the MWP interrogation, the cascaded resonant modes can betransformed into distinctive deep radio-frequency (RF) spectral notches under differentmodulator bias conditions. By using a self-attention assisted CNN processing to leverage boththe local and global features of the RF spectra, a sensing model with improved accuracy can beestablished. As a proof of concept, the proposed scheme is experimentally demonstrated intemperature sensing. Even with a small dataset, the root-mean-square error of the sensing modelestablished after training is achieved to be 0.026 °C, which shows a 10-fold improvement insensing accuracy compared to that of the traditional linear fitting model.

A compact polarization transceiver module for the Fiber Optic Gyroscopes

Xueke Yu, Zuchen Zhang, Wenqi Yu, and Shaobo Zhang

DOI: 10.1364/AO.520236 Received 31 Jan 2024; Accepted 26 Mar 2024; Posted 28 Mar 2024  View: PDF

Abstract: This paper reports a transceiver module with high polarization extinction ratio (PER) for the polarization-maintaining interferometric fiber optic gyroscope (PM- IFOG). This transceiver utilizes a graded-index lens featuring a slanted face coated with a multilayer film, enabling simultaneous collimation and beam splitting. Configurations of the graded-index splitter lens and the corresponding transceiver module are simulated and designed using the ray matrix method and physical optical propagation. The optical loss of the transceiver module is about 3.77 dB, and the PER exceeds 25 dB. The IFOG prototype is implemented by integrating the transceiver module with a PM sensing coil. The bias instability of the IFOG prototype is evaluated to be 0.51°/h by Allan variance. This research contributes to the reduction in size, weight, power consumption, and cost of PM-IFOG systems.

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

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

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

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

Examining the Role of Magnetic Fields in Plasma Behaviour and Surface Evolution of Mg-Alloy with Varied Irradiances in Femtosecond Laser Treatment.

Asadullah Dawood, Muhammad Khan, Shazia Bashir, Ambreen Ayub, Asma hayat, Joseph H. Sanderson, and Navid Chishti

DOI: 10.1364/AO.523804 Received 22 Mar 2024; Accepted 23 Mar 2024; Posted 02 Apr 2024  View: PDF

Abstract: This paper reports the effect of magnetic field on plasma parameters and surface structuring of Mg alloy after laser irradiation. Femtosecond pulses of Ti: Sapphire laser system (800 nm, 35 fs, 1 KHz) are employed as source of irradiation at various irradiances ranging from 0.011 PW/cm2 to 0.117 PW/cm2 to generate ablated Mg-alloy plasma. A Transvers Magnetic Field (TMF) of strength 1.1 Tesla is employed to confine laser generated Mg plasma. All the measurements are performed with & without TMF. The two plasma parameters i.e. excitation temperature (Texc) and electron number density (ne) of Mg plasma have been evaluated by Laser-induced breakdown spectroscopy (LIBS) analysis. It is observed that the values of Texc and ne of Laser Produced Plasma (LPP) of Mg-alloy are higher in the presence of magnetic field as compared to field free case. Both show initially an increasing trend with increasing laser irradiance and after attaining their respective maxima a decreasing trend is observed with the further increase of irradiance. The magnetic confinement validity is confirmed by analytically evaluating thermal beta (βt), directional beta (βd), confinement radius (Rb) and diffusion time (td) for LPP of Mg alloy. To correlate the LPP parameters of Mg-alloy with surface modifications Field Emission Scanning Electron Microscope (FE-SEM) analysis is performed. It was revealed that structures like Laser Induced Periodic Surface Structures (LIPSSs), agglomers, islands, large size bumps along with channels and multiple ablative layers are observed. Distinct and well-defined surface structuring is observed in the presence of TMF as compared to field free case. It is concluded that by applying external magnetic field during laser irradiation, controlled material surface structuring is possible for fabrication of nanogratings and field emitters where spatial uniformity is critically important.

Scaled-laboratory demonstrations of deep-turbulence conditions

Mark Spencer and David Dayton

DOI: 10.1364/AO.520208 Received 28 Jan 2024; Accepted 20 Mar 2024; Posted 02 Apr 2024  View: PDF

Abstract: This paper uses five spatially distributed reflective liquid-crystal phase modulators(LcPMs) to accurately simulate deep-turbulence conditions in a scaled-laboratory environment.In practice, we match the Fresnel numbers for long-range, horizontal-path scenarios usingoptical trombones and relays placed in between the reflective LcPMs. Similar to computationalwave-optic simulations, we also command repeatable high-resolution phase screens to thereflective LcPMs with the proper path-integrated spatial and temporal Kolmogorov statistics.