Photonics

23 pages, 7806 KB

Open AccessArticle

High-Precision Calibration Technology for Laser 3D Projection System Based on Pose Relationship

by Yukun Liu, Xisheng Li, Dabao Lao, Zhengyang Zhang, Xiaojian Wang and Tianqi Chen

Photonics 2026, 13(5), 441; https://doi.org/10.3390/photonics13050441 - 30 Apr 2026

To address the multi-sensor collaborative calibration challenges in laser 3D projection systems, a pose calibration method integrating binocular vision and laser ranging is proposed. A multi-coordinate system fusion framework encompassing the camera coordinate system, galvanometer coordinate system, and workpiece coordinate system is established. [...] Read more.

To address the multi-sensor collaborative calibration challenges in laser 3D projection systems, a pose calibration method integrating binocular vision and laser ranging is proposed. A multi-coordinate system fusion framework encompassing the camera coordinate system, galvanometer coordinate system, and workpiece coordinate system is established. Through the calculation of reference pose matrices and real-time transformations, adaptive calibration under arbitrary workpiece placements is achieved. Experimental results demonstrate that within a working range of 1.5–2.5 m, the calibration error is 45.5

μ

m, meeting the high-precision requirements of aerospace precision machining and assembly. Full article

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24 pages, 8218 KB

Open AccessArticle

Learning to See Around Corners: A Deep Unfolding Framework for Terahertz Radar Non-Line-of-Sight 3D Imaging

by Kun Chen, Shunjun Wei, Mou Wang, Juran Chen, Bingyu Han, Jin Li, Zhe Liu, Xiaoling Zhang, Yi Liao, Pengcheng Gao and Xiaolin Mi

Photonics 2026, 13(5), 440; https://doi.org/10.3390/photonics13050440 - 30 Apr 2026

Abstract

Non-Line-Of-Sight (NLOS) Terahertz (THz) radar 3D imaging leverages electromagnetic wave propagation characteristics such as reflection, diffraction, scattering, and penetration to detect, locate, and image hidden targets in occluded environments. It holds significant potential for applications in autonomous driving, disaster rescue, and urban warfare. [...] Read more.

Non-Line-Of-Sight (NLOS) Terahertz (THz) radar 3D imaging leverages electromagnetic wave propagation characteristics such as reflection, diffraction, scattering, and penetration to detect, locate, and image hidden targets in occluded environments. It holds significant potential for applications in autonomous driving, disaster rescue, and urban warfare. However, uncertainties introduced by reflecting surfaces and occluding objects in practical NLOS scenarios, such as phase errors, aperture shadowing, and multipath effects, lead to issues like blurred imaging and increased artifacts in radar imaging. To address these challenges, this study proposes a 3D learning imaging method for NLOS THz radar based on a holographic imaging operator, leveraging the adaptive optimization properties of deep unfolding networks and prior environmental perception. First, a 3D imaging model for NLOS THz radar in the Looking Around Corner (LAC) scenario is established. A holographic imaging operator is introduced to enhance imaging efficiency and reduce computational complexity. Second, a high-precision NLOS 3D imaging network is constructed based on the Fast Iterative Shrinkage/Thresholding Algorithm (FISTA) framework. Utilizing features specific to NLOS scenes and designing algorithm parameters as functions of network weights, the method achieves high-precision and high-efficiency in the 3D reconstruction of NLOS targets. Finally, a near-field NLOS radar imaging platform operating at 121 GHz (within the sub-THz regime) is developed. Experimental validations in the LAC scenario are performed on targets, including metal letters “E”, a metal resolution chart, and a pair of scissors. The results demonstrate that the proposed method significantly improves 3D imaging precision, achieving a two-orders-of-magnitude increase in computational speed over traditional imaging algorithms. Full article

(This article belongs to the Special Issue Recent Progress in Terahertz Radar Imaging)

14 pages, 2996 KB

Open AccessArticle

Viewing Angle Expansion for Light Field Displays by Crosstalk Suppression with Thin Directional Mesh

by Hsin You Hou and Cheng-Huan Chen

Photonics 2026, 13(5), 439; https://doi.org/10.3390/photonics13050439 - 30 Apr 2026

Abstract

Light field displays offer promising autostereoscopic 3D visualization with continuous parallax, full-color reproduction, and natural depth cues. In practice, ray-tracing simulation is often employed to evaluate the effective viewing zone of light field systems. However, the actual viewing zone in conventional light field [...] Read more.

Light field displays offer promising autostereoscopic 3D visualization with continuous parallax, full-color reproduction, and natural depth cues. In practice, ray-tracing simulation is often employed to evaluate the effective viewing zone of light field systems. However, the actual viewing zone in conventional light field systems is significantly smaller than the simulated value, severely limited by narrow viewing angles due to crosstalk from adjacent elemental images. This study proposes an isolated microlens array (i-MLA) architecture incorporating a custom directional mesh (DM)—a 3D-printed light-blocking barrier grid with tapered pitch—to physically isolate each lenslet and completely suppress crosstalk. Combined with optimized extended coding pitch for a target viewing distance, ray-tracing simulations and experiments on a 13.3-inch 4K display with a 7 mm and 5.4 mm pitch MLA demonstrate dramatic improvement. The conventional light field system provides only a 3.4° margin, which is below the minimum angular separation required for binocular viewing, whereas the i-MLA system achieves a 7.4° margin—twice that of the conventional system. Compared with conventional systems, the i-MLA architecture does not increase overall volume; it simply replaces the glass gap with a single, simple optical element to achieve a wider viewing angle while preserving the compact form factor. Full article

(This article belongs to the Special Issue Recent Advances in Holography and 3D Display)

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11 pages, 2276 KB

Open AccessArticle

Synchronous Mark Design Based on Collinear Holographic Data Storage System to Improve Reconstruction Efficiency

by Ruying Xiong, Lin Peng, Xu Zheng, Junhui Wu, Hongjie Liu and Xiaodi Tan

Photonics 2026, 13(5), 438; https://doi.org/10.3390/photonics13050438 - 29 Apr 2026

Abstract

A collinear holographic data storage system stores two-dimensional information in the three-dimensional spatial domain of the medium, offering features such as high speed, high density, and long lifespan, making it a promising technology for the future of data storage. However, a collinear holographic [...] Read more.

A collinear holographic data storage system stores two-dimensional information in the three-dimensional spatial domain of the medium, offering features such as high speed, high density, and long lifespan, making it a promising technology for the future of data storage. However, a collinear holographic data storage system is limited by the alignment error of the optical system and is also sensitive to environmental noise and external interference, which increases the reading error. When recording and reading holographic storage materials, synchronous marks are used for positioning to correct data misalignment. Therefore, optimizing synchronous mark design of data pages is crucial for improving storage stability and reading accuracy. In this paper, we propose a star-shaped synchronous mark to replace the square-shaped synchronous mark, which improves the holographic grating coupling efficiency. Experimental results show that this method enhances reconstruction strength and reduces reading errors caused by external factors. The star-shaped synchronous mark achieves a better spectral match with the reference pattern, yielding a stronger diffracted signal. Experimental results show that this method reduces the bit error rate by approximately 25% compared to square-shaped synchronous marks under displacement multiplexing. Full article

13 pages, 1744 KB

Open AccessArticle

Direct μJ-Level Femtosecond Laser Welding of Fused Silica to Titanium Foil Without Interlayer

by Haisong Tang, Shuang Liu, Huan Zhan, Guanghua Cheng and Wei Zhang

Photonics 2026, 13(5), 437; https://doi.org/10.3390/photonics13050437 - 29 Apr 2026

Abstract

Direct welding of fused silica to pure titanium (Ti) foil using conventional methods faces significant challenges, such as poor interfacial wettability, insufficient joint strength, and the need for interlayers or surface pretreatments. Existing femtosecond (fs) laser welding techniques for these materials often require [...] Read more.

Direct welding of fused silica to pure titanium (Ti) foil using conventional methods faces significant challenges, such as poor interfacial wettability, insufficient joint strength, and the need for interlayers or surface pretreatments. Existing femtosecond (fs) laser welding techniques for these materials often require high-energy millijoule (mJ)-level pulses or alloy interlayers. Moreover, reports on direct microjoule (μJ)-level fs laser welding of Ti foil to fused silica remain scarce. This study successfully demonstrates a direct welding process for pure Ti foil and fused silica using μJ-level fs laser pulses under ambient conditions, achieving joints with a maximum shear strength of 9.19 MPa. Microstructural analysis revealed an elemental interdiffusion region at the weld interface, supported by mechanical interlocking effects. X-ray photoelectron spectroscopy (XPS) confirmed the occurrence of interfacial chemical reactions, forming titanium silicide (TiSi₂) and titanium oxide (TiO₂). Additionally, a 24 h water immersion test of a square sealed cavity revealed outstanding hermeticity, with no water ingress. This work provides a simple, efficient, and robust solution for high-strength, additive-free bonding of fused silica to Ti foil under low-energy processing conditions. Full article

(This article belongs to the Special Issue Direct Ultrafast Laser Writing in Photonics and Optoelectronics)

23 pages, 3391 KB

Open AccessArticle

Simulating Cascaded Harmonics Generation up to the Sixth Order in β-BBO

by Jozsef Seres, Enikoe Seres and Thorsten Schumm

Photonics 2026, 13(5), 436; https://doi.org/10.3390/photonics13050436 - 29 Apr 2026

Abstract

We simulate the generation of multiple harmonics up to the sixth order extending into vacuum ultraviolet. The harmonics are generated by χ⁽ⁿ⁾:χ^(m) cascades, containing second- or third-order perturbative nonlinear processes. We identify three additional phase-matching conditions beyond standard phase matching, [...] Read more.

We simulate the generation of multiple harmonics up to the sixth order extending into vacuum ultraviolet. The harmonics are generated by χ⁽ⁿ⁾:χ^(m) cascades, containing second- or third-order perturbative nonlinear processes. We identify three additional phase-matching conditions beyond standard phase matching, namely when only the first step or only the second step of the cascades are phase-matched and when the non-phase-matched second or third harmonic produces quasi-phase matching for higher-order harmonics, causing an essential enhancement of the harmonic signals. Full article

(This article belongs to the Special Issue Ultrafast Optics: From Fundamental Science to Applications)

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16 pages, 6198 KB

Open AccessArticle

Characterizing Optical Absorption in Fiber-Structured Media: Integrating Sphere Experiments Coupled with Anisotropic Light-Propagation Monte Carlo Models

by Levin Stolz, Alwin Kienle and Florian Foschum

Photonics 2026, 13(5), 435; https://doi.org/10.3390/photonics13050435 - 28 Apr 2026

Abstract

Accurate determination of the optical absorption coefficient,

μ_{a}

, in turbid media is fundamental to biomedical optics and material characterization. Integrating sphere techniques, which measure total transmittance and reflectance, are a standard method for this purpose. However, the inverse models typically employed [...] Read more.

Accurate determination of the optical absorption coefficient,

μ_{a}

, in turbid media is fundamental to biomedical optics and material characterization. Integrating sphere techniques, which measure total transmittance and reflectance, are a standard method for this purpose. However, the inverse models typically employed rely on the assumption of isotropic light propagation. In fiber-structured materials—a common geometry in biological tissue–this assumption often breaks down, leading to significant quantification errors. In this study, we investigated this effect using Monte Carlo simulations and proof-of-concept experiments on mechanically stretched PTFE tape. The medium was modeled as a slab of aligned dielectric cylinders embedded in an isotropic matrix, and the performance of an isotropic inverse model was compared with that of an anisotropic inverse model. The isotropic model showed substantial systematic errors in

μ_{a}

, with a mean absolute error (MAE) of 19.3%, typical errors between approximately

- 40 %

and 50%, and outliers reaching up to 300%. In contrast, the matched anisotropic model achieved a MAE of 1.2%. Even when the structural parameters of the anisotropic model were perturbed, the MAE remained low at 1.8% for moderate perturbations and 3.9% for severe perturbations. The simulation results therefore indicate that, for the integrating sphere framework considered here, incorporating anisotropic light propagation can improve absorption retrieval more strongly than precise knowledge of all geometric details. Measurements on stretched PTFE tape showed the same qualitative trend and provide proof-of-concept experimental support for the simulation-based findings. Full article

(This article belongs to the Special Issue Emerging Trends in Optical Imaging, Sensing and Wireless Communication Through Random Scattering Media)

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27 pages, 19846 KB

Open AccessReview

Versatile Meta-Devices: Principles and Applications

by Hong Zhang, Zhangyi Du, Yitian Zuo, Yajie Huang, Jinkang Wu, Zhinuo Chen, Yifei Gao, Junbao Hu and Yu Lei

Photonics 2026, 13(5), 434; https://doi.org/10.3390/photonics13050434 - 28 Apr 2026

Abstract

Precise sculpturing of light empowers light with abundant phenomena across fundamental physics and practical applications. The emergence of metasurfaces provides a pivotal solution to the limitations of traditional optical components, which make it difficult to meet the integration requirements of diverse applications, and [...] Read more.

Precise sculpturing of light empowers light with abundant phenomena across fundamental physics and practical applications. The emergence of metasurfaces provides a pivotal solution to the limitations of traditional optical components, which make it difficult to meet the integration requirements of diverse applications, and they are distinguished by their ultra-thin profiles, low optical losses, and high degree of controllability. In this paper, we elucidate the core physical principles to manipulate phase, amplitude, and polarization with meta-optic architecture, along with nonlocal effects. Specifically, we revisit the research progress and typical applications of meta-waveguides, meta-fibers, meta-lasers, meta-spectrometers, and meta-sensing. Finally, it looks forward to the future development direction of meta-optics in exploring the limits of light field control, chip-scale functional integration, and discovering new physical effects, providing theoretical and technical references for the development of metaphotonic devices. Full article

(This article belongs to the Special Issue Metasurfaces and Meta-Devices: From Fundamentals to Applications)

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20 pages, 3472 KB

Open AccessArticle

All-Chalcogenide High-NA Broadband Achromatic Metalens for Long-Wavelength Infrared Regime

by Minsi Lin, Zhenqi Huang, Yue Shen, Haobin Xiao, Yingying Fu, Mingjie Zhang, Yuanzhi Chen, Yi Zhou, Siqi Zhu and Zhenqiang Chen

Photonics 2026, 13(5), 433; https://doi.org/10.3390/photonics13050433 - 28 Apr 2026

Abstract

The long-wave infrared band, which at room temperature covers the infrared radiation of humans and objects, has significant applications across various fields including wireless communication, national defense, military, biomedical, and advanced driver assistance systems. Metalens provides a pathway to lightweight, compact, and integrated [...] Read more.

The long-wave infrared band, which at room temperature covers the infrared radiation of humans and objects, has significant applications across various fields including wireless communication, national defense, military, biomedical, and advanced driver assistance systems. Metalens provides a pathway to lightweight, compact, and integrated solutions for infrared imaging and sensing systems, marking an inevitable trend in future development. This study presents a design for a high numerical aperture of 0.89 in a polarization-insensitive all-chalcogenide metalens operating at 10 µm, utilizing the commercially available chalcogenide glass material As₂Se₃ via a transmission phase approach. Building upon this, we have achieved, for the first time, a high numerical aperture of 0.84 for an all-chalcogenide broadband LWIR achromatic metalens operating in the 9.5–10.5 µm range, with significantly improved focusing performance through the application of particle swarm optimization algorithms. The superior performance of the all-chalcogenide LWIR metalens, combined with the advantages of chalcogenide glass over traditional LWIR materials such as Si or Ge—namely, lower cost, reduced optical loss, and a smaller thermo-optic coefficient—suggests it has significant potential for broader applications. Full article

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35 pages, 2859 KB

Open AccessArticle

Laser Linewidth Effects in Continuous-Variable QKD: Simulation-Based Analysis and Optimization Guidelines for Defense-Grade Secure System

by Seyed Saman Mahjour and Fernando M. Araújo-Moreira

Photonics 2026, 13(5), 432; https://doi.org/10.3390/photonics13050432 - 27 Apr 2026

Abstract

Continuous-Variable Quantum Key Distribution (CV-QKD) offers practical advantages for secure communication, but laser linewidth-induced phase noise remains a critical performance limitation. This work presents a comprehensive simulation-based analysis quantifying the impact of laser linewidth on secret key rate (SKR) in Gaussian-modulated coherent-state CV-QKD [...] Read more.

Continuous-Variable Quantum Key Distribution (CV-QKD) offers practical advantages for secure communication, but laser linewidth-induced phase noise remains a critical performance limitation. This work presents a comprehensive simulation-based analysis quantifying the impact of laser linewidth on secret key rate (SKR) in Gaussian-modulated coherent-state CV-QKD systems. We develop a detailed noise model incorporating detector electronics, Raman scattering, phase recovery, ADC quantization, and laser relative intensity noise. Through systematic parameter sweeps spanning linewidths from 10 Hz to 250 kHz, modulation variances from 1 to 20 SNU, and fiber distances up to 100 km, we identify three distinct operational regimes and optimization strategies for both transmitted local oscillator (TLO) and local–local oscillator (LLO) configurations under homodyne and heterodyne detection. Results show that metropolitan-scale links (50 km) require linewidths below 5 kHz to maintain secure operation, with performance decreasing beyond 25 kHz. We demonstrate that modulation variance must be jointly optimized with laser quality, with optimal values decreasing from 3–4 SNU at narrow linewidths to 2–2.5 SNU at moderate linewidths. The analysis reveals asymmetric sensitivity in LLO systems where local oscillator linewidth degrades performance more strongly than signal laser linewidth. These quantitative findings provide practical design guidelines for achieving secure CV-QKD operation over metropolitan distances with realistic hardware constraints, supporting deployment of defense-grade quantum communication networks. Full article

(This article belongs to the Special Issue Quantum Optics: Communication, Sensing, Computing, and Simulation)

29 pages, 7360 KB

Open AccessReview

Memristors for the Post-Von Neumann Era: Hardware Paradigms, Neuromorphic Perception, and Computing Systems

by Kerui Fu and Tianling Qin

Photonics 2026, 13(5), 431; https://doi.org/10.3390/photonics13050431 - 27 Apr 2026

Abstract

Memristors, as transformative electronic devices designed to transcend the von Neumann architecture, enable the physical unification of information storage and computation, thereby offering a foundational hardware pathway toward energy-efficient, brain-inspired computing. Their intrinsic analog resistive switching, non-volatility, and history-dependent learning capabilities allow them [...] Read more.

Memristors, as transformative electronic devices designed to transcend the von Neumann architecture, enable the physical unification of information storage and computation, thereby offering a foundational hardware pathway toward energy-efficient, brain-inspired computing. Their intrinsic analog resistive switching, non-volatility, and history-dependent learning capabilities allow them to natively implement in-memory computing and emulate synaptic plasticity, addressing the critical bottlenecks of energy and speed in conventional systems. Notably, the evolution from electrically controlled memristors to optoelectronic memristors marks a paradigm shift from pure computing to integrated sensing-processing, opening new dimensions for high-speed, parallel, and adaptive signal processing. In recent years, significant progress has been made in the development of memristor-based neuromorphic vision and tactile systems, on-chip signal processors, and dynamic trajectory trackers, demonstrating their potential in edge intelligence, adaptive robotics, and real-time perceptual tasks. This review systematically summarizes the latest advances in memristor technology, providing a comprehensive analysis of their operating mechanisms, material and structural innovations, and cutting-edge applications in neuromorphic perception and computing. Furthermore, it discusses the key challenges and future directions for the development and integration of memristor-based systems in the post-von Neumann era. Full article

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13 pages, 7521 KB

Open AccessArticle

Design and Diffraction Efficiency Analysis of Field-of-View Deflectors Using Self-Achromatic Grism

by Na Xie, Jingyi Fu, Yunan Wu, Bingqing Xie, Ning Ma and Jun Chang

Photonics 2026, 13(5), 430; https://doi.org/10.3390/photonics13050430 - 27 Apr 2026

Abstract

Field-deflection optical elements play a significant role in various high-resolution and field-of-view (FOV) expandable optical systems. In our previous work, we proposed a self-achromatic double-faced grism (DFG) configuration capable of field deflection. In this paper, to effectively guide the design and fabrication of [...] Read more.

Field-deflection optical elements play a significant role in various high-resolution and field-of-view (FOV) expandable optical systems. In our previous work, we proposed a self-achromatic double-faced grism (DFG) configuration capable of field deflection. In this paper, to effectively guide the design and fabrication of DFG diffractive microstructures, we analyze the diffraction process of light within the DFG based on scalar diffraction theory, establish a diffraction efficiency model for the DFG, and propose an optimization method to improve diffraction efficiency over all FOVs while minimizing diffraction efficiency variations across the waveband. Following this design approach, two DFGs for 3–5 μm were specifically designed: one for a 4.4° FOV with 1.6° deflection, and the other for a 40° FOV with 15° deflection. Further optimization increased the bandwidth-integrated average diffraction efficiency (BIADE) to above 0.93 and reduced the diffraction efficiency variations across the waveband by 23.5% and 48%. And based on a tolerance analysis model, we performed tolerance analysis of the designed DFGs. The BIADE of both elements across all FOVs can exceed 0.86 after adding fabrication errors, maintaining a high diffraction efficiency. The results demonstrate that this method can effectively guide the design and manufacturing of diffractive microstructures. Full article

(This article belongs to the Special Issue Diffractive Optics: From Fundamentals to Applications)

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36 pages, 9864 KB

Open AccessArticle

Orchard-YOLO: A Robust Deep Learning Framework for Fruit Detection Complex Optical and Environmental Degradation

by Yichen Wang, Hongjun Tian, Yuhan Zhou, Yang Xiong, Yichen Li, Manlin Wang, Yijie Yin, Xiaoyin Guo, Jiani Wu, Jiesen Zhang, Ying Tang and Shuai Huang

Photonics 2026, 13(5), 429; https://doi.org/10.3390/photonics13050429 - 27 Apr 2026

Abstract

Accurate target perception in unstructured outdoor environments remains a fundamental challenge in computational imaging and machine vision, primarily due to severe optical degradation caused by variable illumination, specular highlights, and dense foliage occlusion. Existing optical sensing systems often struggle to maintain robustness under [...] Read more.

Accurate target perception in unstructured outdoor environments remains a fundamental challenge in computational imaging and machine vision, primarily due to severe optical degradation caused by variable illumination, specular highlights, and dense foliage occlusion. Existing optical sensing systems often struggle to maintain robustness under these physical constraints, especially when deployed on edge devices with strict computational limits. To address these challenges, this paper proposes Orchard-YOLO, a lightweight, computationally efficient object detection network designed to maintain robustness against environmental and optical noise in complex orchard environments. Unlike generic architectures, Orchard-YOLO introduces three architectural enhancements for robust detection: (1) a High-Resolution P2 Detection Head to preserve high-frequency optical details and fine-grained texture cues often lost during digital downsampling; (2) Coordinate Attention (CA) mechanisms integrated into the feature fusion pathway to filter out background optical interference and enhance spatial discrimination for heavily occluded targets; and (3) a Ghost-convolution-based backbone to optimize the inference pipeline for real-time edge processing. Evaluated on a comprehensive multi-fruit dataset under simulated optical stress (including ±50% illumination variation and up to 70% occlusion), Orchard-YOLO achieves 94.8% mAP@0.5. It shows improved robustness under illumination variation and occlusion compared to baseline models, while achieving up to 25 FPS on an NVIDIA Jetson Nano edge device. These results suggest that Orchard-YOLO offers a detection framework suitable for resource-constrained orchard perception. Full article

(This article belongs to the Special Issue Computational Imaging: Photonics and Optical Applications)

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21 pages, 1866 KB

Open AccessArticle

Mixed-Scene Holographic 3D Display for Film and Television Visual Content Presentation: Zero-Order-Suppressed Single-Hologram Fusion and Parallax-Preserving Digital Resizing

by Pengfei Huang and Tao Wang

Photonics 2026, 13(5), 428; https://doi.org/10.3390/photonics13050428 - 27 Apr 2026

Abstract

Mixed-scene holographic 3D display for film and television visual content presentation remains challenging because recorded digital holograms and computer-generated holograms (CGHs) are produced under different numerical and hardware constraints. Direct hologram superposition typically causes strong zero-order interference, diffraction efficiency degradation, and sampling pitch [...] Read more.

Mixed-scene holographic 3D display for film and television visual content presentation remains challenging because recorded digital holograms and computer-generated holograms (CGHs) are produced under different numerical and hardware constraints. Direct hologram superposition typically causes strong zero-order interference, diffraction efficiency degradation, and sampling pitch mismatch between the recording sensor and the replay panel, while conventional resizing reduces the effective replay aperture and narrows the available parallax. To address these issues, this paper proposes a zero-order-suppressed single-hologram fusion framework with parallax-preserving digital resizing. A recorded digital hologram is first processed by Gaussian high-pass filtering to suppress the dominant zero-order component, then resampled to match the LCOS replay pitch, and finally normalized and fused with a CGH generated through bipolar intensity encoding. On this basis, two resizing routes are developed: a spatial-domain method for aperture-preserving whole-scene scaling and a frequency-domain method for object-selective scaling and translation. Optical validation on a three-channel LCOS prototype shows that the quantitative diffraction efficiency analysis predicts an increase from approximately 10.1% to 20.05% per reconstructed object for the two-hologram fusion case, and the revised experimental results are consistent with this improvement trend. The experiments further verify replay scaling at multiple factors, the selective manipulation of physical and virtual objects, mixed-scene color replay, and occlusion-consistent depth ordering. Together with the distortion analysis, these results demonstrate improved replay visibility after fusion while maintaining geometric controllability and effective replay aperture. By relying on hologram-domain preprocessing and resizing rather than full mixed-scene recomputation, the proposed method also reduces computational burden. The study therefore provides an efficient and controllable mixed-scene holographic replay framework for visually enriched film and television content presentation, although its depth applicability remains bounded and dedicated real-time timing benchmarks are left for future work. Full article

(This article belongs to the Special Issue Recent Advances in Holography and 3D Display)

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16 pages, 6518 KB

Open AccessArticle

Optimization of a Range Walk Error Correction for Underwater Photon Counting LiDAR Under Low-Photon Conditions

by Zunhui Wang, Yicheng Wang, Qingli Ma and Yanhua Wu

Photonics 2026, 13(5), 427; https://doi.org/10.3390/photonics13050427 - 27 Apr 2026

Abstract

Underwater gated time-correlated single-photon-counting (TCSPC) LiDAR is advantageous when weak target echoes coexist with strong backscatter. However, under the first-photon-triggering and SPAD dead-time mechanism, the estimated time of flight becomes dependent on the return strength, thereby producing a range walk error (RWE). This [...] Read more.

Underwater gated time-correlated single-photon-counting (TCSPC) LiDAR is advantageous when weak target echoes coexist with strong backscatter. However, under the first-photon-triggering and SPAD dead-time mechanism, the estimated time of flight becomes dependent on the return strength, thereby producing a range walk error (RWE). This paper develops a condition-calibrated correction framework for accumulated-histogram underwater ranging in the low-photon regime. A non-homogeneous Poisson first-arrival model that jointly includes gate-limited signal photons and in-gate background triggering yields a computable expression for the total trigger probability and the conditional first-arrival time. A first-order expansion around

N_{p e} \approx 0

leads to an approximately linear RWE–

N_{p e}

relation under the present system–water condition. A density-based signal-window localization method and a noise-occlusion-compensated estimator of

N_{p e}

are combined with reference-plane differential calibration. Experiments in a 10 m clear-freshwater tank at 9.11 m show that the mean absolute error is reduced from 39.205 mm to 2.130 mm, corresponding to a 94.57% improvement. Compared with a quadratic model used under higher-photon conditions, the proposed linear model yields an order-of-magnitude smaller residual error in the low-photon region (

N_{p e} < 1.6

). Full article

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11 pages, 2813 KB

Open AccessArticle

Realization of Laser Frequency Stabilization and Continuous Broadband Tuning via Sideband PDH Locking

by Zhuxiong Ye, Shu Liu, Mingkang Han, Jia Feng, Mustafa Shah, Yongze Zhao, Pengjun Wang, Liangchao Chen, Wei Han, Zengming Meng and Lianghui Huang

Photonics 2026, 13(5), 426; https://doi.org/10.3390/photonics13050426 - 26 Apr 2026

Abstract

We demonstrate a sideband Pound–Drever–Hall (SPDH) locking scheme that enables the simultaneous narrow-linewidth stabilization and continuous broadband frequency tuning of a laser referenced to an ultra-stable cavity. The method employs dual-frequency modulation applied to a fiber electro-optic modulator, where high-frequency modulation generates tunable [...] Read more.

We demonstrate a sideband Pound–Drever–Hall (SPDH) locking scheme that enables the simultaneous narrow-linewidth stabilization and continuous broadband frequency tuning of a laser referenced to an ultra-stable cavity. The method employs dual-frequency modulation applied to a fiber electro-optic modulator, where high-frequency modulation generates tunable sidebands and low-frequency modulation provides the error signal. We experimentally stabilize a 922 nm seed laser to the cavity and achieve a laser linewidth of 85(1) kHz with frequency noise suppression of up to 25 dB. The residual amplitude modulation (RAM) remains below 0.08% across the full tuning range. In addition, we demonstrate a continuous frequency tuning range of 1.4 GHz for a frequency-doubled 461 nm laser, with scan rates up to 317 MHz/s, while preserving stable locking to the cavity. This approach avoids complex waveform generation and provides a simple and robust solution for broadband laser frequency control. Full article

(This article belongs to the Special Issue Advanced Research in Quantum Optics)

35 pages, 2102 KB

Open AccessReview

A Review of the Structure of Free-Space Optical Channel Models: Physical Meaning, Assumptions, and Atmospheric Conditions

by Sabai Phuchortham and Hakilo Sabit

Photonics 2026, 13(5), 425; https://doi.org/10.3390/photonics13050425 - 26 Apr 2026

Abstract

Free-space optical (FSO) communication is an attractive high-capacity wireless technology for terrestrial, aerial, and satellite links. However, FSO performance is strongly affected by multiple impairments, including path loss, turbulence attenuation, pointing errors, and equipment loss. Therefore, accurate performance evaluation requires channel modelling that [...] Read more.

Free-space optical (FSO) communication is an attractive high-capacity wireless technology for terrestrial, aerial, and satellite links. However, FSO performance is strongly affected by multiple impairments, including path loss, turbulence attenuation, pointing errors, and equipment loss. Therefore, accurate performance evaluation requires channel modelling that accounts for both deterministic power losses and stochastic channel effects. This paper presents a comprehensive and structured review of FSO channel modelling, covering the transmission, propagation medium, and receiver sections. The composite channel response is represented using a mathematical formulation. Commonly used FSO models are reviewed and organised, including Beer–Lambert and geometrical loss, Kim and Kruse path loss models, Lognormal, Gamma–Gamma, K, and Málaga distributions, along with pointing-error and angle-of-arrival models. Each model is explained in terms of its physical meaning, assumptions, and applicable operating conditions. Lastly, a numerical example is presented to demonstrate how deterministic losses and stochastic channel effects can be combined in FSO performance evaluation. Full article

(This article belongs to the Special Issue Advanced Technologies in Optical Wireless Communications—2nd Edition)

16 pages, 7257 KB

Open AccessArticle

Enhanced Thermal Stability in Compact ASE Sources Enabled by Optimized Erbium-Doped Fiber Design

by Jianming Liu, Wenbin Lin, Wei Liu, Jinjuan Cheng, Chengcheng He, Wei Xu and Jia Guo

Photonics 2026, 13(5), 424; https://doi.org/10.3390/photonics13050424 (registering DOI) - 24 Apr 2026

Abstract

Amplified Spontaneous Emission (ASE) sources are widely employed as highly stable broadband sources in fields such as high-precision navigation and optical detection. Erbium-doped fiber (EDF), as the core active component in ASE sources, has long been a key subject of thermal stability research. [...] Read more.

Amplified Spontaneous Emission (ASE) sources are widely employed as highly stable broadband sources in fields such as high-precision navigation and optical detection. Erbium-doped fiber (EDF), as the core active component in ASE sources, has long been a key subject of thermal stability research. We fabricated a low-doped EDF with an 80 μm-cladding using the vapor phase doping (VPD) technique. This EDF was compared with a commercial 125 μm-cladding EDF using a double-pass forward (DPF) optical path configuration with a narrowband filter. We investigated the temperature-dependent characteristics of the ASE spectra generated by the two EDFs with different parameters. The temperature drift performance of the two EDFs was analyzed based on three critical indicators of the spectrum: mean wavelength, spectral bandwidth, and output power. In comparison with the commonly used EDF, the results show that a properly designed small-cladding EDF with an appropriate length can deliver higher ASE output power and exhibit a lower mean-wavelength temperature drift. This study provides an important guideline for promoting the miniaturization of high-precision fiber-optic sensing devices. Full article

(This article belongs to the Special Issue Advancements in Ultrafast Laser Science and Technology)

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23 pages, 7805 KB

Open AccessArticle

Mie-Scattering-Based Simulation of Underwater Multispectral LiDAR Propagation and Optimal Wavelength Selection

by Zhichao Chen, Zhaoyan Liu, Shi Qiu, Huijing Zhang, Yuwei Chen, Weiyuan Yao, Tong Zhang, Yu Zhang, Hongjia Cheng, Feihong Wang and Zhan Shu

Photonics 2026, 13(5), 423; https://doi.org/10.3390/photonics13050423 (registering DOI) - 24 Apr 2026

Abstract

Multispectral LiDAR can simultaneously obtain distance and spectral information and shows great potential for underwater detection. However, absorption and scattering caused by suspended particles in water lead to energy attenuation and multiple scattering, which affect echo intensity and ranging accuracy, while the propagation [...] Read more.

Multispectral LiDAR can simultaneously obtain distance and spectral information and shows great potential for underwater detection. However, absorption and scattering caused by suspended particles in water lead to energy attenuation and multiple scattering, which affect echo intensity and ranging accuracy, while the propagation characteristics under multi-wavelength conditions remain insufficiently studied. In this study, a simplified underwater propagation simulation model for multispectral LiDAR is established based on the equivalent spherical-particle assumption, combining Mie scattering theory with a semi-analytical Monte Carlo method. The effects of particle size on echo intensity and ranging error are analyzed under fixed concentration conditions. Based on this model, a detection-threshold-constrained optimal wavelength selection criterion is formulated. Multi-distance analysis (3, 5, 8, and 15 m) confirms that the preferred wavelength is primarily governed by particle size and remains stable across depths. The results show that the optimal detection wavelength shifts with particle size, being about 560 nm for fine particles and gradually moving toward the 400–480 nm blue–green band for larger particles. Experimental validation shows that the simulation-based ranging correction reduces RMSE by 9.4–25.9% (average 18.1%) and MAE by 11.8–29.7% (average 22.0%) across five experimental distances. The results provide a preliminary reference for wavelength selection in multispectral LiDAR systems under simplified conditions. Full article

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