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Accepted papers to appear in an upcoming issue

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

3D far-field Lidar sensing and computational modeling for human identification

Alexandar Glandon, Lasitha Vidyaratne, Nibir Dhar, Jide Familoni, Nasrin Sadeghzadehyazdi, Scott Acton, and Khan Iftekharuddin

DOI: 10.1364/AO.508033 Received 13 Oct 2023; Accepted 04 Dec 2023; Posted 04 Dec 2023  View: PDF

Abstract: 3D sensors offer depth sensing that may be used for task-specific data processing and computational modeling. Many existing methods for human identification using 3D depth sensors primarily focus on Kinect data, where the range is very limited. This work considers a novel 3D long range Lidar sensor for far-field imaging of human subjects in 3D Lidar full motion video (FMV). 3D Lidar FMV data for human subject is used to develop computational modeling for automated human silhouette and skeleton extraction followed by subject identification. We propose a generalized matrix completion algorithm to handle missing data in 3D FMV due to self-occlusion and occlusion from other subjects for 3D skeleton extraction. We further study effect of noise in the 3D low resolution far-field Lidar data in human silhouette extraction performance of the model. Moreover, this work addresses challenges associated with far-field 3D Lidar including learning with a limited amount of data and low resolution. Moreover, we evaluate the proposed computational algorithm using a gallery of ten subjects for human identification and show that our method is competitive with the state-of-the-art OpenPose and V2VPose skeleton extraction models using the same dataset for human identification.

Freeform three-mirror anastigmatic large-aperture telescope and receiver optics for CMB-S4

Patricio Gallardo, Roberto Puddu, Kathleen Harrington, Bradford Benson, John Carlstrom, Simon Dicker, Nick Emerson, Jon Gudmundsson, Michele Limon, Jeff McMahon, Johanna Nagy, Tyler Natoli, Michael Niemack, Stephen Padin, John Ruhl, Sara Simon, and CMB-S4 Collaboration

DOI: 10.1364/AO.501744 Received 26 Jul 2023; Accepted 03 Dec 2023; Posted 04 Dec 2023  View: PDF

Abstract: CMB-S4, the next-generation ground-based cosmic microwave background (CMB) observatory, will provide detailed maps of the CMB at millimeter wavelengths to dramatically advance our understanding of the origin and evolution of the universe. CMB-S4 will deploy large and small aperture telescopes with hundreds of thousands of detectors to observe the CMB at arcminute and degree resolutions at millimeter wavelengths.Inflationary science benefits from a deep delensing survey at arcminute resolutions capable of observing a large field of view at millimeter wavelengths. This kind of survey acts as a complement to a degree angular resolution survey. The delensing survey requires a nearly uniform distribution of cameras per frequency band across the focal plane. We present a large-throughput, large-aperture (5-meter diameter) freeform three-mirror anastigmatic telescope and an array of 85 cameras for CMB observations at arcminute resolutions, which meets the needs of the delensing survey of CMB-S4. A detailed prescription of this three-mirror telescope and cameras is provided, with a series of numerical calculations that indicate expected optical performance and mechanical tolerance.

A convenient dual-wavelength digital holography based on orthogonal polarization strategy with Wollaston prism

DIFENG WU, Linna Xie, han xianxin, Chengxin Zhou, Zhengyang Bu, shengde liu, Liyun zhong, and Xiaoxu Lu

DOI: 10.1364/AO.507354 Received 02 Oct 2023; Accepted 02 Dec 2023; Posted 04 Dec 2023  View: PDF

Abstract: Dual-wavelength digital holography effectively expands the measurement range of digital holography, but it increases the complexity of optical system due to non-common-path of two wavelengths. Here, by using orthogonal polarization strategy, we present a dual-wavelength digital holography based on a Wollaston prism (DWDH-WP) to separate the reference beams of two wavelengths and realize common-path of two wavelengths. A Wollaston prism is inset into the reference beam path of off-axis digital holography system, so two orthogonal-polarized reference beams of two different wavelengths separated at different directions are generated. Then a dual-wavelength multiplexed interferogram with orthogonal interference fringes is captured by using a monochrome camera, in which both the polarization orientations and the interference fringe orientations of two wavelengths are orthogonal, so the spectral crosstalk of two wavelengths with arbitrary wavelength difference can be avoided. Compared with the existing DWDH method, the proposed DWDH-WP method can conveniently realize common-path of the reference beams of two wavelengths, so it reveals obvious advantages in spectral separation, spectral crosstalk, system simplification and adjustment flexibility. Both effectiveness and flexibility of the proposed DWDH-WP method are demonstrated by the phase measurement of HeLa cell and vortex phase plate.

Development of a bio-inspired optical system that mimics accommodation and lighting regulation like the human eye

Agustin Santiago Alvarado, Fermin Granados, Brayan Lopez Raymundo, Arturo Hernández Mendez, and Oliver Huerta-Carranza

DOI: 10.1364/AO.506986 Received 26 Sep 2023; Accepted 02 Dec 2023; Posted 04 Dec 2023  View: PDF

Abstract: Bioinspired optical systems have recently been developed using polarizers and liquid or rigid lenses. In this work, we propose a bio-inspired Opto-mechatronic system that imitates the accommodation and regulation of light intensity as the human eye does. The system uses a polymeric lens as a cornea, an adjustable diaphragm as an iris, a tunable Solid Elastic Lens as a crystalline lens, and a commercial sensor as a retina. We also present the development of the electronic control system to accommodate and regulate the amount of light that enters the system, for which two stepper motors, an Arduino control system, and light and movement sensors are used. The characterization of the system is presented together with the results obtained, where it can be seen that the system works in an acceptable range as the human eye does.

Accurate determination of plasma temperature and electron density using a reference target: One-point calibration LIBS elemental analysis of alloy samples

Huiling Zeng, Runhua Li, and Yuqi Chen

DOI: 10.1364/AO.507180 Received 29 Sep 2023; Accepted 02 Dec 2023; Posted 04 Dec 2023  View: PDF

Abstract: Elemental analysis of aluminum alloy samples with calibration-free laser-induced breakdown spectroscopy (CFLIBS) encounters two difficulties: the inconvenience of determining accurate temperature and electron density of the plasma and the influence of self-absorption of the observed aluminum lines. To solve this problem, targetenhanced orthogonal double-pulse laser-induced breakdown spectroscopy (TEODP-LIBS) in reheating regime combined with one-point calibration (OPC) method was proposed in this work. A mixture of copper powders and KHCO3 grains was pressed to a pellet and used as the target. Accurate determination of plasma temperature and electron density can be obtained using a reference target. The plasma temperature could be determined with SahaBoltzmann plot of copper and the electron density of the plasma could be determined according to the Stark broadening of the Hα line of hydrogen. Aluminum alloy samples were analyzed with a relative error of better than 0.02% for major element. This approach provides a convenient way to determine the temperature and electron density of the plasma more accurately and is able to reduce the influence of self-absorption, which is very helpful for realizing quantitative elemental analysis of different samples while using calibration-free algorithm.

An ultra-compact optical full-adder based on directed logic and microring resonators

Feng Chen, Shenghang Zhou, Yuhong Xia, Xing Yu, Jia Liu, Fuhua Li, and Xiubao Sui

DOI: 10.1364/AO.510590 Received 30 Oct 2023; Accepted 01 Dec 2023; Posted 04 Dec 2023  View: PDF

Abstract: Photonic integrated circuits with compact design have opened possibilities for the development of optical computing systems, however, the overly use of photonic components in optical designs has slowed the progress of dense integration. In this paper, we propose an ultra-compact optical full-adder based on directed logic and microring resonators. To the best of our knowledge, the proposed structure requires fewer optical components than any other current designs, resulting in a significantly reduced footprint 38.2 μm × 29.5 μm . Simulation results by finite-difference time-domain (FDTD)demonstrate the effectiveness and feasibility of the proposed optical full-adder.

Effects of excitation wavelength and temperature on the zero-phonon emission line of Cr:YAG

Yuruo Zheng, Yimin Zhou, debao zhang, Jiqiang Ning, Fei Tang, and Changcheng Zheng

DOI: 10.1364/AO.506308 Received 25 Sep 2023; Accepted 30 Nov 2023; Posted 01 Dec 2023  View: PDF

Abstract: In this work, a detailed study of the temperature and excitation wavelength-dependent photoluminescence (PL) spectra of the chromium-doped yttrium aluminum garnet (Cr:YAG) transparent ceramic was conducted. Focusing on the two sets of zero-phonon lines (ZPLs) of the 2E→4A2 transition in this material, the PL spectra are discovered to evolve significantly with respect to temperature and be highly dependent on the excitation wavelength. Compared with the continuous variation behavior with temperature, the increase of excitation wavelength leads to the blueshift of the peak position within the regions of 450 nm to 465 nm, 465 nm to 490 nm, and 490 nm to 500 nm and the sharp change in the PL position at the excitation wavelengths of 465 nm and 490 nm. The electron-phonon coupling (EPC) effect is believed to be more sensitive to the excitation wavelength and different excitation wavelengths involve different electronic levels participating in the light emission processes, which explains the evolution behavior of the PL peak position with respect to excitation wavelength. Moreover, the emergence of weak peaks besides the ZPLs at particular temperatures and excitation wavelengths is also proposed. This work compares the influence of temperature and excitation wavelength to the PL properties of the Cr:YAG transparent ceramic, which promotes the advanced understanding towards the luminescence behavior of the Cr:YAG transparent ceramics.

A High-Temperature Thermal Disturbance Elimination Method Based on Multi-Channel Extraction and Correction

Wanlin Pan, QianShan Liu, Junrui Li, XiaoKang Fan, Lei Zhou, and Yonghong Wang

DOI: 10.1364/AO.509848 Received 19 Oct 2023; Accepted 30 Nov 2023; Posted 01 Dec 2023  View: PDF

Abstract: Digital image correlation (DIC) technology has been widely used in high-temperature measurement fields. However, due to the complexity of high-temperature environments, there are many interference factors that limit the development of high-temperature DIC technology, among which thermal disturbance is one of the most significant factors that severely affects the measurement accuracy of high-temperature DIC. In this paper, a multi-channel separation technique combined with a low-cost laser speckle device is proposed to eliminate thermal disturbance errors in high-temperature DIC measurements. Firstly, a blue laser speckle generation system is independently designed to produce the most suitable speckle particles, and the best laser speckle is determined and projected onto the blue background white spot pattern. Then, a green LED illuminates the sample to provide illumination for the sample's own grayscale characteristics. A color camera collects photos, and the obtained images are processed with channel separation to extract and calculate the displacement of different channels. Finally, the displacement fields of the green and blue channels are subtracted to separate the thermal disturbance error and correct the measurement values. In this paper, a laser speckle projection system is first assembled, followed by comprehensive evaluation of the projected speckle, and finally, a DIC experimental system is constructed for verification experiments at both room temperature and high temperature, and the corrected values are compared with the true values. The results show that the corrected values are highly consistent with the true values, which verifies the reliability of the proposed method.


Clara ABBOUAB, Marie-Alicia Malleville, Baptiste Leconte, Raphael Jamier, Etienne Genier, Philippe Morin, and Philippe Roy

DOI: 10.1364/AO.511239 Received 05 Nov 2023; Accepted 30 Nov 2023; Posted 01 Dec 2023  View: PDF

Abstract: An all-fiber supercontinuum source delivering up to 40 W of average power ranging from 750 nm to 2200 nm is demonstrated. The laser source is based on a self-Q-switched pump-sharing oscillatoramplifier. The self-Q-switched master oscillator generates giant pulses, amplified in the high-power stage. Finally, a passive fiber acts as a nonlinear stage improving the spectrum flatness as well as the spectral broadening. To the best of our knowledge, this is the first time that a pump-sharing oscillatoramplifier is used for SC generation and based on the use of a submeter Ytterbium-doped fiber length inside the oscillator.

LBNH-BNS: Low-frequency Background Estimation and Noise Separation from High-frequency for Background and Noise Subtraction

Yuyao Hu, Peng Wang, Jun Liu, and FU ZHAO

DOI: 10.1364/AO.507735 Received 10 Oct 2023; Accepted 30 Nov 2023; Posted 01 Dec 2023  View: PDF

Abstract: In fluorescence microscopy, background blur and noise are two main factors preventing the achievement of high Signal-to-Noise Ratio (SNR) imaging. Background blur primarily emanates from inherent factors including the spontaneous fluorescence of biological samples and out-of-focus backgrounds, while noise encompasses Gaussian and Poisson noise components. To achieve background blur subtraction and denoising simultaneously, a pioneering algorithm based on Low-frequency Background estimation and Noise separation from High-frequency (LBNH-BNS) is presented, which effectively disentangles noise from the desired signal. Furthermore, it seamlessly integrates lowfrequency features derived from background blur estimation, leading to the effective elimination of noise and background blur in wide-field fluorescence images. In comparisons with other state-of-the-art background removal algorithms, LBNH-BNS demonstrates significant advantages in key quantitative metrics such as Peak Signal-to-Noise Ratio (PSNR), and manifests substantial visual enhancements. LBNH-BNS holds immense potential for advancing the overall performance and quality of wide-field fluorescence imaging techniques.

Comparing Transmission- and Epi-BCARS: A Transnational Round Robin on Solid State Materials

Franz Hempel, Federico Vernuccio, Lukas König, Robin Buschbeck, Michael Rüsing, Giulio Cerullo, Dario Polli, and Lukas Eng

DOI: 10.1364/AO.505374 Received 08 Sep 2023; Accepted 29 Nov 2023; Posted 30 Nov 2023  View: PDF

Abstract: Broadband coherent anti-Stokes Raman scattering (BCARS) is a powerful spectroscopy method combining high signal intensity with spectral sensitivity, enabling rapid imaging of heterogeneous samples in biomedical research and, more recently, in crystalline materials. However, BCARS encounters spectral distortion due to a setup-dependent non-resonant background (NRB).This study assesses BCARS reproducibility through a Round Robin experiment using two distinct BCARS setups and crystalline materials with varying structural complexity, including diamond, 6H-SiC, KDP, and KTP. The analysis compares setup-specific NRB correction procedures, detected and NRB-removed spectra, mode assignment, and the influence of BCARS setup parameters like pump wavelength, pulse width, and detection geometry. These findings are a practical guide for optimising BCARS setups for solid-state applications.

Semi-Analytical Finite Ray-Tracing Through the Quadratic Symmetric GRIN Lens

Conor Flynn and Alexander Goncharov

DOI: 10.1364/AO.504305 Received 31 Aug 2023; Accepted 29 Nov 2023; Posted 30 Nov 2023  View: PDF

Abstract: The propagation of light within a gradient index (GRIN) media can be analysed with the use of differential equations for a given non-homogenous refractive index profile. Numerical methods are often necessary to perform ray-tracing in GRIN media, however, analytical solutions exist for several types of GRIN lenses. In this paper, paraxial and non-paraxial differential equations are derived to calculate the ray path in a GRIN lens. It is shown that the paraxial equation has an analytical solution for a GRIN media with a quadratic profile within the paraxial region. The analytical solution can be obtained by using Legendre polynomials, or the Frobenius method involving a power series. Using the Legendre or Frobenius solution, one can calculate the refractive indices along the ray path. A new recursive relationship is proposed to map the trajectory of light at finite heights. To illustrate the finite ray-tracing method utilizing a non-paraxial differential equation, two lenses (with spherical and elliptical iso-indicial contours) are considered. The lenses' back focal distances, for rays entering the lenses at varying finite heights, are calculated. For each lens, its spherical aberration is estimated. The effective focal length and the shape of the principle surface are also obtained. The accuracy of the results are then compared to the numerical ray-tracing using an optical design software, Zemax OpticStudio. The predicted spherical aberration for the spherical lens differs from numerical ray-tracing by less than λ/14 at the marginal zone, while the error for the effective focal length is less than λ/100.

Improving the multi-functionality of optical tweezers with FPGA integration

Shuo Liu, Xudong Fan, Zhelin Qu, Changfeng Fang, Chao feng, xian zhao, and Junlei Wang

DOI: 10.1364/AO.505998 Received 14 Sep 2023; Accepted 29 Nov 2023; Posted 30 Nov 2023  View: PDF

Abstract: The development of optical tweezers aims to extend their operating function and pattern. However, excessive programming can lead to a decrease in the system's operating speed and introduce bugs or data transmission delays. In this study, we present a time-shared optical tweezers system that allows for parallel operation of multiple functions. To enable efficient data transmission, we employ a queue structure and a buffer. To assess the system's performance, we utilize a biological sample in conjunction with the optical tweezers system and scanning imaging technique. We quantify the trapping parameter while concurrently running power stabilization programs. As a result, the standard deviation of the measured stiffness is reduced by 60% in xy- and 30% in z-directions, respectively, indicating a significant improvement in calibration precision. Throughout the program execution, the system maintains an operating rate of 110 kilohertz, and the data is continuously updated in real-time on the host. The system's performance demonstrates its potential for quantification and morphological reconstruction of biological samples.

Impact of ITO Layer on the Spatial Optical Distribution of Semi-Polar (20-21) InGaN/GaN Multiple Quantum Wells with Surface Morphology

Sheng nie, LINGXIAO SHUAI, maogao gong, Yun Zhang, and Bin Liu

DOI: 10.1364/AO.506774 Received 27 Sep 2023; Accepted 29 Nov 2023; Posted 30 Nov 2023  View: PDF

Abstract: Textured surface with micro-facets have been widely observed in semi-polar and non-polar III-nitride heterostructures, mainly resulted from the anisotropic growth rate in the growth plane. Polarization and intensity distribution of surface emission are both affected by the surface morphology. ITO layer, serving as the current spreading layer, are usually employed to enhance the current injection efficiency and light extraction efficiency in III-nitride emitters. For semi-polar orientation, the introduction of ITO layer could weaken the anisotropic optical property, especially for the spatial intensity distribution. This paper reports the influence of the ITO layer on the spatial intensity distribution of semi-polar (20-21) InGaN/GaN multiple quantum wells. The intensity distribution could be shaped from a rectangular-like pattern to a circular-like pattern with the deposition of an ITO layer. The ITO layer allows more light along the [11-20] direction to emit out at a small angle with respect to the surface normal. With further increasing the ITO thickness, the influence of surface fluctuation of semi-polar sample decreases, leading to an improvement in the proportion of the light at small angles, and a slight decrease in the overall integrated intensity of whole far field. These results will help pave the way to high performance semi-polar emitters with great potential in general illumination and backlighting.

Theoretical and experimental study on stable oscillation of dual-frequency signals in an optoelectronic oscillator

Yuan Chen, ZhiQiang Fan, and Qi Qiu

DOI: 10.1364/AO.509242 Received 13 Oct 2023; Accepted 29 Nov 2023; Posted 30 Nov 2023  View: PDF

Abstract: Due to the gain competition effect, it is hard to simultaneously maintain the oscillation at two frequencies in an optoelectronic oscillator (OEO) loop. In this article, the study about the gain competition effect in dual-frequency OEO is theoretically and experimentally demonstrated. The steady-state conditions in dual-frequency OEO are theoretically analyzed by deriving dynamic equations. A nonlinear time-varying model as well as its calculation methods is carried out to design and study the dynamic process of the dualfrequency OEO. Thanks to this model, the waveform, the spectrum and the amplitude evaluation of generated signals as well as the gain variation in the OEO loop are numerically simulated. Based on the theoretical analysis and numerical simulation results, three schemes that can suppress the gain competition effect are proposed, and the one based on wavelength division multiplexing (WDM) technology is experimentally realized. The experimental results show that the novel independently tunable dual-frequency OEO can generate two-tone RF signals in a range from 1.8 to 18.6 GHz and 1.5 to 18.3 GHz, respectively.

Experimental study and modeling of extreme ultraviolet 4000 lines/mm diffraction gratings coated with periodic and aperiodic Al/Mo/SiC multilayers

AMR HISHAM K. MAHMOUD, DE ROSSI Sébastien, Evgueni Meltchakov, Blandine CAPITANIO, Muriel Thomasset, Maxime Vallet, and Franck Delmotte

DOI: 10.1364/AO.505546 Received 11 Sep 2023; Accepted 28 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: Multilayer coated diffraction gratings are crucial components for extreme ultraviolet (EUV) applications as spectroscopy or spectro-imaging. However, for high groove density, the smoothening of the grating surface profile with multilayer deposition remains a limitation that requires more investigation. In this paper, we report on the design, characterization, and modeling of 4000 lines/mm diffraction gratings coated with periodic and aperiodic Al/Mo/SiC multilayers for EUV radiation. Two types of gratings, with different groove depths are compared. Multilayer coatings were designed using a genetic algorithm to maximize the 1$^{st}$ -order diffraction efficiency in the 17-21 nm and in the 19- nm wavelength ranges at normal incidence. Periodic and aperiodic multilayers with different numbers of layers were deposited by magnetron sputtering on the 2 types of fused silica gratings and the grating groove profile evolution was measured by atomic force microscopy, and by cross-section transmission electron microscopy. The first-order diffraction efficiency was measured in the EUV at 5$^\circ$ incidence using monochromatic synchrotron radiation and modeled using the Rigorous Coupled-Wave Analysis method. The simulation models refined by using the Debye-Waller factor to account for the multilayer interfacial roughness show a good agreement with experimental data. The results reported in this study will allow for designing efficient EUV multilayer gratings for high-resolution spectro-imaging instruments.

Terahertz Tunable Band-stop Filter using Topological Valley Photonic Crystals

Rajesh Kumar, Rohith K M, Shashank Pandey, Sanjeev Srivastava, and Gagan Kumar

DOI: 10.1364/AO.504776 Received 06 Sep 2023; Accepted 28 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: In recent years, there has been a growing interest in the wideband propagation and control of terahertz (THz) radiation due to its potential for a variety of applications, such as 6G communication, sensing, and imaging. One promising approach in this area is the use of valley photonic crystals (VPCs), which exhibit properties like wider band gaps and robust propagation. In this paper, a two-dimensional dielectric silicon-air VPC is studied which is constructed from a unique method of inversion symmetry breaking providing a band gap of 109.4 GHz at a mid-gap frequency of 0.376 THz. We employ an optimized bearded stack interface to construct the VPC waveguide for wideband THz propagation along straight and Z-shaped paths. We demonstrate that a band-stop response can be achieved in a VPC by introducing periodic defects along the domain wall. Furthermore, the stop range can be tuned by varying the refractive index of the defects through incorporating liquid crystal along the domain wall of VPC. Our proposed structure and the techniques employed could be promising for the development of a band stop filter (BSF) and other photonic components having potential applications in 6G communication and beyond.

Design of Compact Off-axis Two-mirror Freeform Optical Antenna for Shaping and Transmitting Elliptical Beam Emitted by Laser Diode

Xin Chen, Lizhong Hu, Jianing Liu, Chaoxiang Wang, chenxi Bai, Yifan Zheng, yan qin, Yang Huajun, and Ping Jiang

DOI: 10.1364/AO.505268 Received 08 Sep 2023; Accepted 28 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: Elliptical Gaussian beams generated by laser diodes (LDs) often exhibit asymmetrical divergence angle distribution, which limits their practical applications. In this study, we propose a novel approach to shape and collimate the elliptical output beam from a laser diode (LD). The design process involves the construction of two freeform reflective surfaces on a reference circle using a three-dimensional point-by-point iterative method, based on the law of conservation of energy, the vector reflection theory, and Fermat’s principle. The output beam’s maximum divergence angle is effectively compressed to 3.1579 mrad. Compact design with folded optical path and antenna size of 368.8cm3. This paper presents a comprehensive design and optimization process, along with an in-depth analysis of the system's performance, thereby offering novel insights for emerging optical design practitioners.

Determination of quantity and volume of Carya cathayensis Sarg by line laser scanning combined with point cloud fusion algorithm

Xing Li, Xiuxiang Chu, Tianze Jia, and Ziyuan Liu

DOI: 10.1364/AO.506332 Received 21 Sep 2023; Accepted 28 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: Optical 3D measurement technology plays a vital role in diverse industries, particularly with the advancements in line laser scanning 3D imaging. In this paper, we propose a line laser scanning-based investigation for detecting Carya cathayensis Sarg. The Carya cathayensis Sarg specimens are scanned using a line laser to achieve three-dimensional reconstruction, enabling the calculation of their volume and quantity based on the acquired point cloud map. Through binocular acquisition and subsequent point cloud alignment and fusion, the error in the three-dimensional reconstruction is significantly reduced. The point cloud map facilitates the automatically identification of the number of scanned areas of Carya cathayensis Sarg areas and accurate volume calculations, with an error control of approximately 0.6% when compared to the actual volume. The application of this research in agriculture allows farmers to classify fruits sizes and optimize their selection, thus facilitating intelligent agricultural practices.

A Fiber-Connectorized Ultrafast-Laser-Inscribed K-Band Integrated Optics Beam Combiner for the CHARA Telescope Array

Jacopo Siliprandi, David MacLachlan, Calum Ross, Tarun Sharma, Lucas Labadie, Kalaga Madhav, Abani Nayak, Aline Dinkelaker, Martin Roth, Nicholas Scott, Vincent du Foresto, Robert Thomson, and Aurélien Benoit

DOI: 10.1364/AO.510293 Received 30 Oct 2023; Accepted 28 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: A fiber-connectorized K-band integrated optics 2-telescope beam combiner was developed for long-baseline interferometry at the CHARA telescope array utilizing the ultrafast laser inscription (ULI) technique. Single-mode waveguide insertion losses were measured to be ~1.1 dB over the 2-2.3 μm window. The development of asymmetric directional couplers enabled the construction of a beam combiner that includes a 50:50 coupler for interferometric combination and two ~75:25 couplers for photometric calibration. The visibility of the bare beam combiner was measured at 87% and then at 82% after fiber-connectorization, by optimizing the input polarization. These results indicate that ULI technique can fabricate efficient fiber-connectorized K-band beam combiners for astronomical purposes.

Effect of sputtering power on the physical properties of amorphous SiO2 doped InZnO transparent conductive oxide

Jin Young Hwang and Sang Yeol Lee

DOI: 10.1364/AO.505798 Received 26 Sep 2023; Accepted 28 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: In order to control the optical and electrical properties of the transparent conductive oxide, the RF (Radio Frequency) sputtering power was changed from 30, 40, 50, to 60 W. To optimize the power condition of SiInZnO (SIZO) layer, we changed sputtering power from 30 to 60 W, systematically. The chemical properties of the SIZO layer were analyzed using XPS (X-ray Photoelectron Spectroscopy). XPS proved that this change is dominant in thickness. In order to fabricate the SIZO transparent conducting oxide (TCO) with the optimized power of 50 W, the transmittance of 99.1 % at 550 nm and the figure of merit of 12.4 × 10-3Ω-1 were obtained.

A portable contactless caliper

Chittanon Buranachai, Konthee Boonmeeprakob, Arinchaya Phanpumnak, Chutintorn Punwong, Sureerat Chana, Panote Thavarungkul, and Proespichaya Kanatharana

DOI: 10.1364/AO.507349 Received 02 Oct 2023; Accepted 27 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: This work presents a portable optical meter for non-contact thickness measurement. The device shines a focused laser light on a thin and transparent sample, resulting in an interference between light reflecting from the top and from the bottom surface and the interfering pattern is recorded by a linear sensor array before data analysis with an Arduino microcontroller. The device produced accurate thickness values from glass cover slips and transparent plastic sheets within a fraction of a second per measurement. Besides, the sample’s refractive index is not required a priori. Therefore, it has a high potential to be of use in real-time quality control in transparent thick-film coating and manufacturing.

Telephoto Achromatic Camera Based on Optical-Digital Co-Design

Jiarui Ji, Lei Yang, and Hongbo Xie

DOI: 10.1364/AO.505630 Received 11 Sep 2023; Accepted 27 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: Due to the difficulty of correcting chromatic aberration (CA) in telephoto cameras, recent studies have combined image algorithms with simple optical structures, such as single-spherical lenses, for high-quality photography, moving away from complex optics. However, this approach often struggles to comprehensively address compounded issues arising from optical aberrations of simple optical systems, including defocus blur and multi-channel misalignment.To tackle this challenge, this manuscript presents an approach for developing a telephoto imaging system by leveraging the distinct characteristics of axial and lateral chromatic aberrations (ACA, LCA) over the visible spectrum. The optical design is limited to a specific wavelength range to preserve high-frequency information of the green channel. A cross-channel fitting method is presented to suppress the LCA. Subsequently, the powerful capabilities of deep learning are utilized to correct ACA, defocus blur, and other residual optical aberrations.Simulation experiments demonstrate the effectiveness of the proposed approach in mitigating the CA inherent in telephoto systems, thereby delivering high-quality imaging results over the whole visible waveband.

Speckle suppression in holographic phase fringe patterns with different level noise based on FFDNet

Yun Liu, Xiaoqiang Wu, Qi Kang, Jiahao Gao, Mingxing Jiao, Junhong Xing, Xian Wang, and Hequn Li

DOI: 10.1364/AO.502343 Received 09 Aug 2023; Accepted 27 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: In this paper, an ANLVENet speckle suppression method in holographic phase fringe patterns with different level noise is proposed based on FFDNet, combined Asymmetric Pyramid Non-local Block with verge extraction module. The experimental results are compared to three network models and several representative algorithms. It is shown that the ANLVENet method not only has better superiority in speckle suppression with different level noise, but also preserves more details of image edge. In addition, another speckle noise model is applied in the phase fringe patterns to prove the stronger generalization of the ANLVENet algorithm. The proposed method is suitable for suppressing the speckle with different levels in large noise range under complex environment conditions.

Thermal process of silicon wafer under CW laser and 100-10000 Hz pulsed laser irradiation

ZhiChao Jia, luanhong sun, Xiang Chen, XinHua Li, zewen li, and lingyun hao

DOI: 10.1364/AO.501947 Received 28 Jul 2023; Accepted 27 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: The thermal process of (001) silicon wafer subjected to continuous-wave (CW) laser and 100-10000 Hz pulsed laser irradiation is investigated experimentally and numerically. The temperature evolution of the spot center is measured using an infrared radiation pyrometer. The waveforms of temperature evolution curves provide valuable information about melting, solidification, vaporization and fracture. To gain a better understanding of the thermal process, a three-dimensional finite element model is established, and numerical simulations are conducted to analyze the temperature, stress, and dislocation field. The results show that the 10 kHz laser exhibits the highest heating efficiency before vaporization, but the lowest ablation efficiency after vaporization due to the shielding effect of vapor. The diffusion time of vapor is found to be more than 50 μs. Fracture occurs during 1 kHz laser irradiation. The motion of liquid may play a significant role, but it cannot be evidenced by simulation due to complex dependence of material parameters on dislocation. This issue should be addressed as a priority in future studies.

From ray optics to modal analysis: using photonics web simulations to build intuition and address common misconceptions

Erik Verlage, Glenda Stump, Anne Marshall, Saif Rayyan, Sajan Saini, and Lionel Kimerling

DOI: 10.1364/AO.502222 Received 07 Aug 2023; Accepted 27 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: The emerging field of silicon photonics has created a large need for PhD photonic integrated circuit design engineers. Developing intuition for electromagnetic waves at the micron scale is a major challenge facing undergraduate and graduate students in photonics. Students often misapply lessons learned from macroscale ray optics to submicron waveguide modes in dielectric structures. In this work, key student misconceptions were identified and addressed in a research study using photonics training simulations. A learning module with interactive 3D vector field visualizations was deployed in a massive open online course to train the next generation of photonics design engineers.

Design and implementation of parallel OTSU algorithmbased on ternary optical computer

kai song, YONGJUN SUN, Huaqiong Ma, and liping yan

DOI: 10.1364/AO.504271 Received 28 Aug 2023; Accepted 27 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: OTSU method (OTSU) is considered to be the best algorithm for threshold selection in image segmentation.The algorithm is easy to calculate and not affected by image brightness and contrast, so it is widely usedin the field of digital image processing. Due to the slow running speed and large storage space of thealgorithm on the traditional electronic computer, this research makes use of the advantages of the ternaryoptical computer (TOC), such as numerous data bits and the reconfigurable processor, through the analysisof OTSU algorithm, finds out the part of the OTSU algorithm that can be calculated in parallel, and putsforward a parallel OTSU algorithm based on TOC. The algorithm first uses a classifier to classify thenumber of pixels. By reconstructing the MSD adder and multiplier, the inter class variance under each grayvalue is calculated in parallel, and the maximum value is selected by comparing the inter class variance.The corresponding threshold is the best segmentation threshold. By analyzing the clock cycle and data bitresources of the algorithm, and then conducting comparative analysis and experimental verification, it isconcluded that this algorithm uses less data bit resources and faster operation speed than the traditionalalgorithm, and gives full play to the advantages of TOC in terms of complex calculation and large amountof data.

Precision tailoring of laser beam propagation in single crystalline diamond tool for in-situ laser-assisted diamond turning

Wangjie Hu, Zengqiang li, xuesen zhao, Tao Sun, and Junjie Zhang

DOI: 10.1364/AO.504587 Received 01 Sep 2023; Accepted 27 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: The characteristics of laser beam propagation within diamond tool critically influence the applied thermal softening capability of in-situ laser-assisted diamond turning (InLAT). In the present work, we perform optical geometric analysis, optical simulation and experimental validation to propose a novel diamond tool configuration for precisely tailoring laser beam propagation in In-LAT. Firstly, the characteristics of laser beam propagation in the current In-LAT diamond tool are theoretically and experimentally explored. Secondly, according to the issues discovered in the current In-LAT diamond tool, an improved tool configuration based on the total internal reflection of laser beam within diamond tool is proposed, aiming for promoting the refraction of laser beam from rake face of diamond tool, as well as eliminating the reflection of laser beam to tool holder. Finally, the optimization of laser beam incident position is carried out for achieving the superior profile and intensity of emitted laser spot. Current work provides a rational laser beam propagation for improving the thermalsoftening capability of In-LAT diamond tool.

Research on underwater motion scene image restoration based on improved U-Net network

liu ye, Hu Luo, and Dawei Tu

DOI: 10.1364/AO.505198 Received 06 Sep 2023; Accepted 27 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: Active underwater polarization imaging is a common underwater imaging method, which uses the polarization difference between the reflected light and the scattered light in the underwater scene to suppress the scattered light, so as to improve the imaging quality of the underwater scene. However, the implementation often requires the acquisition of multiple polarization images, which is not suitable for the restoration of images of underwater motion scenes. To address the above problem, a U-AD-Net deep learning network model based on single polarized image is proposed, taking the polarization information of single polarized image as the feature input, based on the classic U-Net network model, and introducing DenseNet and spatial attention module. The learning ability and generalization ability of the proposed model for deep features are enhanced, and the polarization information that is most helpful to the image restoration is extracted, so as to restore the scene image more comprehensively. IE, AG, UCIQE and SSIM are selected as evaluation metrics to assess the quality of the restored images. Experimental results show that the images restored through this proposed method contain richer detail information, having an obvious advantage to the existing network models. Since only a single polarized image is needed for restoration, this method has dynamic adaptability to underwater moving scenes restoration..

Graphene oxide based optical fiber humidity sensor having linear response throughout a large dynamic range and optimum sensitivity

Sunil Mohan and Sunil Khijwania

DOI: 10.1364/AO.507936 Received 13 Oct 2023; Accepted 27 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: Main objective of the present research is to develop an optical fiber relative humidity (RH) sensor with high sensitivity, linear response over a wide dynamic range and optimum response/recovery times by employing simplest optical fiber sensing configuration. An optical fiber RH sensor that exploits intensity modulation scheme through evanescent wave absorption spectroscopy and fulfils the objective is reported here. The fiber sensor employs graphene oxide (GO) diffused silica nanostructured thin sensing film as the cladding on the centrally decladded straight and uniform multimode optical fiber. Detailed experimental investigations are carried out to analyze the response characteristics of the proposed sensor. In comparison to other GO based optical fiber RH sensors, throughout linear response over the widest dynamic range (15.0% – 95.3%RH) and shortest (fastest) response/recovery times (0.1436s/0.1547s) are observed for the proposed sensor. The linear sensitivity for the developed sensor is observed to be 0.1036 dB/%RH. In addition, proposed sensor exhibits a very good degree of reversibility, reliability and repeatability.

Bilayer and trilayer X-ray mirror coatings containing W, Pt, or Ir in combination with C, C/Co, B4C, or B4C/Ni: X-ray Reflectance, Film Stress, and Temporal Stability

David Windt, Raymond Conley, Eric Gullikson, Christian Gollwitzer, Michael Krumrey, and Christian Laubis

DOI: 10.1364/AO.496511 Received 02 Oct 2023; Accepted 27 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: X-ray reflectance and film stress was measured for twelve bilayer and trilayer reflective interference coatings and compared with a single-layer Ir coating. The interference coatings comprise a base layer of W, Pt, or Ir, top layers of either C or B4C, and, in the case of the trilayer coatings, middle layers of either Co or Ni. The coatings were deposited by magnetron sputtering. Film stress was measured using the wafer curvature technique, while X-ray reflectance was measured at grazing incidence over the ~0.1–10 keV energy band using synchrotron radiation. Re-measurements over a period of more than two years of both stress and X-ray reflectance were used to assess temporal stability. The X-ray reflectance of all twelve bilayer and trilayer coatings was found to be both stable over time and substantially higher than single-layer Ir over much of the energy range investigated, particularly below ~4 keV, except near the B and C K-edges, and the Co and Ni L-edges, where we observe sharp, narrow drops in reflectance due to photo-absorption in layers containing these materials. Film stress was found to be substantially smaller than single-layer Ir in all cases as well, however film stress was also found to change over time for all coatings (including the single-layer Ir coating). The effective area of future X-ray telescopes will be substantially higher if these high reflectance bilayer and/or trilayer coatings are used in place of single-layer coatings. Additionally, the smaller film stresses found in the bilayer and trilayer coatings relative to single-layer Ir will reduce coating-stress-driven mirror deformations. Nevertheless, as all the interference films studied here have stresses that are far from zero (albeit smaller than that of single-layer Ir), methods to mitigate such deformations must be developed nevertheless in order to construct high-angular-resolution telescopes using thin mirror segments. Furthermore, unless film stress can be sufficiently stabilized over time, perhaps through thermal annealing, any such mitigation methods must also account for the temporal instability of film stress that was found in all coatings investigated here.

Aspherical interferometric probe with waveplate array detection: system design and nonlinearity analysis

Xinrui Fan, jianwei wu, xiaofei diao, Achaya Teppitaksak, Yanhui Kang, lihua lei, and Liqin liu

DOI: 10.1364/AO.503098 Received 11 Aug 2023; Accepted 27 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: Optical probes are the preferred choice for high-precision surface metrology, necessitating improved flexibility and a broader range of motion to adapt to the increasing complexity of surfaces. This study introduces an interferometric probe designed for measuring aspheric surfaces, utilizing a waveplate-array detection component. By integrating splitter elements into the detector, the probe improves integration and dynamic scanning performance, while maintaining high precision measurement capability. The system design and working principle are explored, and comprehensive nonlinear models based on the Jones matrix theory are established. These models focus on the nonlinear errors arising from alignment errors in various cases. Moreover, rigorous numerical simulations and optical experiments are conducted to validate the proposed models. When the alignment error reaches 10°, it results in a maximum nonlinear error of 3.02 nm. The experimental results demonstrate the effectiveness of the models in capturing nonlinear errors induced by alignment errors, providing a theoretical foundation for error reduction and compensation.

Super-oscillatory spots with different inhomogeneous linear polarized states

Bhavesh Pant, HEMANT MEENA, and Brijesh Singh

DOI: 10.1364/AO.504695 Received 31 Aug 2023; Accepted 26 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: We present the formation of super-oscillatory (SO) spots by tightly focusing the inhomogeneous linear polarized beam of different polarization states. At the entrance pupil of the focusing lens, a suitable phase manipulation in the incident beam results in a small super-oscillatory spot. Our numerical study based on the vectorial diffraction theory shows that SO spots of controllable size and various polarization combinations are possible. We also discuss the effect of the different polarization patterns of the incident beam on the size and energy distribution of the generated SO spots, which are potentially valuable for the orientation determination of single molecules and polarization-resolved imaging. This study reveals more influence of polarization states on the different components of the focused beam under the utilization of the proposed method rather than the usual tight focusing conditions.

A correction procedure for a tomographic optical setup employing imaging fiber bundles and intensified cameras

Marcel Müller, Florian Bauer, Weiwei Cai, Franz Huber, and Stefan Will

DOI: 10.1364/AO.507266 Received 29 Sep 2023; Accepted 26 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: For reliable tomographic measurements the underlying 2D images from different viewing angles must be matched in terms of signal detection characteristics. Non-linearity effects introduced by intensified cameras and spatial intensity variations induced from inhomogeneous transmission of the optical setup can lead, if not corrected, to a biased tomographic reconstruction result. This paper presents a complete correction procedure consisting of a combination of a non-linearity and flatfield correction for a tomographic optical setup employing imaging fiber bundles and four intensified cameras. Influencing parameters on the camera non-linearity are investigated and discussed. The correction procedure is applied to 3D temperature measurements by two-color pyrometry and compared to results without correction. The present paper may serve as a guideline for an appropriate correction procedure for any type of measurements involving optical tomography and intensified cameras.

In-Situ Measurement of Chromatic Dispersion in an Optical Hypertelescope: A Laboratory Demonstration

Ryan Allured, Jonathan Ashcom, and Philip Chapnik

DOI: 10.1364/AO.496372 Received 25 May 2023; Accepted 26 Nov 2023; Posted 29 Nov 2023  View: PDF

Abstract: Optical interferometry is a technique capable of achieving better spatial resolution than the world's largest monolithic telescopes at a fraction of the cost. Most interferometer architectures split the imaging bandwidth into a number of channels in order to prevent image degradation due to a large spectral bandwidth. An optical hypertelescope permits a much broader spectral bandwidth on a single channel than a conventional interferometer. However, a broader spectral bandwidth becomes more sensitive to differential chromatic dispersion, and this dispersion must be measured and corrected in order to maintain a high signal-to-noise ratio. A prototype dispersion measurement system is presented that is capable of measuring chromatic dispersion in an eight aperture hypertelescope. The optical design, calibration, data acquisition, and dispersion measurement process are described in detail. This system is capable of measuring differential dispersion to better than $\lambda/100$ RMS, and is scalable to a system with an order of magnitude more apertures.

Stimulated emission in CuInS2/ZnS core-shell quantum dot-doped liquid-core optical fiber

Zhe Wang, Mingye Sun, Lai Meng, and Lei Zhang

DOI: 10.1364/AO.507379 Received 04 Oct 2023; Accepted 25 Nov 2023; Posted 28 Nov 2023  View: PDF

Abstract: We fabricated QD liquid-core optical fibers by doping CuInS2/ZnS (CIS/ZnS) core/shell QDs with cladding times of 90 min and 60 min, and compared and analyzed the emission properties with bare core CuInS2 QDs. The emission intensity for fibers doped with CIS/ZnS core/shell QDs with cladding time of 90 min is approximately 4.73 times that of bare-core QD-doped fibers, and the transmission distance in the fiber is the longest. Additionally, the fact that the spectral half-width is narrowing and the spectral intensity is rapidly increasing super-linearly with excitation power indicates that stimulated emission happens in the fiber. The optical performance was compared and showed good agreement with a theoretical two-level system model for the QDs confined in an optical waveguide.

Phase space analysis of two-wavelength interferometry

Robert Leonard and Spencer Olson

DOI: 10.1364/AO.506520 Received 20 Sep 2023; Accepted 24 Nov 2023; Posted 28 Nov 2023  View: PDF

Abstract: Multiple wavelength phase shifting interferometry is widely used to extend the unambiguous range (UR) beyond that of a single wavelength. Towards this end, many algorithms have been developed to calculate the optical path difference (OPD) from the phase measurements of multiple wavelengths. These algorithms fail when phase error exceeds a specific threshold. In this paper, we examine this failure condition. We introduce a "phase-space" view of multi-wavelength algorithms and demonstrate how this view may be used to understand an algorithm's robustness to phase measurement error. In particular, we show that the robustness of the synthetic wavelength algorithm deteriorates near the edges of its UR. We show that the robustness of de Groot's extended range algorithm [Appl. Opt. 33, 5948 (1994)] depends on both wavelength and OPD in a non-trivial manner. Further, we demonstrate that the algorithm developed by Houairi & Cassaing (HC) [J. Opt. Soc. Am. 26, 2503 (2009)] results in uniform robustness across the entire UR. Finally, we explore the effect that wavelength error has on the robustness of the HC algorithm.

One step accurate phase demodulation from a closed fringe pattern with convolutional neural network HRUnet

Rongli Guo, Shuaidong Lu, Zhaoxin Li, Dangjuan Li, Fan Wang, Xiaoying Hu, and Shenjiang Wu

DOI: 10.1364/AO.506877 Received 28 Sep 2023; Accepted 24 Nov 2023; Posted 28 Nov 2023  View: PDF

Abstract: Retrieving phase map from a single closed fringe pattern is a challenging task in optical interferometry. In this paper, a convolutional neural network (CNN) HRUnet, is proposed to demodulate phase from a closed fringe pattern. The HRUnet, derived from the Unet model, adopts a high resolution network (HRnet) module to extract high resolution feature maps of the data and employs residual blocks to erase the gradient vanishing in the network. With the trained network, the unwrapped phase map can be directly obtained by feeding a scaled fringe pattern. The high accuracy of the phase map obtained from HRUnet is demonstrated by demodulation of both simulated data and actual fringe patterns. Compared results between HRUnet and two other CNNS are also provided and the results proved that the performance of HRUnet in accuracy is superior to the two other counterparts.

Progressive Acceleration in Genetic Algorithm by Intragenerational Mutation

Wenjing He, Wenkang Li, Haoyi Zuo, hong zhang, and Lin Pang

DOI: 10.1364/AO.505653 Received 20 Sep 2023; Accepted 24 Nov 2023; Posted 28 Nov 2023  View: PDF

Abstract: The intragenerational mutation of genetic algorithm (IMGA) is proposed to actively broaden the searching space during the optimization process. The searching space is aggressively increased by expanding the variation of mutation rates of all individuals within each generation, leading to the reduction of the required number of iterations, improving the convergence speed and the enhancement factor.

High-fidelity source mask optimization for suppressing line-end shortening

Zhiwei Zhang, Miao Yuan, Zhaoxuan Li, Weichen Huang, Yang he, Zhen Li, and Yanqiu Li

DOI: 10.1364/AO.506473 Received 20 Sep 2023; Accepted 24 Nov 2023; Posted 28 Nov 2023  View: PDF

Abstract: Source mask optimization (SMO) is a widely used computational lithography technique for compensating lithographic distortion. However, line-end shortening is still a key factor that cannot be easily corrected and affects the image fidelity of lithography at advanced nodes. This paper proposes a source mask optimization method that suppresses line-end shortening and improves lithography fidelity. An adaptive hybrid weight method is employed to increase the weights of the line end during the optimization, which adaptively updates the weights in each iteration according to the edge placement error (EPE). A cost function containing a penalty term based on the normalized image log slope (NILS) is established to ensure the fidelity of the overall feature when paying more attention to the line end region. The scope of this penalty term is regulated by widening and extending the split contour to further reduce the line-end shortening. Simulation results show that the proposed method can effectively suppress the line-end shortening and improve the lithography fidelity compared with the traditional SMO method.

Analysis of centroiding algorithms for non-diffracting Structured and Hollow Structured Laser Beams

Martin Dusek, Eva Roiková, Dirk Mergelkuhl, Jean-Christophe Gayde, Krystof Polak, and Miroslav Sulc

DOI: 10.1364/AO.509927 Received 23 Oct 2023; Accepted 24 Nov 2023; Posted 28 Nov 2023  View: PDF

Abstract: This paper explores the potential of optical-based systems, specifically pseudo-non-diffractive beams, as an alternative for alignment. The study focuses on Structured Laser Beams and Hollow Structured Laser Beams, which exhibit lower divergence and enhanced detection capabilities. The research objective is to analyze and compare centroiding algorithms in terms of accuracy and robustness to noise. The study compares the Gamma Center of Gravity, Correlation Template Matching, and Thresholding Center of Gravity. It also introduces a novel Polarization-based algorithm.

Experimental validation and mathematical simulation for laser protection performance of light field imaging

Yangliang Li, Qing Ye, Yunlong Wu, Hao Zhang, Haoqi Luo, Ke Sun, and Xiaoquan Sun

DOI: 10.1364/AO.501097 Received 20 Jul 2023; Accepted 23 Nov 2023; Posted 28 Nov 2023  View: PDF

Abstract: Photoelectric imaging systems typically employ a focal plane detector structure, rendering them vulnerable to laser damage. Laser damage can severely impair or even completely deprive the information acquisition capability of photoelectric imaging systems. A laser damage protection method based on a microlens array light field imaging system is proposed to prevent photoelectric imaging systems from laser damage. The technique utilizes the light field modulation effect of the microlens array to homogenize the spot energy, thereby reducing the maximum single-pixel receiving power at the image sensor. The method's effectiveness has been verified through numerical simulations and experimental validation. Firstly, the laser transmission theoretical model of light field imaging is proposed. Then, an experimental setup is established, and measurements are conducted to capture spot profiles and intensity distributions on the imaging plane across various defocus distances. Finally, the impact of propagation distance on the maximum single-pixel receiving power and suppression ratio of the light field imaging system is experimentally measured. Simulation and experimental results indicate that with the proposed method, the energy suppression ratio can easily reach two orders of magnitude, significantly reducing the probability of laser damage in photoelectric imaging systems.

Evolution of coherence singularities in polarization singular beams

Stuti Joshi, Saba Khan, and Paramasivam Senthilkumaran

DOI: 10.1364/AO.506815 Received 27 Sep 2023; Accepted 22 Nov 2023; Posted 28 Nov 2023  View: PDF

Abstract: The evolution of correlation singularities in partially coherent polarization singular beams (PC-PSBs) is investigated. Since PSBs are the superposition of two orthogonally polarized vortex beams, the occurrence of coherence singularities in PC-PSBs is strongly governed by thetopological charge of the component vortex beams and the spatial coherence length. Coherence singularities appear in the form of ring dislocations in the modulus of the spectral degree of coherence (SDoC) profile, and the number of ring dislocations is equal to the higher value of the topological charge of the superposing vortex beam. Furthermore, the SDoC phase profile can be used to determine the polarity of a PC-PSB. The findings of the study could be valuable in variousapplications that rely on the spatial coherence of beams, such as free-space communication and imaging.

Structured Illumination and Image Enhancement of Three-Dimensional and Moving Objects at a Distance via Incoherent Fourier Ptychography

Shawn Divitt, Samuel Park, Heath gemar, Kyle Judd, Harshil Dave, Di Lin, Dennis Gardner, and Abbie Watnik

DOI: 10.1364/AO.505275 Received 08 Sep 2023; Accepted 22 Nov 2023; Posted 28 Nov 2023  View: PDF

Abstract: We experimentally apply incoherent Fourier ptychography to enhance the resolution of recorded images by projecting known, uncorrelated, random patterns at high speed onto 3-dimensional, moving, and distant objects. We find that the resolution enhancement factor can be greater than 2, depending on the projection and camera optics.

Test station to characterize the emission of a LiDAR

Nelssom Cunha, Manuel Rodrigues, Flávio Ferreira, José Viana Gomes, Joao Linhares, Sandra Franco, Filipe Oliveira, Narciso Soares, Mikhail Vasilevskiy, and L Rebouta

DOI: 10.1364/AO.505749 Received 15 Sep 2023; Accepted 22 Nov 2023; Posted 28 Nov 2023  View: PDF

Abstract: A test station setup to measure the emission characteristics and beam propagation parameters of a Light Detection and Ranging (LiDAR) system is presented. The main blocks of the station are described, to enable the measurement of accessible emission, wavelength peak and FWHM, pulse duration, pulse repetition rate, horizontal and vertical angular resolution, field of view, beam propagation factor M², beam waist size, waist location and divergence. The performance of this test station was demonstrated using a commercial spinning LiDAR, a Velodyne VLP-16, which successfully enables these measurements for a laser beam with a wavelength of 913 nm.

High-quality open-access database for digital lensless holographic microscopy and its application on the improvement of deep-learning-based autofocusing models

Carlos Buitrago, Heberley Tobón, Alejandra Gómez, Samuel Zapata Valencia, Maria Lopera Acosta, Carlos Trujillo, and Jorge Garcia-Sucerquia

DOI: 10.1364/AO.507412 Received 03 Oct 2023; Accepted 22 Nov 2023; Posted 22 Nov 2023  View: PDF

Abstract: Among modern optical microscopy techniques, Digital Lensless Holographic Microscopy (DLHM) is one of the simplest label-free coherent imaging approaches. However, the hardware simplicity provided by the lensless configuration is often offset by the demanding computational postprocessing required to match the retrieved sample information to the user’s expectations. A promising avenue to simplify this stage is the integration of artificial intelligence and machine learning (ML) solutions into the DLHM workflow. The biggest challenge to do so is the preparation of an extensive and high-quality experimental dataset of curated DLHM recordings to train the ML models. In this work, a diverse, open-access, high-quality dataset of DLHM recordings is presented as support for future research, contributing to the data needs of the applied research community. The database is comprised of 11,760 experimental DLHM holograms of bio and non-bio samples with diversity on the main recording parameters of the DLHM architecture. The database is divided into two datasets of 10 independent imaged samples. The first group, named multi-wavelength dataset, includes 8,160 holograms and was recorded using laser diodes emitting at 654 nm, 510 nm, and 405 nm; the second group, named single-wavelength dataset, is composed of 3,600 recordings and was acquired using a 633 nm He-Ne laser. All the experimental parameters related to the dataset acquisition, preparation, and calibration are described in this manuscript. The advantages of this large dataset are validated by re-training an existing autofocusing model for DLHM and as the training set for a simpler architecture that achieves comparable performance, proving its feasibility for improving existing ML-based models and the development of new ones.

Reconstructing highly divergent wavefronts from sparse measurements

Nikolaus Berlakovich, Ernst Csencsics, Damian Senoner, and Georg Schitter

DOI: 10.1364/AO.502824 Received 08 Aug 2023; Accepted 22 Nov 2023; Posted 28 Nov 2023  View: PDF

Abstract: The paper presents a concept for the sparse measurement and reconstruction of highly divergent wavefronts enabling measurements at high throughputs and beyond the dynamic range of the wavefront sensor. In the proposed concept, a direct measurement of the wavefront is carried out, where a few segments of the wavefront are measured with Shack-Hartmann sensors (SHSs). In total about $1\,\%$ of the wavefront is measured and used for the reconstruction of the entire wavefront which makes the concept suitable for applications where low measurement times are needed. A simulation analysis and an experimental validation of the concept are carried out and results show that a wavefront with a divergence of $62^{\circ}$ can be reconstructed with a root-mean-square error of about $200\,nm$.

High-resolution terrain imager development and performance evaluation for lunar exploration

Youngchun Youk, Dongok Ryu, and Jeeyeon Yoon

DOI: 10.1364/AO.504883 Received 05 Sep 2023; Accepted 22 Nov 2023; Posted 30 Nov 2023  View: PDF

Abstract: The Lunar Terrain Imager (LUTI) is a crucial scientific instrument aboard the Korea Pathfinder Lunar Orbiter. We present an analysis of the optical design, assembly, and performance of the LUTI camera system. We outline the key components and technical specifications of the cameras and detail the assembly and alignment process, including collimator calibration and knife-edge scanning technique for modulation transfer function measurements. Furthermore, the thermal vacuum optical performance of the LUTI is evaluated by simulating space conditions. The optical designing, assembly, and thermal vacuum performance testing make the system valuable for high-resolution lunar surface imaging and future space exploration missions.

Nanoporous anti-reflection coating for high temperature applications in the infrared

Lorna Alvarez, Luke J. Currano, Christine Zgrabik, Dajie Zhang, Robert Weiblen, Timothy Montalbano, Noah Talisa, Michael J Purcell, Cavin T Mooer, Michael Thomas, David Young, and Jacob Khurgin

DOI: 10.1364/AO.506714 Received 02 Oct 2023; Accepted 21 Nov 2023; Posted 22 Nov 2023  View: PDF

Abstract: Anti-reflection (AR) coatings are essential to the performance of optical systems, without them surface reflections increase significantly at steep angles and become detrimental to functionality. AR coatings apply to a wide range of applications from solar cells, laser optics to optical windows. Many times, operational conditions include high temperatures and steep angles of incidence (AOIs). Implementation of AR coatings is extremely challenging to operate in both of these conditions. Nanoporous coatings made from high-temperature–tolerant materials offer a solution to this problem. Careful selection of materials is needed to prevent delamination when exposed to high temperatures, and an optimal optical design is needed to lower surface reflections at both normal incidence and steep AOIs. This article presents nanoporous silicon dioxide and hafnium dioxide coatings deposited on sapphire substrate using oblique angle deposition by electron-beam evaporation, a highly accurate deposition technique for thin films. Developed coatings were tested in a controlled temperature environment and demonstrated thermal stability at temperatures up to 800° C. Additional testing at room temperature demonstrated reduction of power reflections near optimal for AOIs up to 70° for a design wavelength of 1550 nm. These findings are promising to help extend the operation of technology at extreme temperatures and steep angles.

High diffraction efficiency varied-line-space concave gratings for Lyman ultraviolet explorer

Tao Ren, Angran Li, Shuhu Huan, Keqiang Qiu, Zheng Lou, Li Ji, ShuangYing Li, and Yilin Hong

DOI: 10.1364/AO.506040 Received 20 Sep 2023; Accepted 21 Nov 2023; Posted 22 Nov 2023  View: PDF

Abstract: In this paper we present the design and fabrication of the reflection varied-line-space concave grating (VLSCG) for the project of CAFE (the Census of warm-hot intergalactic medium, Accretion, and Feedback Explorer), which aims to detect and map the warm-hot circumgalactic medium (CGM) via OVI emission at 103.2 nm, using two Rowland-Circle-like spectrograph channels. High diffraction efficiency at LUV is supposed for VLSCG and an aperture ratio as small as F/3.6 is desired for a compact design. The gratings are fabricated by holographic lithography and ion-beam etching techniques. We introduce an additional lens into the normal holographic exposing system to generate the varied-line-space grating patterns. Grooves with triangle profiles are obtained to increase the diffraction efficiency by oblique ion beam bombardment during etching process. Finally, several VLSCGs with a central line density of 3300 lines/mm have been fabricated successfully. The measured results show that the groove efficiency reaches 51% at 106.4 nm and 31% at 127.4 nm. We imitated the working optical path of the spectrometer and used the -1 order of the VLSCG to measure the image near the exit slit. The results showed that the image of point source has a vertical extent of 0.68 mm, and the aberrations have been corrected.

BRDF modeling and optimization of target surface based on gradient descent algorithm

pengfei yang, Lu Bai, Zifei Zhang, and Yanhui Li

DOI: 10.1364/AO.506672 Received 22 Sep 2023; Accepted 21 Nov 2023; Posted 22 Nov 2023  View: PDF

Abstract: Addressing the current challenges in modeling and optimizing the Bidirectional Reflectance Distribution Function (BRDF) for the target surface, an improved six-parameter semi-empirical model is proposed based on an existing five-parameter semi-empirical model. In comparison with the original five-parameter model, the new model considers reciprocity, and the results demonstrate that as the incident angle increases, the fitting accuracy of the six parameters gradually surpasses that of the five parameters. Additionally, this paper employs a machine learning optimization algorithm, namely the gradient descent method, for optimizing the BRDF. The algorithm was comprehensively compared with other optimization methods, revealing that for the same dataset, the gradient descent method exhibited the smallest fitting errors. Subsequently, utilizing this algorithm for fitting experimental data resulted in errors consistently within 3%, confirming the reliability and accuracy of this optimization algorithm.

Nematic D-shaped Liquid Crystal Voltage Sensor with Enhanced Performance for Power Monitoring and Fault Detection in Extreme Environments

Md Walid Hassan and Md Aslam Mollah

DOI: 10.1364/AO.503706 Received 21 Aug 2023; Accepted 21 Nov 2023; Posted 21 Nov 2023  View: PDF

Abstract: A novel nematic liquid crystal voltage sensor with enhanced performance is proposed in this paper. The sensor is designed D shaped using a single NLC filled core without the presence of air holes, which has made the sensor fabrication very much simpler than previous sensors. The sensor also consists of a circular slot that provides a vast space for the surface plasmon resonance (SPR) phenomenon with a minimum amount of gold. The performance of the proposed sensor is carried out using finite element method (FEM) based simulation. Following this, the sensor has obtained a maximum wavelength sensitivity of 10 nm/V for a wide range of 190V to 250V with 5V increments. The sensor also has a linearity of 0.9926 and a figure of merit (FOM) of 0.2V-1. It has the resolution of 0.01V. The proposed sensor is a promising technology with a wide range of extreme and sophisticated applications. The sensor’s simple structure, high sensitivity, resolution, linearity, and FOM make it perfectly suitable for a variety of sensing applications, including power monitoring, fault detection, medical diagnosis, voltage lines, electronics etc.

Application of Powell algorithm for estimating optical properties of semitransparent medium based on time-domain information

Zhonghao Chang, Shuangcheng Sun, Linyang Wei, and Guangjun Wang

DOI: 10.1364/AO.504903 Received 11 Sep 2023; Accepted 20 Nov 2023; Posted 21 Nov 2023  View: PDF

Abstract: Accurate estimation of the optical properties of a semitransparent medium is crucial in various engineering applications. This study introduces the Powell algorithm to estimate the optical properties of a 2D semitransparent slab. The time-domain radiative transfer equation is solved using the discrete ordinate method. The radiative intensity on the medium's surface serves as the measurement signal for the inverse analysis. The results demonstrate that the Powell algorithm accurately estimates the absorption coefficient, scattering coefficient, and scattering asymmetry factor. For simultaneous reconstruction of these three parameters, it is recommended to use 8 signal detectors on both the left and right sides of the medium. Even when the standard measurement error is increased to 15%, the relative errors for these three parameters remain low, at 1.87%, 1.379%, and 0.194%.

Lens-less single-fiber ghost imaging

Toshitaka Wakayama, Yudai Higuchi, Rikuto Kondo, Yasuhiro Mizutani, and Takeshi Higashiguchi

DOI: 10.1364/AO.507550 Received 06 Oct 2023; Accepted 20 Nov 2023; Posted 21 Nov 2023  View: PDF

Abstract: We demonstrate lens-less single-fiber ghost imaging, which allows illumination and collection using a single optical fiber without a transmission-type system. Speckle patterns with relative coincidence degrees of 0.14 were formed by image reconstruction using improved differential ghost imaging. Employing fiber with a diameter of 105 µm, we achieved a spatial resolution of 0.05 mm in an observing area of 9 mm2 at a working distance of 10 mm. Compared with a conventional neuroendoscope at a power density of 94 mW/cm2, our imaging could be realized by extremely weak illumination at a laser power density of 0.10 mW/cm2. Using our lens-less single-fiber ghost imaging, utilizing 30,000 speckle patterns and implementing a diffuser, we attained an average coincidence degree of 0.45.

Laser Tissue Welding by Using Collagen Excitation at 1,720 nm Near-Infrared Optical Window III

Stefan Thomas, Vidyasagar Sriramoju, and Robert Alfano

DOI: 10.1364/AO.500113 Received 18 Jul 2023; Accepted 20 Nov 2023; Posted 21 Nov 2023  View: PDF

Abstract: Laser tissue welding (LTW) is a method of fusing incised tissues together. LTW has the potential to revolutionize plastic surgery and wound healing techniques by its ability to produce water-tight, scarless seals with minimal foreign body reaction. While using thermal mechanisms to achieve LTW, energy from the incident laser is absorbed by water in the tissue. As the water temperature increases, partial denaturing of the collagen triple helix briefly occurs, which is quickly followed by renaturation of collagen as the tissue cools, thus providing a watertight seal. This research study investigates the efficacy of direct collagen excitation at 1,720 nm to accomplish LTW. This wavelength falls within the near infrared (NIR) optical window III. The tensile strengths of pig skin that have been welded with NIR continuous-wave (CW) diode lasers at 1,455 nm, which promotes thermal mechanisms of tissue welding, and 1,720 nm wavelengths are compared. Near infrared lasers tuned to 1,455 nm and 1,720 nm were used to weld incised pieces of porcine skin together without extrinsic solders or dyes. The tensile force of the welded tissues was measured using a digital force gauge. The average tensile force of the welded pig skin using the 1,720 nm laser was approximately four times greater than that using the CW 1,455 nm laser, suggesting that LTW accomplished through direct collagen excitation in NIR Optical Window III provides greater tensile strengths.

MCNN-DIC: A mechanical constraints-based digital image correlation by a neural network approach

Lu Wang, Yawen Deng, Xianzhi Gao, and Guangyan Liu

DOI: 10.1364/AO.498872 Received 03 Jul 2023; Accepted 18 Nov 2023; Posted 20 Nov 2023  View: PDF

Abstract: Abstract:Digital Image Correlation (DIC) is a widely used photomechanical method for measuring surface deformation of materials. Practical engineering applications of DIC often encounter challenges such as discontinuous deformation fields, noise interference and difficulties in measuring boundary deformations. To address these challenges, a new DIC method called MCNN-DIC is proposed in this study by incorporating mechanical constraints using neural network technology. The proposed method applied compatibility equation constraints to the measured deformation field through a semi-supervised learning approach, thus making it more physical. The effectiveness of the proposed MCNN-DIC method was demonstrated through simulated experiments and real deformation fields of nuclear graphite material. The results show that the MCNN-DIC method achieves higher accuracy in measuring non-uniform deformation fields than a traditional mechanical constraints-based DIC and can rapidly measure deformation fields without requiring extensive pre-training of the neural network.

Impact of Laser Phase Noise on the Ranging Accuracy of Cooperative MIMO FMCW Photonic Radar System

AMITESH KUMAR and Annapurna Kumari

DOI: 10.1364/AO.505755 Received 13 Sep 2023; Accepted 17 Nov 2023; Posted 20 Nov 2023  View: PDF

Abstract: In this article, a FMCW based cooperative 2×2 MIMO photonic radar system using heterodyne detection is presented. The proposed system consists of two separate sensor node that uses linear frequency modulated continuous wave signal which allows simultaneous monostatic and bistatic radar measurement, where the target range and angle of arrival information is extracted. The additional bistatic information enhances the target detection and estimation capabilities with improved accuracy. This accuracy in practical is affected by the laser phase noise which degrades the overall system performance. Here, the analytical laser phase noise model for MIMO system is derived and implemented to analyze its impact on the ranging accuracy of the proposed system. Under the impact of standard white Gaussian laser phase noise assumption, the monostatic and bistatic response of the detected signal is measured and compared using statistics of measurement error. Further, the signal-to-noise ratio and SSB laser phase noise of the monostatic and bistatic response are measured and compared at different target ranges. At last, the phase noise limited ranging accuracy of the system is evaluated and analyzed. The concept shown in this work paves the way for advanced photonic radar system applications such as modern radar systems, electronic warfare systems, metrology, and automotive vehicle radar with multiperspective coherent detection.

Performance of Modulating Retro-reflector-Assisted Ground- HAP-Satellite uplink laser communication system

Xuewen Jiang, Xingyue Guo, and Yi Wang

DOI: 10.1364/AO.507297 Received 29 Sep 2023; Accepted 17 Nov 2023; Posted 20 Nov 2023  View: PDF

Abstract: Satellite-ground laser communication has attracted wide attention due to its advantages of rich spectrum resources, fast communication speed, strong anti-interference ability, and high security. Therefore, this paper proposes to use modulating retro-reflector(MRR) and assemble it on the HAP to improve the performance of the ground-satellite uplink laser communication system.Since the influence of the hovering fluctuation of HAP on the system can not be ignored, this paper introduces the angle of arrival (AoA) jitter to represent the influence of the random jitter of the HAP in the air on the channel and considers the light intensity scintillation, beam wander, atmospheric attenuation, pointing error. The combined effect of the system is analyzed. At the same time, the influence of key factors such as beam width, zenith angle, HAP position distance, wind speed, and cloud visibility on the performance of the ground-HAP-satellite system under different MRR effective areas is simulated and analyzed, and compared with the ground-HAP-satellite system without MRR(G-H-S). The results show that the MRR-assisted ground-HAP-satellite system(G-HM-S) has better communication performance. The work of this paper provides a good theoretical basis for the engineering implementation of the MRR-assisted ground-HAP-satellite laser communication system.

Improving Distance Imaging Accuracy through Temporal Position Correction with Phase Difference Compensation

zengyan wu, Cao Changqing, Zhejun Feng, xiaona wu, chenxuan duan, and Hongyuan liu

DOI: 10.1364/AO.502508 Received 03 Aug 2023; Accepted 17 Nov 2023; Posted 20 Nov 2023  View: PDF

Abstract: This study introduces a time-domain-based phase compensation method to address decoherence effects in optical heterodyne detection, which is critical for remote sensing and distance imaging. Numerical simulations demonstrate a substantial reduction in localization bias (6.56 to 2.85) and an increased probability of bias values below 2 (21.6% to 70.5%). Experiments show significant improvement in whiteboard distance imaging accuracy at 10m from the detector, with 91.7% of data falling within 10-12m compared to a mere 2.3% accuracy before compensation. The method effectively enhances intensity image quality, mitigates decoherence phenomena, and improves detection accuracy and reliability without additional hardware

A Posture Planning Method for Complex Feature Measurement Using a Single-Line Laser Scanner

Feng ShengQuan, Li Qi, Huang JianXin, and Chen Xiang

DOI: 10.1364/AO.506258 Received 21 Sep 2023; Accepted 17 Nov 2023; Posted 20 Nov 2023  View: PDF

Abstract: A single-line laser scanner is commonly utilized for measuring complex surfaces and contours. However, achieving automatic implementation of this scanner poses challenges in terms of designing a measurement posture that considers measurement accuracy, path planning, and the positioning of auxiliary equipment. This ensures non-interference during the measurement process. In this study, we focus on the application of T-SCAN. First, we construct a measurement posture parameter model for T-SCAN and analyze the viewpoint position’s coverage of the measurement area. Second, we propose a measurement path planning method based on scanning posture to minimize overlapping areas. Lastly, we present a measurement station planning method based on scanning postures and analyze the transfer error of the measurement targets to establish a unified measurement field. Experimental results demonstrate that, after the posture adjustment process, the average distance deviation between the measurement data and the theoretical model is significantly reduced.

Quantitative analysis of energy-dispersive X-ray fluorescence spectroscopy based on machine learning and generative data enhancement technique

Wei Zhao, Xianyun Ai, and Hui Zhao

DOI: 10.1364/AO.506027 Received 18 Sep 2023; Accepted 16 Nov 2023; Posted 16 Nov 2023  View: PDF

Abstract: This paper proposes a data enhancement technique to generate expanded datasets for machine learning by developing an X-ray fluorescence spectra simulator based on the physical process. The simulator consists of several modules, including the excitation source, the interaction process, and the detection system. The spectra generated by the simulator are subject to dimension reduction through feature selection and feature extraction algorithms, and then serve as the input for the XGBoost (Extreme Gradient Boosting) model. Six elements of metal samples with various content ranges were selected as the research target. The results showed that for simulated data, the R2 value for elements with concentrations ranging from 0% to 100% is greater than 95%, and for elements with concentrations of <0.3%, the R2 value is greater than 85%. The experimental data were able to be predicted by the model trained by the simulated spectra. Therefore, this approach provides reliable results for practical application and can supply additional datasets to obtain reasonable prediction results for machine learning with inadequate reference materials.

An automatic marker-based alignment method for nano-resolution full-field transmission X-ray microscope

Chenpeng Zhou, Yan Wang, wang shanfeng, jin zhang, Fu Tianyu, Wanxia Huang, Kai Zhang, and Qingxi Yuan

DOI: 10.1364/AO.506046 Received 18 Sep 2023; Accepted 16 Nov 2023; Posted 16 Nov 2023  View: PDF

Abstract: Driven by the development of X-ray optics, the spatial resolution of full-field transmission X-ray microscope (TXM) has reached tens of nanometers and plays an important role in promoting the development of biomedicine and materials science. However, due to the thermal drift and the radial/axial motion error of the rotation stage, the TXM computed tomography (CT) data are often associated with random image jitter errors along the horizontal and vertical directions during CT measurement. A nano-resolution 3D structure information reconstruction is almost impossible without a prior appropriate alignment process. To solve this problem, a fully automatic gold particle marker-based alignment approach without human intervention was proposed in this study. It can automatically detect, isolate, and register gold particles for projection image alignment with high efficiency and accuracy, facilitating a high quality tomographic reconstruction. Simulated and experimental results confirmed the reliability and robustness of this method.

IRS-Assisted Vehicular Visible Light Communication Systems: Channel Modeling and Performance Analysis

Arash Rabiepoor, S. Alireza Nezamalhosseini, and Lawrence Chen

DOI: 10.1364/AO.502663 Received 21 Aug 2023; Accepted 16 Nov 2023; Posted 17 Nov 2023  View: PDF

Abstract: Visible light communication (VLC) is a promising solution as an alternative for the fully-occupied radio frequency bands in the near future. The rear (tail) and front of vehicles have lamps that can be used for vehicular visible light communication (VVLC) systems. On the other hand, one of the main challenges of VLC systems is the line-of-sight (LoS) blockage issue. In thispaper, we propose the installation of intelligent reflecting surfaces (IRSs) (i.e., smart mirrors) on the back of vehicles to overcome the aforementioned issue in VVLC systems. We assume three different patterns of angular distribution for the radiation intensity, such as a commercially available LED with an asymmetrical pattern (Philip Luxeon Rebels), a symmetrical Lambertian pattern, and an asymmetrical Gaussian pattern. In the first section of this paper, we obtain the channel model for the IRS-Assisted VVLC systems, then the evaluation of the path loss results versus link distance under different conditions such as weather type (clear weather, rainy, moderate fog, and thick fog) and radiation patterns are investigated. Moreover, the impact of system parameters such as the aperture size of the photodetector (PD), side-to-side and front-to-front distances, number of IRS elements, and IRS area are studied. In the second part, we derive a closed-form expression for the maximumachievable link distance versus the probability of error for the IRS-Assisted VVLC systems. In addition, the impact of parameters in single-photon avalanche diode (SPAD), background noise, as well as the system parameters evaluated for the path loss is analyzed in this section.

Liquid crystal wavefront correction based on improved machine learning for free space optical communication

HONGYANG GUO, wei tang, zihao wang, liangzhu yuan, yang li, Dong He, Qiang Wang, and Yongmei Huang

DOI: 10.1364/AO.505697 Received 13 Sep 2023; Accepted 15 Nov 2023; Posted 15 Nov 2023  View: PDF

Abstract: In order to suppress the impact of atmosphere turbulence on the space laser communication link, the wavefront correction technology of liquid crystal spatial light modulator (LCSLM) is studied. Combining with the control mode of LCSLM, we propose an improved deep learning approach that restores the input image features into wavefront and then controls LCSLM to compensate for the phase distortion. This method does not have Zernike coefficient truncation and does not require the calculation of coefficient matrices, thus improving the accuracy and efficiency of the algorithm. At the same time, as for its powerful phase fitting ability, LCSLM can be used as a turbulence simulator to construct datasets. During the training process of the neural networks, a calibration between LCSLM and deep learning is established. Finally, a spatial optical coupling experimental system is built. The results show that under different atmospheric conditions, the liquid crystal wavefront correction method has a significant improvement in terminal coupling efficiency, and has certain application prospects in the field of free space optical communication.

A robust point light source calibration method for near field photometric stereo using feature points selection

Long Ma, Xu Liu, Yuzhe Liu, Xin Pei, and Shengwei Guo

DOI: 10.1364/AO.505234 Received 06 Sep 2023; Accepted 15 Nov 2023; Posted 16 Nov 2023  View: PDF

Abstract: In this paper, we present a robust method for non-isotropic point light source calibration through feature points selection. By analyzing the relationship between the observed surface and its image intensity under near field lighting, the feature points selection method is firstly developed to effectively address the noisy observations and improve the calibration robustness. Afterwards, to enhance the efficiency and the accuracy of calibration, a cost function of lp-norm is established based on the above relationship and an improved Newton method based iteration process is applied to calculate the light source parameters. The simulations demonstrate that the proposed method is capable of achieving robust calibration results with the estimation error less than 2.7mm and 0.8deg even the image intensities are corrupted by Gaussian white noise with standard deviation up to 0.4. The experimental validation is performed using a self-designed photometric stereo system, where the calibration of point light sources is conducted and measurements are taken on both a standard sphere and a compressor blade based on the obtained calibration results, which demonstrates the effectiveness of the new method.

A Deep Learning Approach to Predict Optical Attenuation in Additively Manufactured Planar Waveguides

Keno Pflieger, Andreas Evertz, and Ludger Overmeyer

DOI: 10.1364/AO.501079 Received 20 Jul 2023; Accepted 12 Nov 2023; Posted 13 Nov 2023  View: PDF

Abstract: The booming demand for efficient, scalable optical networks has intensified the exploration of innovative strategies that seamlessly connect large-scale fiber networks with miniaturized photonic components. Within this context, our research introduces a novel neural network, specifically a \ac{CNN}, as a trailblazing method for approximating the nonlinear attenuation function of centimeter-scale multimode waveguides. Informed by a raytracing model that simulated many flexographically printed waveguide configurations, we cultivated a comprehensive dataset that laid the groundwork for rigorous \ac{CNN} training. This model demonstrates remarkable adeptness in estimating optical losses due to waveguide curvature, achieving an attenuation standard deviation of 1.5 dB for test data over an attenuation range of 50~dB. Notably, the \ac{CNN} model's evaluation speed, at 517 microseconds per waveguide, starkly contrasts the used ray tracing model that demands 5 to 10 min for a similar task. This substantial increase in computational efficiency accentuates the model's paramount significance, especially in scenarios mandating swift waveguide assessments, such as optical network optimization. In a subsequent study, we test the trained model on actual measurements of fabricated waveguides and its optical model. All approaches show excellent agreement in assessing the waveguide's attenuation within measurement accuracy. Our endeavors elucidate the transformative potential of machine learning in revolutionizing optical network design.

A robust structured light 3D measurement method based on polarization-encoded projection patterns

Zhenmin Zhu, Yumeng Zhou, Wenquan Lu, Jing Zhang, lisheng zhou, and Haoran Liu

DOI: 10.1364/AO.502522 Received 03 Aug 2023; Accepted 08 Nov 2023; Posted 09 Nov 2023  View: PDF

Abstract: Fringe projection profilometry (FPP) is widely used in 3D vision measurement because of its high robustness and measurement accuracy. In the case of HDR objects, due to the problem of surface reflectivity, the obtained image will be overexposed. This will cause the sinusoidality of the fringes projected on the surface of the object in the acquired image to be interfered, resulting in a phase error in the calculated wrapped phase. Therefore, a polarization-encoded sinusoidal structured light is proposed to enhance the sinusoidality of the fringe. Because the phase information contained in the polarized sinusoidal structured light fringe is only related to the polarization state, but not to the light intensity. Polarization coding assisted structured light measurement strategy (PASM) is proposed. This method uses polarization coding assisted polarization phase-shifting fringes for phase unwrapping. This method does not need to rotate the polarizer, and only needs a single exposure to improve the fringe quality and obtain a more stable unwrapping phase. The experimental results show that the obtained polarization fringes have better sinusoidality, and the phase unwrapping can be more accurate. The reconstructed 3D point cloud also does not appear missing and has better accuracy. It is a reliable method for visual measurement of HDR objects.

Design and Analysis of Photonic Crystal Nano-Cavity based Bio-Sensor

Akash Pradhan, KEERTHAN MADAGALA, Kalamcheti Naga Sravya, and chandra prakash

DOI: 10.1364/AO.503892 Received 23 Aug 2023; Accepted 07 Nov 2023; Posted 08 Nov 2023  View: PDF

Abstract: A new, design of photonic crystal nanocavity-based bio-sensor has been proposed to detect different blood components. A finite difference time domain (FDTD) numerical technique has been used to characterize the sensor by evaluating its frequency response. The shift in resonating wavelength of the proposed cavity is utilized to detect blood refractive index fluctuation due to the presence of various components. The obtained numerical findings show that the maximum sensitivity for a shift in resonating wavelength is reported as 760 nm/RIU for various blood components. Moreover, the fabrication of PhC is always prone to the fabrication induced disorders. Hence, the impact of fabrication imperfections on the sensor’s performance also has been included in the analysis.

Broadband free space 2 × 4 90° optical hybrid for satellite laser communication

lingling xu, Jianfeng Sun, Qian Xu, Haisheng Cong, Weijie Ren, Zhang kun, Yuxin Jiang, Chaoyang Li, and hanrui pan

DOI: 10.1364/AO.505607 Received 12 Sep 2023; Accepted 03 Nov 2023; Posted 06 Nov 2023  View: PDF

Abstract: We design a broadband free space 2 × 4 90° optical hybrid over a spectral window of 1000 – 1200 nm and 1470 – 1650 nm, and verify the feasibility of the scheme experimentally. The hybrid consists of three broadband polarization beam splitters, an achromatic λ/4 wave plate, and three achromatic λ/2 wave plates. The fabricated hybrid exhibits a good quadrature phase response with an interchannel imbalance of 0.93~1.07 and a low phase deviation of less than 0.2° under the typical communication wavelengths of 1064 nm and C-band. The experimental results of the heterodyne method show that the proposed hybrid can effectively solve the wavelength incompatibility problem in satellite laser communication and realize interconnection at different wavelengths. The designed hybrid (without coupling) has a measured insertion loss of no more than 7.34 dB at 1064 nm and Cband. A high-speed transmission experiment with BPSK format has been conducted to verify the performance of the assembled device.

Design and Implementation of Dual Aspheric Integrated Shaping Element for High-Power Fiber Laser

Yupeng Xiong, Shiyu Chen, Yanxing Ma, Yang Ou, Wenwen Lu, Yifan Dai, Shanyong Chen, and Cheng HUANG

DOI: 10.1364/AO.503237 Received 16 Aug 2023; Accepted 30 Oct 2023; Posted 17 Nov 2023  View: PDF

Abstract: A novel dual aspheric integrated beam shaper suitable for high-power laser situation was first designed and realized. The model for this new lens was derived theoretically and the performance is evaluated using detailed simulation. The ultrasonic vibration assisted cutting and the high precision grinding and polishing technology were used for the processing. The surface accuracy was less than 200nm measured with a profiler, and the roughness was smaller than 20nm with the help of the white light interferometer. And the shaping experiments were carried out, which verified the Gaussian beam has uniform intensity distribution with the uniformity of 85.13% in the near field and converges to a point in the far field which is exactly expected. It thus provides an actual selection for the high-power laser shaping.

High sensitivity optical fiber probe for simultaneous measurement of chloride ions and temperature

Xia Li, Yu Li, Chenxiao Wang, Wa Jin, Guangwei Fu, Xinghu Fu, and Weihong Bi

DOI: 10.1364/AO.500182 Received 10 Jul 2023; Accepted 02 Oct 2023; Posted 22 Nov 2023  View: PDF

Abstract: A fiber optic probe for the simultaneous measurement of chloride ions and temperature is presented. The Ag/alginate composite film is used as the reflective surface of the Fabry-Perot interferometer(FPI) and is also a sensitive film for the adsorption of chloride ions. The experimental results show that the Fabry-Perot (FP) response sensitivity is approximately 1.4689 nm/μM as the chloride ion concentration changes from 1µM to 9µM, but the fiber Bragg grating(FBG) is insensitive to chloride ions. When the temperature is changed from 35℃ to 80℃, the response sensitivities of the FP and the FBG are about 0.7nm/℃ and 0.01115nm/℃, respectively.