Photonics

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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16 pages, 14066 KB

Open AccessArticle

Joint Modulation Format Identification and OSNR Monitoring Based on Amplitude-Analytic Complex Planes for Digital Coherent Receivers

by Ruyue Xiao, Ming Hao, Shuang Liang, Weigang Hou and Jianming Tang

Photonics 2026, 13(5), 422; https://doi.org/10.3390/photonics13050422 - 24 Apr 2026

Abstract

Joint modulation format identification (MFI) and optical signal-to-noise ratio (OSNR) monitoring constitutes one of the most critical functions integrated in digital coherent receivers, ensuring high flexibility and stability in elastic optical networks (EONs). Since signal amplitude information captures inherent characteristics associated with modulation [...] Read more.

Joint modulation format identification (MFI) and optical signal-to-noise ratio (OSNR) monitoring constitutes one of the most critical functions integrated in digital coherent receivers, ensuring high flexibility and stability in elastic optical networks (EONs). Since signal amplitude information captures inherent characteristics associated with modulation formats and fluctuations induced by OSNR variations, a simple and effective optical performance monitoring (OPM) scheme based on an amplitude-analytic complex plane is proposed. By employing a multi-task learning algorithm incorporating the multi-order gated aggregation (MOGA) module, the proposed scheme enables simultaneous MFI and OSNR monitoring for polarization division multiplexed (PDM)-QPSK/-16QAM/-32QAM/-64QAM/-128QAM signals. The performance of the proposed scheme is numerically verified in 28 GBaud coherent optical communication systems of various configurations. Numerical simulation results show that 100% identification accuracy is obtainable for all five modulation formats, even at OSNR values lower than the corresponding theoretical 20% forward error correction (FEC) limit. Meanwhile, the mean absolute error (MAE) of OSNR monitoring for QPSK, 16QAM, 32QAM, 64QAM, and 128QAM are 0.16 dB, 0.15 dB, 0.17 dB, 0.28 dB, and 0.33 dB, respectively. Furthermore, simulation results show that the proposed scheme is robust to residual chromatic dispersion (CD) and the nonlinear effects with strong generalization capability. These results suggest that the proposed scheme is promising for applications in next-generation EONs. Full article

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17 pages, 6779 KB

Open AccessArticle

Polarization Fading Noise Suppression in Phase-Sensitive OTDR Using Variational Mode Decomposition

by Ruotong Mei, Weidong Bai, Xinming Zhang, Junhong Wang, Yu Wang and Baoquan Jin

Photonics 2026, 13(5), 421; https://doi.org/10.3390/photonics13050421 - 24 Apr 2026

Abstract

To address the polarization fading noise in coherent detection phase-sensitive optical time-domain reflectometry (Φ-OTDR) for distributed low-frequency vibration sensing, a Φ-OTDR sensing scheme integrating polarization diversity reception and the variational mode decomposition (VMD) algorithm is proposed. The mechanism of polarization fading induced by [...] Read more.

To address the polarization fading noise in coherent detection phase-sensitive optical time-domain reflectometry (Φ-OTDR) for distributed low-frequency vibration sensing, a Φ-OTDR sensing scheme integrating polarization diversity reception and the variational mode decomposition (VMD) algorithm is proposed. The mechanism of polarization fading induced by fiber birefringence and external perturbations is systematically analyzed. A signal–noise mathematical model for polarization diversity reception is established, and the adaptive decomposition capability of the VMD algorithm for non-stationary phase signals is elaborated. This scheme can accurately separate the additional noise introduced by polarization diversity reception from the target low-frequency vibration signals. Experimental results demonstrate that, compared with the single-path detection scheme, the proposed method eliminates the amplitude attenuation of beat frequency signals caused by polarization mismatch at the optical path level. Meanwhile, it effectively suppresses both the additional noise introduced by polarization diversity and the low-frequency phase drift resulting from unstable laser frequency. It achieves precise phase restoration of vibration signals excited at 50 Hz under three typical sensing distances of 5 km, 10 km, and 30 km. Additionally, it successfully restores low-frequency vibration signals as low as 0.6 Hz at the sensing distance of 30 km. Full article

(This article belongs to the Section Lasers, Light Sources and Sensors)

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31 pages, 3239 KB

Open AccessReview

Ultrafast Fiber Lasers in the 2 μm Band: Mode-Locking Techniques, Performance Advances and Applications

by Silun Du, Tianshu Wang, Bo Zhang, Shimeng Tan and Tuo Chen

Photonics 2026, 13(5), 420; https://doi.org/10.3390/photonics13050420 - 24 Apr 2026

Abstract

Ultrafast fiber lasers operating near 2 μm have emerged as a critical platform for advancing mid-infrared photonics due to their narrow pulse durations, high peak powers, and broad tunability. These sources exploit the rich energy-level structures of Tm³⁺ and Ho³⁺ doped [...] Read more.

Ultrafast fiber lasers operating near 2 μm have emerged as a critical platform for advancing mid-infrared photonics due to their narrow pulse durations, high peak powers, and broad tunability. These sources exploit the rich energy-level structures of Tm³⁺ and Ho³⁺ doped fibers and reside within an atmospheric transmission window, enabling applications spanning nonlinear microscopy, precision micromachining, optical frequency metrology, biophotonics, and free-space optical communication. Recent progress in low-loss fiber fabrication, dispersion-engineered cavity design, and mode-locking technologies has significantly expanded the performance boundaries of 2 μm ultrafast fiber lasers. This review systematically examines the underlying pulse-formation mechanisms and categorizes state-of-the-art mode-locking approaches. Representative laser architectures are compared with respect to pulse duration, energy scalability, repetition-rate enhancement, spectral characteristics, and environmental stability. Key application pathways in high-resolution spectroscopy, biomedical diagnostics, and mid-IR supercontinuum generation are highlighted. Finally, the remaining challenges and prospective research directions are discussed to inform the development of next-generation ultrafast photonic sources in the 2 μm band. Full article

(This article belongs to the Special Issue Advancements in Mode-Locked Lasers)

11 pages, 1306 KB

Open AccessArticle

Polarization Control Methods for Mitigating Four-Wave Mixing Effect in NG-EPON Networks

by Yan Xu and Shuai Wang

Photonics 2026, 13(5), 419; https://doi.org/10.3390/photonics13050419 - 24 Apr 2026

Abstract

In order to meet the growing traffic demands, the formulation of the NG-EPON protocol is under heated discussion. Therefore, for the wavelength allocation scheme of NG-EPON, adopting O-band wavelengths is considered as the potentially feasible solution. While the low dispersion property of O-band [...] Read more.

In order to meet the growing traffic demands, the formulation of the NG-EPON protocol is under heated discussion. Therefore, for the wavelength allocation scheme of NG-EPON, adopting O-band wavelengths is considered as the potentially feasible solution. While the low dispersion property of O-band would induce four-wave mixing (FWM) to NG-EPON networks. In this paper, polarization control methods are used to mitigate the FWM effect in NG-EPON networks, including orthogonal linear polarization, circular polarization and left to right circular polarization alternation methods. For the 4-channel NG-EPON networks, the FWM-induced sensitivity penalty is achieved as 0.3 dB by using the orthogonal linear polarization method, compared with the FWM-induced sensitivity penalty of 4.1 dB, which is the worst-case scenario. By adopting the circular polarization method, the FWM-induced sensitivity penalty is measured as 0.2 dB, which is 3.9 dB better than that of the worst-case scenario. By employing the left to right circular polarization alternation method, the FWM-induced sensitivity penalty is 0.2 dB, compared with the worst-case scenario, whose sensitivity penalty is 4.1 dB induced by the FWM. In addition, for the 8-channel NG-EPON networks, the FWM-induced sensitivity penalty is decreased to 0.4 dB for minimum, compared with the worst-case scenario, whose sensitivity penalty is unpredictable. Full article

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11 pages, 889 KB

Open AccessArticle

Competing Built-in Electric Fields in Au/MoS₂/WSe₂ Dual Junction Photodetectors for Broadband VIS-IR Detection

by Haoxuan Li, Xuhao Fan, Qirui Sun, Shian Mi, Changyi Pan, Huiyong Deng, Ning Dai and Yufeng Shan

Photonics 2026, 13(5), 418; https://doi.org/10.3390/photonics13050418 - 24 Apr 2026

Abstract

Van der Waals (vdW) heterostructures are attractive for optoelectronic devices due to their lattice-mismatch tolerance and tunable band structures. Here, we report a gate-tunable Au/MoS₂/WSe₂ dual junction photodetector featuring competing asymmetric built-in electric fields. Spatially resolved photocurrent measurements reveal that [...] Read more.

Van der Waals (vdW) heterostructures are attractive for optoelectronic devices due to their lattice-mismatch tolerance and tunable band structures. Here, we report a gate-tunable Au/MoS₂/WSe₂ dual junction photodetector featuring competing asymmetric built-in electric fields. Spatially resolved photocurrent measurements reveal that selective utilization of these built-in electric fields decouples the transport dynamics of dark and photogenerated carriers. Such decoupling allows for independent modulation of the dark current and photocurrent, enabling the concurrent realization of the ultralow dark current and high photocurrent. Moreover, gate-voltage modulation enhances the photoresponse by ~245%, yielding a detectivity of 1.98 × 10¹² Jones over the 532–940 nm range. Imaging and optical communication further verify the device’s practical potential. These results provide a viable route toward high-sensitivity and electrically reconfigurable broadband photodetectors. Full article

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

11 pages, 1503 KB

Open AccessArticle

A Terahertz Permittivity Sensor Based on an SSPPs–SRR Coupled Structure

by Ting Zeng, Chunyang Bi, Zhichao Bi, Jun Zhou and Sen Gong

Photonics 2026, 13(5), 417; https://doi.org/10.3390/photonics13050417 - 24 Apr 2026

Abstract

Accurate permittivity characterization at terahertz frequencies is important for material analysis and device design, yet it remains challenging for small-volume samples and compact test structures. In this work, a terahertz permittivity sensor based on a spoof surface plasmon polariton (SSPPs) transmission line coupled [...] Read more.

Accurate permittivity characterization at terahertz frequencies is important for material analysis and device design, yet it remains challenging for small-volume samples and compact test structures. In this work, a terahertz permittivity sensor based on a spoof surface plasmon polariton (SSPPs) transmission line coupled to a backside split-ring resonator (SRR) is proposed and numerically studied. The SSPPs line is patterned on the top side of the substrate, while the SRR is etched on the backside, with the sample loaded into the SRR gap. The SSPPs mode penetrates through the substrate and excites the SRR, producing a pronounced transmission notch. Changes in the sample permittivity modulate the effective capacitance of the resonator, resulting in a monotonic shift in the notch center frequency. For relative permittivities from 1 to 8, the notch center frequency decreases from 152.1 GHz to 117.8 GHz, corresponding to a total shift of 34.3 GHz and an average sensitivity of about 4.90 GHz/ε_r. The minimum S₂₁ remains within approximately −23.80 to −21.56 dB, while the Q-factor stays in the range of 94.33–108.23, indicating good spectral readability. Tolerance analysis further shows that the resonance frequency is sensitive to critical structural dimensions and layer alignment, and practical implementation is therefore more suitable for single-device calibrated frequency-shift sensing. These results demonstrate the feasibility of the proposed dual-layer SSPPs–SRR configuration for compact permittivity sensing in the terahertz regime. Full article

(This article belongs to the Special Issue New Perspectives in Biomedical Optics and Optical Imaging)

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20 pages, 4990 KB

Open AccessArticle

Curvature Radius Measurement Based on Interferogram Analysis and Deep Learning Model

by Yan-Yi Li, Chuen-Lin Tien, Hsi-Fu Shih, Han-Yen Tu and Chih-Cheng Chen

Photonics 2026, 13(5), 416; https://doi.org/10.3390/photonics13050416 - 24 Apr 2026

Abstract

Accurate estimation of curvature radius from interference fringes is critical in optical metrology and precision manufacturing. Conventional interferogram analytical approaches often require manual intervention and are sensitive to fringe variations related to noise and environmental vibrations. To address these limitations, we combine an [...] Read more.

Accurate estimation of curvature radius from interference fringes is critical in optical metrology and precision manufacturing. Conventional interferogram analytical approaches often require manual intervention and are sensitive to fringe variations related to noise and environmental vibrations. To address these limitations, we combine an improved Twyman–Green interferometer with different artificial intelligence (AI) deep learning models and utilize a self-developed MATLAB analysis program to propose a non-destructive and rapid measurement system for optical coating substrates. The proposed AI-assisted Twyman–Green interferometric system differs fundamentally from conventional wavefront sensing techniques in both principle and implementation. This paper utilizes the Twyman–Green interferometer to generate interference fringe datasets on B270 glass and sapphire substrates, and employs convolutional neural network (CNN), ResNet-18, and VGG-16 models for training and evaluation. The proposed method integrates image enhancement, fringe pattern clustering, and analysis and validation based on fast Fourier transform (FFT). Experimental results show that ResNet-18 outperforms other models, with a mean absolute percentage error of 5.44% on sapphire substrates and 3.40% on B270 glass substrates. These findings highlight the effectiveness and robustness of deep learning models, especially residual networks, in automatic ROC prediction for optical measurement applications. Full article

(This article belongs to the Special Issue Recent Developments in Fringe Pattern Analysis for Optics and Photonics)

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12 pages, 1913 KB

Open AccessArticle

Femtosecond Laser-Induced Ultrafast Electron Redistribution near a Microscale Metallic Filament

by Dacai Liu and Bin Li

Photonics 2026, 13(5), 415; https://doi.org/10.3390/photonics13050415 - 24 Apr 2026

Abstract

In this study, a femtosecond laser beam is delivered to metal wire targets to generate suprathermal electron jets reaching energies of several hundreds of keV. During the process, it is observed that the mirror-imaging distribution of the beam focus with respect to the [...] Read more.

In this study, a femtosecond laser beam is delivered to metal wire targets to generate suprathermal electron jets reaching energies of several hundreds of keV. During the process, it is observed that the mirror-imaging distribution of the beam focus with respect to the surface of the target displays highly asymmetric features and different dynamic responses. Especially, the exterior focus exhibits an extraordinary polarity reversal of the macroscopic current, while the interior focus behaves ordinarily. The former is attributed to the strong field at the focal point outside the surface, causing the secondary ionization and driving electrons back to the target, thereby reshaping the distribution of these high-energy hot electrons and the morphology of plasma jets. A numerical model is proposed to simulate the experimental observation and interpret the unexpected phenomenon. Furthermore, the particle-in-cell algorithm is also implemented to verify the results and present more details. This study seeks to emphasize the role of focal position in regulating the photoemission process, which may offer a fresh perspective for research in laser–material interactions and dynamics. Full article

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17 pages, 43020 KB

Open AccessArticle

Cascaded Fiber-Optic Time Synchronization System with Different Modulation Format

by Zhaohui Wang, Jiameng Dong, Shangsu Ding, Song Yu and Bin Luo

Photonics 2026, 13(5), 414; https://doi.org/10.3390/photonics13050414 - 23 Apr 2026

Abstract

High-precision time and frequency transfer plays a pivotal role in metrology, geodesy, deep-space exploration and other scientific applications. Based on current time synchronization research, extending point-to-point schemes into a wide-area network can significantly increase the range of applications. Therefore, we design and implement [...] Read more.

High-precision time and frequency transfer plays a pivotal role in metrology, geodesy, deep-space exploration and other scientific applications. Based on current time synchronization research, extending point-to-point schemes into a wide-area network can significantly increase the range of applications. Therefore, we design and implement a cascaded fiber optic time synchronization system, which is the most basic form of networking. This paper considers two different modulation formats for time synchronization systems from the perspective of fiber nonlinear effects, namely the intensity modulation with direct detection (IMDD) scheme and the phase modulation with self-coherent detection (PMSCD) scheme. The analysis indicates that the constant-envelope characteristic of the PMSCD scheme provides superior transmission performance. Accordingly, we deployed the PMSCD system over a 500 km intercity fiber link and the IMDD system over a 68 km metropolitan fiber link, forming a 568 km cascaded system that achieves time synchronization precision better than 50 ps. This work offers a practical reference for the future development of high-precision fiber-optic time and frequency synchronization networks. Full article

16 pages, 11599 KB

Open AccessArticle

Dual-Mode Tunable Near-Perfect Terahertz Absorber Based on GST Micro-Cavity

by Dongjing Li, Chenyang Cui, Fan Guo and Pingping Min

Photonics 2026, 13(5), 413; https://doi.org/10.3390/photonics13050413 - 23 Apr 2026

Abstract

A micro-cavity based on phase-change material is a very important strategy for the realization of tunable absorption and conversion of terahertz waves. In this work, a tunable terahertz metamaterial absorber based on the phase-change material germanium–antimony–tellurium (GST) is demonstrated. The device features a [...] Read more.

A micro-cavity based on phase-change material is a very important strategy for the realization of tunable absorption and conversion of terahertz waves. In this work, a tunable terahertz metamaterial absorber based on the phase-change material germanium–antimony–tellurium (GST) is demonstrated. The device features a metal–insulator–metal triple-layer structure, where the dynamic switching of absorption characteristics is achieved via thermally controlled GST phase transition. In the amorphous state, the absorber exhibits a single absorption peak at 7.7 THz. Upon crystallization, the absorption switches to dual peaks at 5.1 THz and 8.3 THz, achieving near-perfect absorption in both states. Full-wave electromagnetic simulations and theoretical analysis based on a multiple-reflection interference model indicate that this performance tuning originates from the GST-phase-transition-induced change in the equivalent optical cavity length. This corresponds to a switch between two resonant modes: coupled inner–outer ring resonance and independent outer ring resonance. These results provide a foundation for developing dynamically tunable terahertz devices with promising applications in terahertz communications, imaging, and sensing. Full article

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12 pages, 5834 KB

Open AccessArticle

Quantitative Phase Factor Retrieval from Single-Shot Off-Axis Interferograms for Object Reconstruction

by Jialing Chen, Zixi Yu, Jianglong Lei, Yuanxiang Wang and Qingli Jing

Photonics 2026, 13(5), 412; https://doi.org/10.3390/photonics13050412 - 23 Apr 2026

Abstract

In the far-field approximation, an object’s diffraction field can be expressed as its Fourier transform multiplied by a phase factor. Here, we present a simple method with which to directly retrieve this phase factor from a single-shot off-axis interference pattern. By exploiting and [...] Read more.

In the far-field approximation, an object’s diffraction field can be expressed as its Fourier transform multiplied by a phase factor. Here, we present a simple method with which to directly retrieve this phase factor from a single-shot off-axis interference pattern. By exploiting and adjusting its unique two-dimensional quadratic form, the quadratic contribution from the object’s Fourier transform can generally be neglected, particularly for amplitude-only objects and slowly varying phase objects. The phase factor is extracted by fitting a quadratic surface to the unwrapped phase obtained via Fourier-transform-based phase retrieval. Removing this factor enables precise reconstruction through a straightforward inverse Fourier transform, without requiring iterative computations. Compared with conventional far-field diffraction setups, our approach reduces system length and allows the use of smaller CCD sensors. Experimental validation using a modified Mach–Zehnder interferometer demonstrates high reconstruction accuracy and robustness. Overall, this method provides an efficient, practical, and real-time solution for object reconstruction, with the potential to simplify and miniaturize optical setups, offering an alternative approach to standard coherent diffraction imaging techniques. Full article

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

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24 pages, 2353 KB

Open AccessReview

Pulsed Diode-Pumped Alkali Vapor Lasers: State of the Art, Open Challenges, and Future Architectures

by Wenning Xu, Rongqing Tan and Zhiyong Li

Photonics 2026, 13(5), 411; https://doi.org/10.3390/photonics13050411 - 23 Apr 2026

Abstract

Diode-pumped alkali vapor lasers (DPALs) offer high quantum efficiency, low thermal loading, excellent beam quality, and emission wavelengths matched to important application scenarios. Extending DPALs toward pulsed regimes is of particular interest for applications such as lidar, free-space optical communication, and precision material [...] Read more.

Diode-pumped alkali vapor lasers (DPALs) offer high quantum efficiency, low thermal loading, excellent beam quality, and emission wavelengths matched to important application scenarios. Extending DPALs toward pulsed regimes is of particular interest for applications such as lidar, free-space optical communication, and precision material processing, where high peak power and flexible temporal control are required. This review surveys the key technologies underlying DPAL systems and summarizes the progress in pulsed-generation approaches. The pulsed techniques reported to date are systematically reviewed, including pump modulation, intracavity modulation, cavity dumping, and mode-locking, together with a comparison of their performance. The current status indicates that pulsed DPALs remain at an early stage, with limitations in parameter space exploration and performance scaling. Future developments are expected along several directions, including further exploration of mode-locked DPALs, burst-mode pulse generation for structured temporal output, power scaling through MOPA architectures, and spectral extension via nonlinear frequency conversion. These directions collectively define the pathway toward high-performance pulsed DPAL systems. Full article

(This article belongs to the Special Issue Laser Technology and Applications, 2nd Edition)

11 pages, 14513 KB

Open AccessArticle

Design and Co-Simulation of an Integrated Thin-Film Lithium Niobate Optical Frequency Comb for SDM Interconnects

by Haichen Wang, Jiahao Si, Jingxuan Chen, Zhaozheng Yi, Shuyuan Shi, Mingjin Wang and Wanhua Zheng

Photonics 2026, 13(5), 410; https://doi.org/10.3390/photonics13050410 - 23 Apr 2026

Abstract

We propose a monolithically integrated optical frequency comb (OFC) generation platform on thin-film lithium niobate (TFLN), featuring cascaded dual-drive Mach–Zehnder modulators (DDMZM) and a

{S i}_{3} N_{4}

-assisted spot size converter (SSC). To capture microscopic mode mismatches and spatial phase accumulation [...] Read more.

We propose a monolithically integrated optical frequency comb (OFC) generation platform on thin-film lithium niobate (TFLN), featuring cascaded dual-drive Mach–Zehnder modulators (DDMZM) and a

{S i}_{3} N_{4}

-assisted spot size converter (SSC). To capture microscopic mode mismatches and spatial phase accumulation often overlooked in idealized scalar simulations, we establish a multi-physics co-simulation framework integrating finite-difference time-domain (FDTD) analysis with macroscopic transmission modeling. Based on this framework, the cascaded modulator architecture generates 25 highly stable comb lines with a dense 2 GHz spacing and an envelope flatness within 2 dB. Tolerance analysis indicates that the comb generation is highly resilient to typical manufacturing and environmental variations, including thermal bias drift, RF phase mismatch, and half-wave voltage (

V_{π}

) dispersion. Furthermore, physical-layer modeling shows that the integrated SSC reduces fiber-to-chip coupling loss to 0.55 dB per facet, preserving the necessary optical power budget. To validate the platform’s viability as a multi-wavelength continuous-wave source for spatial-division multiplexed (SDM) interconnects, a parallel transmission over a 20 km standard single-mode fiber is modeled. Using a digital signal processing (DSP)-free 10 Gb/s non-return-to-zero (NRZ) scheme, the 25-channel system maintains a worst-case bit error rate strictly below the forward error correction (FEC) threshold. This work offers a practical, physics-based evaluation framework for high-density co-packaged optics (CPO). Full article

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

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17 pages, 2160 KB

Open AccessArticle

Research on Coal and Rock Identification by Integrating Terahertz Time-Domain Spectroscopy and Multiple Machine Learning Algorithms

by Dongdong Ye, Lipeng Hu, Jianfei Xu, Yadong Yang, Zeping Liu, Sitong Li, Jiabao Li, Longhai Liu and Changpeng Li

Photonics 2026, 13(5), 409; https://doi.org/10.3390/photonics13050409 - 22 Apr 2026

Abstract

Aiming to address the problems of low accuracy in coal–rock identification during coal mining, which lead to energy waste and safety hazards, a high-precision coal–rock medium identification method combining terahertz time-domain spectroscopy technology and multiple machine learning algorithms is proposed. By preparing coal–rock [...] Read more.

Aiming to address the problems of low accuracy in coal–rock identification during coal mining, which lead to energy waste and safety hazards, a high-precision coal–rock medium identification method combining terahertz time-domain spectroscopy technology and multiple machine learning algorithms is proposed. By preparing coal–rock samples with a gradient change in coal content, terahertz time-domain spectroscopy data of coal–rock mixed media are collected, and optical parameters such as the refractive index and absorption coefficient are extracted. Principal component analysis is used to reduce the dimensionality of the terahertz data, and machine learning algorithms such as support vector machine, least squares support vector machine, artificial neural networks, and random forests are adopted for classification and identification. The study found that terahertz waves are more sensitive to coal–rock media in the 0.7–1.3 THz frequency band, and that the refractive index and absorption coefficient of coal–rock mixed media are significantly positively correlated with coal content within the range of 0–30%. After feature extraction and K-fold cross-validation, the random forest model achieved a coal–rock classification accuracy of over 96% on the test set, significantly outperforming other comparison algorithms. The research verifies the efficiency and practicality of terahertz technology combined with multiple machine learning algorithms in coal–rock identification, providing a new method for fields such as mineral separation. This method has, to a certain extent, broken through the accuracy bottleneck of traditional coal–rock identification technologies within its applicable range, providing a new solution for real-time detection of coal–rock interfaces and is expected to further reduce the risks of ineffective mining and roof accidents in the future. Full article

(This article belongs to the Special Issue Advancements in Terahertz Metamaterial Optics, Devices, and Applications)

21 pages, 4914 KB

Open AccessReview

Recent Progress in Multimode Fibers

by Ming-Jun Li

Photonics 2026, 13(5), 408; https://doi.org/10.3390/photonics13050408 - 22 Apr 2026

Abstract

Multimode fibers (MMFs) have been a key component in short-reach transmission systems for over 50 years and remain the predominant transmission medium for Vertical Cavity Surface-Emitting Laser (VCSEL)-based short links in data centers. To meet the growing demand for higher data rates, MMFs [...] Read more.

Multimode fibers (MMFs) have been a key component in short-reach transmission systems for over 50 years and remain the predominant transmission medium for Vertical Cavity Surface-Emitting Laser (VCSEL)-based short links in data centers. To meet the growing demand for higher data rates, MMFs have continuously evolved to enhance bandwidth performance. This paper provides an overview of the fundamental properties of MMFs, with an emphasis on fiber parameters that influence bandwidth capabilities. We discuss trends in increasing data rates for MMF transmission systems in data centers and review recent progress in MMF technology aimed at boosting bandwidth. In particular, we highlight innovative fiber designs, including high-bandwidth 50 μm MMFs, large-core MMFs, long-wavelength MMFs, universal fibers, MMF bundles, and multicore fibers. Full article

(This article belongs to the Special Issue Advances in Multimode Optical Fibers and Related Technologies)

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18 pages, 24765 KB

Open AccessArticle

Field-Transformation-Based Light-Field Hologram Generation from a Single RGB Image

by Xiaoming Chen, Xiaoyu Jiang, Yingqing Huang, Xi Wang and Chaoqun Ma

Photonics 2026, 13(5), 407; https://doi.org/10.3390/photonics13050407 - 22 Apr 2026

Abstract

We propose a field-transformation-based framework for generating phase-only light-field holograms from a single RGB image. The method establishes an explicit pipeline from monocular scene inference to holographic wavefront synthesis, without requiring multi-view capture or task-specific hologram-network training. First, we construct a layered occlusion [...] Read more.

We propose a field-transformation-based framework for generating phase-only light-field holograms from a single RGB image. The method establishes an explicit pipeline from monocular scene inference to holographic wavefront synthesis, without requiring multi-view capture or task-specific hologram-network training. First, we construct a layered occlusion RGB-D model from the input image using monocular depth estimation, connectivity-based layer decomposition, and occlusion-aware inpainting, which provides a lightweight 3D prior for sparse-view rendering in the small-parallax regime. Second, we transform the rendered sparse RGB-D light field into a target complex wavefront on the recording plane through local frequency mapping, thereby bridging explicit scene geometry and wave-optical field construction. Third, we optimize the phase-only hologram under multi-plane amplitude constraints using a geometrically consistent initial phase and an error-driven adaptive depth-sampling strategy, which improves convergence stability and reconstruction quality under a limited computational budget. Numerical experiments show that the proposed method achieves better depth continuity, occlusion fidelity, and lower speckle noise than representative layer-based and point-based methods, and improves the average PSNR and SSIM by approximately 3 dB and 0.15, respectively, over Hogel-Free Holography. Optical experiments further confirm the physical feasibility and robustness of the proposed framework. Full article

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21 pages, 5606 KB

Open AccessArticle

Tip–Tilt Aberration Compensation for Laser Array Atmospheric Propagation Based on Cooperative Beacons

by Xiaohan Mei, Yi Tan, Ce Wang, Jiayao Wu, Ping Yang and Shuai Wang

Photonics 2026, 13(5), 406; https://doi.org/10.3390/photonics13050406 - 22 Apr 2026

Abstract

Laser beam combining is essential for achieving high-power and high-radiance output. However, atmospheric turbulence induces independent tip–tilt aberrations across discrete sub-beams in laser array systems, which severely degrades the concentration of far-field energy. Traditional wavefront sensing techniques are primarily designed for the continuous [...] Read more.

Laser beam combining is essential for achieving high-power and high-radiance output. However, atmospheric turbulence induces independent tip–tilt aberrations across discrete sub-beams in laser array systems, which severely degrades the concentration of far-field energy. Traditional wavefront sensing techniques are primarily designed for the continuous wavefront of a single laser and are not directly applicable to laser array, whereas indirect optimization-based methods often suffer from slow convergence and limited real-time performance. To address these limitations, this study introduces a tip–tilt aberration compensation system for laser array propagation based on cooperative beacons with a shared-aperture transmit–receive configuration. The primary innovation consists of a modified Shack–Hartmann wavefront sensor (SHWFS) tailored to a discrete multi-beam layout, which facilitates the direct, independent, and simultaneous measurement of tip–tilt aberrations for each sub-beam. In conjunction with a segmented deformable mirror (SDM), the architecture can facilitate real-time closed-loop correction with high bandwidth and high precision. Numerical simulations of a 7-, 19-, and 37-beam laser array, together with validation experiments utilizing a 30-beam configuration, demonstrate that the proposed approach effectively suppresses tip–tilt error induced by turbulence. After closed-loop correction, the Strehl ratio (SR) increases above 0.92 (

r_{0} = 5 cm

), while the beam quality factor β reduces below 1.37 (

r_{0} = 5 cm

). Furthermore, the system retains performance stability as the number of sub-beams increases, demonstrating the scalability of the proposed method. In contrast to conventional approaches designed for a continuous wavefront, the proposed method offers a feasible approach for a discrete laser array system, providing robust and scalable tip–tilt correction under varying atmospheric conditions. Full article

(This article belongs to the Special Issue Challenges and Future Directions in Adaptive Optics Technology, 2nd Edition)

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20 pages, 3318 KB

Open AccessArticle

Fast Decomposition of Single Excitation–Emission Matrix Fluorescence Spectrum via Encoder–Decoder Model

by Zhenjie Zhou, Qingtao Wu and Xiaoping Wang

Photonics 2026, 13(5), 405; https://doi.org/10.3390/photonics13050405 - 22 Apr 2026

Abstract

Three–dimensional excitation–emission matrix (3D–EEM) fluorescence spectroscopy is widely applied for the rapid characterization of dissolved organic matter (DOM) in aquatic environments. However, conventional decomposition based on parallel factor analysis (PARAFAC) requires multiple spectra and manual intervention, limiting its applicability for rapid analysis and [...] Read more.

Three–dimensional excitation–emission matrix (3D–EEM) fluorescence spectroscopy is widely applied for the rapid characterization of dissolved organic matter (DOM) in aquatic environments. However, conventional decomposition based on parallel factor analysis (PARAFAC) requires multiple spectra and manual intervention, limiting its applicability for rapid analysis and future online implementation. The purpose of this study is to develop an efficient data–driven method capable of decomposing fluorescence components from a single 3D–EEM spectrum. We propose a conditional single–spectrum decomposition network (CSSD–Net) based on the encoder–decoder model. The encoder extracts fluorescence features from the input spectrum, while the decoder combines these features with conditional information on component count to generate up to five component maps. The component count can be automatically predicted by CSSD–Net or manually specified to support flexible application scenarios. CSSD–Net was trained using publicly available component spectra from the OpenFluor database without PARAFAC preprocessing. Validation on natural water samples demonstrates that the results obtained from CSSD–Net using a single sample are highly consistent with those from PARAFAC using multiple parallel samples, with a mean Tucker’s congruence coefficient (TCC) of 0.9615. These results show that CSSD–Net provides a fast and practical solution for decomposing single 3D–EEM spectra under constrained aquatic scenarios, and it has potential for future near–real–time and in situ applications. Full article

(This article belongs to the Special Issue Advanced Optical Metrology Technology)

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