Photonics

21 pages, 2975 KB

Open AccessArticle

Misalignment-Induced Aberration Compensation for Off-Axis Reflective Telescopes Based on Fusion of Spot Images and Zernike Coefficients

by Wei Tang, Yujia Liu, Weihua Tang, Jie Fu, Siheng Tian and Yongmei Huang

Photonics 2026, 13(2), 212; https://doi.org/10.3390/photonics13020212 - 23 Feb 2026

Viewed by 485

Abstract

Off-axis reflective telescopes are prone to component misalignment due to external environmental factors and mechanical vibrations. This misalignment introduces low-order aberrations, which severely degrade imaging quality. Thus, active misalignment correction is crucial for maintaining the imaging performance of off-axis reflective telescopes. Current computer-aided [...] Read more.

Off-axis reflective telescopes are prone to component misalignment due to external environmental factors and mechanical vibrations. This misalignment introduces low-order aberrations, which severely degrade imaging quality. Thus, active misalignment correction is crucial for maintaining the imaging performance of off-axis reflective telescopes. Current computer-aided alignment technologies for optical systems mostly rely on wavefront sensors to acquire aberrations at multiple fixed fields of view (FOVs) or even the full FOV. This significantly increases system complexity and hinders practical engineering applications. To address this issue, this study first conducts sensitivity analysis of misaligned degrees of freedom (DOFs) using a mode truncation algorithm based on singular value decomposition (SVD). A compensation strategy is proposed to avoid the aberration coupling effect. Furthermore, two novel misalignment aberration compensation methods for off-axis reflective telescopes are presented. These methods require only a single focal spot image and eliminate the need for aberration detection and iterative calculations. One method directly solves component misalignment errors using a convolutional neural network (CNN) based on the system’s point spread function (PSF). To further improve compensation performance, an improved method fusing spot images and Zernike coefficients is proposed. In practical misalignment correction, both methods input a single acquired focal spot image into a well-trained model to obtain the misalignment compensation amount. Simulation experiments demonstrate that the improved method, which uses Zernike polynomial coefficients as an intermediate feature bridge, effectively establishes the mapping relationship between spot images and misalignment amounts. It achieves higher solution accuracy and better aberration compensation effect compared to the direct CNN method. This verifies the necessity of extracting Zernike polynomial coefficient features from spot images. Comparative experiments with the traditional sensitivity matrix method show that the two proposed methods outperform the sensitivity matrix method in aberration compensation accuracy over a large misalignment range. Comprehensive simulation results confirm the feasibility and effectiveness of the proposed methods. They overcome the limitations of existing methods, such as complex structure, high cost, and low efficiency, to a certain extent. Full article

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25 pages, 896 KB

Open AccessArticle

Sequential Deep Learning with Feature Compression and Optimal State Estimation for Indoor Visible Light Positioning

by Negasa Berhanu Fite, Getachew Mamo Wegari and Heidi Steendam

Photonics 2026, 13(2), 211; https://doi.org/10.3390/photonics13020211 - 23 Feb 2026

Viewed by 1105

Abstract

Visible Light Positioning (VLP) is widely regarded as a promising technology for high-precision indoor localization due to its immunity to radio-frequency interference and compatibility with existing Light-Emitting Diode (LED) lighting infrastructure. Despite recent progress, current VLP systems remain fundamentally limited by nonlinear received [...] Read more.

Visible Light Positioning (VLP) is widely regarded as a promising technology for high-precision indoor localization due to its immunity to radio-frequency interference and compatibility with existing Light-Emitting Diode (LED) lighting infrastructure. Despite recent progress, current VLP systems remain fundamentally limited by nonlinear received signal strength (RSS) characteristics, unknown transmitter orientations, and dynamic indoor disturbances. Existing solutions typically address these challenges in isolation, resulting in limited robustness and scalability. This paper proposes SCENE-VLP (Sequential Deep Learning with Feature Compression and Optimal State Estimation), a structured positioning framework that integrates feature compression, temporal sequence modeling, and probabilistic state refinement within a unified estimation pipeline. Specifically, SCENE-VLP combines Principal Component Analysis (PCA) and Denoising Autoencoders (DAE) for linear and nonlinear observation conditioning, Gated Recurrent Units (GRU) for modeling temporal dependencies in RSS sequences, and Kalman-based filtering (KF/EKF) for recursive state-space refinement. The framework is formulated as a hierarchical approximation of the nonlinear observation model, linking data-driven measurement learning with Bayesian state estimation. A systematic ablation study across multiple scenarios, including same-dataset evaluation and cross-dataset generalization, demonstrates that each component provides complementary benefits. Feature compression reduces redundancy while preserving dominant signal structure; GRU significantly improves robustness over static regression; and recursive filtering consistently reduces positioning error compared to unfiltered predictions. While both KF and EKF improve performance, EKF provides incremental refinement under mild nonlinearities. Extensive simulations conducted on an indoor dataset collected from a realistic deployment with eight ceiling-mounted LEDs and a single photodetector (PD) show that SCENE-VLP achieves sub-decimeter localization accuracy, with P50 and P95 errors of 1.84 cm and 6.52 cm, respectively. Cross-scenario evaluation further confirms stable generalization and statistically consistent improvements. These results demonstrate that the structured integration of observation conditioning, temporal modeling, and Bayesian refinement yields measurable gains beyond partial pipeline configurations, establishing SCENE-VLP as a robust and scalable solution for next-generation indoor visible light positioning systems. Full article

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

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14 pages, 4023 KB

Open AccessArticle

Dual-Resonance Plasmonic Nanocavity with Differential Thermo-Optic Response for Enhanced Fiber-Optic Thermal Flowmeters

by Yekun Cao, Lei Sun, Min Li, Ming-Yu Li, Xiaoyan Wen, Shuo Deng, Sisi Liu, Hongyun Gao, Haifei Lu and Dengzun Yao

Photonics 2026, 13(2), 210; https://doi.org/10.3390/photonics13020210 - 23 Feb 2026

Viewed by 410

Abstract

Optic-fiber-based thermal flowmeters have the merits of compact size and high sensitivity, which typically require two light beams separately acting as a pump for heating the sensing unit and a probe for sensing temperature with the variation of external flow. Here, we propose [...] Read more.

Optic-fiber-based thermal flowmeters have the merits of compact size and high sensitivity, which typically require two light beams separately acting as a pump for heating the sensing unit and a probe for sensing temperature with the variation of external flow. Here, we propose a metallic nanostructure with multiple plasmonic resonance modes for the application of an optic-fiber-based thermal flowmeter. The optical properties of a nanostructure comprised of a double-width gold grating, a poly (methylmethacrylate) (PMMA) layer, and a gold film are numerically simulated in the spectral range of 600–1800 nm. The optical resonances of different modes are systematically investigated with the variation of the structural parameters. Interestingly, two optical resonance modes with distinct spectral shift under the same temperature variation, i.e., 21.34 pm/°C vs. 269.2 pm/°C, are obtained after the strategic optimization of the nanostructure. Finally, the sensitivity of the flowmeter with the proposed nanostructure is investigated by adopting the low-temperature sensitivity mode for optical pumping and the high-temperature sensitivity mode for temperature sensing, proving its significant potential as an optic-fiber-based thermal flowmeter. Full article

(This article belongs to the Special Issue Advances in Optical Sensors and Applications)

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10 pages, 1113 KB

Open AccessArticle

Pump-Enhanced Idler-Resonant 1626 nm Optical Parametric Oscillator

by Yanyan Liu, Chaozhe Hu, Guodong Zhao, Chihua Zhou, Jian Xia, Jie Ren, Wei Tan and Hong Chang

Photonics 2026, 13(2), 209; https://doi.org/10.3390/photonics13020209 - 23 Feb 2026

Viewed by 422

Abstract

The 1626 nm laser is an essential component for conducting superlattice research on the strontium atomic clock platform. The superlattice constructed with the 1626 nm and 813 nm lasers will facilitate cutting-edge quantum information research focused on topological quantum states transport. We demonstrate [...] Read more.

The 1626 nm laser is an essential component for conducting superlattice research on the strontium atomic clock platform. The superlattice constructed with the 1626 nm and 813 nm lasers will facilitate cutting-edge quantum information research focused on topological quantum states transport. We demonstrate an idler-resonant optical parametric oscillator that achieves 1626 nm laser output based on pump enhancement technology. Through a well-designed external cavity, a laser output of 127 mW at 1626 nm has been achieved, with a corresponding pump quantum conversion efficiency of 50% and a pump threshold of 110 mW. The long-term power stability of the output laser is ±1.5% per hour. Variations in the pump cavity modes under different experimental conditions have been measured, and the impedance matching process of the pump light within the cavity has been discussed. The 1626 nm laser and the associated technologies reported in this manuscript will provide optical support for the investigation of superlattice physics on the strontium optical lattice clock platform. Full article

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17 pages, 14773 KB

Open AccessArticle

AI-Based 2D Phase Unwrapping Under Rayleigh-Distributed Speckle Noise and Phase Decorrelation

by Aidan Soal, Juergen Meyer, Bryn Currie and Steven Marsh

Photonics 2026, 13(2), 208; https://doi.org/10.3390/photonics13020208 - 22 Feb 2026

Viewed by 549

Abstract

Phase unwrapping is a critical step in interferometric imaging modalities such as holography and synthetic aperture radar, yet conventional analytical algorithms struggle in low signal-to-noise and high-speckle environments. This study presents an artificial intelligence (AI)-based phase-unwrapping framework using a Pix2Pix conditional generative adversarial [...] Read more.

Phase unwrapping is a critical step in interferometric imaging modalities such as holography and synthetic aperture radar, yet conventional analytical algorithms struggle in low signal-to-noise and high-speckle environments. This study presents an artificial intelligence (AI)-based phase-unwrapping framework using a Pix2Pix conditional generative adversarial network (cGAN). A model was designed for robustness under Rayleigh-distributed speckle noise and phase decorrelation, conditions representative of realistic interferometric measurements. Trained on synthetically generated wrapped–unwrapped phase pairs, the AI approach was compared against established analytical phase-unwrapping methods, a quality-guided unwrapping algorithm (Herraez)and a minimum-norm network-flow optimization method (Costantini). Quantitative evaluation using the root mean square error (RMSE), structural similarity index measure (SSIM), and a composite performance index demonstrated that the cGAN was superior under noisy conditions, successfully recovering phase information beyond its training noise range at

σ = 10

, and accurately unwrapping phases up to

σ = 20

. This was under a pure unwrapping performance analysis, utility performance was also tested comparing all images to clean noiseless phase. The Pix2Pix model also proved resilient to detector artifacts, despite not being explicitly trained on them, and its worst performance yielded RMSE and SSIM values of 0.089 and 0.927, respectively, with perfect values being 0 and 1. The proposed framework simultaneously unwraps and denoises the phase, offering a simple, open-source, and highly adaptable alternative for phase unwrapping in noisy interferometric systems. Future work will focus on extending the framework to experimental datasets. 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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13 pages, 2754 KB

Open AccessArticle

Improved Sensitivity of Brain Cancer Detection Using 2D Photonic Crystal Sensor

by Sarra Bendib, Nadhir Djeffal, Abderrahim Yousfi, Okba Saidani and Abdallah Hedir

Photonics 2026, 13(2), 207; https://doi.org/10.3390/photonics13020207 - 22 Feb 2026

Viewed by 522

Abstract

This study investigates the influence of cavity configuration on the performance of two-dimensional (2D) photonic crystal (PhC) sensors, with particular emphasis on the effect of doubling the number of cavities. A comparative analysis between single-cavity and dual-cavity configurations is conducted to evaluate their [...] Read more.

This study investigates the influence of cavity configuration on the performance of two-dimensional (2D) photonic crystal (PhC) sensors, with particular emphasis on the effect of doubling the number of cavities. A comparative analysis between single-cavity and dual-cavity configurations is conducted to evaluate their impact on key sensing parameters. In the dual-cavity configuration, two resonant cavities are introduced between coupled waveguides, enabling strong optical mode coupling and enhanced electromagnetic field confinement within the sensing region. This coupling leads to sharper resonance peaks, reduced linewidths, and increased interaction between the optical field and the infiltrated analyte. As a result, the dual-cavity sensor exhibits significantly improved sensing performance, achieving a high sensitivity of 9261.54 nm/RIU, a quality factor of 15,352.38, a figure of merit exceeding 4.5 × 10⁷, and a detection limit below 1.7 × 10⁻⁷ RIU. These results demonstrate that doubling the cavity number effectively amplifies light–matter interaction and resonance stability, making the proposed dual-cavity 2D PhC sensor a highly promising platform for precise refractive index sensing in biomedical applications. Full article

(This article belongs to the Section Biophotonics and Biomedical Optics)

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13 pages, 1360 KB

Open AccessArticle

Investigating the Interplay of Absorption and Scattering in Phosphor-Converted LEDs Using a GPU-Accelerated Monte Carlo Framework

by Philip Gelbing, Joachim Jelken, Florian Foschum and Alwin Kienle

Photonics 2026, 13(2), 206; https://doi.org/10.3390/photonics13020206 - 21 Feb 2026

Viewed by 458

Abstract

Optimizing phosphor-converted light-emitting diodes is challenging due to the complex interplay of absorption, elastic scattering and luminescence. Unlike previous studies that focused on characterizing optical parameters, this work isolates their individual contributions in order to derive fundamental design limits. We present a comprehensive [...] Read more.

Optimizing phosphor-converted light-emitting diodes is challenging due to the complex interplay of absorption, elastic scattering and luminescence. Unlike previous studies that focused on characterizing optical parameters, this work isolates their individual contributions in order to derive fundamental design limits. We present a comprehensive analysis using a GPU-accelerated Monte Carlo framework that solves the luminescent radiative transfer equation, including the full luminescence cascade. We systematically investigate the influence of the absorption (

μ_{a}

) and scattering (

μ_{s}

) coefficients by varying them over a range of 0.1 to 10 times the reference values of a standard phosphor (0.8 wt%). We found that transmitted luminescence saturates when absorption exceeds approximately three times the reference value (

μ_{a} \approx 1.2 {mm}^{- 1}

) and peaks at an optimal

μ_{s}

before backscattering losses dominate. In high-concentration regimes, mirror-assisted geometries are shown to enhance backward emission by a factor of 2.1 compared to open boundaries. Our findings provide model-based predictions for luminescence transport in phosphor–polymer composites. Full article

(This article belongs to the Special Issue Advancements in Optical Measurement Techniques and Applications)

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14 pages, 3153 KB

Open AccessArticle

Hybrid Graphene—VO₂ Reconfigurable Terahertz Metamaterial Absorber for Broadband RCS Reduction and High-Performance Sensing

by Kunxuan Su, Yingwen Long and Wenhao Yang

Photonics 2026, 13(2), 205; https://doi.org/10.3390/photonics13020205 - 21 Feb 2026

Viewed by 785

Abstract

A hybrid graphene-VO₂ reconfigurable terahertz metamaterial absorber is proposed for broadband radar cross-section (RCS) reduction and high-performance sensing. The designed structure leverages the phase transition property of VO₂ and the electrostatic tunability of graphene to achieve dynamic switching between ultra-broadband and [...] Read more.

A hybrid graphene-VO₂ reconfigurable terahertz metamaterial absorber is proposed for broadband radar cross-section (RCS) reduction and high-performance sensing. The designed structure leverages the phase transition property of VO₂ and the electrostatic tunability of graphene to achieve dynamic switching between ultra-broadband and narrowband absorption states. When VO₂ is in the metallic state and graphene is unbiased, the absorber exhibits over 90% absorption across 0.82~3.50 THz, corresponding to a relative bandwidth of 124%. In the narrowband mode, with VO₂ in the insulating state and graphene biased (E_f = 1 eV), a sharp absorption peak exceeding 60% is achieved at 1.48 THz. The symmetrical design ensures polarization insensitivity and wide-angle stability. Applications in broadband RCS reduction higher than 10 dB and refractive index sensing with a sensitivity of 24.86 GHz/RIU are demonstrated, surpassing conventional terahertz sensors. This work provides a promising platform for adaptive terahertz stealth and sensing systems. Full article

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15 pages, 2313 KB

Open AccessArticle

The Phenomenon of Focal Shift Induced by Interface Reflection Loss in Microsphere-Assisted Imaging

by Heying Zhang, Cong Zhai, Heming Jia, Yuzhen Guo, Bin Yao, Menghui Xiang, Danfeng Cui, Yongqiu Zheng, Yonghua Wang and Chenyang Xue

Photonics 2026, 13(2), 204; https://doi.org/10.3390/photonics13020204 - 19 Feb 2026

Viewed by 454

Abstract

Microsphere-assisted super-resolution imaging technology, due to its ability to break through the diffraction limit, has become a powerful tool for achieving optical observations at the micro-nano scale. However, there remains a significant discrepancy between the simulation results of microsphere focusing behavior and experimental [...] Read more.

Microsphere-assisted super-resolution imaging technology, due to its ability to break through the diffraction limit, has become a powerful tool for achieving optical observations at the micro-nano scale. However, there remains a significant discrepancy between the simulation results of microsphere focusing behavior and experimental observations in existing studies, necessitating a more precise physical explanation. This study proposes that the interface reflection characteristics are a key factor influencing the focusing behavior of microspheres. We constructed a numerical simulation model based on ray optics theory using MATLAB, explicitly considering the reflection and transmission of light at the microsphere-medium boundary, and systematically analyzed the imaging process and focal position of the microsphere. Experimental results demonstrate that after accounting for energy loss due to reflection, the focal position obtained from the simulation calculations shows a high degree of consistency with the experimental results. The average deviation of our model from experimental results is reduced by 76% compared to conventional paraxial theory and by 86% compared to Finite-Difference Time-Domain (FDTD) simulations. Additionally, the findings validate the reliability of determining microsphere focusing theory using irradiance. Full article

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14 pages, 2367 KB

Open AccessArticle

Efficient Multipartite Energy Transfer Based on Strongly Coupled Topological Cavities

by Jun Ren, Jinhua Li, Ya Liu and Yujing Wang

Photonics 2026, 13(2), 203; https://doi.org/10.3390/photonics13020203 - 19 Feb 2026

Viewed by 356

Abstract

Efficient and robust energy transfer is fundamental to quantum information processing and light-harvesting technologies. However, conventional systems are often limited by short interaction ranges and high susceptibility to environmental disorder. In this study, we propose and theoretically investigate a topologically protected tripartite energy [...] Read more.

Efficient and robust energy transfer is fundamental to quantum information processing and light-harvesting technologies. However, conventional systems are often limited by short interaction ranges and high susceptibility to environmental disorder. In this study, we propose and theoretically investigate a topologically protected tripartite energy transfer system based on photonic crystal nanocavities. By utilizing topological corner states as localized interaction nodes and edge states as robust transmission channels, we construct a platform that mediates energy exchange among three distinct quantum emitters. Using the Lindblad master equation formalism, we analyze the spectral dependence of coupling strengths and transfer dynamics. Our results demonstrate that coherent coupling between nearest neighbors is the dominant mechanism driving high-efficiency transport, whereas next-nearest-neighbor interactions can induce destructive interference. Furthermore, compared to bipartite systems, the tripartite configuration exhibits an enhanced cumulative probability for charge separation. Crucially, numerical simulations confirm that the energy transfer efficiency and time remain virtually unaffected by random structural disorder or sharp interface bends, unequivocally validating the topological protection of the system. These findings establish a robust blueprint for scalable quantum interconnects and integrated photonic circuitry. Full article

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

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19 pages, 4940 KB

Open AccessArticle

Numerical Calculations of Fiber Bragg Gratings with Intensity-Dependent Refractive Index

by Christos Lazakis and Nikolaos A. Stathopoulos

Photonics 2026, 13(2), 202; https://doi.org/10.3390/photonics13020202 - 18 Feb 2026

Viewed by 462

Abstract

Modified discrete transfer matrix and transmission line models were applied to nonlinear refractive index fiber Bragg gratings (FBG). The methods were validated against analytical solutions for Kerr-type uniform FBG, evaluating accuracy, convergence, and computational time. Spectral reflectivity, bistability, index distribution, and group delay [...] Read more.

Modified discrete transfer matrix and transmission line models were applied to nonlinear refractive index fiber Bragg gratings (FBG). The methods were validated against analytical solutions for Kerr-type uniform FBG, evaluating accuracy, convergence, and computational time. Spectral reflectivity, bistability, index distribution, and group delay were computed for various FBG types, with results discussed for each grating, particularly regarding reflectivity and bistability. Full article

(This article belongs to the Special Issue Specialty Optical Fibers: Advances in Design, Fabrication, Performance and Applications)

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13 pages, 5462 KB

Open AccessArticle

Mode Exchange and Phase Jumps at Exceptional Points in Anisotropy-Driven Layered Non-Hermitian Structures

by Milena Ramanovich, Andrey Novitsky and Denis V. Novitsky

Photonics 2026, 13(2), 201; https://doi.org/10.3390/photonics13020201 - 18 Feb 2026

Viewed by 450

Abstract

Exceptional points (EPs) are the most intriguing features of non-Hermitian systems associated with symmetry- and topology-driven applications. In this paper, we study the influence of tunable anisotropy on the topological properties of EPs in

PT

-symmetric layered structures. In particular, the eigenvalues exchange [...] Read more.

Exceptional points (EPs) are the most intriguing features of non-Hermitian systems associated with symmetry- and topology-driven applications. In this paper, we study the influence of tunable anisotropy on the topological properties of EPs in

PT

-symmetric layered structures. In particular, the eigenvalues exchange and eigenmodes exchange are predicted at the points of equal light transmission of different polarizations. We also unveil that the EPs tailored by the anisotropy parameters are associated with the

π

phase jump of reflection coefficients. Indirect evidence for the topological nature of EPs shown here is important for deeper insights into behaviors of anisotropic non-Hermitian systems and can be further used for the development of tunable non-Hermitian sensors and other applications. Full article

(This article belongs to the Section New Applications Enabled by Photonics Technologies and Systems)

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11 pages, 1724 KB

Open AccessArticle

On-Chip Optical Signal Enhancement in Micro-Ring Resonators Using a NaYF₄:Er³⁺-Doped Polymer Nanocomposite

by Zheng Wang, Changlong Li, Guanlin Li, Hengyuan Han, Shaozhi Gu, Fei Wang and Daming Zhang

Photonics 2026, 13(2), 200; https://doi.org/10.3390/photonics13020200 - 18 Feb 2026

Viewed by 465

Abstract

This study develops a micro-ring resonator that provides optical amplification based on NaYF₄:5%Er³⁺ nanoparticles doped with SU-8. By utilizing the frequency selection properties of the micro-ring resonator, a filter with amplification capabilities is successfully developed. The device features a quality [...] Read more.

This study develops a micro-ring resonator that provides optical amplification based on NaYF₄:5%Er³⁺ nanoparticles doped with SU-8. By utilizing the frequency selection properties of the micro-ring resonator, a filter with amplification capabilities is successfully developed. The device features a quality factor of 5.72 × 10⁴ and a free spectral range of 0.081 nm. Operating at an on-chip power of 108 mW, the micro-ring resonator amplifier exhibits a relative gain of 8.92 dB within a size of 2.3 cm × 1.5 cm. To the best of our knowledge, the amplification of optical signals in micro-ring resonators using erbium-doped polymers has not been reported. This technology highlights the significant potential of using erbium-doped materials to fabricate various integrated devices for on-chip optical amplification. Full article

(This article belongs to the Section Optoelectronics and Optical Materials)

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14 pages, 4032 KB

Open AccessArticle

An 850 nm Grating Coupler on Thin-Film Lithium Niobate Enabled by Topological Unidirectional Guided Resonance

by Yuan Fan, Haihua Yu, Hao Yu, Haoran Wang, Yi Zuo and Chao Peng

Photonics 2026, 13(2), 199; https://doi.org/10.3390/photonics13020199 - 17 Feb 2026

Viewed by 850

Abstract

The inherently high-voltage-length product (V_πL) of thin-film lithium niobate (TFLN) modulators in the O-, C-, and L-telecom bands restricts further scaling of photonic integrated circuits’ bandwidth density, driving their migration toward shorter operating wavelengths. Nevertheless, the corresponding grating couplers, [...] Read more.

The inherently high-voltage-length product (V_πL) of thin-film lithium niobate (TFLN) modulators in the O-, C-, and L-telecom bands restricts further scaling of photonic integrated circuits’ bandwidth density, driving their migration toward shorter operating wavelengths. Nevertheless, the corresponding grating couplers, as critical optical input/outputs (optical I/Os) interfaces, remain largely undeveloped. Here, we demonstrate an 850 nm TFLN grating coupler designed based on topological unidirectional guided resonance (UGR). By breaking C₂ symmetry of the unit cell and precisely tailoring its geometry, we achieve unidirectional upward radiation with a 63.7 dB up/down intensity ratio. Subsequent apodization of groove widths and periods enables precise control of the electrical field distribution in both real and momentum spaces. This yields a vertical-cavity surface-emitting laser (VCSEL)-matched, highly fabrication-tolerant TFLN grating coupler that attains, to the best of our knowledge, the highest simulated coupling efficiency of −0.6 dB without mirrors or hybrid materials. This work delivers a high-efficiency, layout-flexible, and complementary metal oxide semiconductor (CMOS)-compatible optical I/Os solution for short-wavelength TFLN modulators with low V_πL. It offers substantial engineering value and broad applicability for on-chip light source integration and high-bandwidth-density short-reach optical interconnects. Full article

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9 pages, 1830 KB

Open AccessCommunication

Adaptive Routing for Meshed QKD Networks of Flexible Size Using Deep Reinforcement Learning

by Tim Johann, Sebastian Kühl and Stephan Pachnicke

Photonics 2026, 13(2), 198; https://doi.org/10.3390/photonics13020198 - 16 Feb 2026

Viewed by 479

Abstract

Quantum Key Distribution (QKD) networks guarantee information-theoretical security of exchanged keys, but key rates are still limited. This makes efficient and adaptive routing a critical challenge, especially in meshed topologies without quantum repeaters. Conventional shortest path routing approaches struggle to cope with dynamic [...] Read more.

Quantum Key Distribution (QKD) networks guarantee information-theoretical security of exchanged keys, but key rates are still limited. This makes efficient and adaptive routing a critical challenge, especially in meshed topologies without quantum repeaters. Conventional shortest path routing approaches struggle to cope with dynamic key store filling levels and changes in network topologies, which leads to load imbalance and blocked connections. In this work, we propose an adaptive routing framework based on Deep Reinforcement Learning (DRL) for hop-wise end-to-end routing in unknown meshed QKD networks. The agent leverages Graph Attention Networks (GATs) to process the network states of varying topologies, enabling generalization across previously unseen meshed networks without topology-specific retraining. The agent is trained on random graphs with 10 to 20 nodes and learns a routing policy that explicitly balances key consumption across the network by utilizing a reward function that is based on the entropy of key store filling levels. We evaluate the proposed approach on the 14-node NSFNET topology under time-varying traffic demands. Simulation results demonstrate that the DRL-based routing significantly outperforms hop-based and weighted shortest path benchmarks, achieving up to a 18.7% increase in mean key store filling levels while completely avoiding key store depletion. These results highlight the potential of graph-based DRL methods for scalable, adaptive, and resource-efficient routing in future QKD networks. Full article

(This article belongs to the Special Issue Machine Learning and Artificial Intelligence for Optical Networks)

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11 pages, 3142 KB

Open AccessArticle

Processing Maps and Nano-IR Diagnostics of Type I Modifications in Mid-IR Germanate-Based Optical Glass

by Paul Mathieu, Nadezhda Shchedrina, Florence De La Barrière, Guillaume Druart and Matthieu Lancry

Photonics 2026, 13(2), 197; https://doi.org/10.3390/photonics13020197 - 16 Feb 2026

Viewed by 577

Abstract

Mid-IR flat/integrated optics require low-loss, programmable phase control. We investigate femtosecond laser direct writing (FLDW) in aluminogermanate glass (Corning 9754), first mapping the processing landscape to delineate no modification, Type I index increase, and spatial broadening regimes. We then operate in a non-accumulating [...] Read more.

Mid-IR flat/integrated optics require low-loss, programmable phase control. We investigate femtosecond laser direct writing (FLDW) in aluminogermanate glass (Corning 9754), first mapping the processing landscape to delineate no modification, Type I index increase, and spatial broadening regimes. We then operate in a non-accumulating regime that provides a broad, stable writing window. Quantitative-phase microscopy yields Δφ and a monotonic Δn with optically limited cross-sections compatible with low loss. Transmission spectroscopy shows high values (about 90% up to 4 µm) and no additional absorptions across the near-IR and mid-IR range. FTIR reveals a redshift of the Ge–O–(Ge/Al) stretching envelope from ≈1 µJ, correlating with the high Δn onset. s-SNOM at 925 cm⁻¹ resolves the written line as reduced near-field amplitude and decreased phase, confirming a local complex permittivity change consistent with densification-driven Type I tracks. Together, these results define practical conditions for on-demand mid-IR flat/GRIN/Fresnel optics by FLDW in this commercial mid-IR transparent glass. Full article

(This article belongs to the Special Issue Advances in Micro-Nano Optical Manufacturing)

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17 pages, 556 KB

Open AccessArticle

Performance Evaluation and Aggregation Curve Modelling for Multimedia Services in the GPON

by Kamil Trubák, Jan Látal, Jiří Štípal, Petr Šiška, Jan Nedoma and Josef Vojtěch

Photonics 2026, 13(2), 196; https://doi.org/10.3390/photonics13020196 - 16 Feb 2026

Viewed by 464

Abstract

With the increasing number of end users that are using multimedia services, demand for access network high-bitrate systems with sufficient quality of services is also increasing. However, this might not always be ensured by telecom operators, as they must optimize networks according to [...] Read more.

With the increasing number of end users that are using multimedia services, demand for access network high-bitrate systems with sufficient quality of services is also increasing. However, this might not always be ensured by telecom operators, as they must optimize networks according to the Quality of Service (QoS) and multimedia data transmission. In this work, we tested Gigabit Passive Optical Network (GPON) performance with the help of various tools (iPerf, RFC 6349 or ITU-T Y.1564). The Grafana software v7.3.3 tool is used to monitor data streams. Measurements were made to limit the downstream bitrate of up to 20 end users at 1 Gbit/s, 500 Mbit/s, 300 Mbit/s and 100 Mbit/s. Based on repeated measurements, an aggregation curve was modelled, indicating the available bitrate with respect to the network load. Full article

(This article belongs to the Section Optical Communication and Network)

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21 pages, 5796 KB

Open AccessArticle

Analysis and Design of a Hybrid Graphene/Vanadium-Dioxide Terahertz Metasurface with Independently Reconfigurable Reflection Phase and Magnitude

by Eric Amoateng, Ellis Mubarak Sani, Kingsford Sarkodie Obeng Kwakye and Alexandros Pitilakis

Photonics 2026, 13(2), 195; https://doi.org/10.3390/photonics13020195 - 15 Feb 2026

Viewed by 533

Abstract

A reconfigurable THz metasurface (MS) capable of independent reflection amplitude and phase modulation is designed and analyzed. The tunability is achieved in a simple few-layer structure by control over the chemical potential of a graphene monolayer patterned in square patches and over the [...] Read more.

A reconfigurable THz metasurface (MS) capable of independent reflection amplitude and phase modulation is designed and analyzed. The tunability is achieved in a simple few-layer structure by control over the chemical potential of a graphene monolayer patterned in square patches and over the bulk conductivity of an overlying vanadium dioxide (VO₂) patch array; these impart control over the reflection phase and magnitude, respectively. To design and analyze the MS unit cell, we employ intuitive equivalent circuit and transmission line modeling, which is validated against full-wave simulations, showing good agreement in the regime of interest, i.e., on the first resonance for normal plane wave incidence. The simulated phase modulation approaches

250 °

, enabling binary-encoded digital metasurface designs, while the magnitude modulation spans more than 20 dB, from

- 3

dB almost down to perfect absorption. The flexibility of dynamic phase and amplitude control can unlock the full potential of such THz MS hybrid designs for future wireless communications (6G and beyond) and for sensing applications. Finally, the analytical modeling can be extended to polarization-dependent, anisotropic, or non-local EM responses and/or to include aspects of the multiphysical control mechanisms. Full article

(This article belongs to the Special Issue Photonics Metamaterials: Processing and Applications)

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22 pages, 3200 KB

Open AccessReview

Advances in Carbon Dots-Integrated Optical Fiber Sensors

by Kandasamy Sasikumar, Gyeongchan Lee, Ramar Rajamanikandan and Heongkyu Ju

Photonics 2026, 13(2), 194; https://doi.org/10.3390/photonics13020194 - 15 Feb 2026

Viewed by 677

Abstract

Carbon dots (CDs) have enormous potential in optical sensing applications due to their remarkable physicochemical properties. Benefiting from high specific surface area, rich active sites, bright photoluminescence, high photostability, and biocompatibility, CDs have been widely used as functional layers in optical fiber sensors, [...] Read more.

Carbon dots (CDs) have enormous potential in optical sensing applications due to their remarkable physicochemical properties. Benefiting from high specific surface area, rich active sites, bright photoluminescence, high photostability, and biocompatibility, CDs have been widely used as functional layers in optical fiber sensors, resulting in notable improvements in sensitivity, response speed, and environmental stability. This review describes recent advances in CD-integrated optical fiber sensors, with a focus on CD synthesis techniques and their integration with optical fibers for the sensing of diverse analytes, including heavy metal ions, biomarkers, and dyes. CD-integrated fiber sensors exhibit significantly enhanced detection performance in terms of sensitivity, selectivity, repeatability, response time, and recovery time, compared with their CD-free counterparts. Finally, current challenges and future perspectives are discussed. This review aims to provide valuable insights for the design and development of novel CD-integrated optical fiber platforms for sensing chemically and biologically relevant analytes. Full article

(This article belongs to the Special Issue Advancements in Fluorescent Materials and Applications)

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26 pages, 3560 KB

Open AccessArticle

Resilient Optical Wireless Communication Through WDM-Based RIS-Assisted Multi-Connectivity

by Sarah O. M. Saeed, Ahmad Qidan, Taisir Elgorashi and Jaafar Elmirghani

Photonics 2026, 13(2), 193; https://doi.org/10.3390/photonics13020193 - 15 Feb 2026

Viewed by 523

Abstract

The susceptibility of a Line-of-Sight (LOS) link in Optical Wireless Communication (OWC) to blockage is a major challenge affecting its deployment for next generation networks. Another challenge is the random orientation of the receiving device which also affects the amount of received optical [...] Read more.

The susceptibility of a Line-of-Sight (LOS) link in Optical Wireless Communication (OWC) to blockage is a major challenge affecting its deployment for next generation networks. Another challenge is the random orientation of the receiving device which also affects the amount of received optical power when the incidence angle is high. Reflecting Intelligent Surfaces (RIS) is a promising technology for using non-LOS (NLOS) communication. This work aims to study the effect of these LOS link impairments on Wavelength Division Multiplexing (WDM)-based resource allocation in OWC with and without the use of RIS elements and the effect on resilience. In this work, we adopt the state-of-the-art Orientation-based Random Way-Point (ORWP) model for mobility and random orientation of the User Equipment (UE) and calculate blockage geometrically assuming human objects since OWC links are not independent in contrast to RF-based communication. We propose multi-connectivity with physical path disjointness using multiple Angle Diversity Receiver (ADR) designs to evaluate the system performance using both a Mixed Integer Linear Program (MILP) and a low-complexity algorithm. Full article

(This article belongs to the Special Issue Recent Advances in Optical Wireless Communication Systems and Networks)

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15 pages, 3524 KB

Open AccessArticle

Simultaneous Compensation for Perturbed Dual-Channel Telescopes Based on Nodal Aberration Theory

by Mengmeng Xu, Qihao Wang, Yuan Yao, Weijie Deng, Donglin Xue and Xuejun Zhang

Photonics 2026, 13(2), 192; https://doi.org/10.3390/photonics13020192 - 14 Feb 2026

Viewed by 381

Abstract

Active compensation for perturbed space telescopes is an effective means of improving the image quality. In common-aperture dual-channel telescopes, compensation can only be achieved by adjusting a shared secondary mirror (SM), making it difficult to balance the inconsistent misalignment aberrations between the channels. [...] Read more.

Active compensation for perturbed space telescopes is an effective means of improving the image quality. In common-aperture dual-channel telescopes, compensation can only be achieved by adjusting a shared secondary mirror (SM), making it difficult to balance the inconsistent misalignment aberrations between the channels. To address this issue, an analytic method for simultaneous compensation of dual-channel aberrations is proposed. Based on the improved Nodal Aberration Theory (NAT), the analytic solution for the common compensation position of the SM is derived by establishing a geometric balance between the aberration field nodes in the image planes of the two channels. On this basis, the theoretical conditions required for the consistency of the optimal compensation positions in the dual channels are also presented. The robustness of the method is validated through Monte Carlo simulations under conditions of random noise and surface figure errors. The results show that the average RMS wavefront error (WFE) of each channel is reduced to less than λ/16 (λ = 632.8 nm) after compensation under various misalignment conditions. Compared with the traditional Sensitivity Matrix Method (SMM), the proposed method exhibits superior compensation accuracy and decoupling capability in correcting dual-channel aberrations, thereby significantly improving the optical performance of the system. Full article

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13 pages, 2375 KB

Open AccessArticle

Attosecond Visible Pulse Generation via Hollow-Core Fiber Broadening and Light Field Synthesis: The Role of Second- and Third-Order Dispersion

by Jiayi Ma, Jiahui Huang, Meng Yue, Peng Xu, Gaiyan Chang, Guanghua Cheng, Guodong Zhang, Dandan Hui and Yuxi Fu

Photonics 2026, 13(2), 191; https://doi.org/10.3390/photonics13020191 - 14 Feb 2026

Viewed by 700

Abstract

The attosecond (10⁻¹⁸ s) light pulse represents the fastest time scale currently mastered by the scientific community, which enables the observation of electron dynamics within atoms and molecules, offering powerful tools to probe chemical reaction mechanisms and advance research in photovoltaic materials [...] Read more.

The attosecond (10⁻¹⁸ s) light pulse represents the fastest time scale currently mastered by the scientific community, which enables the observation of electron dynamics within atoms and molecules, offering powerful tools to probe chemical reaction mechanisms and advance research in photovoltaic materials and biological processes. In this work, we investigate the generation of visible attosecond optical pulses via spectral broadening in Hollow-Core Fiber (HCF), followed by coherent recombination using a Three-Channel Light Field Synthesizer (TCLFS). The influence of the input pulse duration on Group Delay Dispersion (GDD), Third-Order Dispersion (TOD), and spectral broadening is systematically analyzed. Furthermore, the effects of GDD, TOD, and the carrier–envelope phase (CEP) on waveform synthesis are quantitatively examined for the first time. These findings provide valuable insights into dispersion management strategies essential for developing high-quality visible attosecond light sources, paving the way for future applications in ultrafast spectroscopy and light field-driven electron dynamics. Full article

(This article belongs to the Special Issue Lightwave Electronics)

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20 pages, 912 KB

Open AccessArticle

Distributed Probabilistic Data Association Feedback Particle Filter for Photoelectric Tracking System

by Chang Qin, Yikun Li, Jiayi Kang, Xi Zhou, Yao Mao and Dong He

Photonics 2026, 13(2), 190; https://doi.org/10.3390/photonics13020190 - 14 Feb 2026

Viewed by 370

Abstract

A photoelectric tracking system is a typical bearing-only target tracking system that faces significant challenges arising from measurement origin uncertainty due to clutter and the discrepancy between continuous-time target dynamics and discrete-time optical sampling, as well as the inherent nonlinearity of bearing-only tracking. [...] Read more.

A photoelectric tracking system is a typical bearing-only target tracking system that faces significant challenges arising from measurement origin uncertainty due to clutter and the discrepancy between continuous-time target dynamics and discrete-time optical sampling, as well as the inherent nonlinearity of bearing-only tracking. This paper addresses these issues by proposing a novel distributed probabilistic data association feedback particle filter (DPDA-FPF) framework. To resolve the tracking ambiguity at the local level, we extend the feedback particle filter to a continuous-discrete setting integrated with probabilistic data association. Subsequently, the local state estimates and covariances from spatially separated tracking systems are transmitted to a fusion center and integrated using an optimal linear covariance-weighted fusion rule to improve global observability and mitigate biases of individual systems. Numerical simulations in a 3D scenario with moderate clutter density demonstrate that while individual sensor tracks suffer from fluctuations, the proposed fused estimate achieves substantially lower root mean square errors in both position and velocity. The results validate the efficiency of the proposed architecture as a robust solution for photoelectric tracking applications. Full article

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22 pages, 6553 KB

Open AccessArticle

Integrated Wavefront Detection for Large-Aperture Segmented Planar Mirrors: Concept & Principle

by Rui Sun, Qichang An and Xiaoxia Wu

Photonics 2026, 13(2), 189; https://doi.org/10.3390/photonics13020189 - 14 Feb 2026

Viewed by 506

Abstract

Planar mirrors play a crucial role in autocollimation testing and optical beam relay systems of telescopes and other fields. However, for the next-generation large-aperture telescopes, typical monolithic planar mirrors fall short in meeting anticipated performance requirements, owing to their high costs and fabrication [...] Read more.

Planar mirrors play a crucial role in autocollimation testing and optical beam relay systems of telescopes and other fields. However, for the next-generation large-aperture telescopes, typical monolithic planar mirrors fall short in meeting anticipated performance requirements, owing to their high costs and fabrication limitations. Here, a new integrated multimodal testing method for 3–4 m-class segmented planar mirrors is proposed. The presented system utilizes an innovative keystone architecture with a central mirror and keystone-shaped segments, which is superior to the traditional hexagonal architecture. To facilitate rapid coarse alignment, a machine vision system based on edge detection is investigated. Furthermore, the dispersed fringe technique is used for robust co-phasing. By using a segmented planar mirror designed with sub-aperture stitching strategy and combining local apertures, the system cost was reduced and high-precision measurement was achieved. Eventually, the alignment, co-focus and co-phasing measurements based on the proposed concept were completed, and the transfer characteristics were determined by analyzing the Optical Transfer Function (OTF). Test data shows co-phasing accuracy of better than 30 nm RMS (root-mean-square) and alignment accuracy less than 10 arcseconds. In addition, the system uses small-aperture mirrors in autocollimation testing to facilitate flexible alignment and testing of individual segments. The test optical path is configured to match the effective focal length of the system under test, and the optical lever effect of reflectors enhances the alignment sensitivity. The method combines autocollimation and wavefront sensing which allows the approach to provide high-precision control of co-focus, co-phasing, and surface errors correction. Full article

(This article belongs to the Special Issue Advances in Optical Fiber Sensing Technology)

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17 pages, 2451 KB

Open AccessArticle

Design of a Combined-Freeform-Surface Diffuse-Reflection System for High-Uniformity, Compact LED Inspection Illumination

by Jianghua Rao, Xin Xu, Riquan Zhou, Xiaowen Liang, Zhenmin Zhu, Yuanyuan Peng and Mingke Xu

Photonics 2026, 13(2), 188; https://doi.org/10.3390/photonics13020188 - 14 Feb 2026

Viewed by 361

Abstract

LED diffuse-illumination systems are widely used in industrial inspection and real life because of their scattering properties. However, there has been little research on secondary optical designs for diffuse illumination. Considering the need for diffuse light in real life and work, combined with [...] Read more.

LED diffuse-illumination systems are widely used in industrial inspection and real life because of their scattering properties. However, there has been little research on secondary optical designs for diffuse illumination. Considering the need for diffuse light in real life and work, combined with existing specular-reflection technology, this study proposes a design method for a combined-freeform-surface illumination system with specular and diffuse reflections. Considering that a separate diffusing device cannot effectively control the diffusion area of the light source, the unique properties of the specular-reflective device were utilized in this study. First, the specular-reflection device directs the light from the central portion of the LED to the diffuse-reflection device, and the light collected is then redistributed by the diffuse-reflection device. Two mathematical models were established according to the light-emitting angle of the LED, which corresponded to two freeform surfaces. In addition, when evaluating the uniformity of the target-plane illumination, a set of constraint equations was added to obtain the diffuse freeform surface contour of the target plane. Finally, the ratio of the diameter to the thickness of the resulting illumination system exceeded six, and the illumination uniformity increased to over 56% (with a uniformity improvement ratio of ≥6% compared to traditional single-freeform-surface systems and ≥10% compared to integrating sphere systems). It is specifically designed for industrial precision inspection scenarios, has higher illumination uniformity than other diffuse illumination systems, and has better compactness, making it suitable for high-precision inspection lighting applications. Full article

(This article belongs to the Special Issue Recent Advances in Imaging and Non-Imaging Optical Technologies)

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13 pages, 779 KB

Open AccessArticle

Enhanced Signal of Sum Sideband via Parametric Interactions in a Mechanical PT-Symmetric System

by Hui Zheng, Zihan Du and Aixi Chen

Photonics 2026, 13(2), 187; https://doi.org/10.3390/photonics13020187 - 13 Feb 2026

Viewed by 336

Abstract

We investigate a double-probe-field-driven cavity optomechanical system with a degenerate optical parametric amplifier (OPA). When the system is in a mechanical PT-symmetric case, we study the generation mechanism of the sum sideband and how to enhance the generation efficiency of the sum sideband [...] Read more.

We investigate a double-probe-field-driven cavity optomechanical system with a degenerate optical parametric amplifier (OPA). When the system is in a mechanical PT-symmetric case, we study the generation mechanism of the sum sideband and how to enhance the generation efficiency of the sum sideband by controlling parametric interactions. Our model consists of two directly coupled PT-symmetric mechanical resonators, which are coupled to a Fabry–Pérot cavity equipped with an optical parametric amplifier. Research indicates that in a PT-symmetric mechanical resonator, there exist special exceptional points (EPs). Near EPs, the generation efficiency of the sum sideband is significantly enhanced. Notably, the introduction of an OPA can remarkably boost the efficiency of sum sideband generation (SSG) and establish a new sideband matching condition for the upper sum sideband. We conduct a detailed analysis of the dependence of SSG on system parameters, such as mechanical coupling strength, OPA nonlinear gain, OPA pump light field phase, and probe field frequency detuning. The research reveals that even with a weak driving field, a significantly enhanced efficiency of SSG can be achieved by adjusting the OPA gain coefficient and phase. This research offers new insights into enhancing or regulating light propagation in nonlinear optomechanical devices and holds potential for applications in high-precision measurement and optical communication. Full article

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

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14 pages, 1363 KB

Open AccessArticle

Comparative Study of PLSR and SVR Using MLP Feature Extraction for Quantitative Analysis of Steel Alloy Elements by Laser-Induced Breakdown Spectroscopy

by Weifeng Chen and Yu Ding

Photonics 2026, 13(2), 186; https://doi.org/10.3390/photonics13020186 - 13 Feb 2026

Cited by 1 | Viewed by 385

Abstract

With the rapid development of the steel industry, the accurate detection of alloy element contents is of great significance for the evaluation of material properties and quality control. This study aims to establish a rapid, stable, and highly accurate quantitative detection method based [...] Read more.

With the rapid development of the steel industry, the accurate detection of alloy element contents is of great significance for the evaluation of material properties and quality control. This study aims to establish a rapid, stable, and highly accurate quantitative detection method based on handheld LIBS to achieve effective analysis of key elements such as Fe, Cr, Mn, Ni, and Cu. To meet the demand of the steel industry for rapid, stable, and high-accuracy quantification of key alloy elements such as Cr, Mn, Ni, and Cu, this study was carried out on 20 types of standard steel spectral samples. Support Vector Regression (SVR) and Partial Least Squares Regression (PLSR) models were constructed, respectively. The SVR penalty factor C (0.1–10) and loss parameter ε (0.001–1), as well as the PLSR latent variable number Lv (1–20), were optimized using five-fold cross-validation repeated 100 times. Model performance was evaluated using the coefficient of determination (R²), root-mean-square error (RMSE), and mean relative error (MRE). In the comparison of quantitative performance, excellent predictive ability for major elements such as Fe and Cr was achieved by both models; test-set R² values exceeded 0.92, meeting the detection requirements for high-content alloy elements. For low-content Ni, Cu, and Mn, PLSR gives R² values of 0.92, 0.93, and 0.89, while SVR yields 0.85, 0.49, and 0.36, showing clear limitations, especially for Cu and Mn. After introducing Multilayer Perceptron feature extraction, the R² of Ni, Cu, and Mn increases to 0.99, 0.99, and 0.97 for PLSR and to 0.99, 0.93, and 0.94 for SVR, with RMSE and MRE markedly reduced. In summary, the integration of LIBS with MLP feature extraction and PLSR offers both rapid processing capabilities and high precision, significantly improving the quantification of low-concentration elements, and is well-suited for real-time online monitoring in steel production, facilitating quality control and process optimization. Full article

(This article belongs to the Special Issue Advanced Lasers and Their Applications, 3rd Edition)

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13 pages, 2598 KB

Open AccessArticle

Efficiency of Self-Injection Locked Lasers

by Haipeng Liu, Tianyu Sun, Jijun Feng, Anwei Zhou, Zheng Xing, Zhongming Zeng and Baoshun Zhang

Photonics 2026, 13(2), 185; https://doi.org/10.3390/photonics13020185 - 13 Feb 2026

Viewed by 580

Abstract

The integrated III-V self-injection locked (SIL) laser exhibits excellent linewidth compression, noise reduction, and frequency stability. However, the laser’s low efficiency and fluctuating output power severely limit its applications in optical coherent transmission, light detection and ranging (LiDAR), spectroscopy, and so on. Based [...] Read more.

The integrated III-V self-injection locked (SIL) laser exhibits excellent linewidth compression, noise reduction, and frequency stability. However, the laser’s low efficiency and fluctuating output power severely limit its applications in optical coherent transmission, light detection and ranging (LiDAR), spectroscopy, and so on. Based on the rate equations for a semiconductor laser coupled to counter-propagating fields in a micro-ring resonator (MRR), we systematically investigate the laser power and linewidth compression under self-locking conditions. We improve the slope efficiency by adjusting the injection phase, diode–MRR coupling efficiency, the normalized mode-coupling rate between clockwise (CW) and counter-clockwise (CCW) modes, and the MRR Q-factor. The results show that the enhanced diode–MRR coupling efficiency effectively increases the laser slope efficiency and improves the stability of the injection phase and feedback intensity. The injection phase significantly influences the range of the self-injection locked state. The normalized mode-coupling rate effectively affects the locking bandwidth and maintains stable power transfer. The MRR intrinsic Q-factor has a positive correlation with improving the laser slope efficiency and compressing the linewidth. Full article

(This article belongs to the Special Issue Semiconductor Lasers: Innovations, Challenges, and Future Perspectives)

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12 pages, 2133 KB

Open AccessArticle

A Compact and Robust Polarization-Entangled Photon Source Towards Application in Mobile Platforms

by Jing Wang, Peng Li, Luyi Sun, Pengcheng Wang, Nachuan Li, Xiao-Tian Zhang, Yan-Xiao Gong, Hua-Ying Liu, Shi-Ning Zhu and Zhenda Xie

Photonics 2026, 13(2), 184; https://doi.org/10.3390/photonics13020184 - 13 Feb 2026

Viewed by 794

Abstract

Entangled-photon sources are indispensable components in free-space quantum key distribution (QKD) systems. Here, we present a compact, lightweight, and robust airborne entangled-photon source (AEPS) based on a Sagnac loop structure with single-mode fiber coupling. To meet the drone requirements for miniaturization, lightweight design, [...] Read more.

Entangled-photon sources are indispensable components in free-space quantum key distribution (QKD) systems. Here, we present a compact, lightweight, and robust airborne entangled-photon source (AEPS) based on a Sagnac loop structure with single-mode fiber coupling. To meet the drone requirements for miniaturization, lightweight design, and high robustness, we developed a highly integrated entangled-photon source using customized miniature optical components and an adhesive bonding technique. The total volume and weight are only 38 × 40 × 24 mm³ and 58 g, respectively. Entangled-photon pairs at 810 nm are generated via Type-II spontaneous parametric down-conversion (SPDC) in a periodically poled KTiOPO₄ (PPKTP) crystal. We achieve a quantum state fidelity of F = 0.986 ± 0.0017, a photon-pair generation rate of 3.03 × 10⁶ pairs/s/mW, and a CHSH Bell parameter of S = 2.764 ± 0.082. Owing to its excellent size, weight, performance, and stability, the proposed entangled-photon source is particularly well suited for drone-based free-space mobile quantum communication. Full article

(This article belongs to the Special Issue Advancements in Optical Devices for Quantum Information Processing and Communication)

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21 pages, 7192 KB

Open AccessArticle

Expectation–Maximization Method for RGB-D Camera Calibration with Motion Capture System

by Jianchu Lin, Guangxiao Du, Yugui Zhang, Yiyan Zhao, Qian Xie, Jian Yao and Ashim Khadka

Photonics 2026, 13(2), 183; https://doi.org/10.3390/photonics13020183 - 12 Feb 2026

Viewed by 525

Abstract

Camera calibration is an essential research direction in photonics and computer vision. It achieves the standardization of camera data by using intrinsic and extrinsic parameters. Recently, RGB-D cameras have been an important device by supplementing deep information, and they are commonly divided into [...] Read more.

Camera calibration is an essential research direction in photonics and computer vision. It achieves the standardization of camera data by using intrinsic and extrinsic parameters. Recently, RGB-D cameras have been an important device by supplementing deep information, and they are commonly divided into three kinds of mechanisms: binocular, structured light, and Time of Flight (ToF). However, the different mechanisms cause calibration methods to be complex and hardly uniform. Lens distortion, parameter loss, and sensor degradation et al. even fail calibration. To address the issues, we propose a camera calibration method based on the Expectation–Maximization (EM) algorithm. A unified model of latent variables is established for the different kinds of cameras. In the EM algorithm, the E-step estimates the hidden intrinsic parameters of cameras, while the M-step learns the distortion parameters of the lens. In addition, the depth values are calculated by the spatial geometric method, and they are calibrated using the least squares method under an optical motion capture system. Experimental results demonstrate that our method can be directly employed in the calibration of monocular and binocular RGB-D cameras, reducing image calibration errors between 0.6 and 1.2% less than least squares, Levenberg–Marquardt, Direct Linear Transform, and Trust Region Reflection. The deep error is reduced by 16 to 19.3 mm. Therefore, our method can effectively improve the performance of different RGB-D cameras. Full article

(This article belongs to the Special Issue Next-Generation Optical Transmission Systems: Breakthroughs, Technologies, and Emerging Frontiers)

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