High-energy nanosecond thin-rod laser amplifier based on Yb:YAG

Jinsheng Liu, Jiaqi Zhang, Lailin Ji, Xianghe Guan, Yvkun Zhu, Wei Feng, Tianxiong Zhang, TAO WANG, Yong Cui, Xiaohui Zhao, Zhan Sui, and YAN-QI Gao

DOI: 10.1364/AO.558335 Received 30 Jan 2025; Accepted 06 Mar 2025; Posted 07 Mar 2025  View: PDF

Abstract: Nanosecond pulsed lasers with repetition rates that can deliver tens of millijoules of energy have significant application prospects as seed sources for Joule-scale laser amplifiers to achieve high energy output. In contrast to the high average power and high repetition-rate scheme, we have developed a compact thin-rod amplifier to obtain high-energy pulses with lower repetition rates. A maximum pulse energy of 30 mJ was generated at a repetition rate of 5 Hz with a pulse duration of 10 ns. To the best of our knowledge, these experimental results represent the highest pulse energy outputs obtained using thin Yb:YAG-based rods at 293 K. The output light exhibits linear polarization because of the quarter-wave plate (QWP) and polarization beam splitter (PBS). This study paves the way for a simple and cost-efficient method to achieve compact tens of millijoules of energy output.

Optimizing the shape of the microstructure and the number of dots on the front light guide plate to reduce manufacturing energy consumption

Chia Hung Yeh and Wei Min Chen

DOI: 10.1364/AO.547448 Received 08 Nov 2024; Accepted 06 Mar 2025; Posted 07 Mar 2025  View: PDF

Abstract: The front light guide panel is situated in front of the liquid crystal display (LCD) layer. The light guide plate plays a crucial role in uniformly distributing the emitted light across the entire screen surface. This technology is particularly advantageous for e-book readers or in environments with limited illumination, as it provides front lighting that helps to alleviate eye fatigue. The objective of this study is to enhance the design of the dot microstructure of the light guide plate within the front light module while simultaneously reducing the number of dots. The optimization of the dot microstructure shape is conducted using a randomized algorithm, employing the root mean square as the target metric. This aims to improve the uniformity of light output from the front light module while decreasing the number of dots. The results of the research indicate that transforming the conventional crater shape, widely used in the industry, into a triangular dot microstructure could potentially reduce the number of dots from 1,694,459 to 746,125. This innovative approach has the capacity to streamline the manufacturing process, minimize costs, and decrease manufacturing energy consumpt. Overall, the cost reduction is projected to reach as high as 44%.

LFE-Net: a low-light fringe patterns enhancement method based on convolutional neural networks

Linlin Wang, Wenke Xue, Qian Gao, Chuanyun Wang, Li Bingzhen, and wanyi guo

DOI: 10.1364/AO.551276 Received 11 Dec 2024; Accepted 06 Mar 2025; Posted 07 Mar 2025  View: PDF

Abstract: Fringe Projection Profilometry (FPP) technique has always been the focus of research and attention from numerous scholars. However, when obtaining the fringe pattern, the light of the image may be too low due to environmental factors, which may affect the smooth progress of subsequent work. To address this issue, this paper proposes a LFE-Net network for enhancing low-light fringe patterns. In this method, RGB fringe patterns and grayscale fringe patterns are used as the input feature map of the entire network. The wavelet transformation preprocessing (WTP) module is designed to process the input feature map and obtain rich feature information. At the same time, the residual convolution transformer parallel (RCTP) module is designed to ensure the network model's ability to connect contextual information and enhance the flexibility of the network. The experimental results on our dataset show that our proposed method outperforms the multi-scale Retinex (MSRCR) algorithm, MIRNet, and HWMNet in processing low-light fringe pattern. Moreover, under the same hardware environment, the computational time of the proposed method is only 8.66% of MIRNet and 40.49% of HWMNet.

MEMS-based beam steering system with metalens and cone lens

Run Lyu and Zhouping Su

DOI: 10.1364/AO.553150 Received 16 Dec 2024; Accepted 06 Mar 2025; Posted 07 Mar 2025  View: PDF

Abstract: A micro-electro-mechanical system (MEMS) -based LiDAR beam steering system composed of a cone lens and a metalens is proposed. The cone lens has three key optical surfaces, through which the incoming beam passes and then outputs along the horizontal direction. When the MEMS rotates around the Y-axis, the output beam can scan across a 360° field of view in the horizontal direction. Additionally, the output beam can also scan in a vertical field of view (FoV) of 12.3°. The metalens can further reduce the divergence of the output beam in both the horizontal and vertical directions. The output beam has a narrow divergence with a divergence angle of 0.313° × 0.287°. The system provides a design solution for a solid-state LiDAR beam steering system that meets the design requirements of a large FoV with a more miniaturized structure, while being easier to integrate and assemble.

Knife-edge Lateral Scanning Method for Automated Measurement of Wavefront Power Spectral Density

Zhang Ze, Zhaoyang Jiao, yuxin Liu, kaiqi zhang, Ziming Dong, and Jianqiang Zhu

DOI: 10.1364/AO.553324 Received 09 Jan 2025; Accepted 06 Mar 2025; Posted 07 Mar 2025  View: PDF

Abstract: In high-power laser systems, mid to high frequency errors in optical elements can significantly degrade beam quality, potentially leading to filamentation, which increases the risk of damage to optical components and reduces system efficiency. Power Spectral Density (PSD) is a critical metric for assessing surface wavefront errors in optical elements. The PSD of the components was ensured through iterative processing and measurements. Currently, PSD detection primarily relies on interferometry. However, its stringent stability requirements make efficient online measurements challenging. A knife-edge lateral scanning method is proposed that enables an efficient and automated PSD detection of large-aperture optical components. The equivalent relationship between shadowgraph PSD and wavefront PSD is established in this method, which allows for the direct calculation of the PSD curve from the shadowgraphs. It reveals periodic modulation by laterally blocking higher-order light spots. The scanning and image-stitching techniques in this approach eliminate the need to determine the knife-edge cutting position, thus making the measurement process more convenient. The experimental results indicate an RMS error of approximately 5%. This method lays the foundation for rapid online PSD detection, which has significant implications for the efficient iterative processing of high-precision optical components.

Equal-intensity beam splitter fabricated by tandem dielectric beam splitters for passive laser speckle reduction

Zhaomin Tong, JINJIA LIU, Bin Xie, YIFEI MA, mei wang, Suotang Jia, and Xuyuan Chen

DOI: 10.1364/AO.554544 Received 02 Jan 2025; Accepted 06 Mar 2025; Posted 07 Mar 2025  View: PDF

Abstract: An equal-intensity beam splitter that utilizes tandem dielectric beam splitters with specific splitting ratios and high-reflectivity mirror for passive laser speckle reduction is presented. The beam splitter coatings of the dielectric beam splitters were designed using Essential Macleod software simulations. Specifically, three laser sub-beams of equal optical intensities were generated. The optical path delay between laser sub-beams destroyed the temporal and spatial coherence of the laser. The objective speckle contrasts decreased from 0.75 to 0.47 when all three laser sub-beams were utilized. The study demonstrates the potential of equal-intensity beam splitter, which can be designed to be compact and static, for laser projection.

Wind Vector Retrieval Algorithm for Coherent Doppler Lidar Based on KNN-COOKS

Jie Yu, Pan Guo, Chen Siying, He Chen, Rongzheng Cao, Yixuan Xie, Zhengfeng Zou, Shengli Yin, Yinghong Yu, Junshuai Liu, and Mengjun Feng

DOI: 10.1364/AO.553700 Received 26 Dec 2024; Accepted 05 Mar 2025; Posted 05 Mar 2025  View: PDF

Abstract: The wind vector retrieval from a coherent Doppler lidar system in Plan Position Indicator (PPI) scanning mode often suffers from high inversion errors in horizontal wind speed and direction at farrange gates due to "erroneous" radial wind speed. To address this, we propose a weighted sine wave fitting algorithm that combines K-Nearest Neighbors and Cook's distance (KNN-COOKS). Numerical simulation experiments show KNN-COOKS achieves higher accuracy than Direct Sine Wave Fitting (DSWF) and Adaptive Iterative Reweighted sine wave fitting (AIR) and performs comparably to filtered sinusoidal wave fitting (FSWF). Validation with real-world data shows KNN-COOKS increases valid data by 22.5% and 12.5% over DSWF and AIR, respectively, while reducing computation time by 62% compared to FSWF and 38% compared to AIR.

Portable Low-Coherence Digital Holographic Microscope

Miguel Leon-Rodriguez, Juan Rayas, Amalia Martinez-Garcia, Adrian Martínez Gonzalez, Israel Miguel-Andrés, Alejandro Téllez-Quiñones, and Fernando Mendoza Santoyo

DOI: 10.1364/AO.554697 Received 03 Jan 2025; Accepted 04 Mar 2025; Posted 05 Mar 2025  View: PDF

Abstract: We present a compact and portable setup in digital holographic microscopy that works in a slightly off-axis configuration. It has a monolithic structure and a minimal number of elements, a feature that makes it insensitive to vibrations. The design uses an LED as an illuminating source and a single cube beam splitter interferometer to avoid parasitic interferences and coherent noise reduction. We get imaging phase reconstruction in one shot, which is valuable for dynamical inspection and portability due to its high stability and compactness. We present experimental results of the system tested in a harsh environment.

Multifocal waveguide-based display in augmented reality

Seong-Hyeon Cho, Jae-Sang Lee, Do-Hun baek, Woo Choi, and Young-Wan Choi

DOI: 10.1364/AO.553392 Received 18 Dec 2024; Accepted 04 Mar 2025; Posted 05 Mar 2025  View: PDF

Abstract: We present a waveguide-based near-eye display designed to enhance immersive augmented reality (AR) experiences. Unlike conventional waveguide displays that project virtual content onto a single focal plane, the proposed waveguide optics can display images across an extended depth range. This extension of focus cues is achieved through a multifocal holographic optical element (M-HOE) that functions as an output coupler on the waveguide. The M-HOE is fabricated by spatially implementing three distinct volume holographic gratings within a single photopolymer film, thereby serving as a trifocal miniaturized flat lens. Consequently, virtual content via the output coupler is spatially focused at three focal distances (−300, −600, and −1000 mm from the output coupler), as experimentally demonstrated in the AR display. Although this approach is in its early stages, it holds promise for alleviating the vergence-accommodation conflict, a significant challenge in head-mounted AR displays, thereby providing users with a truly immersive AR experience.

The robustness of phase modulated holographic vibrometry in the presence of parasitic vibrations

Florian Dötzer, Johannes May, and Stefan Sinzinger

DOI: 10.1364/AO.545135 Received 17 Oct 2024; Accepted 04 Mar 2025; Posted 04 Mar 2025  View: PDF

Abstract: Holographic vibrometry is a family of interferometric methods for contactless, full-field vibration measurements. Harmonic surface vibrations with high frequencies (Hz to upper MHz regime) and low displacements (sub-nm to µm) as found in micro-electromechanical systems can be characterized. Using frequency shifting, it is possible to obtain static interference or low-frequency beats between the reference arm and one of the sidebands generated by the vibration in the object arm. This enables the use of regular video-rate camera detectors which offers high lateral spatial resolutions at short measurement times compared to scanning methods. The key to achieving high signal-to-noise ratios is a narrow-band lock-in evaluation at the beat frequency captured by a sequence of holograms. However, in many scenarios the signal of interest is superimposed by parasitic low-frequency vibrations from the environment. The beat signal is then Doppler-broadened and narrow-band filtering is no longer possible. In phase modulated holographic vibrometry, the lock-in signal is given by a beat between the object vibration and an artificial reference vibration. In this work we exploit that acoustic signals are less susceptible to Doppler-broadening as their absolute frequencies are orders of magnitude smaller compared to optical signals. This leads to a stable narrow-band beat frequency even in the presence of perturbations. The superior robustness is demonstrated experimentally by successfully recovering displacements below 10 nm in the face of perturbations up to three orders of magnitude larger. We discuss limiting factors and show that cutting exposure times allows to tolerate even larger perturbations.

Adaptive image selection method for focus stacking based on low-level vision task-driven network and liquid lens

Jiale Wei, Shanshan Wang, Qun Hao, MENGYAO LIU, and Yang Cheng

DOI: 10.1364/AO.555601 Received 14 Jan 2025; Accepted 04 Mar 2025; Posted 04 Mar 2025  View: PDF

Abstract: An all-in-focus (AIF) image has been employed broadly in various fields, such as microscopy imaging, medical imaging, and high-level vision tasks. Focus stacking is a key technology for merging AIF images. Considerable efforts have been made to reconstruct AIF images accurately. However, little attention has been paid to capturing focal stack images effectively. This paper proposes an adaptive image selection method for capturing focal stack images based on a low-level vision task-driven network and liquid lens. The proposed method can maintain the integral quality using the minimum number of focal stack images. The lowlevel vision task-driven network termed FocalAIF-Net consists of two-branch FocalNet and auxiliary low-level vision task AIFNet. The FocalNet can estimate the blur map and the focal map from a defocused image with its depth map. Various quantitative and qualitative evaluation results on three benchmark datasets show that our FocalAIF-Net network achieves acceptable generalization performance. Additionally, we employ a liquid lens to zoom swiftly under the guidance of the proposed decision algorithm during real-world experiments to verify the effectiveness of the proposed method. The results show that the focal stack acquired by our method has a strong ability to merge a more accurate AIF image and consume less running time compared to that achieved with a common average interval by mechanical movement.

Study of high-order LG modes in off-axis pumped solid-state lasers with the thermal lens effect

mengmeng xu, Miao Hu, Qin Li, Jiaxin Fu, Xue Wang, Haozhen Li, Meihua Bi, xuefang zhou, Sunqiang Pan, and Chong Liu

DOI: 10.1364/AO.546540 Received 30 Oct 2024; Accepted 03 Mar 2025; Posted 03 Mar 2025  View: PDF

Abstract: This study investigates the influence of the thermal lens effect on the characteristics of LG modes in an off-axis pumped solid-state laser. The experiment directly generated LG01-LG07 laser modes through off-axis pumping. The results indicate that the thermal lens effect leads to mode hopping in the LG modes. As the crystal temperature increases, the LG modes transition from a single mode to a mixed mode and then hop to adjacent lower-order LG modes, with the temperature of the crystal heat sink being negatively correlated with the mode order during hopping. This phenomenon is attributed to the off-axis pumping causing a shift in the optical axis of the resonator, which reduces the effective off-axis amount. By adjusting the off-axis amounts of each LG mode, the mode hopping issue was effectively resolved, resulting in a stable output of 1.52 W for the LG07 beam at a pump power of 6.5 W.

Integrated Single Cell Metabolism Monitoring Platform

Le Roi du Plessis, Gurthwin Bosman, and Pieter Neethling

DOI: 10.1364/AO.549455 Received 18 Nov 2024; Accepted 03 Mar 2025; Posted 03 Mar 2025  View: PDF

Abstract: Recent work has shown that incorporating Optical Tweezers with microfluidic devices is useful for monitoring cell properties for diagnostic and sorting purposes. The integration of optical tweezers with microfluidic devices presents the potential to monitor single-cell response to changing extracellular conditions. We present a simple system using laminar flow in a microfluidic chip to generate two different and distinct environments between which a trapped cell is moved. As examples of the capabilities of our system, cellular responses to increased salinity and ethanol are monitored by measuring cell morphology and endogenous fluorescence from co-enzymes Nicotinamide-adenine dinucleotide (NADH) and Flavin adenine dinucleotide (FAD).

Effects of vacancy defects on the electronic structure and optical properties of ZnSe and ZnSe:Cr

Jianpu Xi, Shicong Liu, Lijuan Deng, Yonggao Yue, and Qiudong Li

DOI: 10.1364/AO.551343 Received 10 Dec 2024; Accepted 03 Mar 2025; Posted 04 Mar 2025  View: PDF

Abstract: Transition metal Cr-doped ZnSe semiconductor is a kind of material which can realize laser output in mid-infrared band, and has broad application prospects in the fields of air monitoring, surgery, optical communication, industrial production and national defense. In this work, the electronic structure, optical properties and stability of ZnSe and ZnSe:Cr with vacancy defects introduced in Zn and Se sites were investigated. The vacancy affected the internal structure of ZnSe crystal and increased the band gap, but the direct band gap semiconductor characteristics of ZnSe were not changed. Affected by the vacancy of adjacent atoms, the impurity bands (IBs) produced by Cr2+ in ZnSe:Cr crystals have undergone changes in their position and degeneracy in the band gap. In terms of optical properties, Zn vacancy leads to more significant redshift (5375.37 nm  6447.55 nm) of ZnSe:Cr absorption peak position than Se vacancy (620.48 nm). Zn vacancy and Se vacancy made the absorption peak of ZnSe red shift (2250.84 nm) and blue shift (144.71 nm) respectively. Meanwhile, Zn vacancy improved the refractive index and reflectivity of the crystal significantly. Our results suggest that vacancy defects can affect the luminescence range of ZnSe and ZnSe:Cr crystals. This phenomenon can be used to judge whether the crystals are defective or not, and long wave laser can also be obtained from the defective crystals.

Design of polarization beam splitter based on thin-film lithium niobate and grating-assisted contra-directional coupler

Yuling Shang, Shujie Deng, Hui Jiang, Chunquan Li, H gt, Shaoyun Wu, and Zhuofan Song

DOI: 10.1364/AO.552017 Received 13 Dec 2024; Accepted 03 Mar 2025; Posted 04 Mar 2025  View: PDF

Abstract: Polarization Beam Splitter (PBS) is a key device for controlling the polarization state of light in photonic integrated circuits (PICs). In this paper, a compact TM-pass/TE-divide PBS based on thin-film lithium niobate (TFLN) and grating-assisted contra-directional coupler (GACDC) is proposed.The device makes the contra-directional coupling satisfy the phase matching condition by etching the corrugated strip waveguide on the waveguide sidewall. It is able to separate the two polarization modes of TE and TM, thus realizing the function of polarization beam splitting. The coupling length of the PBS designed in this paper is 209.5 μm. The simulation results at the center wavelength of 1555nm show that, the extinction ratio of the TE(TM) polarization mode is 42.8 dB (21.0 dB), and the insertion loss is 0.56 dB (0.01 dB). Besides, the crosstalk of the contra-directional coupling is-21 dB. The waveguide grating is employed in the device, which makes it independent of the free spectral range and has a large tuning range. The design has the advantages of high extinction ratio and low crosstalk, which is expected to be an indispensable component in future TFLN PICs.

High Performance Self-Powered Ultraviolet Ti3C2Tx/GaN Schottky photodiode with Interdigitated Electrode

Zhang Xiaodong, Kang Xiang, Muyang Ye, Shuang Gao, and Xiujuan Wang

DOI: 10.1364/AO.553364 Received 17 Dec 2024; Accepted 03 Mar 2025; Posted 04 Mar 2025  View: PDF

Abstract: Ultraviolet Photodetectors (UV-PDs) with high sensitivity and fast optical response have become an important part of modern optoelectronic information transmission and communication systems. In this paper, a high performance self-powered titanium carbide (Ti3C2Tx)/gallium nitride (GaN) Schottky UV-PD with an interdigitated electrode was developed. Schottky-contacted Ti3C2Tx interdigitated electrode was successfully fabricated on GaN substrates using a spray coating method. The as-prepared Ti3C2Tx/GaN UV-PD exhibits very excellent optoelectronic properties. Under zero bias and weak UV illumination (48 μW/cm2), the device demonstrates an ultra-low dark current (2.818 × 10-10 A) and a fast response speed (tr =170 μs/tf=210 μs), the responsivity and detectivity could reach 120.78 mA/W and 1.272 × 1012 Jones, respectively. Furthermore, to demonstrate the feasibility of practical application, the device was applied in a UV point-of-view imaging system and highquality images were successfully obtained under low-light conditions. This work presents an effective approach to develop high-performance UV-PDs for optical imaging applications.

Configurable conditional-logic multi-functional logic gate in a linear three-core optical fiber coupler

Francisco Leonardo Bezerra Martins, João Paulo Rodrigues, and José Cláudio do Nascimento

DOI: 10.1364/AO.557449 Received 23 Jan 2025; Accepted 03 Mar 2025; Posted 04 Mar 2025  View: PDF

Abstract: Digital conditional logic functions are essential for decision-making algorithms and computational deductive reasoning. However, research on all-optical fiber-based conditional-logic devices is very scarce, and generally proposes single-function nonlinear devices that are expensive and hard to manufacture. In this paper, we present the first numerical acquisition of a configurable conditional-logic multi-functional OR/AND-IMPLY logic gate using a linear optical fiber-based device. Our device consists of a single piece of planar three-core optical fiber linearly propagated by low-powered amplitude-modulated light pulses. This result provides further evidence that the complex logical processing of optical signals within functional fibers is not only possible, but can be achieved with even the simplest linear fiber devices.

Infrared adaptive camouflage with thermal switch enabled by dual-band metamaterial emitter based on VO2

Han Chen and han wang

DOI: 10.1364/AO.547018 Received 11 Nov 2024; Accepted 03 Mar 2025; Posted 04 Mar 2025  View: PDF

Abstract: With the rapid development of infrared (IR) detection technology, the demand for IR camouflage in military reconnaissance and civilian fields is increasing. Although low emissivity materials can accommodate for high-temperature objects, they cannot allow temperature-variable objects to maintain infrared camouflage at all times in their environment. In this paper, a simple metamaterial structure is used to realize IR adaptive camouflage with thermal switch. VO2 is used as a temperature-adaptive switch due to its dynamic modulation properties, which increases the emissivity of the tunable emitter to match the environment in cold state. The metal phase, in contrast, decreases the emissivity to reduce the radiant energy and achieve IR camouflage. The emission enhancement mechanism of the tunable metamaterials is analyzed and revealed by the magnetic field diagram. The study of radiation power and temperature shows that the tunable infrared camouflage metamaterials can determine their high emissivity/reflectivity optical properties through thermal switch, thereby displaying a temperature that is proximate to the ambient and integrating into the environment in an infrared camera. In conclusion, the emitter provides a certain degree of assistance for the development and application of tunable emission devices with broadband in the mid-infrared.

Design of 4-channel DeMUX based on photonic crystals with ultra-high quality factor

guanhua chen, JunChi Yao, huifang zhu, Ting Zhi, Jin Wang, Xue Junjun, lin chen, Tao Tao, and Zhikuo Tao

DOI: 10.1364/AO.549235 Received 15 Nov 2024; Accepted 03 Mar 2025; Posted 04 Mar 2025  View: PDF

Abstract: In this paper, a four-channel DeMUX based on photonic crystals with ultra-high quality factor is realized by simulation. Plane wave expansion (PWE) and finite element method (FEM) were used to characterize the photonic band structure and investigate the optical propagation behaviors, respectively. Through optimizing the resonator structure and material parameters, the quality factor Q of the designed resonator can be as high as 9082. Furthermore, the average quality factor Q of the four-channel DeMUX using the designed resonator can be as high as 8901. In addition, several key performances have been investigated, such as transmission, crosstalk level, foot print and channel spacing. The footprint of the device is 324 µm2, the average channel spacing is 0.77 nm, and crosstalk between channels values from -9.9 to 30.1 dB. Particularly, the average Δλ is 0.17 nm which leads ultra-high quality factor. The ultra-high quality factor makes the proposed DeMUX a promising for future on-chip optical integration.

Chiral Topological Bound States in Continuum within Photonic Crystal Cavities

bilin ye and Ran Hao

DOI: 10.1364/AO.550858 Received 02 Dec 2024; Accepted 03 Mar 2025; Posted 04 Mar 2025  View: PDF

Abstract: We propose a magneto-optically tunable platform for chiral topological bound states in the continuum (BICs) within photonic crystal cavities. By integrating magneto-optical (MO) materials into topological bulk-state cavities, we demonstrate dynamic control over circular polarization states through external magnetic fields, a capability unattainable with conventional symmetry-breaking approaches. Initially, doubly degenerate topological quadrupole BICs are formed in a hexagonal lattice photonic crystal. Applying a vertical magnetic field breaks timereversal symmetry, lifting the degeneracy and generating paired chiral BICs with opposite pseudo-spin and orbital angular momentum. These chiral BICs manifest as circularly polarized beams in the far-field, with handedness directly controlled by the magnetic field direction. The synergy of topological BICs and MO materials achieves a high quality factor (Q~109) and tunable polarization states, enabling applications such as reconfigurable lasers and polarizationsensitive sensors. This work bridges topological photonics and magneto-optics, offering a paradigm for dynamically controlled photonic devices in quantum optics and telecommunications.

Tunable-Density Sharp Spot Array Projection for 3D Sensing Using a Single DOE

Yanbo Zhao, Zijie Zhao, and Qiaofeng Tan

DOI: 10.1364/AO.551886 Received 17 Dec 2024; Accepted 02 Mar 2025; Posted 03 Mar 2025  View: PDF

Abstract: Structured light projection has been widely used for high-precision 3D sensing in computer vision. Diffractive approaches based on fixed components offer unique advantages over DLP or LCD projectors due to their compact size and low energy consumption. However, current methods using components like diffractive optical elements (DOEs) or metasurfaces are typically designed to generate fixed patterns, limiting adaptability in practical applications, such as adjusting sensor resolution and frame rates. In this paper, a novel single DOE based projector with tunable spot array density is proposed. The property of tunable-density is achieved through adjusting the distance between the laser source and DOE while no need to replace fixed components during 3D measurements. Simulation and experimental results of tunable-density sharp spot array structured light projection are presented, demonstrating the effectiveness of the proposed method.

A Non-Reference Convolutional Weighted Entropy Method for ISAR Imaging Evaluation

Xulang Zhou, Yongdiao Wen, Beijia Ning, wenhui hu, Gang Li, and Tie Li

DOI: 10.1364/AO.554084 Received 26 Dec 2024; Accepted 01 Mar 2025; Posted 03 Mar 2025  View: PDF

Abstract: To effectively assess the quality of Inverse Synthetic Aperture Radar (ISAR) imaging which are characterized by a strong scattering point distribution, this paper proposes a nonreference evaluation method based on convolutional weighted image entropy. The method combines sliding convolution operations to independently calculate the image entropy for different regions of the ISAR image. Within each region, differentiated entropy weights are assigned based on the scattering intensity of individual points, emphasizing key scattering point distribution structures. This enables precise quality scoring of a single image. The quality score differences between different images are then compared to provide a percentage-based evaluation of image differences, which serves as the final similarity assessment. Experimental results show that this approach can finely capture subtle structural changes in radar images and demonstrating higher sensitivity to key structural differences, compared with traditional evaluation methods. It effectively mitigates the impact of image geometric transformations on the results and exhibits greater robustness to environmental noise. The proposed method demonstrates significant advantages in ISAR image quality evaluation tasks in multi-precision fusion acceleration processing, with broad application potential.

Research on QR steganographic image transmission system based on multimode fiber and deep learning

Houru Zhao, Yiqiang Zhang, Zhang Leihong, and Dawei Zhang

DOI: 10.1364/AO.544443 Received 11 Oct 2024; Accepted 01 Mar 2025; Posted 03 Mar 2025  View: PDF

Abstract: In this paper, we propose a QR steganographic image transmission system based on multimode optical fiber with deep learning, aiming to solve the image distortion problem due to mode dispersion in multimode optical fiber transmission and to improve the security and reliability of transmission. The system adopts the PIES-Net model to generate visually imperceptible steganographic images by embedding secret images in equal proportions into camouflaged images. Subsequently, the steganographic image is converted into a QR code, which utilizes its error correction capability to ensure that the original data can be recovered through redundant information even if some of the information is lost or corrupted during transmission.After the QR code is transmitted over a multimode optical fiber, a scattered image is formed at the receiving end. In this paper, an improved SFNet model based on U-Net architecture is proposed for reconstructing QR codes and recovering the original information from the scattered image. The experimental results show that the system generates a covertly written image with high steganography, and the extracted secret image excels in visual quality, peak signal-to-noise ratio, and image correlation, and demonstrates excellent robustness and security in a variety of noise environments. This study provides new ideas and important support for the development of multimode fiber communication and image steganography.

InGaAs Trap Detector: Advancing Toward a Short-Wave Infrared Standard with 1% Uncertainty

Anouar Rahmouni, Alan Migdall, Ping-shine Shaw, Oliver Slattery, Joseph Rice, and Thomas Gerrits

DOI: 10.1364/AO.557268 Received 16 Jan 2025; Accepted 28 Feb 2025; Posted 03 Mar 2025  View: PDF

Abstract: Trap detectors are effective as transfer or secondary standards in radiometry, with silicon photodiodes commonly used for visible and germanium for near-infrared (NIR) light detection due to their cost-effectiveness and simple fabrication. However, indium gallium arsenide (InGaAs) photodiodes offer superior performance in the short-wave infrared, 1 µm to 1.65 µm range, featuring high internal quantum efficiency, excellent temperature stability, and enhanced durability. In this work, we developed and evaluated a small-footprint InGaAs-based trap detector using an optical trap configuration with two off-the-shelf InGaAs photodiodes and a high-reflectivity mirror. The prototype trap demonstrated excellent stability, low polarization dependence, and good responsivity stability, achieving system detection efficiency exceeding 98% for wavelengths between 1 and 1.3 µm. This versatile detector is suitable for both free-space and optical fiber measurements. The near-unity efficiency of the optical trap configuration shows potential as a primary standard with 1% uncertainty for short-wave infrared standard (SWIR) detection. While the 1% uncertainty of the trap detector is two orders of magnitude higher than the 0.01% possible with cryogenic radiometry, the latter requires an expensive cryostat and very specialized expertise. The trap detector, with its robust design and its 1% uncertainty, provides a practical and cost-effective alternative as a standard detector for a wide range of applications, offering a viable solution to reduce reliance on cryogenic radiometry for photodiode calibration.

TS-DENet: A Transferable Self-Supervised Learning Method for Multi-Modal Fluorescence Image Denoising

Liangliang Huang, Zhong Wen, zhaokai wang, Quanzhi Li, Qilin Deng, Xu Liu, and Qing Yang

DOI: 10.1364/AO.547303 Received 08 Nov 2024; Accepted 28 Feb 2025; Posted 28 Feb 2025  View: PDF

Abstract: Recent fluorescence diagnostic tools have demonstrated effectiveness in detecting early-stage neoplasmatic tissue and monitoring therapy, allowing rapid non-invasive live imaging diagnosis. However, varying light conditions in in-vivo environments and modalities of observation systems often introduce multi-level noises to acquired images, causing degraded image quality. Deep learning (DL) has shown great potential in improving image quality, but its perfomance may be limited when dealing with insufficient labelled training data and the challenges of acquiring high-quality multi-modality fluorescence images in specific biomedical tasks. To address this problem, we propose two-stage deep denoising and edge enhancement framework (TS-DENet), including large-dataset-based pre-training and domain-specific fine-tuning. The pre-training stage learns contextual features and complex data distribution via masked reconstruction task. The fine-tuning stage further focuses on denoising and applies edge enhancement to eliminate the image blur induced by denoising. Through extensive experiments, TS-DENet demonstrates state-of-the-art perfomance in diversified data regimes. Compared with other DL-based methods, TS-DENet shows better generalizability and transferability. For in-vivo experiment, we apply TS-DENet to a multimode fiber endoscopic system to observe gastric tissues in a rat. The results suggest that TS-DENet provides a potential solution for fluorescence image quality improvement in clinic practice.

Laser-Induced Fluorescence Spectroscopy Combined with Multivariate Analysis for Rice Seeds and Grains discrimination

Rabbi Boateng, Jerry Opoku-Ansah, Charles Amuah, Peter Adueming, Andrew Huzortey, Kingsley Taah, and Moses Jojo Eghan

DOI: 10.1364/AO.546627 Received 30 Oct 2024; Accepted 28 Feb 2025; Posted 28 Feb 2025  View: PDF

Abstract: Rice is a staple food in sub-Saharan Africa, including Ghana. Local production is hindered by the use of rice grains for cultivation, which directly affects both yield and grain quality. This study employed laser-induced fluorescence spectroscopy (LIFS) combined with multivariate analysis to rapidly and non-destructively discriminate between rice seeds and grains, offering an alternative to conventional methods. Fluorescence spectra from rice seeds and grains from six locally cultivated rice varieties were analyzed using three pre-processing techniques (Z-score, 1st derivative, 2nd derivative) across four machine learning models: Linear Discriminant Analysis (LDA), Quadratic Discriminant Analysis (QDA), K-nearest neighbor (KNN), and Support Vector Machine (SVM). Principal Component Analysis (PCA) score plots and the Davies-Bouldin index (DBI) were used to assess the separation between seeds and grains. The 1st and 2nd derivatives outperformed Z-score, with SVM performing well under the 2nd derivative and KNN excelling under the 1st derivative. LDA and QDA varied depending on the rice variety and pre-processing method. The best accuracy was achieved using the 1st derivative with KNN, confirming LIFS combined with multivariate techniques is an effective method for discriminating rice seeds and grains.

SPECKLE SUPPRESSION BY HYBRID DUAL SOLIDSTATE LIGHT-SOURCE IN LASE PROJECTION

Guo ZQ, Xiaoyuan Guo, Jiaqi Gu, Yi Li, and Changzheng Sun

DOI: 10.1364/AO.555631 Received 21 Jan 2025; Accepted 28 Feb 2025; Posted 28 Feb 2025  View: PDF

Abstract: Laser projection has become a new trend for display technology. However, speckle noise due to laser coherence poses a notable hurdle in achieving high image quality. We propose a new architecture of Hybrid Dual Solid-State Light-source (HDSSL), which composes of RGB lasers and RGB light emitting diodes (LEDs). Theoretical analysis and experimental results confirm that HDSLL can effectively suppress the speckle noise. Speckle contrast below 4% on a 70-inch projection image at a distance of 1.5 meters has been demonstrated, resulting in a lower speckle contrast than the human speckle perception threshold. Furthermore, the research indicates that increasing the proportion of LEDs in the light source reduces speckle noise.

Development of a Standard Maritime Cn2 Model Using Satellite Measurements

Gregory Anderson, Steven Fiorino, and David Meier

DOI: 10.1364/AO.557266 Received 17 Jan 2025; Accepted 27 Feb 2025; Posted 28 Feb 2025  View: PDF

Abstract: The Hufnagel-Valley (H-V) 5/7 model was developed to characterize optical turbulence (Cn2) as it varies with height over land. While the H-V 5/7 model is not meant to predict precise values, observations over land will likely be in a range close to the models prediction. H-V 5/7 is not suitable for modeling turbulence over the ocean, and to date no standard ocean profiles have been developed. The primary objective of this research is to develop a H-V-like standard maritime model of optical turbulence based on oceanic conditions from the surface layer up to the stratosphere. Maritime temperature climatology data are obtained from the Atmospheric Infrared Sounder (AIRS) aboard NASA’s Aqua satellite. Three maritime profiles are proposed as alternatives to the H-V 5/7 profile. Results show that maritime profiles generally do not exhibit the surface spike in turbulence seen in H-V 5/7. Additionally, a strong latitudinal variation in the height of the Cn2 inflection associated with the tropopause is observed, motivating the need for separate models for polar and tropic regions.

Research on mechanistic of pressure and temperature in lithium-ion cells based on fiber Fabry-Pérot cavity and Bragg grating

Liu QinPeng, xing hua, YANG DI, Yan Cheng, and Liu Bo

DOI: 10.1364/AO.553581 Received 23 Dec 2024; Accepted 26 Feb 2025; Posted 26 Feb 2025  View: PDF

Abstract: A compact gas pressure and temperature sensor based on a Fabry-Pérot interferometer (FPI) and fiber Bragg grating (FBG), which is used inside a cell, is proposed and experimentally demonstrated, and it is successfully applied to the measurement of internal temperature and pressure of cells. The experimental results show that the sensor's sensitivity for pressure and temperature is 5.026 nm/MPa and 0.17 nm/°C, respectively. The sensor designed has a good linearity and stability, and it enables the measurement of pressure and temperature under different working conditions of the cell, which provides data support for cell health management and early danger warning.

Signal-to-noise ratio of thermoelectric sensor of single-photon detector operating at different temperatures

Astghik Kuzanyan, Vahan Nikoghosyan, Artur Davoyan, and Armen Kuzanyan

DOI: 10.1364/AO.555644 Received 15 Jan 2025; Accepted 26 Feb 2025; Posted 27 Feb 2025  View: PDF

Abstract: In this paper, we present the results of modeling and simulation of heat propagation processes in the thermoelectric sensor operating in the temperature range of 0.5 – 1.5 K. The detection of single photons with energies ranging from 0.8 to 7.1 eV is considered. The multilayer thermoelectric sensor, with a square surface area of 1 μm², consists of a tungsten absorber, a thermoelectric layer of lanthanum-cerium hexaboride, a molybdenum heat sink, and a sapphire substrate. Heat transfer processes in the sensor, designed as specified, were investigated using the three-dimensional matrix method based on the equation of heat propagation from a limited volume. The temporal dependencies of the average temperature of the layers' surfaces were calculated, and the temporal dependence of the signal caused by the absorbed photon, specifically the voltage induced at the boundaries of the thermoelectric layer were determined. The total noise equivalent power of the thermoelectric sensor was calculated and compared with signal power, allowing us to determine the signal-to-noise ratio for all considered photon energies and operating temperatures. We found that the signal-to-noise ratio increases with an increase in the energy of the absorbed photon and a decrease in the operating temperature of the sensor. We demonstrated that a signal-to-noise ratio greater than unity can be achieved for detecting photons with an energy of 0.8 eV and significantly exceeds than unity for detecting photons with higher energies.

Ability and limitations of effective medium theory in terms of filling fraction and number of layers for hyperbolic metamaterials

shahnawaz shah, Beenish Ansari, Arbab Kalhoro, Farida Memon, ayaz ali, and Sharjeel Afridi

DOI: 10.1364/AO.555864 Received 16 Jan 2025; Accepted 26 Feb 2025; Posted 27 Feb 2025  View: PDF

Abstract: Hyperbolic metamaterials (HMM) exhibit unusual electromagnetic properties, attracting significant interest from both fundamental physics and applied engineering perspectives. At subwavelength dimensions, effective medium theory (EMT) is commonly used to simulate and describe HMMs, which are typically composed of metal-dielectric multilayers, due to their simplicity in design. Inthiscontext, theEMTapproximationmustalignwithexactelectromagnetic solutions. However, significant discrepancies in the predicted properties sometimes arise due to these approximate methods. This article compares the EMT with exact solution (i.e., transfer matrix method) and demonstrates that the transmission and reflection in the HMM multilayer structure are sensitive to variations in the number of layers and filling fraction, even when the layer thicknesses are much smaller than the operating wavelength. Understanding this dependence of EMTonnumber of layers and filling fraction is crucial for reducing errors and improving the design of plasmonics and photonics devices.

Laser damage of UV hafnia-based multilayer dielectric coatings at 355 nm wavelength

Maxwell Weiss, Carmen Menoni, Aaron DAVEPORT, Samuel Castro Lucas, Dovile Pamedytyte, Justinas Galinis, Andrius Melninkaitis, Walter Siehien, Justin Siehien, Francois Schiettekatte, and Martin Chicoine

DOI: 10.1364/AO.557518 Received 20 Jan 2025; Accepted 25 Feb 2025; Posted 26 Feb 2025  View: PDF

Abstract: Mega-Joule ultraviolet nanosecond pulses were used to demonstrate ignition at the National Ignition Facility in Dec. 2022 [Nat. 601, 542 (2022]. Scaling in energy and repetition rate this and other solid state laser fusion drivers relies in part on significant improvements in the UV multi-layer dielectric (MLD) coatings which are the weakest component in critical optics in the lasers’ architecture. This paper reports on the laser damage behavior of MLD coatings based on HfO₂, SiO₂, and Al₂O₃ designed for operation at λ=355 nm. Two-layer anti-reflection (AR) coatings were designed and fabricated by reactive biased target deposition (BTD) using mixtures of HfO₂ and SiO₂, HfO₂ and Al₂O₃, as the high index layer. The laser damage response was assessed from 1-on-1 and S-on-1 tests from which the laser induced damage threshold (LIDT) fluence was determined. It is shown that in BTD ARs which contain mixtures, Hf(1-y)Si(y)Ox and Hf(1-y)Al(y)Ox in the high index layer, the 1-on-1 LIDT increases with respect to BTD HfO₂/SiO₂ AR. However, this increase is below the one predicted by the scaling of the 1-on-1 LIDT with optical bandgap from LIDT values of HfO₂/SiO₂ and HfO₂/ Al₂O₃. The S-on-1 LIDT of BTD ARs decreases by ~25% for S=10 and remains unchanged to S=10⁴ laser shots, indicating no accumulation fatigue. Neither UV preconditioning or etching of the substrate prior to coating deposition caused a major improvement in the 1-on-1 LIDT of the BTD ARs.

Optimal design of splicing structure for metal working molds of cube corner retroreflective sheeting

Qilu Huang, Chao Zhang, Shaobin Yan, Zeqiang Chen, Juan Zhang, and Tingdi Liao

DOI: 10.1364/AO.547281 Received 08 Nov 2024; Accepted 25 Feb 2025; Posted 26 Feb 2025  View: PDF

Abstract: The principle of retroreflection and the formation characteristics of the effective area in metal cube corner retroreflectors (CCRs) based on geometric optics are analyzed. The design formulas for the optimal splicing structure of metal CCR molds in the direction of 90° direction are derived. Optical simulation software is utilized to simulate the impact of different cutting positions on the effective area of CCRs, and nickel working molds are fabricated for experimental validation of the splicing process. Splicing at the optimal position can achieve the maximum retroreflective efficiency at the joint and weaken the traces of splices.

Designing an off-axis freeform imaging system with a wide field of view based on an equal optical path surface extension method

Yi Shi, Rina Wu, Lu Xing, and Qin Dai

DOI: 10.1364/AO.547566 Received 08 Nov 2024; Accepted 25 Feb 2025; Posted 26 Feb 2025  View: PDF

Abstract: An off-axis imaging system with a wide field of view (FOV) has significant applications, and combined with freeform surfaces can achieve excellent optical performance. However, the initial structure of the off-axis freeform imaging system with a wide FOV is rare, and its optimization requires extensive experience, which brings a challenge to the system design. In this paper, a design method is proposed to effectively construct the initial structure of an off-axis freeform imaging system with a wide FOV. In this method, with the expansion of the FOV, the freeform surfaces of a system are extended according to the equal optical path condition. Then, an iterative process exists for improving the imaging quality of the system. To verify the effectiveness of the method, an off-axis freeform imaging system with a 40° × 1° FOV is designed as an example. Using the proposed method, a good initial structure of the system is constructed, and after optimization, the system achieves good imaging quality.

Infrared stray radiation analysis for antireflective film coated on the optical window

chuang sun, Yinan Wang, Xiaohao Cheng, ren Zeng, and Xue Chen

DOI: 10.1364/AO.551181 Received 26 Dec 2024; Accepted 25 Feb 2025; Posted 26 Feb 2025  View: PDF

Abstract: Antireflective films are extensively employed in various optical detection systems to provide protection, increase signal transmittance, and suppress infrared (IR) stray radiation on optical windows. Drawing on thin film optical theory, several types of antireflective films are presented, and their optical properties are modeled. Additionally, directional spectral reflectivity at the interface between the film and optical window is determined sequentially. Subsequently, an analysis of IR stray radiation in optical systems utilizes an equivalent model of optical windows coated with antireflective films, thoroughly investigating their corresponding effects. Results demonstrate that applying high-quality antireflective films can achieve a normal reflectance at the interface of less than 1.0%, effectively suppressing stray radiation originating from thermal emissions of the optical window itself. A contrast coefficient for each point on the entrance pupil surface is introduced to quantify the contribution of the target signal. The analysis reveals a significant increase in the contrast coefficient with the application of antireflective films, thereby substantially improving detection precision.

Enhancing magnetic dipole emission with hollow hybrid metal-dielectric-metal nanostructure

Xun Zhou, wei wang, Jing Du, Shiyu Shen, and Qi Ding

DOI: 10.1364/AO.554969 Received 14 Jan 2025; Accepted 25 Feb 2025; Posted 26 Feb 2025  View: PDF

Abstract: We propose hollow metal-dielectric-metal hybridized nanodisks to enhance both the magnetic dipole (MD) and electric dipole (ED) emission. By employing a hollow hole in the center of the hybridized nanodisks, the maximum magnetic field intensity is obtained in the hole and can be used to fully improve MD emitter emission. It indicates that radiative decay rate (RE) of MD emitter is 965-fold enhanced in the hole, and has been achieved with a large quantum efficiency (QE) over 80%. Due to it can access to the magnetic hot spot. It can achieve greater MD emission, which has more advantages than without holes. By forming the dimer, it can be further enhanced with an enhancement factor of the RE of ED emitter over 130 within the dimer. The achieved enhancement factor significantly exceeds the results obtained in the literature using the metal-dielectric-metal structures. We also investigated the directional properties of the hybridized nanodisks. We show that hybrid structure exhibits a strong capability for forward far-field scattering, and changing the thickness of Al2O3 film that can control the directionality of scattering. Our study offers a viable option for selective excitation of magnetic and electric emissions in light-matter interactions and open up new possibilities for enhancing the radiation from MD emitter.

Material-discriminating Chromatic X-ray Stationary Intraoral Computed Tomography Technology

Zihan Zhao, Peng Ran, Yuzhang Gu, Shiming Deng, Jingrui Wu, and Yang Yang

DOI: 10.1364/AO.549388 Received 18 Nov 2024; Accepted 25 Feb 2025; Posted 26 Feb 2025  View: PDF

Abstract: Three-dimensional (3D) dental imaging, such as cone-beam computed tomography (CBCT), is essential for diagnosing dental conditions but is limited by high costs, prolonged examination times, and increased radiation exposure. Additionally, standard CBCT lacks the ability to capture spectral X-ray information, which is crucial for distinguishing different dental materials. To address these issues, we propose a novel, low-cost, low-dose dental CT method, Chromatic X-ray Stationary Intraoral Computed Tomography (S-IDECT), which integrates a multi-source X-ray array with dual-energy CT technology. This approach not only generates quasi-3D images with depth information but also produces multiple monochromatic images across a broad energy range (20–90 keV), enhancing material discrimination. The S-IDECT system uses a mechanical arm to emulate a source array, enabling precise positioning of seven X-ray sources. The method employs the Adaptive Steepest Descent Projection onto Convex Sets (ASD-POCS) algorithm for image reconstruction and material decomposition. Experimental results demonstrate significant improvements in material differentiation, particularly for calcified tissues and restorative materials, with a relative error of less than 10%. This study demonstrates the feasibility of S-IDECT as a promising dental imaging tool, offering enhanced diagnostic accuracy and material differentiation for improved dental care and treatment planning. This approach has the potential to improve the early detection of dental diseases, reduce radiation exposure to patients, and provide more accurate treatment planning, ultimately benefiting both patients and dental professionals.

Bessel beams with adjustable focal position

keren zhalenchuck and Alon Bahabad

DOI: 10.1364/AO.551861 Received 12 Dec 2024; Accepted 24 Feb 2025; Posted 25 Feb 2025  View: PDF

Abstract: We present a simple method to dynamically modify the focal position of a Bessel beam without changing its non-diffraction length and intensity profile. This method can be useful for various applications such as microscopy and optical trapping.

Transformer-based wavefront sensing for atmospheric turbulence aberration correction

Xiaohan Liu, Wen Luo, peng hu, Jianzhu Zhang, Feizhou Zhang, and Hua Su

DOI: 10.1364/AO.554333 Received 29 Dec 2024; Accepted 24 Feb 2025; Posted 25 Feb 2025  View: PDF

Abstract: We propose a novel Transformer-based wavefront sensing method that employs a cross-task pretraining strategy to establish strong global dependencies. Compared to the CNNbased approach, this method significantly improves the aberration estimation accuracy, reducing test set loss by 70.5% and RMS by 45.7%. Notably, the attention maps of different Zernike output terms in this method exhibit remarkable consistency with the PSFs corresponding to individual aberrations. The results demonstrate that the method effectively decouples individual aberrations via the self-attention mechanism, capturing complex physical relationships and enhancing model interpretability, thus highlighting its potential as a unified methodology for advancing wavefront sensing.

A broadband absorption spectrum processing method based on end-to-end deep learning networks

Haoyong Li, HAI ZHONG, Dahua Gao, Mengmeng Tao, ZHONGQIANG ZHANG, LINDE JI, and GAN LI

DOI: 10.1364/AO.546273 Received 29 Oct 2024; Accepted 23 Feb 2025; Posted 25 Feb 2025  View: PDF

Abstract: Absorption spectrum technology are ideal candidates for engine temperature measurement, new material design and chemical reaction mechanism analysis. However, to date, the existing methods for processing absorption spectrum are very cumbersome and timeconsuming, typically involving steps such as filtering and denoising, baseline correction, and comparison of measured data with theoretical databases, which results in poor real-time performance. Herein, we propose an absorption spectrum processing method based on the neural network that can directly input the collected unprocessed absorption spectrum and obtain the test results without other steps. To address the challenge of limited spectral features in absorption spectrum and enhance processing accuracy, a novel neural network is designed by combining long short-term memory (LSTM) network and fully convolutional network (FCN). The final experimental results show that the proposed method achieves accurate temperature processing results for H2O and CO2, with an accuracy rate of 98.4%, and the average processing time for each absorption spectrum is 0.00036 seconds, which, to the best of our knowledge, is the fastest processing speed for absorption spectrum. As a result, the presented method shows great potential for real-time online processing of absorption spectrum.

Scanning imaging through the scattering medium based on the guided filter assisted by WTMMandNLM

Yiyang Zhang, Zhewen Ding, Runze Li, Xianglei Meng, Yan Shi, Chunlian Zhan, and Chunliu Zhao

DOI: 10.1364/AO.550068 Received 26 Nov 2024; Accepted 23 Feb 2025; Posted 25 Feb 2025  View: PDF

Abstract: In a scanning imaging system through a scattering medium, the quality of the imaging result is related to the energy distribution of the focusing point. In actual imaging, the energy of the focusing point cannot be perfectly concentrated. The scattering noise is always surrounded around the focusing point, which reduces the signal-to-noise ratio and results in poor image quality. To improve the quality of imaging, further processing of the imaging results is required, while most conventional image processing methods can only achieve one of the goals such as noise reduction, image smoothing, edge sharpening, or maintaining image integrity. In this paper, a scanning imaging system through the scattering medium based on an adaptive guided f ilter assisted by wavelet transform modulus maximum(WTMM) and non-local mean(NLM is proposed for the first time, which can make the imaging results simultaneously have the advantages of low noise, high contrast and clear details. To verify the validity of the proposed method, a scanning imaging system through the scattering medium was set up. Transmissive imaging was performed at different positions from the focal plane. The experimental results show that the background noise is significantly restrained, single-pixel response and edge continuity are good, and details are clear with the proposed method. Compared with the traditional methods and the deep learning methods, the proposed method can improve the PSNR and SSIM by up to 10.68 dB, 0.75 and 5.34 dB, 0.72, respectively. Finally, the performance of the proposed method in high-noise environments, its application in the field of real-time imaging, and the future improvements are discussed. The method proposed in this paper can effectively improve the quality of scanning imaging results through the scattering medium, which is expected to promote the application of this technology in endoscopic imaging and other fields.

Enhancing performance of saturable absorbers through excellence in parameters extraction

Meng Wang, Qing-Yu Liu, Chen Zhang, Yonggang Wang, Nan Lin, and Xiaoyu Ma

DOI: 10.1364/AO.543527 Received 27 Sep 2024; Accepted 23 Feb 2025; Posted 25 Feb 2025  View: PDF

Abstract: Individual fitting methods traditionally struggle to accu-rately and uniquely extract the parameters of saturableabsorbers (SAs), leading to inconsistencies. To overcomethis, we introduce a novel simultaneous fitting method,unprecedented in optics, along with a new set of fit-ting functions derived from a two-level system bandstructure model. These functions enable the simultane-ous reproduction of experimental data for both R(Fout)or T(Fout) versus Fout and R(Fin) or T(Fin) versus Fin,ensuring parameter consistency across diverse charac-terization techniques.We have successfully applied thismethod to characterize three distinct 1064 SESAM sam-ples using an open-aperture Z-scan device, obtainingconsistent parameter results. Furthermore, by generat-ing a mode-locking laser with 1064 SESAM-sample3,we have independently evaluated the saturation flu-ence parameter under mode-locking threshold condi-tions. The findings provide substantial evidence that ourmethod extracts parameters with higher accuracy. Forexample, for 1064 SESAM-sample3, our method deter-mined Fs = 245µJ/cm2 from the open-aperture Z-scan,significantly closer to the independently estimated valueof 253.6µJ/cm2 from the mode-locking threshold, thanthe 87µJ/cm2 obtained by other methods.These resultsstrongly suggest that our method not only offers a moreprecise parameter extraction but also has the potentialto be generalized for the characterization of all types ofSAs. This approach promises to enhance the accuracyand reliability of SA parameter extraction, advancingthe field of ultrafast photonics.

A New Infrared Emissivity Measurement Approach for Solid and Opaque Materials

Umut Kayikci, Ovul Arslan, Emrah Oduncu, Mustafa Sivaslıgil, and Kivanc Uluca

DOI: 10.1364/AO.550393 Received 05 Dec 2024; Accepted 23 Feb 2025; Posted 26 Feb 2025  View: PDF

Abstract: Accurate emissivity assessment is crucial for real temperature measurements using remote sensing methods and infrared signature modeling. This study introduces a novel method for determining the emissivity of materials. Samples were stabilized at 343.15 K in a temperature-controlled scientific chamber, and the emitted infrared radiation was measured using a hyperspectral infrared imaging camera and a broadband longwave infrared imaging camera, covering the wavelength ranges of 3.5–5.5 𝜇𝑚 and 7.5–14 𝜇𝑚, respectively. The novelty of this approach is the integration of an infrared mirror and a blackbody within the chamber, which reduces ambient interaction effects on the emissivity measurement. This method was applied to material samples including aluminum (6061), copper, steel (316), and plastic (PA66 GF30), with results compared to those from a previously established method and available literature data.

Miniaturized optical system for a chip based cold atom inertial sensor

Soizic Hello, Henk Snijders, Benjamin Wirtschafter, Aurelien Boutin, Ludovic FULOP, Frederic SEGUINEAU, Christoph Westbrook, Arnaud Brignon, and Matthieu Dupont-Nivet

DOI: 10.1364/AO.551283 Received 10 Dec 2024; Accepted 23 Feb 2025; Posted 03 Mar 2025  View: PDF

Abstract: We miniaturized the complex optical system responsible for the cooling, pumping and imaging of an on-chip based cold atom inertial sensor. This optical bench uses bonded miniature optics and includes all the necessary optical functions. The bench has a volume of 35x25x5 cm3. We developed a laser frequency lock adapted to the optical bench using saturated absorption in a rubidium cell. The entire laser source based on frequency doubling of 1.56 𝜇m f iber lasers, including the control system and the saturated absorption module, fits in a 5𝑈-rack. Using the miniaturized bench, we realized two and three dimensional magneto optical traps for Rubidium 87 atoms.

Perfect absorber due to optical Tamm states based on α-MoO

Jingkai Miao, Jingwen Li, Xueyi Sang, Maozhu Wang, Yaoyao Shi, Changdong Chen, Youwen Liu, Jinbin Li, and Daxing Dong

DOI: 10.1364/AO.553872 Received 27 Dec 2024; Accepted 22 Feb 2025; Posted 25 Feb 2025  View: PDF

Abstract: α-phase molybdenum trioxide (𝛼-MoO3) is a biaxial van der Waals semiconductor with anisotropic phonon polaritons that exhibit elliptical and hyperbolic dispersion, enabling strong mid-infrared (mid-IR) light absorption. This study presents a composite structure consisting of an -MoO3 layer and two one-dimensional photonic crystals (1DPCs). Perfect absorption is achieved for transverse electric (TE) and transverse magnetic (TM) polarized light at the longitudinal and transverse optical phonon frequencies, respectively, due to the excitation of optical Tamm states (OTSs) at the 𝛼MoO3/1DPC interface. Using the transfer matrix method (TMM) and electromagnetic field simulations, we analyze the mechanisms of absorption enhancement and investigate the effects of 𝛼-MoO3 thickness, top-layer photonic crystal width, and polarization angles. This study provides important theoretical guidance for designing 𝛼-MoO3-based advanced mid-IR detectors and sensing applications.

Fabry-Perot interference optical fiber biosensor based on ECF for label-free DNA detection

lina wang, Chong Li, Jun Cai, Shenbing Wu, Mingxia Dai, Youfu Geng, Duo Yi, and Xuejin LI

DOI: 10.1364/AO.550754 Received 17 Dec 2024; Accepted 21 Feb 2025; Posted 21 Feb 2025  View: PDF

Abstract: A fiber-optic biosensor based on self-reference parallel Fabry-Perot interference (FPI) is proposed for label-free DNA detection. The parallel FPI is formed by fusing a section of exposed core fiber (ECF) in SMF and MMF. Adopting the air cavity of the ECF as reference, a self-reference differential phase demodulation technique is designed to improve the stability and resolution of the demodulation system. By functionalizing the fiber surface, the sensor can efficiently capture complementary DNA (cDNA). In addition, the specific selection performance of the sensor is explored using non-complementary DNA (N-cDNA). The experimental results show that the sensor exhibits a good linear response in the concentration range of 1 µM to 5 µM and the sensitivity reaches 5.53 °/µM. The detection concentration can be as low as 1 µM. The proposed fiber-optic DNA biosensor has good sensitivity and selectivity, and has broad prospects in the fields of biomedical diagnostics, environmental monitoring, and food safety detection.

An Optical Fiber-Based Surface Plasmon Resonance Sensor for Simultaneous Measurement of Relative Humidity and Temperature

Qi Zhang, Xiaopeng Chen, Xinghui Li, huifang wang, Xue Zhou, xin yan, and Tonglei Cheng

DOI: 10.1364/AO.553828 Received 29 Dec 2024; Accepted 21 Feb 2025; Posted 21 Feb 2025  View: PDF

Abstract: In this work, an optical fiber sensor based on surface plasmon resonance (SPR) is proposed for simultaneous measurement of relative humidity (RH) and temperature, which is constructed by connecting two segments of thin-core fibers (TCFs) using a multimode fiber (MMF). The gold film was coated on TCFs to excite the SPR effect, and one was further coated with carboxymethyl cellulose (CMC) to sense RH (Channel Ⅰ), the other with gold nanoparticles (AuNPs) and polydimethylsiloxane (PDMS) to sense temperature (Channel II). The detection was performed via the resonance wavelength shift induced by refractive index (RI) change of CMC and PDMS in relation to RH variation and temperature variation, respectively. The incorporation of gold nanoparticles enhances the sensitivity of the SPR sensor and shifts the resonance wavelengths, allowing for more outstanding demarcation of the SPR dip between the two portions inside the equivalent spectrum. Experimental results show that the sensor has a RH sensitivity of -1.053 nm/%RH and a temperature sensitivity of -2.306 nm/°C. Moreover, the cross-sensitivity issue present in RH and temperature measurements was resolved by introducing a dual-wavelength matrix method. The proposed sensor offers high sensitivity, a simple structure, and ease of manufacturing, making it broadly applicable in fields such as food safety, biotechnology, and manufacturing.

Super-resolution terahertz imaging algorithm based on blind deconvolution

Guanwen Wang and Feng Qi

DOI: 10.1364/AO.551837 Received 12 Dec 2024; Accepted 21 Feb 2025; Posted 25 Feb 2025  View: PDF

Abstract: Although terahertz imaging is gaining popularity, achieving a high resolution is crucial. Diffraction-limited imaging remains a significant practical challenge. In synthetic aperture radar (SAR) imaging, antennas are typically treated as point sources, and the effect of the antenna aperture is ignored. This study illustrates the impact of the applied antenna on imaging performance and provides a method for achieving super-resolution imaging through compensation within the aperture field of the antenna. Simulation experiments verify the robustness of the algorithm under low SNR conditions. The image contrast can be used for deconvolution kernel iteration. Iteration over target distances and deconvolution kernels accelerates the process of blind deconvolutional imaging. Distinguishing from time-domain pulse and step-frequency continuous wave measurements, the proposed algorithm allows accurate calculation of the distance to the target under single-frequency measurement conditions. The algorithm improves the resolution from a theoretical value of 5.5 mm to 3 mm.

Structured Light Residual Channel Attention Network for Super-Resolution Enhancement of 3D Measurement Images

YuFan Yang, Yichun Tai, zhenzhen huang, Peng Tao, and Zhijiang Zhang

DOI: 10.1364/AO.549610 Received 05 Dec 2024; Accepted 21 Feb 2025; Posted 25 Feb 2025  View: PDF

Abstract: The precision of structured light 3D measurements is often limited by resolution degradation during image acquisition and processing, leading to blurred edge features. Interpolation based upsampling methods are insufficient, and improving camera resolution in large f ield of view systems is costly. We propose the Structured Light Residual Channel Attention Network (SLRCAN), a super-resolution network tailored to the characteristics of structured light imagery. The network integrates residual architectures with a designed channel attention mechanism. We apply SLRCAN to measurement for continuous casting billet in an industrial environment. Dedicated datasets tailored to this scenario are used to train and evaluate the model, offering insights applicable to other scenarios. To address the limitations of conventional image quality metrics in evaluating geometric fidelity, we propose the Assessment of Smoothness (AS), a task-specific metric designed to quantify edge continuity. Experimental results at scale 2× and 4× show that SLRCAN outperforms existing methods, achieving state-of-the-art performance. Additionally, laser measurement experiments on standardized small objects validate its applicability, emphasizing its potential for precise measurement in structured light.

Analytical analysis of phase modulation-based frequency-tunable optoelectronic oscillators

Sajad Jahanbakht

DOI: 10.1364/AO.551566 Received 10 Dec 2024; Accepted 20 Feb 2025; Posted 21 Feb 2025  View: PDF

Abstract: Optoelectronic oscillators (OEOs) can realize ultra-low noise radio frequency (RF) oscillations in the microwave or millimeter wave range. Most frequency-tunable OEOs (FTOEO) use an electro-optic phase modulator, a narrow band optical filter, and a frequency-tunable laser source to tune the RF oscillation frequency. This paper presents an analytical approach to signal and noise analysis of such FTOEOs which is named here the phase transfer approach (PTA). The numerical conversion matrix approach (CMA) is also generalized to the analysis of FTOEOs. The PTA has the advantage of being simple and fast. The CMA has more abilities such as considering the amplitude to phase conversion but at expense of more complicated implementation. The validity of PTA is verified by comparing its results with the CMA and other presented formulations in the literature.

High Conversion Efficiency Mode Converters Based on Cubic Spline Interpolation

jiaqi Yuan, yanwei Wang, Zhe Han, Yan Qi, Boxia Yan, Yuanyuan Fan, Mi Zhou, and qian wang

DOI: 10.1364/AO.555092 Received 07 Jan 2025; Accepted 20 Feb 2025; Posted 21 Feb 2025  View: PDF

Abstract: Mode converters play a critical role in mode-division multiplexing (MDM) systems. This paper proposed and demonstrated a design method of high conversion efficiency (CE) and broadband mode converters based on cubic spline interpolation curves. The cubic spline interpolation method provides great flexibility in defining the boundaries of irregularly shaped waveguide curves. This design effectively improves the mode conversion efficiency and can be applied to different mode orders and multimode waveguide widths. For the TE0-TE1 converters, we designed different multimode waveguide widths (1um,1.1um) to demonstrate the flexibility of the design approach. At 1550nm, the conversion efficiency of the TE0-TE1 converters and TE0-TE2 converters are -0.026 dB (99.4%) and -0.039 dB (99.1%), respectively, according to the theoretical results. The mode converters were manufactured using a standard process provided by the foundry, and the measured conversion efficiency for all devices was above -0.419dB (90.8%) in the wavelength range of 1530 to 1565nm.

A Wide-angle Orthogonal Polarized Metalens for Near-infrared Imaging

Xiaofan Liu, yang qiu, Xingyan Zhao, Shaonan Zheng, Yuan Dong, Qize Zhong, and Ting Hu

DOI: 10.1364/AO.541798 Received 10 Sep 2024; Accepted 20 Feb 2025; Posted 21 Feb 2025  View: PDF

Abstract: Polarized imaging is capable of displaying the morphological and physicochemical properties of a target and detecting invisible targets, while a large field-of-view (FOV) is of great beneficial to obtain more information of the scene in one imaging session. In this study, we propose a wide-angle orthogonal polarized metalens (WOPM) that works in the nearinfrared (NIR) band using a single metasurface-based optical component. The WOPM can focus two orthogonal polarizations at different positions on the focal plane, maintaining high focusing performance even at an off-axis angle over ±30°. The average focusing efficiency are over 60% for both two polarizations. In order to further improve focusing ability of the WOPM, particle swarm optimization (PSO) algorithm is adopted to optimize the phase profiles, thereby reducing errors between the ideal phase distribution and actual phase in the metalens. The PSO-optimized WOPM attains higher average focusing efficiency of 69.0% and 65.3%, while achieving better average polarization extinction ratio of 22.03 dB and 19.72 dB for x-polarization (x-pol) and y-polarization (y-pol), respectively.

Study on the underwater Phaeocystis globosa monitoring based on ACE-DCP enhancement and YOLOv8 detection

Yanbin Tong, Wenjie Zhang, Weihong Zhang, Mingda Che, Xiaobo Li, Haofeng Hu, Jingsheng Zhai, Rongxin Su, Rongyong Zhang, and Qi Li

DOI: 10.1364/AO.549613 Received 21 Nov 2024; Accepted 19 Feb 2025; Posted 21 Feb 2025  View: PDF

Abstract: Occurrence of Phaeocystis globosa blooms poses a potential hazard to both human society and the ecological environment, particularly concerning the safety of cooling systems in coastal nuclear power plants. However, current ecological monitoring techniques fail to dynamically detect the densities of solitary cells of Phaeocystis globosa prior to the blooms, thus hindering timely interventions. This study proposes a framework for harmful algae monitoring by integrating underwater microscopic imaging, image processing, and object detection. Flume experiments were conducted using Phaeocystis globosa as the case study for monitoring object. The results indicate that the proposed framework exhibits favorable performance in recognizing different types of algae, particularly in distinguishing between Phaeocystis globosa and Chlorella. Despite their similar morphology observed from the underwater imaging device under dark-field illumination, the false detection rate between Phaeocystis globosa and Chlorella is approaches 0 % when using the YOLOv8 object detection model. Adaptive Contrast Enhancement (ACE) amplifies the color discrepancies among algae and eliminates virtual focus interference, thus improving precision of algae classification. Subsequently, Dark Channel Prior (DCP) reduces the noise caused by image scattering and limits missed detection. Consequently, the precision of Phaeocystes globosa recognition using the YOLOv8 model in increased from 74 % to 91 %. This study presents an effective solution for in-situ monitoring of specific harmful algae, which has the potential to enhance the capabilities for dynamic detection and early warning of Phaeocystis Globosa blooms.

Recombobulate Correction Method for Oscillating Scene Change Artifacts in Longwave Infrared Fourier Transform Spectroscopy Spectra

Kody Wilson, Covadonga Diaz Ruiz, Shannon Young, Michael Dexter, and Anthony Franz

DOI: 10.1364/AO.553544 Received 23 Dec 2024; Accepted 19 Feb 2025; Posted 21 Feb 2025  View: PDF

Abstract: Fourier transform spectroscopy (FTS) is commonly used to measure spectra because of its accuracy and versatility. When the irradiance changes in the field of view (FOV), the spectra are contaminated by scene change artifacts (SCAs). SCAs caused by an oscillatory obscuration affecting multiple target spectra were examined through simulations and experiments. This work demonstrates that SCAs are hundreds of times larger than the spectra. These SCAs are dependent on the oscillation phase and appear in real and imaginary parts of the spectra. The Recombobulate Correction Method (RCM) is introduced which removes SCAs by separating interferogram points into target and background interferograms. RCM demonstrated a 99% reduction in spectral SCAs. Future work will address other non-oscillatory SCAs.

Quantitative density retrieval of gas jets for high harmonic generation EUV sources with a compact schlieren imaging system

Sven Weerdenburg, Roland Horsten, and Wim Coene

DOI: 10.1364/AO.553974 Received 02 Jan 2025; Accepted 19 Feb 2025; Posted 25 Feb 2025  View: PDF

Abstract: We demonstrate the implementation of a compact schlieren imaging technique for quantitatively measuring atomic density profiles in a gas jet-based high harmonic generation EUV source. This technique compares high harmonic generation light sources and the optimization, considering different nozzle geometries, backing pressures, and vacuum systems. The simplicity of schlieren imaging could make it a suitable standardized inspection tool for gas jet-based high harmonic generation sources. Several gas jet profiles at different backing pressures were analyzed, enabling the retrieval of the peak pressure within the gas jet and the impact of the vacuum system to the jets shape.

Focusability of femtosecond laser pulse after Treacy compressor with diffraction gratings with small-scale and large-scale non-flatness

Anton Kochetkov and Efim Khazanov

DOI: 10.1364/AO.553580 Received 20 Dec 2024; Accepted 18 Feb 2025; Posted 21 Feb 2025  View: PDF

Abstract: Analytical expressions for the focal intensity (Strehl ratio) were obtained for both small-scale and large-scale variations in the surface profile of the compressor diffraction grating substrate. The validity of the expressions is confirmed by numerical simulations. It is shown that for small (much less than beam diameter) scale variations with rms of less than 10 nm their influence can be neglected. Numerical simulations confirmed that quadratic aberrations (defocus, astigmatism) can be suppressed by rotating the fourth grating, and therefore grating manufacturers should ignore these aberrations to minimize higher orders.

Shock hydrodynamic analysis and shadowgraphic measurements of laser-ablated carbon-reinforced silicon carbide (C/SiC)

Quincy Zawadzky and Ashwin Rao

DOI: 10.1364/AO.554540 Received 02 Jan 2025; Accepted 18 Feb 2025; Posted 21 Feb 2025  View: PDF

Abstract: Shadowgraphic measurements of pulsed laser ablation of a C/SiC target at laser energies from 50 - 110 mJ and ambient pressures of 100 and 760 Torr were performed to study ablation shockwave dynamics. Laser shadowgraph images of the expanding shock front between 0.1 - 10 μs after the ablation were used to characterize hydrodynamics of the shock front and shock heated gas flow using Sedov-Taylor theory and Rankine-Hugoniot relations. The shockwave transitioned to an acoustic wave around 5 - 6 μs, limiting the validity of the Rankine-Hugoniot relations to this range of delay times. The shock expansion and Sedov-Taylor fit coefficients were used to calculate initial blast energy as well as the energy expended on laser-target interaction. This energy was used to calculate an upper estimated of the mass ablated by the laser across all experimental conditions. A maximum mass removal of 0.568 μg was calculated for a 110 mJ pulse at atmospheric pressure. Increasing the ambient pressure was found to have minimal effect on the amount of mass ablated; this is attributed to the effects of inverse Bremsstrahlung coupling between the 1064 nm probe laser and the plume limiting the amount of laser energy deposited directly into the target.

Performance of Four Deep Neural Network Architectures in Moiré Fringe Intelligent Analysis

xinyu zeng, Yunyun Chen, Wei-hao Cheng, Wen-zhuo Xie, and Cui-hong Yang

DOI: 10.1364/AO.554689 Received 02 Jan 2025; Accepted 18 Feb 2025; Posted 21 Feb 2025  View: PDF

Abstract: Moiré fringe analysis holds significant importance in the assessment of flow fields, while deep learning is advancing rapidly and frequently employed to handle complex computational challenges. In this paper, the performance of U-net, Unet++, Unet 3+, and ResUnet in moiré fringe analysis is compared and analyzed. The results indicate significant differences among the four neural networks in terms of training time, prediction speed, prediction accuracy, and generalization capability regarding positional variations. Among the four neural networks, ResUnet exhibits the shortest training times, highest accuracy and fastest prediction speeds, while also exhibiting good generalization capability. Consequently, in practical applications, using ResUnet for moiré fringe analysis is an excellent choice. In a word, this paper provides a basis for selecting network models in the application of deep learning within the field of moiré tomography.

Optical design of a freeform f-θ lens for 405nm laser scanning unit

Chen Xu, Dongjie Ye, Haizhou Liu, Zexiang Cheng, Hongze Lin, and Weitao Song

DOI: 10.1364/AO.551643 Received 26 Dec 2024; Accepted 18 Feb 2025; Posted 21 Feb 2025  View: PDF

Abstract: In this study, we propose an effective design method for a compact freeform f-θ lens system in 3D printing laser scanning units. A two-phase design strategy based on surface type upgrade is put forward for the design of the f-θ lens, where different models and design targets are adopted at different stages in optimization. An automatic linearity error correction procedure is proposed to replace the tedious manual adjusting process. A design result comparable with A4 scanning width was obtained following the proposed design strategy. The relative difference in diffraction spot size is below 3% in ±100mm scanning width, and the linearity error stays below 0.3%, indicating a good optical performance. Tolerance performance and thermal analysis show that the design result meets the requirement in real usage conditions.

Research on 3D Reconstruction of Strongly-reflective Surface Based on Binocular Line-structured Light

Siying Heng, Guohe LI, HaoZhen Huang, Qing CHANG, Zhongxuan ZHANG, and Yuanzhen Wang

DOI: 10.1364/AO.553374 Received 23 Dec 2024; Accepted 18 Feb 2025; Posted 21 Feb 2025  View: PDF

Abstract: This paper proposes a 3D reconstruction method utilizing binocular line-structured light to address accuracy errors caused by strong light reflection on the surfaces of objects in industrial production. This method involves an algorithm that employs image transformation, threshold segmentation, and image fusion to extract the center of the laser stripe from multiple images. Two monocular systems, each consisting of a light plane and a single camera, as well as a binocular system, were established. These systems generated three sets of point cloud data, which were then integrated using point cloud integration technology to obtain a set of high-dynamic range 3D data. Experimental results showed that within a measurement range of 600 mm by 800 mm and at a measurement distance of 1200 mm, the root-mean-square (RMS) value was calculated to be 0.041 mm, with an overall error spread of ±0.07 mm(0.13%). The point cloud data in the experimental scenarios presented in this paper demonstrated a maximum improvement of 121% compared to traditional methods.

Wide Bandgap Photoconductor (SiC:V)-based Optically Addressed Light Valve for High Fluence Operation

Bikramjit Chatterjee, Soroush Ghandiparsi, Miranda Gottlieb, Qinghui Shao, Clint Frye, Sara Harrison, and Lars Voss

DOI: 10.1364/AO.540656 Received 29 Aug 2024; Accepted 18 Feb 2025; Posted 21 Feb 2025  View: PDF

Abstract: Optically addressable light valves based on wide bandgap 4H- and 6H-SiC as photoconductors were designed to withstand higher operational laser fluences than the stateof-the-art bismuth silicon oxide (BSO; Bi12SiO20) based devices. Vanadium-doped SiC was selected as the photoconductors due to their reasonable photoresponsivity while many fold improvement in laser induced damage threshold as compared to BSO. The laser induced damage threshold values of the materials were measured after exposing ~200 sites on the samples to increasing levels of fluence of a gaussian pulsed Nd: YAG laser system (1064 nm) with a 5 Hz repetition rate. The measured damage threshold values for BSO, 4H- and 6H-SiC were 0.4 J/cm2, 1.75 J/cm2 and 1.8 J/cm2, respectively. Photoconductive switches based on 4H- and 6H-SiC samples were characterized at wavelengths of 380 nm, 405 nm, and 447 nm. The peak photoresponsivity values of the 4H- and 6H-SiC materials were measured to be under 380 nm and 405 nm, respectively. The photoconductor was bonded to a BK7 optical window with 5 μm diameter microspheres as spacers. A twisted nematic type E7 liquid crystal (LC) was filled in the 5 μm gap in a vacuum chamber. The desired alignment of the liquid crystal was achieved by mutually orthogonal orientation of LC alignment layers on the two mating faces (SiC and BK7). The fabricated devices were modulated using address beams of wavelengths 380 nm, 405 nm, and 447 nm. Required transmission levels of > 90% was achieved for the fabricated OALVs for a sinusoidal voltage waveform that meets the lifetime requirement of the device.

Counter-propagating fs/ps CARS for sub-100 µm resolution gas-phase thermometry

Alan Kim, Ryan Thompson, Andrew Cutler, and Chloe Dedic

DOI: 10.1364/AO.559674 Received 17 Feb 2025; Accepted 18 Feb 2025; Posted 25 Feb 2025  View: PDF

Abstract: A novel counter-propagating (CoP) phase-matching configuration of hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) for ultra-highresolution measurements of gas temperature is demonstrated. The resulting spatial resolution is on the order of 10’s of µm—more than an order of magnitude improvement over traditional phase matching geometries—which enables spatially-resolved gas-phase measurements of steep temperature gradients and minimizes effects of spatial averaging. Two additional advantages of this measurement system are the ability to simply control the location of the measurement volume by adjusting relative pulse timing and the size of the probe volume by adjusting pulse duration. The spatial resolution of the counter-propagating CARS system (CoPCARS) was quantified and the gas temperature across a high-velocity microscale jet emanating from a hypodermic needle was measured using CoPCARS.

Ultra-large bandwidth fully polarization photodetectors based on displacement-induced chiral dielectric metasurfaces

bo cheng, Yuxiao Zou, and Guofeng Song

DOI: 10.1364/AO.550178 Received 26 Nov 2024; Accepted 17 Feb 2025; Posted 19 Feb 2025  View: PDF

Abstract: The polarization of light is enriched with many important structural information of materials, but it cannot be directly accessed by photodetectors. Although fully polarized photodetectors based on chiral metasurface filters can solve the challenge of polarized light signal acquisition, the effective bandwidth in the near-infrared band generally does not exceed 80 nm, which severely limits the overall signal capacity of the devices. We have investigated the equivalent the electric field direction distribution pattern of the metasurface by varying the distance between two short arms in the unit cell of the metasurface, realizing a great leap in spectrum from zero chirality to very large chirality with a bandwidth of 200 nm. In addition, combining this chiral metasurface with an InGaAs/InP intensity photodetector enables average errors of the degree of circular polarization of -18 dB. The large bandwidth fully polarized photodetectors are expected to lead the way of the field of polarization modulation on nanophotonic platforms.

Scaling laws for the noise-equivalent angle and C-tilt, G-tilt anisoplanatism due to scintillation

Eric Mitchell, Derek Burrell, Milo Hyde, Ronald Driggers, and Mark Spencer

DOI: 10.1364/AO.553861 Received 06 Jan 2025; Accepted 17 Feb 2025; Posted 19 Feb 2025  View: PDF

Abstract: In this paper, we derive single-integral solutions, applicable in the weak-to-moderate scintillation regime, for both the noise equivalent angle (NEA) due to scintillation and the scintillation-induced root mean squared error (RMSE) between gradient tilt (G-tilt) and centroid tilt (C-tilt). In practice, the NEA due to scintillation gives a measure for the scintillation-induced track error, whereas the scintillation-induced RMSE between C-tilt and G-tilt gives a measure of the C-tilt, G-tilt anisoplanatism due to scintillation. Assuming spherical-wave propagation, we fit closed-form expressions to the numerically integrated solutions. These closed-form expressions serve as "scaling laws,'' and we validate their use with wave-optics simulations. At large, we determine that the one-axis NEA due to scintillation scales as a function of aperture size, propagation distance, wavelength, and Rytov number, whereas the one-axis scintillation-induced RMSE between C-tilt and G-tilt scales proportionally to the Rytov number when normalized by the diffraction angle. These findings will aid in the design of active electro-optical systems, which inevitably experience the effects of scintillation when imaging through distributed-volume turbulence.

Super-resolution reconstruction of off-axis digital holography based on Real-ESRGAN

geng chen, Jie Chen, houzhang liao, keke liu, Xin Tang, and Yong Kong

DOI: 10.1364/AO.545914 Received 23 Oct 2024; Accepted 16 Feb 2025; Posted 19 Feb 2025  View: PDF

Abstract: Off-axis digital holography plays a crucial role in high-precision three-dimensional imaging. However, high-resolution phase images are often affected by the limited pixel size of the sensor. To address this issue, this study proposes, for the first time, the application of Real-ESRGAN for the super-resolution reconstruction of off-axis digital holography images. By directly applying super-resolution processing to the hologram, the limitations of the sensor’s pixel size are overcome, followed by phase reconstruction to obtain high-resolution phase images. The Real-ESRGAN model enhances the texture information of the hologram through a deep network with multiple Residual-in-Residual Dense Blocks (RRDB), restoring a clearer hologram. A U-Net architecture with spectral normalization, offering stronger discriminative capability, is used to provide detailed per-pixel feedback for the generator. Experimental results demonstrate that this method can be applied to multi-scale holograms, outperforming comparison networks in both visual quality and quantitative metrics, thus providing an innovative solution for super-resolution reconstruction of off-axis digital holography.

Time-shifted Pseudonoise Multibeam Lidar Array using Acousto-optic Deflectors

Kai-Ting Ting and Kelvin Wagner

DOI: 10.1364/AO.552082 Received 16 Dec 2024; Accepted 14 Feb 2025; Posted 19 Feb 2025  View: PDF

Abstract: We present a new multibeam lidar system that simultaneously transmits an array of K low-crosstalk codes and detects the backscattered returns from all directions on a single large-area high-speed detector. We generate a series of long, time-shifted, self-synchronizing maximum-length pseudo-noise (PN) codes in the form of an array of K intensity-modulated beams from a single aperture using only two traveling-wave acousto-optic deflectors (AODs). The cascaded AODs driven by counter-propagating acoustic waves are imaged onto each other, and the diffracted fields are imaged onto the far-field target where the interferometric product between two Binary-Phase-Shift Keying (BPSK) encoded ML sequences generates time-permuted versions of the PN-code as an array of K beams highly identifiable as well-separated peaks in the cross-correlation. The time delay of the various beams are optimized to be well separated across the multibeam array and are delayed by many milliseconds even though the acoustic propagation time is only a few microseconds. This relies on the shift-and-add property of ML codes to produce time-shifted versions of a PN-code from the bitwise mod-2 addition (XOR) of 2 relatively delayed copies of the code, where XOR is instead implemented as the interferometric product between BPSK bipolar PN-codes. This PN-encoded interferometry enables a new type of multibeam lidar imaging system that does not require scanning mirrors at the transmitter or multi-pixel wideband detector arrays at the focal plane of the receiver. A preliminary experimental demonstration of a 32-beam array parallel ranging using a N=2¹⁵−1 maximal length code shows how a single cross-correlator computationally identifies the different time-shifted beams and their time-of-flight (TOF) range delays.

Passively Q-switched Erbium-doped Fiber Laser based on Nickel ferrite (Ni-Fe2O4) Nanoparticles Saturable-absorber: Synthesis, Theoretical Modeling and Experimental Demonstration

Muhammad Hamza, Mamoon Asghar, Shahid Sadiq, Anwar Haq, Tahani A. Alrebdi, Asma Noor, Izhar Saghir, Rizwan Ahmed, M. Aslam Baig, and Haroon Asghar

DOI: 10.1364/AO.555056 Received 08 Jan 2025; Accepted 14 Feb 2025; Posted 14 Feb 2025  View: PDF

Abstract: In this study, the detailed experimental investigations of passively Q-switched (PQS) erbium-doped fiber laser (EDFL) were reported based on nickel-ferrites (Ni-Fe2O4) thin-film saturable absorber (SA). The Ni-Fe2O4 nanoparticles (NPs) were synthesized using a sol-gel auto-combustion mechanism and its thin film was prepared in polyvinyl alcohol (PVA) via the solution casting method. The crystallographic structure, surface morphology, and particle size of Ni-Fe2O4 NPs were characterized using the X-ray Diffraction (XRD) technique, high-resolution transmission electron microscopy (HR-TEM), and scanning electron microscopy (SEM). The obtained findings indicate that the prepared Ni-Fe2O4 SA yielded a 12.6% modulation depth (ΔT), saturation intensity (Isat) of 34.04 MW/cm2 and non-saturable losses (Tns) of 48.2%. The prepared SA upon integration within laser cavity resulted in an emission wavelength of 1563.87 nm at a input pump power (Pin) of 104.6 mW. With increased Pin, a stable passive Q-switched pulse operation initiated by incorporating a Ni-Fe2O4 thin film as an SA in the EDFL ring cavity yielded a repetition-rate (RR) ranges from 34.18 to 69.44 kHz, and pulse-width (PW) varied from 10.94 to 4.74 µs upon tuning a Pin to 284 mW. Additionally, the pulse energy (PE), peak power, and average output power (Pavg) were 167.33 nJ, 35.30 mW, and 11. 62 mW, respectively. Finally, the stability of EDFLs with Ni-Fe2O4-SA was thoroughly examined by operating the SA continuously 1 hour at a Pin of 104.6 mW, that maintained constant PW of 8.43  0.08 µs and RR 43.89  0.10kHz. This study shows that ferrite composites serve as highly effective SAs in EDFLs, enabling stable and controllable generation of ultrashort pulses due to their high damage threshold and nonlinear optical properties that are critical for variety of uses in photonics.

Adjustable dual-wavelength soliton complexes in a mode-locked Tm-doped figure-of-eight fiber laser

Ze Xin Du, Jinjer Huang, Ying Sun, Bo Yang Su, and Xin Lu Zhang

DOI: 10.1364/AO.544165 Received 04 Oct 2024; Accepted 14 Feb 2025; Posted 19 Feb 2025  View: PDF

Abstract: Dual-wavelength rectangular and step-like soliton complexes, mixed by single solitons and soliton molecules, and a coexistence of soliton bundle and soliton rain, were observed in a Tm-doped fiber laser, mode-locked by the technique of nonlinear amplifying loop mirror. Increasing pump power and tuning the polarization can adjust the pulse energy ratio and switch the pulse wavelength between two generated wavelengths. In such a process, the pulse can either resonate in the rectangular form or locally expand in the step-like form and even break up. Spectral fringes of the soliton complexes and involved pulse dynamics reflected a varying interaction among solitons.

Localised measurement of ultrasonic waves using a Fabry-Perot sensor illuminated by a Bessel beam

Dylan Marques, Oliver Sheppard, Edward Zhang, Peter Munro, and James Guggenheim

DOI: 10.1364/AO.548048 Received 21 Nov 2024; Accepted 14 Feb 2025; Posted 19 Feb 2025  View: PDF

Abstract: Fabry-Perot (FP) ultrasound sensors are a class of optical ultrasound sensors used in photoacoustic tomography (PAT) and other applications. Conventionally, an FP ultrasound sensor comprises an ultrasonically compressible planar microcavity, locally interrogated by a focussed Gaussian beam. One way to increase the sensitivity could be to replace this beam with a Bessel beam. The rationale is two-fold. First, as a Bessel beam’s wavefront better matches the modes of the planar microcavity, this could increase the Q-factor leading to higher sensitivity. Secondly, as a Bessel beam provides a focussed spatial structure- a central core surrounded by concentric rings- it might retain the ability to locally interrogate the sensor. To explore this idea, we developed an experimental system featuring a custom FP ultrasound sensor interrogated by a Bessel beam, and evaluated its on-axis sensitivity, directivity and image resolution when performing PAT. For comparison, we measured the same characteristics using a conventional focussed Gaussian beam with a spot size similar to the core of the Bessel beam. As anticipated, the Bessel beam provided a higher ultrasonic on-axis sensitivity. However, the directivity and spatial resolution were degraded, suggesting that the Bessel beam yielded a larger acoustic element. We conclude that it is feasible to increase the sensitivity of an FP ultrasound sensor using a Bessel beam. Further work is required to establish whether differently designed Bessel beams could concurrently offer a smaller acoustic element.

Highly sensitive surface plasma resonance refractive index sensor composed of micro-grooved single-mode fibers

Yu Zhang, Yanshu Zeng, Jianxin Wang, Xinping Song, Zheng Li, Chuanlong Lv, Miao Liu, Chao Liu, and Paul Chu

DOI: 10.1364/AO.550849 Received 05 Dec 2024; Accepted 12 Feb 2025; Posted 19 Feb 2025  View: PDF

Abstract: Surface plasmon resonance (SPR) has many applications in optical fiber sensing. Herein, a single-mode optical fiber refractive index sensor based on SPR is designed and analyzed. The sensor comprises micro-grooves etched on a single-mode optical fiber to fix the gold wire. This structure facilitates the energy exchange between the fiber core and the plasmonic mode on the metal, and the mechanical strength is improved. The influence of structural parameters on the sensing properties is assessed by the finite element method. The sensor can detect sodium chloride solutions with concentrations between 120 g/L to 360 g/L and hemoglobin solutions in the concentration range between 0 g/L and 100 g/L. The sensor can determine refractive indexes in the range between 1.33 and 1.41 with a maximum wavelength sensitivity of 28,200 nm/RIU. Since the sensor offers high-sensitivity refractive index detection for multiple liquids, it has immense potential in bio-sensing.

Replacing Toxic Dyes with Photonic Crystals for Printing Applications: Simulation Study

Zaky Zaky, S Alamri, M. Al-Dossari, D. Mohamed, V. D. Zhaketov, and Arafa H.Aly

DOI: 10.1364/AO.545462 Received 21 Oct 2024; Accepted 11 Feb 2025; Posted 19 Feb 2025  View: PDF

Abstract: Photonic crystals are proposed for colored printing due to the fantastic geometrical and optical features of the photonic crystal structures. This study proposes multilayers to replace toxic dyes with safe, durable printers in the range of visible light. The color of each point on the printed model can be changed by changing the thickness of the layers. Furthermore, the suggested models have demonstrated the practical ability of PhC to manipulate the structural color by tuning the thickness of layers. These properties make it a good choice for 3D printing. Hence, we can substitute dangerous dyes with periodic optical structures in 3D printers to cover 3D shapes.

Birefringence Enhancement in Terahertz Photonic Crystal Fiber via Liquid-Filled Holes

João Paulo Lebarck Pizzaia, Gabriel Bastida, Marcelo Segatto, Elizandra Coelho, Arnaldo Leal Junior, and Carlos Schmidt Castellani

DOI: 10.1364/AO.546784 Received 18 Nov 2024; Accepted 11 Feb 2025; Posted 12 Feb 2025  View: PDF

Abstract: This study investigates the enhancement of birefringence in terahertz (THz) Porous Core Photonic Crystal Fibers (PC-PCFs) through strategic filling of their cladding air-holes with various liquid configurations. Terahertz PCFs are crucial for applications requiring precise control over polarization states and low losses, such as in telecommunications and sensing. By employing Ansys Lumerical Finite Element Method simulations, three distinct liquid-filling strategies (Cases A, B, and C) were analyzed. Each case focused on filling different sections of the PCF’s structure: outer ring, outer ring limited to outermost holes, and inner ring, respectively. The simulations evaluated the influence of these configurations on the PCF’s refractive indices, birefringence, and attenuation within the THz frequency range. The results demonstrate that f illing specific air-holes with water can effectively increase the PCF’s birefringence, offering tailored polarization characteristics for THz applications. For instance, Case C, in which the inner ring holes are filled, showed a substantial increase in birefringence from 0.0274 to 0.0447, highlighting its potential for polarization-sensitive devices.

Robust and Precision Small Angle Measurement through Speckle Pattern Imaging Technique

Vikas ., kuldeep kumar, Mukesh Jewariya, Rajesh Kumar, S.S.K TITUS, and Surya Gautam

DOI: 10.1364/AO.530304 Received 16 May 2024; Accepted 11 Feb 2025; Posted 12 Feb 2025  View: PDF

Abstract: With technological advancements in various fields, the precision estimation of small angles is becoming crucial in many scientific, engineering, and metrology applications. Traditional mechanical and electromagnetic methods face limitations in automation and precision when measuring small angles while optical techniques such as interferometers and autocollimators offer high sensitivity and accuracy but suffer from complexity and instability. On other hand speckle pattern-based techniques have emerged as promising alternatives, offering high sensitivity and non-contact measurement capabilities. This paper presents a robust and precision small-angle measurement technique utilizing speckle patterns. For demonstration, an experimental setup, featuring a CMOS (complimentary metal-oxide-semiconductor) camera and a motorized stage, is developed alongside an algorithm for angle estimation. Mathematical analysis and experimental validation confirm the method’s accuracy and preciseness, while repeatability testing confirms its robustness. The method's simplicity, reliability, and compatibility with small spaces make it a valuable tool for various applications where precision small angle measurement is required.

Design of multilayer terahertz metasurface with dual control of amplitude and phase for multifunctional applications

yuping zhang, siyu zheng, Xiaoyu Hao, Yu Wang, Yang Liu, Min Zhang, Meng Liu, and huiyun zhang

DOI: 10.1364/AO.551370 Received 09 Dec 2024; Accepted 07 Feb 2025; Posted 07 Feb 2025  View: PDF

Abstract: Current metasurfaces encounter challenges in achieving precise control over transmittance-reflection mode conversion. This study presents a multilayer metasurface structure that incorporates dual amplitude and phase control, utilizing vanadium dioxide (VO2) and Dirac semimetals (DSMs) as tunable materials. By adjusting the orientation angle of the split-ring resonator (SRR) in the intermediate layer and modifying the Fermi energy level of the DSMs, precise dual control of amplitude and phase can be attained. When VO2 is in its insulating phase with a DSM Fermi level of 500 meV, the metasurface exhibits notable transmission characteristics, enabling refraction and focusing at 0.6 THz, while demonstrating remarkable insensitivity to the incident angle. Conversely, when VO2 transitions to the metallic phase and the Fermi level of the DSMs is decreased to 1 meV, the metasurface displays significant reflection characteristics, facilitating the generation of vortex beam and multi-beam vortices. This multifunctional metasurface holds considerable promise for applications in terahertz and optical technologies.

Ballistic Range Experimental Results of Radiating Wakes of Spheres at Mach 10

Robert Macdermott, Nicholas Mueschke, Naibo Jiang, Paul Hsu, and Sukesh Roy

DOI: 10.1364/AO.546959 Received 06 Nov 2024; Accepted 07 Feb 2025; Posted 05 Mar 2025  View: PDF

Abstract: Hypersonic flow field can experience significant thermochemical nonequilibrium effects. However, very little ground test data exists for nonequilibrium hypersonic wake flows. Simulating air chemistry in nonequilibrium is highly challenging due to the lack of validation data. To address a lack of validation data, ballistic free flight tests were conducted examining nonequilibrium chemistry effects in the wake of 25 mm spheres at Mach 10. A laser-based diagnostic system was used to measure naturally-produced nitric oxide concentrations. Measurements were made up to 53 body diameters behind the sphere. NO concentrations in the wake peaked between 4.1 +/- 0.8% and 6.8 +/- 1.3%.

High-Performance Gate-All-Around Phototransistor with Ga2O3/NiO Heterojunction for Advanced Ultraviolet Detection

Xiaoxi Li, Zhifan Wu, Shuo Li, Shuqi Huang, Yuan Fang, Yuchun Li, cizhe fang, Xiangyu Zeng, Yue Hao, and Genquan Han

DOI: 10.1364/AO.551210 Received 05 Dec 2024; Accepted 06 Feb 2025; Posted 07 Feb 2025  View: PDF

Abstract: Abstract: Gallium oxide (Ga2O3), with its ultra-wide bandgap of 4.9 eV, excellent thermal stability, and availability in large native substrates, is an ideal material for solar-blind ultraviolet (UV) detection. In this study, we present a high-performance gate-all-around (GAA) phototransistor based on a p-NiO/n-Ga2O3 heterojunction, specifically designed for advanced UV detection applications. The incorporation of p-NiO as the gate material provides a strong built-in electric field, which significantly improves carrier separation, suppresses dark current, and enhances overall photoresponse. The constructed GAA phototransistor exhibits superior optoelectronic properties, including a responsivity of 8.64 × 106 A/W, an external quantum efficiency of 4. × 109 %, a detectivity of 9.92 × 1019 Jones, and rise/fall times both of 5 μs. Comprehensive simulation and experimental analyses reveal that the enhanced performance stems from the favorable type-II band alignment at the NiO/Ga2O3 interface, which facilitates efficient photocarrier generation and transport. This work not only establishes a pathway for developing high-sensitivity and fast-response UV photodetectors but also lays the foundation for further advancements in solar-blind optoelectronics for environmental monitoring, space exploration, and other critical applications.

Analysis of probability density functions for aperture-averaged fluctuations of laser radiation power for extended range of atmospheric propagation conditions

Valeriy Kolosov and Grigory Filimonov

DOI: 10.1364/AO.551004 Received 05 Dec 2024; Accepted 23 Jan 2025; Posted 24 Jan 2025  View: PDF

Abstract: This study presents a detailed analysis of probability density functions (PDFs) for aperture-averaged fluctuations of laser radiation power under various atmospheric turbulence conditions. Using numerical simulations, we examined an extensive range of Rytov parameter values, representing different turbulence strengths, and analyzed propagation across all primary diffraction regimes, including geometric optics (plane wave), Fresnel, and Fraunhofer (spherical wave) approximations. Four key PDF models—log-normal, gamma, fractional exponential, and fractional gamma—were evaluated for their ability to model intensity fluctuations, with the optimal PDF identified for various propagation scenarios. The results highlight a continuous transition between PDF models as the scintillation regime changes, offering guidelines to selecting the best-fit PDF under specific propagation conditions. Comparison of simulation results with experimental data confirms that the selected PDFs accurately represent atmospheric turbulence effects on laser beam propagation across various aperture sizes and distances.

Spatial Filter Pinhole Image Acquisition Based on Beam Self-fold Back in Multi-pass Amplifier Laser System

Xin Zhang, Liang Lingxi, Xibo Sun, Lanqin Liu, Deen Wang, Ying Yang, Yilin Yao, Xuewei Deng, and Qiang Yuan

DOI: 10.1364/AO.547736 Received 14 Nov 2024; Accepted 21 Jan 2025; Posted 22 Jan 2025  View: PDF

Abstract: The energy in a multi-pass amplifier laser system can reach ten thousand joules high or more nowadays. The high-precision alignment of beams is one of the prerequisite conditions for the operation of multi-pass amplification laser systems. High-precision alignment reference acquisition is the base for a precise alignment emphasizing the precise far-field reference imaging of the final pass pinhole. A new method of imaging the reference pinhole based on the total reflection of a rectangular prism was proposed in this study. By plugging a rectangular prism into the laser beam line, the laser beam will be reflected and folded back directly to the final pass line while the middle stage amplification beam line is missed. The pilot beam will directly light up the reference pinhole, and a clear boundary pinhole image will be obtained. In a multi-pass laser amplification system, employing this method for laser collimation eliminates the necessity of introducing a new light source and mitigates the effects of multi-stage pinhole collimation, which significantly assists the rapid and precise collimation of multi-path optical systems within intense laser systems.

A generalised physics-based correction for adjacency effects

Alexandre Castagna and Quinten Vanhellemont

DOI: 10.1364/AO.546766 Received 04 Nov 2024; Accepted 15 Jan 2025; Posted 16 Jan 2025  View: PDF

Abstract: Atmospheric scattering occurs over a horizontal scale of several kilometres. This results in influence from neighbouring surface features on the signal recorded over a given position, reducing contrast and the accuracy of quantitative retrievals of surface reflectance from satellite imagery. This atmospheric blurring, or adjacency effect, must be accounted for when both contrast in surface reflectance and magnitude of atmospheric scattering are significant. Taking into account the adjacency effect is of particular importance for aquatic remote sensing of inland and coastal waters, due to the high contrast between water and different land cover types, and the small spatial scale of most inland water bodies. In this paper, we present a physics-based processor to retrieve surface reflectance over all surface types, regardless of the subscene composition and the sensor waveband configuration. The processor is implemented in the free and open source ACOLITE software and is composed of two modules: (1) TSDSF for the estimation of aerosol properties, and (2) RAdCor, for the retrieval of surface reflectance. We demonstrate the performance of the TSDSF+RAdCor processor for the Operational Land Imager (OLI) on board of Landsat 8 and the Multispectral Instrument (MSI) on board of Sentinel-2A and 2B over a set of small (< 1 km2 ) inland waters in Belgium, and compare the performance with other common processors for these sensors (C2RCC, POLYMER, Sen2Cor, iCOR, ACOLITE/DSF, LaSRC). For clear sky matchups, the relative deviation against in situ data in the visible wavebands ranged between 6 % and 18 % for OLI, and between 14 % and 31 % for MSI, except for the MSI waveband centred at 443 nm where the relative deviation was 70 %. In the near-infrared wavebands, the relative deviation varied from 70 % to 150 % depending on the waveband and sensor, with the exception of the MSI waveband centred at 704 nm, for which the performance was of the same magnitude as for the visible wavebands. Overall, the new processor outperformed the other evaluated processors in the visible range, with the exception of the MSI waveband centred at 443 nm, and was outperformed by C2RCC and POLYMER in the near-infrared wavebands. Sen2Cor and LaSRC had the lowest performance in all wavelengths, while ACOLITE/DSF provided accurate retrievals in the visible range, with large errors in the near-infrared wavebands. iCOR had intermediate performance in the visible and NIR. Results are also presented for MSI cloudy sky scenes, with larger relative deviation values across all wavebands. Only one cloudy sky matchup was available for OLI, and hence no performance statistics were computed. The use of RAdCor to correct imagery when in situ reflectance data are available is demonstrated by estimation of aerosol properties through optimisation. Recommendations on how to use TSDSF and RAdCor in ACOLITE are provided.

3D imaging method based on binary encoded fringes with complementary symmetric error diffusion paths

YuQi Yang, Jiangping Zhu, Jun Luo, Fan Yang, and Pei Zhou

DOI: 10.1364/AO.547430 Received 07 Nov 2024; Accepted 17 Dec 2024; Posted 18 Dec 2024  View: PDF

Abstract: Digital micromirror device (DMD) projectors, with their high-speed switching capabilities, have enabled the widespread application of binary fringe projection in dynamic 3D imaging. The Floyd-Steinberg (FS) dithering technique, widely regarded for its high measurement accuracy, is one of the mainstream methods for this purpose. However, binary fringe patterns generated using FS error diffusion exhibit intensity deviations compared to standard sinusoidal fringe patterns, introducing phase extraction errors in subsequent calculations. We observed that these phase errors are closely related to the selection of the error diffusion path. To address this issue, we propose a 3D imaging method based on binary coded fringes generated using complementary symmetric diffusion paths for accurate 3D surface measurement. Our approach utilizes two sets of binary fringe patterns: one generated with the traditional FS error diffusion kernel and path, and the other with the symmetric FS kernel and path. By applying three-frequency temporal phase unwrapping to each set of binary fringes, we obtain two absolute phase maps. These maps are then fused using the IRM-SR algorithm to yield an optimized absolute phase for 3D reconstruction. xperimental results on plane and object reconstructions demonstrate that this method significantly enhances 3D reconstruction accuracy.

Femtosecond laser-induced damage threshold of a thermo-optically addressed spatial light modulator due to giant third harmonic generation

Loîc Ramousse, Vincent Femy, Cyrille Claudet, Gilles Chériaux, Nicolas Forget, and Aurelie Jullien

DOI: 10.1364/AO.545125 Received 22 Oct 2024; Accepted 01 Dec 2024; Posted 12 Dec 2024  View: PDF

Abstract: We investigate the laser flux resistance of a complex multilayer structure containing liquid crystals, focusingon the behavior of a spatial light modulator operating in the femtosecond regime. Our findings showthat exceptionally high third-harmonic generation at the interfaces between the liquid crystal layer andits substrates, driven by the discontinuity in third-order nonlinear susceptibility, is the primary cause oflaser damage. A quantitative study of the chromatic laser-induced damage threshold (LIDT) of a thermo-optically addressed spatial light modulator (TOA-SLM) is then presented. Since the device operates overan ultra-broadband spectrum, the study focuses on the behavior in the ultra-short pulse regime. Thechromatic analysis offers a detailed map of the device’s operational limits in ultrafast optics. With amaximum LIDT value of 500 GW/cm2, the modulator is an excellent candidate for integration into ultrafastoptical systems. Additionally, the study examines the laser-induced functional threshold (LIFT), showingthat the device remains fully operational up to its damage threshold.