Photonics doi: 10.3390/photonics13090850

Authors: Jianfeng Chen Ming Zhao He Tian

The performance of a laser detection system is commonly summarized by a limited set of instrumental figures of merit, including detection range, sensitivity, spatial or spectral resolution, ranging precision, and temporal response [...]

Photonics doi: 10.3390/photonics13090849

Authors: Jingyu Zhang Hengli Feng Chang Liu Dongchao Fang Jincheng Wang Lehui Wang Zuoxin Zhang Lingling Ran Yang Gao

In the original publication [...]

Photonics doi: 10.3390/photonics13090848

Authors: Mario Angel Rico-Mendez Romeo de Jesús Selvas-Aguilar Ricardo Chapa-Garcia Manuel García-Méndez Juan M. Sierra-Hernandez Norma Patricia Puente-Ramirez Eloisa Gallegos-Arellano Carlos Adrián Calles-Arriaga Arturo Alberto Castillo-Guzmán Leonardo Arévalo Sheila Bazavilvazo-Azua

The design of a temperature sensor employing the Mach–Zehnder configuration with an offset in the fiber core represents a novel methodology that leverages the distinctive attributes of optical fibers to attain enhanced sensitivity and precision in temperature measurement. Ambient temperature measurement is enabled by aluminum-doped zinc oxide (AZO) thin films deposited via sputtering onto optical fiber structures for sensing applications in the 1540–1555 nm wavelength range. This work explores the potential of this interferometer to measure the effective temperature variation within the core offset of an optical fiber in a cost-effective manner.

Photonics doi: 10.3390/photonics13090847

Authors: Jiangbo Feng Yun Chen Ming Li Jiayi Zhang Yanqian Lu Huaquan Su Zhenhong Wang

In this work, we demonstrate the generation and evolution behaviors of rectangular pulses in a passively mode-locked erbium-doped fiber laser incorporating a molybdenum disulfide (MoS2) saturable absorber. At lower pump power, the laser operates in a stable fundamental mode-locked state. With the gradual increase in pump power and appropriate polarization adjustment, the rectangular-pulse regime with spectral sidebands emerges, and the corresponding temporal width broadens from 0.34 ns to 0.85 ns as the pump increases. The dispersive Fourier transform measurements are employed to capture the real-time spectral evolution of the generated rectangular pulses, revealing that such pulses consist of chaotic sub-pulses with randomly varying intensities. Furthermore, the numerical simulations are in reasonable agreement with the experimental observations. Our results shed light on the pulse dynamics governing the transition and temporal width broadening, contributing to a deeper understanding of the underlying nonlinear phenomena in ultrafast fiber lasers.

Photonics doi: 10.3390/photonics13090846

Authors: Lifeng Duan Shuchen Guo Yejun Xu Guoping Shi

Hong–Ou–Mandel (HOM) interference is a standard method for characterizing photon indistinguishability. However, in practical systems, the measured interference visibility is not solely determined by intrinsic spectral–temporal mismatch but can also be affected by photon-number statistics, multiphoton background, and technical noise. Here, we establish an observable-level framework for HOM interference in practical sources. Within this framework, under stationary-field and fixed-detection conditions, the measured coincidence rate is decomposed into a delay-dependent interference term and a stationary background term that is independent of the HOM interference. The former is described by an effective spectral overlap function G(τ), while the latter mainly originates from source statistics and additive imperfections. This separation shows that the dip profile and intrinsic depth are governed by overlap-limited interference, whereas the observed visibility can be further degraded by statistical and external backgrounds. For single-photon and weak coherent-state inputs, G(τ) represents the effective temporal–spectral mode overlap. For the stationary thermal field, its delay-dependent envelope can be related to the first-order temporal coherence function g(1)(τ), whereas photon bunching contributes separately through g2(0). We further clarify that post-selection suppresses background and reveals intrinsic interference more directly but cannot improve the intrinsic spectral–temporal overlap. This framework provides a unified basis for distinguishing genuine indistinguishability loss from measurement-induced visibility degradation.

Photonics doi: 10.3390/photonics13090845

Authors: Chenglong Ding Zhigang Li Dapeng Zhao Zongsheng Chen Xiangyin Lv Jinhua Zhang Jiangming Shi

Photonic crystal films possess excellent capability for infrared radiation modulation, and applying them to the surfaces of high-temperature targets represents an effective approach to achieving mid-far infrared stealth. In this work, a novel photonic crystal heterostructure film comprising 12 layers and constructed from Al2O3, Ge, and HfO2 is designed based on the transfer matrix method. Experimental measurements show that, at room temperature, the film exhibits an emissivity of 0.181 in the mid-infrared band and 0.144 in the far-infrared band, demonstrating favorable infrared stealth performance; meanwhile, an emissivity of 0.475 in the non-detection band (5–8 µm) enables effective radiative cooling. As the temperature is gradually elevated up to 500 °C, the film structure maintains favorable infrared stealth and radiative cooling performance. Furthermore, both theoretical simulations and experimental measurements demonstrate that the film exhibits excellent wave-transmission performance in the 2–18 GHz range, which enables its potential integration with radar-absorbing materials to achieve multi-band compatible stealth. The designed and fabricated photonic crystal heterostructure film provides a new reference for infrared stealth strategies under high-temperature conditions.

Photonics doi: 10.3390/photonics13090844

Authors: Yuanqing Wang Feng Chen Jinghao Zhang Tong Li Lianqing Dong Leran Wang Yang Zhang Jinhui Yang Xiaoju Men Dongmeng Wei Jicun Feng Xueliang Lü Bin Xu Likuan Zhu Kun Liang

The rapid development of quantum technology and single-photon LiDAR places ever-increasing demands on detection systems in terms of bandwidth suppression, wavelength stability, and transmission accuracy. Conventional narrowband filters, however, are falling short in sub-nanometer bandwidth control and high-temperature stability, thereby failing to support high-precision measurements. Consequently, ultra-narrowband optical filters have emerged as a key approach to overcoming these performance bottlenecks. This paper focuses on the core technologies involved in the development of such filters. In the design phase, multiparameter optimization based on a Fabry–Perot (F-P) etalon translates application requirements into fabrication parameters. For substrate fabrication, a sequential process of computer numerical control (CNC) milling, lapping, chemical–mechanical polishing (CMP), and ion-beam polishing, followed by atomic layer deposition (ALD) step formation and ion-beam evaporation coating, is employed to achieve a nanometer-level surface figure and roughness. Temperature control is achieved via a dual-tank hybrid circulation system, maintaining stability within ±0.1 °C. Test results show that the fabricated filter exhibits a full width at a half maximum (FWHM) of 30–60 pm, a free spectral range (FSR) of 260 ± 5 pm, a temperature stability of ≤3 pm/°C, and a peak transmittance of ≥80%. These results preliminarily confirm the device’s excellent overall performance.

Photonics doi: 10.3390/photonics13090843

Authors: Xin Chen Nikolay Ledentsov Jason E. Hurley Oleg Makarov Hao Dong Ming-Jun Li Alexander Ledentsov Nikolay Ledentsov

Self-injection-locked (SIL) VCSELs enable optically coupled apertures to operate as a coherent supermode, combining the high power of multi-aperture devices with quasi-single-mode emission, narrowing far-field beam divergence, and simplified coupling into single-mode fiber. In this work, SIL VCSELs operating near 850 and 880 nm were characterized to examine whether their chirp properties remain favorable for extended-reach short-wavelength transmission. More than 29% coupling efficiency into a single-mode fiber (Hi780) was achieved for both devices using a two-lens coupling arrangement. Link transfer functions were measured over a long single-mode fiber, Hi780, and via a standard single-mode fiber, with fundamental-mode launch via a spliced section of Hi780. The dominant transient chirp behavior is preserved for SIL VCSEL similar to the case of single-mode (SM) VCSEL and multi-aperture (MA) SM VCSELs. A unified full-spectrum fitting method was introduced to extract physically consistent chirp and chromatic-dispersion parameters from shallow transfer-function dips, with the chirp parameter determined to be approximately −2.6. Using the 850 nm SIL VCSEL, 32G NRZ transmission over 1 km of a standard single-mode fiber with fundamental mode launch was demonstrated. Compared with the back-to-back configuration, the 1 km link produced a more open eye and improved 7-tap FFE equalized Gaussian BER from 7.56 × 10−13 to 1.53 × 10−14, confirming chirp–dispersion-induced bandwidth enhancement.

Photonics doi: 10.3390/photonics13090842

Authors: Raushan Z. Aimagambetova Aigul N. Seraly Ali D. Mekhtiyev Aliya D. Alkina Ruslan A. Mekhtiyev Dinara T. Mukasheva

The development of accurate, robust and adaptive methods for monitoring the optical parameters of fiber-optic sensors (FOS) is one of the priority tasks in the field of precision measurements, especially in the context of rapidly growing requirements for intelligent monitoring systems. This paper presents a comprehensive approach to the use of modern deep learning algorithms for analyzing and processing spectral data coming from FOS. The proposed solution is based on the use of convolutional neural networks (CNN) for the automatic extraction of informative features, as well as autoencoders for noise suppression and signal restoration. A hybrid architecture combining CNN and recurrent neural networks (RNN) was developed. The experiments conducted confirmed the effectiveness of the evaluated models. On the independent regression test set, the CNN-only model achieved a macro-averaged R2-based prediction score of 98.2% without added noise and 89.4% under high-noise conditions; on a separate temporal test sequence, the hybrid CNN + RNN model achieved 95.0% compared with 88.0% for CNN alone. The presented approach has high resistance to noise and the ability to scale to various types of FOS. At the conclusion, the prospects for the practical applications of the proposed system are discussed: the structural monitoring of buildings and structures and the automation of processes in industry and energy, with an emphasis on reliability, autonomy and integration with existing platforms.

Photonics doi: 10.3390/photonics13090841

Authors: Qi Cheng Feng Pan Binjie Gu

Fringe projection profilometry (FPP) is widely used for high-precision three-dimensional measurement, but its performance is severely degraded when measuring non-Lambertian surfaces with complex reflectance. In such cases, fringe saturation and low modulation lead to unreliable phase values and missing reconstructed data. To address this problem, this paper proposes a photometric feature-guided phase inpainting method for high-dynamic-range FPP. Within the proposed FPP with multi-illumination framework, fringe images are used for phase calculation, while multi-illumination images provide additional photometric cues for phase inpainting. A photometric feature-guided phase inpainting network is designed to extract photometric features from multi-illumination images and fuse them with the damaged phase map. The network is trained on a large-scale synthetic dataset and validated using a prototype system. In standard copper-sphere measurements, PF-PINet increases the average reliable reconstruction ratio from 81.85% to 84.31% while maintaining comparable reconstruction accuracy, indicating that the proposed method can improve measurement completeness for non-Lambertian surfaces.

Photonics doi: 10.3390/photonics13090840

Authors: Erem Ujah Gymama Slaughter

Gold-sputtered tapered optical fiber (Au-TOF) sensors were developed for high-sensitivity refractive-index (RI) detection using aqueous glucose standards spanning 1.33–1.41 RIU (0–50% w/v). The sensors were fabricated using a reproducible workflow combining flame-brushing tapering, plasma surface preparation, and rotational magnetron sputtering, enabling azimuthally uniform gold coatings without the use of thiol or sulfur linker chemistries. Two sputtering durations (24 s and 30 s) were investigated to examine thickness-dependent plasmonic coupling and sensing performance. Optical measurements were conducted using a broadband supercontinuum source and compact spectrometer within a 20 µL microfluidic sensing chamber. Increasing glucose concentration produced a monotonic decrease in transmission intensity and a systematic red shift of the resonance minimum, consistent with enhanced evanescent-field interaction at the gold–dielectric interface. Across four independent trials, the 24 s Au-TOF sensor exhibited sensitivities up to 985 nm/RIU, while the 30 s device achieved sensitivities up to 1590 nm/RIU with strong linearity (R2 = 0.99). The enhanced sensitivity observed for the longer sputtering duration is attributed to improved gold film continuity and stronger plasmonic coupling. These results demonstrate a scalable, linker-free fabrication strategy for plasmonically enhanced tapered fiber sensors and establish the Au-TOF platform as a promising approach for label-free optical biosensing in compact microfluidic environments.

Photonics doi: 10.3390/photonics13090839

Authors: Lianghao Wang Zhiping Song Zihao Liu Yunzhi Wu Feng Wang Li Li Zhengqiang Li

Polarization–spectral imaging information has theoretical advantages for camouflage-net recognition. However, for specific observation targets, the selection of effective spectral bands and polarization parameters, which are critical issues in engineering applications, remains unclear. Using a newly developed hyperspectral polarization imaging prototype, we conducted systematic outdoor observations of a grassland camouflage net placed over green grass at multiple viewing angles and different times of day. After evaluating and confirming the net’s camouflage effectiveness, we analyzed polarization-parameter spectral images of both the net and background under varying observation and solar altitude angles. The Fisher criterion was used to identify discriminative polarization parameters and spectral ranges. Effective bands were determined as S1 at 675–696 nm, S2 at 673–677 nm, S3 at 607–616 nm and 625–700 nm, DoP at 676–700 nm, and DoLP at 668–680 nm. Grayscale images in these bands further verified the separability of the camouflage net from grass. These results provide experimental evidence and wavelength-parameter references for engineering applications of polarization–spectral imaging in camouflage-net detection.

Photonics doi: 10.3390/photonics13090838

Authors: Hyeonah Lee Hyeran Noh

Reliable multicomponent analysis has become an increasingly important objective in surface-enhanced Raman spectroscopy (SERS) as analytical applications extend beyond the detection of predefined molecular targets. However, identifying and quantifying multiple constituents within chemically complex samples remains challenging because SERS spectra do not directly represent the underlying chemical composition. Instead, they are shaped by selective molecular representation at plasmonic interfaces and the superposition of signals from multiple constituents, creating an inherently ambiguous relationship between spectral observations and the chemical system being analyzed. This review examines the physicochemical origins of these challenges and the analytical strategies developed to overcome them. Existing approaches are organized according to their primary analytical function, encompassing selective interfacial control, physical separation, computational analysis, and integrated analytical workflows. Each strategy is evaluated in the context of the specific analytical limitations it addresses, providing a structured perspective on the capabilities and remaining challenges of multicomponent SERS analysis.

Photonics doi: 10.3390/photonics13090837

Authors: Zhenzhen Chen Sen Wang Chao Ma Shaowen Jing Jiayu Huang Mingkun Zhang

Multispectral imaging systems often exhibit wavelength-dependent focus responses because of chromatic aberration, nonuniform spectral sensitivity, and scene-dependent reflectance, making single-band autofocus inadequate for optimizing the complete spectral image cube. This study proposes a nonlinear band-correlation guided deep reinforcement learning framework for autofocus optimization in a self-developed 31-band multispectral imaging system covering 360–980 nm. Through-focus response curves are first extracted from all spectral channels, and a nonlinear inter-band dependency matrix is constructed to characterize complementary and redundant focusing information across wavelengths. A compact subset of representative bands is then selected and embedded into the reinforcement-learning state to guide closed-loop motor control. The agent jointly determines focusing direction, displacement, and stopping time while maximizing a global multispectral focus objective that considers mean sharpness, inter-band consistency, and worst-band degradation. During online autofocus, only the representative bands are acquired at intermediate positions, whereas the complete 31-band cube is captured after convergence. The framework is evaluated using standardized optical targets and spectrally heterogeneous scenes through focal-position error, MTF, full-band focus retention, acquisition time, and robustness. The proposed design provides an efficient computational-photonics solution for broadband multispectral autofocus with reduced band-acquisition and motor-search overhead.

Photonics doi: 10.3390/photonics13090835

Authors: Shiqi Yang Daohong Xiao Jiaqi Ma Bowen Wei Kui Wang Haishan Tian Leyong Jiang

Optical bistability (OB) is a fundamental nonlinear effect enabling all-optical switching, logic, memory, and sensing, yet its practical implementation is often hindered by high power thresholds and limited tunability. Here, we theoretically demonstrate low-threshold and widely tunable OB in a Fabry-Pérot (FP) cavity integrated with a Weyl semimetal (WSM) layer. By combining the strong local-field enhancement of the FP cavity with the large third-order nonlinear conductivity of the WSM, we achieve pronounced bistable hysteresis loops in both the transmitted field amplitude and transmittance at terahertz frequencies, with incident field switching thresholds on the order of 106 V/m. The bistability thresholds and hysteresis width can be flexibly controlled by tuning the WSM Fermi energy, the position of the WSM layer inside the cavity, and the mirror transmittance. Moreover, exploiting the strong dependence of the down-switching threshold on small changes in the refractive index of the cavity medium, we propose a terahertz refractive-index sensor with a maximum sensitivity of 296.8 GW·m−2·RIU−1. These results provide a theoretical basis for tunable nonlinear photonic devices and threshold-based terahertz refractive-index sensors using topological semimetals.

Photonics doi: 10.3390/photonics13090836

Authors: Shudi Zhang Jianyun Lin Jingru Huang Zhisen Liang Fangfang Chen Guihong Wang

The unauthorized addition of silver nitrate and excessive heavy metals in hair dye creams poses serious health risks, creating an urgent need for rapid and sensitive detection methods. In this study, a spark discharge-enhanced fiber laser-induced breakdown spectroscopy (SD-LIBS) system was developed for the simultaneous detection of five heavy metal ions (Ag, Pb, Cd, As, Hg) in hair dye creams. The system combines three signal enhancement strategies: high-voltage spark discharge, Au nanoparticle-assisted enhancement, and analyte enrichment on a superhydrophobic–hydrophilic patterned graphite substrate. Hydrophilic micro-pits were fabricated by laser scanning on the superhydrophobic graphite surface to confine and concentrate analyte residues upon droplet drying. Under optimized discharge conditions of 22 nF and 2000 V, the spark discharge achieved a signal-to-noise ratio enhancement of approximately 12 times. Au nanoparticle enhancement further increased the signal intensity by 49.3%. The optimal pit diameter and sample loading volume were 0.1 mm and 30 μL, respectively. The calibration curves for all five elements showed good linearity with R2 values close to 0.99, and the lowest detection limits reached 1.9 μg/L for Ag. The average spiked recovery was 82.07%. The method was applied to four commercial hair dye cream samples, and the Ag detection results agreed well with ICP-OES reference values with an average error of 16.41%. This work demonstrates that the proposed SD-LIBS system offers a low-cost, rapid, and sensitive approach for on-site screening of heavy metals in cosmetics and other consumer products.

Photonics doi: 10.3390/photonics13090834

Authors: Yongchao Zou Minzheng Sun Kang Lou Pan Xu Zhengliang Hu Min Zhu

Interferometric fiber-optic hydrophones are widely deployed in passive underwater acoustic detection systems due to their high sensitivity, broad dynamic range, and immunity to electromagnetic interference. However, their adoption in active acoustic systems has been largely constrained by the limited operational bandwidth of conventional architectures. Through coupled acoustic–structural simulations, this work identifies the operational bandwidth bottleneck as arising from low-order mechanical resonance of high-modulus mandrel structures combined with acoustic cavity resonance and near-field scattering within the enclosed cylindrical geometry. To address these limitations, an optimized push–pull mandrel structure, featuring a miniaturized profile of φ16 mm × 16 mm and a low-acoustic-impedance material, is implemented to eliminate mechanical resonance within the operational bandwidth. Additionally, an integrated acoustic metamaterial liner is incorporated into the inner tube to mitigate scattering and reverberation effects. Experimental characterization of the prototype demonstrates an average phase sensitivity of −132 dB re 1 rad/μPa, with fluctuations below ±1.5 dB across 20 Hz to 31.5 kHz. Utilizing a custom-built demodulation system, the hydrophone achieves a minimum detectable pressure of 42 dB re 1 µPa/√Hz (126 µPa/√Hz) at 20 Hz and 27 dB re 1 µPa/√Hz (22 µPa/√Hz) above 2 kHz, remaining over 28 dB below the deep-sea state zero ambient noise floor at 20 Hz, and hence confirms its passive acoustic monitoring capability. In the high-frequency regime above 2 kHz, the hydrophone maintains ±1.5 dB sensitivity flatness with a horizontal directivity variation of only ±1.51 dB at 30 kHz, providing a robust platform for broadband active acoustic applications, including active sonar, underwater acoustic imaging and acoustic communications. These results establish a viable pathway toward next-generation dual-mode underwater acoustic systems requiring both high-fidelity passive listening and broadband active detection.

Photonics doi: 10.3390/photonics13090833

Authors: Bin Wang Yue Liu Kaifeng Zheng Xiaoyan Shi Bing Liu Siyao Ma Shuo Yang Jinguang Lv Yanxiu Jiang Enzhu Hou

A plasmonic metamaterial absorber (MA) exhibits unique capabilities of electric field enhancement and spectral modulation and plays a critical role in biosensing applications. In this work, a narrowband MA structure based on a plasmonic square nanoring is designed and numerically investigated. The proposed MA exhibits a high narrowband absorption of 99.77% at a 1216 nm wavelength with a 10 nm full width at half maximum (FWHM). Furthermore, the MA has sensitive absorption properties to the refractive index (RI) of the environment and shows a sensitivity of 186 nm/RIU with a stable value of quality factor around 120 in the RI ranging from 1.0 to 1.4. The sensing and recognition ability of the MA on actual biological cells is also numerically verified. Meanwhile, the proposed MA exhibits excellent capability of fluorescence amplification, achieving 3438–fold fluorescence enhancement at a 1216 nm wavelength with a high directivity of ~590. These properties make the proposed MA structure an excellent candidate for application in an optical sensing system.

Photonics doi: 10.3390/photonics13090832

Authors: Daniel A. Nolan

In this study, we simulate the propagation of a single photon propagating within a modal group, traversing a multimode optical fiber in the presence of mode coupling. We illustrate the propagation graphically on a group of higher-order Poincaré spheres. The spheres display the propagation of the light within the modal group, including polarization in time and in distance as the transmission proceeds. Thus, the amplitudes and the relative phases within the group can be visualized throughout the transmission, which is novel and very useful for understanding the propagation. At the fiber output, we show how to recover the input of a classical state using the simulated propagation information displayed on such multiple spheres. Once a classical modal path is established from end to end, one can transmit quantum states, for example, as a spatial–time binned QKD (quantum key distribution) code. A quantum state follows this classical path. Hence, it can include binary or qudit information. Accessing a photon output state is important for many quantum network applications, including quantum key distribution, routing, and entanglement swapping. In addition, one can use other modal groups within the same fiber to multiplex other quantum channels as well as multiplex within a modal group using principal modes, a more complicated communication method. This situation is also discussed while considering the use of the spheres. Their applications include higher-dimensional quantum communications, quantum cryptography, and quantum networks. It is important to point out that in today’s commercial quantum communications systems, quantum states are single-photon states, but they are not entangled. This includes both discrete and continuous variable quantum key distribution systems. Thus, in this report, we also address single-photon systems that are not entangled, unless stated otherwise.

Photonics doi: 10.3390/photonics13090831

Authors: Hexi Liang Wenzheng Ni Kangle Wang Jingwei Zhou Jinlin Liu Yong Ai

To mitigate the delay spread and inter-symbol interference (ISI) induced by optical scattering in underwater wireless optical communication (UWOC), this paper introduces long short-term memory (LSTM), a convolutional block attention module (CBAM), and residual connections into a convolutional neural network autoencoder (CNN-AE), and proposes a CNN-LSTM-AE-based end-to-end DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) system. In the proposed system, convolutional layers in the encoder serve to extract local features; CBAM adaptively weights salient features along the channel and spatial dimensions; LSTM layers model the temporal dependencies of signal sequences; and residual connections are incorporated to improve the learning capability for subtle signal features, thereby enhancing the robustness of the system against multipath channels. A symmetric structure is adopted at the receiver, ultimately enabling end-to-end signal recovery. Simulation results show that, under typical clear ocean and coastal ocean channel conditions, the proposed system outperforms end-to-end systems based on a fully connected autoencoder (FC-AE) and a CNN-AE at different modulation orders, i.e., different numbers of bits per symbol. For example, under strong scattering conditions in the coastal ocean channel, when the number of bits per symbol is 2 and the bit error rate (BER) is 10−3, the proposed system achieves signal-to-noise ratio (SNR) gains of approximately 3.33 dB and 1.82 dB over the two baselines. In terms of block error rate (BLER), SNR gains of approximately 4.52 dB and 2.19 dB are achieved over the two comparison systems, which substantiates the superior end-to-end transmission reliability of the proposed system.

Photonics doi: 10.3390/photonics13090830

Authors: Hezu Bai Xulei Qin Yayu Dai Zhou Ge Chengze Li Chaofan Wu Feng Shi Hongchang Cheng Haibo Fan Shilong Li

Neuromorphic imaging sensors (event cameras) offer a promising paradigm for computational imaging and human pose estimation (HPE) under extreme illumination conditions. Nevertheless, dark-scene background activity originating from photodiode dark current and circuit thermal noise, together with hot-pixel noise, severely corrupts event streams and impedes reliable HPE in low-light scenarios. To address this issue, we propose an adaptive event denoising framework built upon a spatiotemporal Gaussian-weighted neighborhood model with a dynamic thresholding mechanism. It can effectively suppress background activity and hot-pixel noise while preserving edge and motion details critical for pose estimation. Leveraging this denoising front-end, we construct a complete dark-scene neuromorphic HPE pipeline by transferring the pre-trained MediaPipe model onto event-based time-surfaces. Quantitative and qualitative evaluations on public and self-collected datasets demonstrate that our approach outperforms state-of-the-art denoising methods with an improvement of over 20% on public benchmarks and over 30% on self-collected dark-scene data. We expect our work to pave the way toward reliable dark-scene human–robot interaction through robust neuromorphic pose estimation.

Photonics doi: 10.3390/photonics13090829

Authors: Yuxing Bai Xiaolei Hao

Nonsequential double ionization is a fundamental process in ultrafast strong-field physics, containing rich information about electron correlation dynamics. In few-cycle laser fields, the carrier-envelope phase becomes a key parameter for controlling electron behavior. However, this process involves nonlinear mechanisms such as multi-electron stochastic dynamics and complex Coulomb interactions, posing significant challenges to traditional analytical theories. To address this, this work develops a numerical experimental approach based on a Monte Carlo stochastic algorithm, transforming the quantum problem into a computable stochastic sampling task. Through statistical sampling and final-state analysis of tens of millions of quantum trajectories, the central regulatory role of the carrier-envelope phase is systematically revealed. The computational results show that this phase can not only independently regulate the yields of double ionization and frustrated double ionization but also control the branching ratio between them, with a peak-to-peak modulation amplitude of approximately 20%. Additionally, it enables fine-tuning of the electron momentum correlation distribution. The physical mechanisms underlying these regulatory effects are clearly elucidated through analysis of the quantum trajectories. This work not only clarifies the critical role of the carrier-envelope phase in few-cycle intense-field double ionization but also demonstrates the powerful capability of the Monte Carlo stochastic algorithm in revealing the intrinsic stochasticity of strong-field physics and achieving precise physical control, providing an example for the deep integration of intense-field physics and computational science.

Photonics doi: 10.3390/photonics13090828

Authors: Pengcheng Yan Lingzhi Jiang Yufei Liu Cong Zhang Yangchen Cai Tianjun Wang Kai Zhu Bindi Xu Xian Zuo Shaowei Jiang Liming Yang

Lensless coded ptychography (CP) acquires multiple intensity measurements by translating either the object or the coded sensor, and reconstructs high-resolution, large field-of-view images via iterative phase retrieval. However, conventional CP suffers from slow reconstruction due to two limitations. First, periodic uniform scanning requires dense measurements to maintain sufficient measurement diversity, resulting in a large number of raw measurements and a high computational burden for iterative phase retrieval. Second, random initialization leads to slow convergence and increases the risk of stagnation in local minima. To address these limitations, we propose a joint optimization of the sampling strategy and initialization for accelerated reconstruction. Specifically, a continuous non-uniform scanning strategy preserves measurement diversity while reducing the number of required measurements. In addition, a low-resolution regularized ptychographic iterative engine (rPIE)-based initialization provides a more accurate starting point and accelerates the convergence of iterative phase retrieval. Both simulations and experiments demonstrate that the proposed approach achieves approximately three-fold faster convergence while maintaining high reconstruction fidelity. The proposed approach offers an effective solution for high-throughput imaging applications, including digital pathology and label-free quantitative phase imaging.

Photonics doi: 10.3390/photonics13090827

Authors: Jie He Yanan Zhao Tianli Wu

Optical force originates from momentum transfer in the interaction between light and matter, forming the physical foundation of optical tweezers. Over the past four decades, optical tweezers have undergone a paradigm shift from passive manipulation tools to active platforms for bioregulation. This review focuses on the latest advances in the field of optical mechanics, ranging from fundamental theory to biological applications. First, the fundamentals of light scattering in different regimes and the resultant optical forces, as well as the conditions for stable trapping, are described. Then, advanced optical tweezer platforms developed on various physical principles are reviewed. Applications from single-molecule force spectroscopy to optical probing of cellular mechanosensitive responses are discussed. At the end, future directions toward in vivo manipulation, high-throughput automation, and intelligent real-time feedback control are provided.

Photonics doi: 10.3390/photonics13090826

Authors: Qirui Shu Miaomiao Yan Ying He Yanfang Yang

Hyperbolic mode resonance (HMR) has emerged as a solution to conventional sensing limitations. HMR is introduced into a photonic crystal fiber (PCF) for sensing in this research. A dual-channel PCF sensor for both refractive index (RI) and temperature measurement is proposed. The device has two independent sensing channels on the dual-side polishing surface of the PCF. An Au/SnO2 bilayer in the RI channel induces HMR, while the SnO2/PDMS bilayer in the temperature channel induces lossy mode resonance, with PDMS as the temperature-sensitive material. By introducing an Au layer to excite the HMR effect, the sensor achieves enhanced resonance depth and figure of merit (FOM) without sacrificing sensitivity. When the RI ranges from 1.34 to 1.426 and the temperature varies from 25 °C to 100 °C, the optimized sensor achieves a maximum RI sensitivity of 9100 nm/RIU and a maximum temperature sensitivity of −5 nm/°C. A 2×2 sensitivity-matrix analysis confirms effective dual-parameter decoupling, with demodulation errors below 5×10−4 RIU and 0.1 °C under simultaneous RI and temperature variations. This PCF sensor offers high sensitivity, quantifiably low crosstalk, wide detection range and good fabrication tolerance, making it promising for biomedical, environmental and industrial applications.

Photonics doi: 10.3390/photonics13090825

Authors: Jincheng Wang Ronghui Yang Shiqi Yang Qijun Ma Wei Huang

In this paper, we systematically investigate the near-field radiative heat transfer in a two-body system described by the Lorentz model. Under a finite temperature difference between the upper and lower plates, we explore the evolution of radiative heat transfer by tuning the longitudinal and transverse optical phonon frequencies. Our results show that the near-field radiative heat flux can exceed the blackbody radiation limit by several orders of magnitude. Both the longitudinal and transverse phonon frequencies serve as effective control knobs, enabling broadband modulation of the spectral heat transfer. Furthermore, mirror-symmetric modulation of these phonon frequencies in both plates significantly enhances the tunability of the near-field heat transfer. The findings obtained in this study provide valuable theoretical insights and a fundamental reference framework for designing actively tunable near-field thermal devices based on similar material platforms.

Photonics doi: 10.3390/photonics13090824

Authors: Wen Xie Xinyu Jin Cheng Gu Boxiong Cui Chenyang Ma Yilun Ma Zhengyang Xie Xin Zhao Pengwei Gong Zheng Zheng

We propose and experimentally demonstrate a broadband photonic approach to linear frequency-modulated (LFM) microwave signal generation based on segmented parabolic phase modulation and synchronous recirculating phase accumulation. The expanded quadratic-phase trajectory is mapped onto an amplitude-limited segmented parabolic waveform, thereby increasing the effective phase slope without requiring a proportional increase in the peak driving voltage. Meanwhile, a synchronized optical recirculating loop enables coherent phase accumulation over multiple modulation passes, providing further bandwidth enhancement. Numerical simulations verify the bandwidth scalability enabled by this two-stage phase-enhancement mechanism, achieving a maximum simulated bandwidth of 39.04 GHz. Experimentally, an LFM microwave signal with a duration of 512 ns and a bandwidth of 10.6 GHz is successfully generated, corresponding to a chirp rate of 20.70 GHz/μs, a time–bandwidth product (TBWP) of 5427.2, and a pulse-compression ratio (PCR) of 5429.48. The frequency-sweep linearity of the generated signal is measured to be 0.868%, demonstrating excellent linear frequency-sweep performance. Multiple independent measurements further confirm the stability and repeatability of the system, with the maximum measured bandwidth reaching 11.65 GHz. These results demonstrate that the proposed approach provides a scalable route to broadband LFM microwave signal generation, combining high chirp rates, large TBWPs, excellent frequency-sweep linearity, and good repeatability.

Photonics doi: 10.3390/photonics13090823

Authors: Tong Xu Huatao Zhu Yongcheng Li Xin Zhang Xing Yang Haijing Li Yibo Liu Feng Jiang

A simple scheme for generating a broadened and flattened optical frequency comb (OFC) based on a directly modulated distributed feedback (DFB) semiconductor laser cascaded with electro-optic modulators is proposed. The initial OFC is generated by a gain-switched DFB semiconductor laser. The number of comb lines is further expanded by cascading a phase modulator and the flatness performance is optimized by adjusting the power of the radio frequency (RF) and bias voltage of the cascaded Mach–Zehnder modulator. Moreover, the spacing of comb lines can be flexibly tuned by varying the frequency of the RF signal, and an OFC with seven spectral lines within a 4.72 dB power variation is demonstrated. The 17-line OFC with a frequency spacing of 12.5 GHz and a flatness of 3 dB is further applied to an optical transmission system, which enables error-free transmission of a non-return-to-zero data signal. The proposed scheme is expected to find applications in flexible-grid optical transmission systems.

Photonics doi: 10.3390/photonics13090822

Authors: Jiyeon Baek Yuna Lee Hyunchae Chun

Free-space optical (FSO) communication provides high-capacity wireless transmission but suffers from reduced optical coupling efficiency when compact Cassegrain telescopes are employed because the secondary mirror blocks the central portion of the incident beam. This paper proposes a vortex beam-assisted FSO communication system that combines aperture-matched optical coupling with machine learning-based signal recovery. Unlike a conventional Gaussian beam, the annular intensity distribution of a Laguerre–Gaussian vortex beam is matched to the unobstructed annular aperture of a centrally obscured Cassegrain telescope, thereby reducing obstruction-induced optical loss. The coupling characteristics are analyzed using an annular aperture overlap model and experimentally validated in a 100 m free-space optical link employing Cassegrain transmitter and receiver front-ends. The mean measured telescope-output power is increased by more than 30% over the Gaussian reference. To overcome the system-induced signal aliasing, Transformer and long short-term memory (LSTM) equalizers are optimized and applied. Both models substantially outperform optimized threshold detection, while the LSTM achieves the lowest observed error rate with markedly fewer multiply–accumulate operations than the Transformer. These results show that aperture-matched optical coupling and computationally efficient sequence equalization, such as LSTM is a crucial component of compact, high-performance telescope-assisted FSO systems.

Photonics doi: 10.3390/photonics13090821

Authors: Samuel Lawman Keri McLean Sharon Mason Yao-Chun Shen Stephen B. Kaye Yalin Zheng

Microbial keratitis requires rapid diagnosis to enable timely, pathogen-directed treatment. Corneal confocal microscopy (CCM) is used to quickly diagnose acanthamoeba and fungal keratitis, but it relies on undesirable contact objective lenses, and no supplier has maintained a stable market presence. Here, we present a preliminary investigation of the potential abilities of non-contact optical coherence microscopy (OCM) as an emerging alternative technology. Using an ex vivo porcine corneal infection model, we demonstrate that OCM can produce en face images, similar to CCM, as well as 3D visualizations of fungi hyphae in stromal tissue. Like CCM, bacterial pathogens were too small to directly identify in OCM images. Nevertheless, OCM detected that ex vivo infection induced systematic changes to the stroma that are consistent with the breakdown of keratocyte structure. These proof-of-concept results demonstrate the potential of non-contact OCM for imaging microbial keratitis and support further in vivo evaluation.

Photonics doi: 10.3390/photonics13090820

Authors: Meng Zhu Mengmeng Liu Mingqian Xu Ru Yao Rongxuan Liang De Song Weijun Chen

To analyze the performance stability of image intensifiers under humid and hot environments, this study systematically investigates the device performance degradation mechanism through a combination of natural exposure tests, humidity variation aging tests, disassembly analysis of failed samples, and a vacuum decay model. Experimental and simulated results reveal that moisture ingress in a humid and hot environment serves as the dominant factor governing the luminance gain stability of devices. Microscopic defects such as cracks, voids, and delamination phenomena existing at the sealing interface between ceramic rings and metal gaskets are the direct cause of moisture penetration. The attenuation of luminance gain accelerates significantly with increasing ambient humidity, demonstrating that water vapor is the primary driving factor for the environmental failure of devices. This study clarifies the attenuation mechanism of luminance gain for image intensifiers under humid and hot environments, providing a theoretical basis for the optimization of packaging processes and the improvement in environmental adaptability of image intensifiers.

Photonics doi: 10.3390/photonics13090819

Authors: Zhen Zhang Zhigang Li Zhichao Bu Yaru Dai

Multi-wavelength Raman–Mie polarization lidar provides vertically resolved measurements of aerosol scattering, particle shape, and wavelength-dependent response. We analyzed quality-controlled clear-sky observations from June 2025 to March 2026 to identify aerosol optical regimes at inland Beijing Nanjiao and coastal Beihai. The feature space comprised log10(β532), δp,532, and AEβ,355/532, representing scattering intensity, particle nonsphericity, and size-sensitive spectral response. K-means was applied independently at each site. At Beijing Nanjiao, a high-depolarization, strong-scattering, and low-AEβ,355/532 regime was concentrated in the lowest observed layer, whereas lower-loading regimes occurred more frequently aloft. Its dust AOD and dust fraction were descriptively 37.5% and 19.2% above the site means, although the dust-related inter-regime differences were not significant after FDR correction. Beihai was dominated by low-depolarization regimes separated mainly by scattering intensity and wavelength response. Its strongest-scattering regime showed total, sea salt, OC, and sulfate AOD enhancements of 33.0%, 15.5%, 24.5%, and 41.9%, respectively; the inter-regime differences were significant for total and sulfate AODs but not for sea salt AOD. MERRA-2 aerosol diagnostics and trajectory analyses provided auxiliary regional context for interpreting the lidar-defined optical states and were not used as clustering inputs or direct chemical validation. These results demonstrate that combined polarization and multi-wavelength lidar sensing can distinguish vertically varying aerosol optical states that are obscured in surface or column-integrated observations.

Photonics doi: 10.3390/photonics13090818

Authors: Hailiang Liu

Non-radiative carrier loss originating from material defects together with slow charge migration act as two major limiting factors heavily suppressing the power output performance of organic bulk-heterojunction (BHJ) photovoltaic devices (OSCs). Herein, two-dimensional (2D) CdSe nanoplatelets (NPLs) are introduced into conventional P3HT:PCBM and high-efficiency PBDB-T:PCBM photoactive films as multi-purpose doping additives, aiming to finely tune film microstructures, refine interfacial energy level matching, accelerate carrier migration, and eliminate native trap sites inside the active layer. Statistical measurement data verify that 3 mg CdSe NPLs as the ideal doping dosage can realize concurrent performance upgrades for the two distinct OSC architectures investigated here. As for devices built on P3HT:PCBM blend films, the component modified with optimized CdSe NPL additives delivers boosted charge mobility rising from 0.87 × 10−5 cm2/V·s up to 2.58 × 10−5 cm2/V·s. Meanwhile, trap site concentration drops markedly from 7.41 × 1015 cm−3 to 4.33 × 1015 cm−3, which lifts the device power conversion efficiency (PCE) from 2.77% to 3.15%. Even more noticeable improvements are observed in PBDB-T:PCBM photovoltaic units; the refined doping recipe raises carrier mobility from 5.42 × 10−4 cm2/V·s to 8.69 × 10−4 cm2/V·s and cuts trap density down from 7.13 × 1016 cm−3 to 3.15 × 1016 cm−3, thus bringing about a substantial PCE boost ranging from 6.52% to 9.14%. The present study confirms the bifunctional advantages possessed by 2D CdSe NPLs, which can adjust BHJ microphase separation and electronic characteristics simultaneously. This research offers a simple and broadly applicable route to fabricate organic photovoltaic cells with superior efficiency.

Photonics doi: 10.3390/photonics13090817

Authors: Chengyao Pu Ge Yin Yanqing Wang Zeyu Zhao Changtao Wang Xiaoliang Ma Mingbo Pu Weijie Kong

Near-field lithography (NFL) can achieve super-resolution patterning beyond the optical diffraction limit by using evanescent waves. However, optical proximity effects in NFL can lead to nonuniform near-field intensity distribution, reduced image contrast, and increased pattern distortion in the photoresist. Optical proximity correction (OPC) is required to improve pattern fidelity. This work investigates a pixelated OPC method based on the covariance matrix adaptation evolution strategy (CMA-ES) under a rigorous coupled-wave analysis (RCWA) imaging model. The mask pixels are iteratively optimized according to the photoresist pattern error obtained from RCWA forward simulation. To improve mask regularity, total variation and connected component area penalties are introduced into the objective function to suppress fragmented pixel structures. Representative 110 nm half-pitch axis-symmetric layouts and additional two-dimensional layouts are used for evaluation. The results show that optimization driven only by pattern error can reduce proximity-induced pattern distortion, but it tends to generate fragmented pixel structures in the corrected mask layouts. After the regularization terms are introduced, the optimized mask layouts become more regular, with a moderate tradeoff in pattern error. The proposed method shows potential applications in mask layout optimization and hotspot repair for NFL in the future.

Photonics doi: 10.3390/photonics13090816

Authors: Achyutesh Dixit Praveen Chandra Pandey Subhashish Tiwari

A three-ring hexagonal photonic crystal fiber (PCF), with its pitch Λ = 28 µm and airhole radius varied between 7 and 12.6 µm, was analyzed for efficient delivery of a 980 nm EDFA pump and guidance of 1.48–1.55 µm signal wavelengths. Using FEM solutions of the vector wave equation, the effective index, effective area (Aeff), and core region power fraction were obtained across all wavelengths. At 0.98 µm, the PCF exhibits tight confinement, small Aeff, and negligible confinement loss, confirming its suitability for efficient pump absorption. Increasing the airhole radius produced smooth, monotonic growth in Aeff and a slight decrease in the doped region overlap, while pump confinement remained consistently strong. Variations in the core refractive index demonstrated an effective means to control confinement, with Aeff decreasing and loss monotonically reducing as ncore increases. Mode field profiles validated a stable, centered fundamental mode at both the pump and signal wavelengths, with only moderate expansion at longer wavelengths. Overall, the PCF design showed predictable, fabrication-friendly behavior with stable confinement and very low leakage, making it highly suitable for EDFA pump delivery applications. The obtained modal characteristics and confinement behaviors indicate that the proposed structure can provide reliable pump delivery while maintaining stable guidance of signal wavelengths in the EDFA operating band. The simple three-ring geometry also offers a structural advantage for fabrication compared with more complex PCF configurations.

Photonics doi: 10.3390/photonics13090815

Authors: Shengbing Zhang Ming Li Huaizhu Yuan Xin Tu

Currently, acoustic sensing technology is widely applied in military defense, non-destructive testing (NDT), and biomedical imaging, and it is increasingly penetrating various aspects of daily life. However, traditional piezoelectric acoustic sensors are highly susceptible to performance degradation when operated in harsh environments. In contrast, optical microcavities-a class of optical resonant cavities with characteristic dimensions on the micrometer scale—offer distinct advantages, including compact footprints, immunity to electromagnetic interference (EMI), and ultra-high sensitivity. Leveraging these exceptional properties, researchers have extensively explored acoustic sensing technologies based on optical microcavity platforms. This paper reviews recent research progress in optical microcavity-based acoustic sensing, categorized by the structural configurations of the microcavities. First, we introduce the key performance specifications of different optical microcavities in acoustic sensing, such as sensitivity and frequency response bandwidth. Second, we categorically discuss the structural designs of various optical microcavities alongside corresponding optimization methods to improve sensing performance. Finally, we summarize the current applications of optical microcavity-based acoustic sensing across multiple fields and outline future development trends in this research area.

Photonics doi: 10.3390/photonics13090814

Authors: Liuxin Qian Zean Shen Zhiheng Li Mengjiao Ren Leyong Jiang Jiao Tang

Three-dimensional Dirac semimetal (3D DSM), characterized by linear band dispersion and strong terahertz nonlinear optical responses, has attracted increasing interest as promising materials for compact nonlinear photonic devices. Optical bistability (OB), which enables two stable output states under the same input condition, is of particular importance to all-optical switching, optical logic gates, and optical memory. However, achieving OB with a sufficiently low switching threshold remains a key challenge. Here, we propose a prism-coupled multilayer structure incorporating 3D DSMs to realize low-threshold, tunable OB by exciting the surface plasmon polaritons (SPPs). The prism-coupling configuration enables efficient excitation of SPPs, producing strong local-field enhancement around the nonlinear 3D DSM layer. This enhanced light–matter interaction, together with the large nonlinear refractive index of the 3D DSM, substantially reduces the electric-field threshold required for bistable switching. Numerical results show that OB can be achieved with an incident electric-field threshold on the order of 105 V/m through optimizing the material and structural parameters. Moreover, the switching threshold and hysteresis loop width can be flexibly controlled by varying the Fermi energy, relaxation time, and geometric parameters of the 3D DSM multilayer structure. These results suggest that SPP-assisted 3D DSM structures provide an effective platform for low-threshold, actively tunable optical bistable devices in integrated terahertz photonic systems.

Photonics doi: 10.3390/photonics13090813

Authors: Rong Wang Xinlei Qian Chongpeng Huang Ailing He Nanruo Chen

Leveraging advances in artificial intelligence algorithms, distributed acoustic sensing (DAS) based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) has achieved high event-recognition accuracy through a variety of learning models. Nevertheless, further improving the accuracy of multi-event recognition remains a persistent challenge. In this paper, we propose a Dual-Domain Fusion Network (DD-FusNet) for vibration event recognition in Φ-OTDR sensing systems. To fully capture signal dynamics, the model simultaneously processes time- and frequency-domain representations, employing a crucial cross-attention mechanism to bridge these branches and enable dynamic, learnable interactions. Experimental results based on a six-class field engineering vibration event dataset collected by Φ-OTDR, containing car events, manual tapping, road breaker, excavation, leaking and noise, demonstrate that the proposed method achieves an average accuracy of 99.12%, significantly outperforming baseline methods by approximately 3 to 10 percentage points in accuracy, thereby ensuring the accuracy of multi-event recognition. We believe the proposed DD-FusNet will advance the recognition capabilities of Φ-OTDR systems in complex industrial sensing applications.

Photonics doi: 10.3390/photonics13090812

Authors: Eugen Smolkin Yury Shestopalov

The complex dispersion and modal sensitivity of an axisymmetric transverse magnetic surface wave supported by a dielectric-coated perfectly conducting cylinder are investigated. Starting from Maxwell’s equations, the boundary-value problem is reduced to a nonlinear complex dispersion equation for the longitudinal propagation constant β. A numerical framework combining zero-level localization of the real and imaginary parts of the dispersion function, nonlinear root refinement, numerical clustering, and adaptive continuation in the complex coating permittivity is used to identify and track a selected spectral branch. One- and two-parameter computations characterize the mapping ε↦β(ε) over prescribed subsets of the complex-permittivity plane. At fixed ℑε, increasing ℜε increases ℜβ and decreases ℑβ, whereas at fixed ℜε, increasing ℑε increases both components of β in the investigated parameter range. The rectangular-grid, concentric-circle, and radial-beam experiments show that the spectral response is smooth on the considered parameter sets but non-affine, coupled, and direction-dependent. The corresponding longitudinal electric field is reconstructed, normalized, and phase-aligned along the tracked branch. Difference fields, radial localization measures, a global modal distance, and a normalized correlation coefficient show that the same qualitative radial TM mode is retained throughout the sampled parameter domain, while its propagation constant and spatial localization vary continuously with the complex coating permittivity.

Photonics doi: 10.3390/photonics13090811

Authors: Yihao Wang Yang Gao Yuxin Fan Maofu Gao Jiabing Shen

We propose a gold-graphene-vanadium dioxide (VO2) hybrid metasurface capable of reversibly switching among three operational modes in the terahertz regime. The device achieves flexible polarization control by combining the insulator-to-metal phase transition of VO2 with the electrical tunability of graphene. Simulation results reveal three distinct behaviors depending on the biasing conditions of the materials. With graphene held at a chemical potential of 0 eV and VO2 in the insulating state, the metasurface acts as a linear-to-linear polarization converter. The polarization conversion ratio (PCR) exceeds 0.9 over the frequency range from 5.5 to 8.6 THz. When the graphene chemical potential is raised to 0.9 eV while VO2 remains insulating, the metasurface switches to linear-to-circular conversion. Notably, the handedness of the outgoing wave depends on the polarization of the incoming signal. Over the 6.45–8.36 THz band, the axial ratio (AR) remains below 3 dB. A third functional state emerges when VO2 switches to its metallic phase. In this state, the device simply operates as a broadband co-polarized reflector, covering the terahertz communication band from 0.1 to 10 THz. This switchable, multifunctional behavior should prove useful for terahertz communications, polarization imaging, and sensing applications.

Photonics doi: 10.3390/photonics13090810

Authors: Asma Mimouni Younès Messaddeq Bora Ung

We present a nanovoid-assisted ring-core fiber designed for low-bending-loss few-mode transmission. We develop a 3D model that accounts for stress-induced perturbation in bending-loss evaluation. Our model yields closer agreement with published experimental data compared to conventional conformal mapping. The nanovoid ring-core fiber reduces the bending loss of higher-order modes by up to three orders of magnitude, and improves the degeneracy of the higher-order pair nearly ten-fold at tight bending radii. The proposed fiber maintains the same intermodal separation compared to the reference ring-core fiber across all investigated radii.

Photonics doi: 10.3390/photonics13090809

Authors: Pei-Di Yang

With the rapid advancement of terahertz technology, electromagnetic interference has become a critical issue that compromises device performance, creating an urgent demand for high-performance terahertz absorbers. Three-dimensional (3D) printing, characterized by flexible structural design, monolithic fabrication, and mold-free processing, has emerged as a promising technique for producing terahertz absorbers. In this work, inspired by the structural and functional characteristics of deep-sea sponges, we propose a bioinspired absorber design that integrates a porous topology with 3D printing. By optimizing the rotation angle and the hollowed array, the absorber establishes multiple internal reflection paths, which, combined with the structural matrix and the graphene conductive coating, enable highly efficient dissipation of electromagnetic energy. Experimental results show that the fabricated sample achieves an absorptivity exceeding 99% over the 0.5–2.0 THz frequency range, while also exhibiting wide-angle absorption and polarization-insensitive performance. The influence of pore size and graphene concentration on the absorption properties is systematically revealed. This work further enhances the performance of 3D-printed terahertz absorbers and provides a novel technical pathway for the design and fabrication of high-performance terahertz absorbers.

Photonics doi: 10.3390/photonics13090808

Authors: Jian Zhou Bolin Li Yicong Feng Qi Wang Xiaoming Nie

To address the contradiction between high resolution and structural complexity in traditional laser self-mixing measurement systems, the co-axial dual-wavelength solid-state laser self-mixing technology was proposed and studied. An LD pumped the Nd:YVO4 crystal and doubled the frequency to generate two wavelengths laser at 1064 nm and 532 nm, and a co-axial dual-wavelength laser self-mixing measurement system was constructed. A 90° phase difference between the two wavelengths was produced by adjusting the angle of incidence of the parallel glass plate. As a result, a set of orthogonal signals was built to distinguish the direction of the displacement for the target. The experiments showed that the measuring system can adapt to different vibration waveforms and that the frequency measurement upper limit can be reached at 7 kHz. The system exhibited high precision in displacement measurement, with an RMS displacement noise of 8.64 nm and a cumulative error of 33.66 nm at a peak-to-peak amplitude of 5000 nm, along with a short-term resolution better than 2 nm.

Photonics doi: 10.3390/photonics13090807

Authors: Qinyi Zhang Jianjun Yu Hanyu Zhang Zhongxiao Pei Jiali Chen Xin Lu Jianyu Long Yifan Chen Ye Zhou

Hybrid terahertz (THz)/free-space optical (FSO) systems offer a promising paradigm for high-capacity, all-weather wireless communication, yet their deployment is often hindered by the bulky size and high complexity of discrete transceivers. This paper experimentally demonstrates a low-complexity hybrid THz/FSO transmission architecture featuring a unified photonic transmitter. By leveraging a polymethyl methacrylate (PMMA) plate serving as a dichroic beam combiner—which reflects the 1550 nm optical signal while transmitting the 300 GHz THz signal—we realize simultaneous signal propagation over a shared aperture and link. Photonics-aided techniques are employed to generate both carriers, ensuring system integration and coherence. The experimental results verify that both the THz and FSO links independently support 30-GBaud quadrature phase-shift keying (QPSK) transmission over a 10-m wireless distance, achieving a net data rate of 60 Gbps per link while satisfying the 7% hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10−3. This work validates the feasibility of shared-transmitter designs and provides a compact, cost-effective solution for future high-speed fronthaul/backhaul networks.

Photonics doi: 10.3390/photonics13090806

Authors: Yi Gong Ying Gu Min Jiang Shan Jin Lifeng Bian Shulong Lu

Size-dependent efficiency degradation remains a major challenge for deeply scaled GaN-based micro-light-emitting diodes (micro-LEDs) because etched sidewalls increasingly influence carrier recombination. Here, square micro-LEDs with lateral dimensions of 5–50 μm were fabricated from the same plasma-assisted molecular-beam-epitaxy-grown wafer containing a five-period InGaN quantum-well/1 nm Al0.1Ga0.9N interlayer/InGaN quantum-dot-like (QW–QD) composite active region. Contrary to the conventional size effect, the light output power density and external quantum efficiency (EQE) increased as the mesa size decreased. The peak EQEs were 3.66%, 4.53%, 5.63%, 7.04%, and 7.27% for the 50, 40, 30, 10, and 5 μm devices, respectively, corresponding to an approximately 98.6% increase from 50 to 5 μm. The favorable scaling is consistent with localization-mediated suppression of lateral carrier loss combined with size-dependent light extraction. The present measurements do not quantitatively separate injection, internal efficiency, and extraction contributions. These results demonstrate the potential of ultrathin-interlayer QW–QD active-region engineering for scaled GaN micro-LEDs.

Photonics doi: 10.3390/photonics13090805

Authors: Guanliang Chen Zhenyun Tang Wanzhi Zhang Yantong Wu Li Xiang Yinxian Luo Sanjie Liu Dongmei Li Huiyun Wei Mingzeng Peng Zhigang Song Xinhe Zheng

To address the challenges encountered in the simulation and parameter extraction of distributed-feedback (DFB) lasers, this work starts from the rate equations of the DFB laser to derive its steady-state formulation and small-signal model, and constructs steady-state and transient KAN-based physics-informed neural-network (KAN-PINN) architectures. In the steady state, accurate L-I/I–V curves and −3 dB bandwidth results are obtained. In addition, the parameters of the DFB laser are systematically organized, and a KAN-PINN inverse mode, an adaptive moment estimation (Adam) optimizer-based physical inversion, and a hybrid strategy combining the two are proposed. The extracted parameters show small deviations from the true values, and the simulation results agree well with the actual data. The proposed methodology can be extended to other lasers and even to a broader class of optoelectronic devices for parameter extraction and simulation.

Photonics doi: 10.3390/photonics13090804

Authors: Jiahui Chen Ziling Chen Xitong Liang Yuan Wang Lin Xu Chi Zhang

Non-mechanical beam steering technology based on liquid crystal optical switches and liquid crystal polarization gratings holds significant application prospects in fields such as laser communication, radar detection, and optical information processing. Traditional nematic liquid crystal optical switches exhibit slow response speeds, whereas novel ferroelectric liquid crystal optical switches, despite their fast response, are hampered in engineering applications by complex fabrication processes, the large number of devices required for cascading, and substantial module thickness. To address these issues, this paper proposes and demonstrates a fast non-mechanical beam steering scheme by cascading a stressed polymer network liquid crystal (SPNLC) optical switch with a liquid crystal polarization grating. The SPNLC is fabricated by mechanically shearing a polymerized liquid crystal–polymer composite, enabling sub-millisecond response and continuous linear phase modulation without the need for an alignment layer. A 30-μm-thick SPNLC half-wave plate was prepared, which introduces a phase retardation of 3.6 μm under a driving voltage of 300 V, and the rise time and fall time are measured to be approximately 526 μs and 560 μs at a driving voltage of 20 V with a 1 kHz square wave, and 470 μs and 538 μs at 27 V under the same waveform conditions. Cascaded with a passive polarization grating, the waveplate enables fast electrical switching of the beam between the ±1st diffraction orders. Furthermore, a two-dimensional multi-angle beam deflector was constructed based on a supra-binary cascade scheme. Experimental results confirm that the system possesses sub-millisecond response, large phase retardation, continuous tunability, and an alignment-layer-free fabrication process, demonstrating its feasibility for large-range fast beam scanning.

Photonics doi: 10.3390/photonics13090803

Authors: Xiaohui Wang Enbing Qi Yifan Wu Xuan Sun Xiuhua Men Jianbin Wang Junjie Zhang

This paper comprehensively investigates the ultraviolet (UV) nanosecond laser fabrication of micro-groove textures on planar PEEK surfaces, as well as their surface performance in terms of wettability and frictional properties. Firstly, a three-dimensional finite element model, incorporating a moving Gaussian heat source, solid heat transfer and deformed geometry, was established to simulate the transient temperature field and ablation profile of PEEK during UV laser ablation. The predicted groove depth agreed with the experimental value with a low deviation of 11.19%. Based on the simulation and single-factor experiments, the optimized single-track laser parameters were determined as a laser power of 1.2 W, a scanning speed of 100 mm/s and a repetitive frequency of 100 kHz. Secondly, systematic single-factor and multi-pass laser ablation experiments of PEEK surfaces were conducted to fabricate micro-groove textures with precisely tailored geometric parameters. Furthermore, ablated surface characterization showed that the laser-textured surfaces exhibited increased roughness, apparent crystallinity up to 22.13%, and pronounced anisotropic wettability, with improved liquid spreading along the groove direction and restricted spreading across the grooves. Finally, fretting wear tests under simulated body fluid lubrication were carried out. The research findings reported in this paper provide a systematic theoretical and experimental basis for the application of UV nanosecond laser surface texturing in the fabrication of PEEK-based components.

Photonics doi: 10.3390/photonics13090802

Authors: Jun Zeng Chunqiu Xia Hongwei Zhang Hongchao Gao Ziyang Wang Mingjun Li

Existing LiDAR super-resolution methods primarily aim to increase point cloud density or improve coordinate reconstruction accuracy. However, they tend to introduce blurred edges and distorted planar surfaces during reconstruction, making it difficult to preserve the consistency of local scene geometry. To address this issue, this paper proposes a structural consistency-aware LiDAR super-resolution method that aims to preserve the local geometric relationships of the reconstructed point cloud with respect to the ground-truth point cloud in edge and planar regions. Specifically, complementary observations from adjacent frames are first fused using multi-scale dilated convolutions. An anisotropic Swin Transformer and a Coordinate-Aware Structure Enhancement (CASE) module are then employed to accommodate the horizontally dense and vertically sparse sampling pattern of LiDAR, strengthen long-range geometric modeling, and reduce the loss of critical structural information. During training, a local curvature-based structural consistency loss is designed to separately constrain edge sharpness and planar smoothness. During inference, prediction uncertainty and point cloud height are combined to adaptively remove low-confidence points, further improving the geometric reliability of the reconstructed point cloud. Experiments on the KITTI dataset show that the proposed method achieves an MAE of 0.4916 and an IoU of 0.4633, outperforming the representative comparison methods on both metrics. When the reconstructed point clouds are applied to A-LOAM, the average RTE and RRE values are reduced by 34.6% and 31.2%, respectively. In addition, experiments on the self-collected CSU-SLAM dataset provide preliminary evidence of the applicability of the proposed method to indoor and outdoor scenes under a different LiDAR configuration.

Photonics doi: 10.3390/photonics13090801

Authors: Zhaoyu Li

This paper proposes Non-Uniform Adaptive Acquisition (NUAA), a signal-feature-matched non-uniform adaptive photonic sampling framework that recovers broadband radio-frequency (RF) waveforms from highly sparse programmable non-uniform photonic sampling points. A 200 MHz mode-locked laser together with five electrical optical delay lines (EDLs; motor-actuated optical delay units) arranges the non-uniform sampling instants. Benefiting from the joint design of the photodetector/track-and-hold amplifier (PD/THA) response model and programmable non-uniform optical pulse spacing, a low-bandwidth PD infers neighboring pulse amplitudes from their deterministic superposition at the readout. In numerical simulations of this physical forward operator, that construction corresponds to a 1 THz equivalent sampling rate on the 1 ps EDL grid, while the electrical front end operates at a 1 GHz average sampling rate (cascaded PD–THA analog 3 dB bandwidth ≈0.676 GHz). Under severe blocking interference and low signal-to-noise ratio (SNR), the numerical simulations show that the strongest broadband chirplet result uses a scene prior with support locking: with the NUAA–MU (Mamba–Unfolding) reconstructor at 0.1% multi-coset sparsity, all Ntrial=50 Monte Carlo trials succeed within 200 ms (Wilson 95% CI [93, 100]%; cumulative-best NMSE −28.0 dB), whereas the configuration without a scene prior is substantially weaker in the same window. A scene prior may come from known radar or communication waveform families, coarse occupancy reported by a companion sensor, or accumulation across related tasks.

Photonics doi: 10.3390/photonics13090800

Authors: Pavel Parchinsky Abdumanap A. Nasirov Shavkat U. Yuldashev Azamat Arslanov Rafael A. Nusretov Natalia A. Kulagina Sultan Kh. Suleymanov Peng Li Sergei A. Khakhomov Alina V. Semchenko Vitali V. Sidski Vladimir E. Gaishun Konstantin D. Danilchenko

This study examines the effect of graphene oxide nanoparticle (NGO) doping (0.12–0.24 mol%) on the properties of FTO layers obtained by spray pyrolysis. The results show that nanographene promotes phase separation within the FTO bulk. The resulting inhomogeneities are fluorine-depleted relative to the surrounding film. NGO doping also modifies the position and intensity of peaks in the absorption spectra, indicating a change in the nature of the absorbing centers. The optical bandgap of the FTO layers decreases with the increase in NGO from 4.4 eV (undoped samples) to 3.95 eV (samples with the highest NGO concentration). Furthermore, NGO doping reduces the sheet resistance of the FTO layers. This reduction is attributed to increased charge carrier mobility resulting from passivation of nanocrystallite interfaces by graphene oxide nanoparticles. However, the sheet resistance depends non-monotonically on NGO content, with the lowest value observed at 0.16 mol% NGO. The increase in resistance at higher NGO concentrations is due to enhanced carrier scattering caused by the growing size and number of the phase inhomogeneities within the FTO layer.

Photonics doi: 10.3390/photonics13080799

Authors: Lingli Li Lianzheng Su Bingxing Zhang Kaiyue Xie Xuyang Feng Meihua Yan Xueling Yang Zhimei Wang Jun Ma Hang Zhao Tianyu Mao Xinxin Shang Bingying Pan

Three layered rare-earth hydrogenselenite–selenite hydrates, Ln(HSeO3)(SeO3)·2H2O (Ln = Yb, Dy, Eu), were synthesized under hydrothermal conditions and systematically characterized. Single-crystal and powder X-ray diffraction show that the compounds are isostructural and crystallize in the non-centrosymmetric orthorhombic Sohncke space group P212121, featuring LnO8 polyhedra and SeO3/HSeO3 units assembled into hydrogen-bonded layered frameworks. Two-component inversion-twin refinements gave Flack x values of 0.06(4), 0.27(3), and 0.22(3) for the Yb-, Dy-, and Eu-containing crystals, respectively; the Yb crystal is dominated by one inversion domain, whereas the Dy and Eu crystals contain appreciable inverted-domain fractions. Because L/D/DL descriptors conventionally refer to the absolute configuration of chiral molecular entities, they are not assigned to these extended inorganic frameworks. Under the present achiral synthesis conditions, crystals dominated by the opposite, inversion-related framework hand cannot be excluded. Photoluminescence measurements reveal characteristic Dy3+ and Eu3+ emissions, while the Yb analogue exhibits a broad visible band tentatively related to host-framework states. Magnetic measurements show no long-range ordering above 2 K; the Yb and Dy phases display dominant antiferromagnetic correlations, whereas the Eu phase is governed mainly by Van Vleck paramagnetism. These results identify Ln(HSeO3)(SeO3)·2H2O as a layered Sohncke-symmetry platform with lanthanide-dependent optical and magnetic behavior. The observed lanthanide emissions and non-centrosymmetric framework suggest prospective photonic and nonlinear-optical applications, although device-level performance remains to be established.

Photonics doi: 10.3390/photonics13080796

Authors: Yong Wang Li Pei Zhenyu Gu Wei Jiang Wensheng Wang Jing Li Jingjing Zheng Tigang Ning

High-port-count fiber pump combiners are important passive components for scalable laser diode (LD) power combining in high-power fiber-laser systems. However, increasing the number of input ports from 19 to 37 narrows the fabrication window because fiber bundle packing, taper uniformity, splice matching, thermal management, and backward-light tolerance must be controlled simultaneously. In this work, a 37 × 1 tapered fiber bundle pump combiner was fabricated by a tubing-based method using thirty-seven 135/155 µm multimode input fibers and an 800/880 µm output fiber. The input fibers were weakly etched to improve bundle compactness, and the glass-tube-assisted fiber bundle was tapered, cleaved, and fusion-spliced with a tapered output fiber. The fabricated combiner was characterized using thirty-seven 915 nm fiber-coupled LDs. At a total injected power of 4.89 kW, the combiner delivered 4.80 kW output power, corresponding to an overall transmission efficiency of 98.16%. The single-port transmission efficiencies were approximately in the range of 97.3–98.1%, indicating good port-to-port uniformity for the dense 37-fiber bundle. During full-power operation, the highest temperature appeared in the tapered fiber bundle region and reached 103.8 °C, while the fusion-splice region reached 76.2 °C. In addition, the device withstood 500 W backward-propagating light without observable damage, indicating its practical tolerance to reverse-power loading. These results show that the proposed 37 × 1 fiber pump combiner provides an effective all-fiber solution for multi-kilowatt LD power combining.

Photonics doi: 10.3390/photonics13080798

Authors: Jianing Yu Luwei Li Yichong Liu

A one-dimensional Thue–Morse graphene photonic crystal (1D TMGPC) composed of alternating identical dielectric layers and graphene sheets is proposed, in which two distinct graphene chemical potentials are arranged according to a Thue–Morse quasi-periodic sequence. Using the transfer matrix method, we demonstrate that this structure effectively modulates terahertz waves and generates multiple abundant photonic bandgaps at both 20 K and 300 K. Notably, a novel splitting of low-frequency bandgaps produces two additional omnidirectional and polarization-insensitive bandgaps centered at approximately 1.45 THz and 1.95 THz. By analyzing the dispersion relations, reflection phase, photonic density of states, and electric field distributions, the boundary-driven modulation mechanism associated with the quasi-periodic chemical potential is elucidated. Furthermore, the proposed structure exhibits excellent multi-dimensional tunability. The bandgap properties can be dynamically tuned via the electrical control of graphene chemical potentials without altering the physical geometry. Structural tailoring provides an additional degree of freedom, as increasing the Thue–Morse sequence order induces passband splitting. Additionally, increasing the number of repeating periods yields comb-like multi-channel narrowband filtering responses. At a cryogenic temperature of 20 K, two distinct multi-channel narrowband comb filtering responses appear in the frequency ranges of 1.20–1.33 THz and 4.10–4.80 THz, with a minimum full width at half maximum (FWHM) of 1.10 GHz. At a room temperature of 300 K, the higher-frequency comb filtering response remains in the range of 4.10–4.80 THz, with a minimum FWHM of 5.70 GHz. Moreover, we evaluate the performance and stability of the structure when employed as filters and electro-optic switches, thereby providing useful insights for terahertz applications. With its simple geometry, abundant bandgaps, and flexible electro-structural tunability, the proposed 1D TMGPC is highly promising for broadband and electrically tunable terahertz devices.

Photonics doi: 10.3390/photonics13080797

Authors: Yiting Li Jun Chang Xuehui Zhao Yue Zhong Xianzhu Liu

By analyzing the semantic information of Direct Current (DC, the zeroth-order spherical harmonic coefficient) gradients during 3D Gaussian Splatting (3DGS) optimization, this paper achieves unsupervised state classification in scenes with discrete appearance states under the proposed State-Discovery Gaussian Splatting (SD-GS) framework via SVD dimensionality reduction and K-means clustering. To improve the stability of the clustering results, an appearance-difference-weighted refinement mechanism is further proposed to confirm high-confidence labels. To address the difficulty of distinguishing similar states when the number of states exceeds two, a sequential peeling strategy is proposed that decomposes a multi-class partition into several two-class separations. On four real-world scene datasets, SD-GS achieves 100% classification accuracy with reconstruction quality of 31.98–38.83 dB PSNR. Ablation studies validate the effectiveness of the gradient direction mode and the SVD dimensionality reduction strategy.

Photonics doi: 10.3390/photonics13080795

Authors: Tingting Qin Yang Tu

Overlapping optical cells create geometry-dependent inter-cell interference, while receiver-geometry and channel-estimation errors can reverse the effective non-orthogonal multiple access (NOMA) decoding order and increase successive interference cancelation (SIC) failures. This paper develops a chance-constrained receiver–scheduler co-design framework for a multi-cell NOMA visible-light communication network with an asymmetrically clipped DC-biased optical orthogonal frequency-division multiplexing physical layer. Correlated position, photodetector-orientation, and channel-estimation errors are propagated through nonlinear geometry-based scenarios. For each SIC direction, a joint three-SINR event defines a layer-, resource-, and direction-labeled probabilistic decodability graph. Candidate NOMA and orthogonal modes are screened on optimization scenarios, admitted by independent one-sided confidence bounds, and selected through resource-constrained mixed-integer linear programming. With the matching fixed, hierarchical powers are adapted under empirical conditional-value-at-risk constraints using trust-region sequential quadratic programming. Because candidate-edge certificates need not remain valid after global matching and power redistribution, the frozen complete assignment is independently recertified before held-out testing. Under the specified uncertainty generator, the proposed method maintains selected-pair outage probabilities of approximately 2.7×10−3–3.3×10−3 over the half-power-angle sweep, compared with 0.027–0.060 for nominal-CSI allocation. Additional experiments quantify network-wide outage, model misspecification, unbalanced deployments, feasibility, and computational cost. The results support reliable slow-timescale scheduling under the adopted link and uncertainty models, without implying distribution-free, waveform-level, or real-time guarantees.

Photonics doi: 10.3390/photonics13080794

Authors: Meet Kumari Satyendra K. Mishra Jyoteesh Malhotra

Light fidelity (LiFi) is progressively evolving as a highly promising communication technology because of its unique benefits, available spectrum, low implementation costs, and adaptive beamforming capabilities. Despite their advantages, existing LiFi networks remain constrained by limited data rates, coverage area, and information security in practical environments. Therefore, a high-speed, high-capacity, and secure quantum key distribution (QKD)-assisted integrated multi-wavelengths (450/532/620 nm) LiFi system using mode division multiplexing (MDM) is proposed. The results demonstrate that the proposed system achieves maximum transmission distances of 20.5–22 m and 19–22 m using different Laguerre–Gaussian (LG) and Hermite–Gaussian (HG) mode indices {[0,0], [0,10], [0,20], [0,30]}, at an aggregate data rate of 40 Gbps. Furthermore, the minimum acceptable transmitter angles of 30–90° for irradiance angles of 20–80° are required to maintain the target bit error rate (BER) of 10−9. The minimum photodetector detection areas required at transmission distances of 20–30 m are 1–2 cm2 at the minimum BER limit. Moreover, the proposed system exhibits optimum performance, achieving an optical loss of −39.47 dB, −49.03 dBm received power, and 45.39 dB signal-to-noise ratio for 1–10 photons/pulse. Compared with existing studies, the proposed system demonstrates enhanced overall performance across various communication metrics.

Photonics doi: 10.3390/photonics13080793

Authors: Linbin Zheng Junheng Pan Jau Tang

Quantum tunneling is conventionally described by the Schrödinger wave equation with a prescribed static potential barrier, providing accurate transmission probabilities but offering limited insight into the microscopic dynamics of particle–barrier interactions. In this work, we develop a dynamical second-quantization framework in which the barrier is modeled as an ensemble of quantized internal modes rather than as an externally imposed classical potential. The tunneling particle interacts directly with these microscopic barrier excitations through a coupled particle–barrier Hamiltonian, from which the Heisenberg equations of motion are derived. Collective coherent excitations of the barrier modes give rise to an emergent effective barrier that naturally recovers the conventional rectangular barrier and the WKB transmission limit under appropriate conditions. Unlike standard treatments, the present formulation explicitly incorporates microscopic barrier dynamics and provides a unified description of particle–barrier coupling within a second-quantized formalism. The framework further suggests that repeated tunneling events may experience different microscopic interaction histories, motivating a statistical interpretation of tunneling times. Because both the particle and barrier are treated within the same operator formalism, the theory provides a natural foundation for extension to relativistic quantum transport, photonic barriers, cavity quantum electrodynamics, and other structured quantum media.

Photonics doi: 10.3390/photonics13080792

Authors: Zhirong Li Wenyan Jia Fuzhong Bai Xiaojuan Gao Zhaoxin Xu Yuetao Sun Xiulan Wen

To address the challenges of unreliable feature matching, high mismatch rates, and limited reconstruction accuracy in binocular stereo vision applied to asphalt pavement with inherent weak texture features, this paper proposes a speckle-assisted binocular 3D reconstruction method based on a multi-scale adaptive feature fusion algorithm. Infrared speckle patterns are actively projected to enrich the pavement surface features, and a multi-scale matching framework is developed by integrating Laplacian pyramid representations, feature-driven adaptive regularization, and Softmax-based nonlinear fusion. This design can achieve stable and accurate disparity estimation, even in weak texture regions, and produce high-quality 3D point clouds that faithfully represent both macro-scale undulations and micro-scale texture details. Ablation experiments validate the effectiveness of the proposed modules, showing that the relative errors of the arithmetic mean height (Sa) and root-mean-square height (Sq) are reduced to below 2.3%. When aligned with 3D scanner data using the iterative closest point (ICP) algorithm, the reconstructed point clouds achieve sub-millimeter mean error and an overlap rate exceeding 97%. The results indicate that the proposed method offers a reliable technical solution for efficient texture-depth analysis and practical pavement condition assessment.

Photonics doi: 10.3390/photonics13080791

Authors: Yuan Song Kewei Chai Qipeng Lu Xuepeng Gong Yang Bai Zhen Zhang

Tin contamination on extreme ultraviolet (EUV) collector mirrors significantly degrades mirror reflectivity. Hydrogen-based plasma cleaning is currently the standard method for removing the tin layer. However, to prevent substrate damage from over-cleaning, real-time monitoring of the tin layer thickness is critical. It has been established that the secondary electron yield (SEY) induced by high-energy primary electron bombardment correlates with the tin layer thickness. Thus, SEY can serve as a thickness indicator to determine the optimal cleaning endpoint. In this study, the evolution of secondary electrons during the cleaning process is simulated using a Particle-in-Cell (PIC) model combined with the Monte Carlo method, and the relationship between SEY and tin layer thickness is established. The simulation results indicate that under the specified conditions, H3+ is the dominant ionic species generated. Primary electrons account for nearly 24% of the incident particles, with an average energy of approximately 47 eV. Most secondary electrons possess energies below 30 eV, and their yield increases monotonically with the tin layer thickness, ranging from 0.60 to 1.05. These findings provide a novel approach for in situ detection of tin contamination layer evolution on EUV collector mirrors.

Photonics doi: 10.3390/photonics13080790

Authors: Meng Qi Ruiyang Li Yuanji Li Jinxia Feng Kuanshou Zhang

We demonstrate a high-power broadband continuous mode-hop-free (MHF) tunable singly resonant optical parametric oscillator (SRO). To obtain broadband continuous MHF operation, a synchronous etalon-angle locking technique and a feedback-optimized temperature controller were developed based on theoretical investigation. At a pump power of 21 W that was eight times the pump threshold, the measured signal was tuned from 1551.9087 nm to 1568.6549 nm, and the corresponding idler was tuned from 3384.3030 nm to 3307.3073 nm simultaneously. A continuous MHF tuning bandwidth of 2.064 THz was achieved at a tuning speed of 4.7 GHz/s. Continuous MHF operation in the whole tuning band was verified by high-resolution absorption spectroscopy of acetylene and methane, and by the continuous sinusoidal transmission through a Fabry–Perot etalon. The measured powers of the signal at 1560 nm and idler at 3346 nm were 4.12 W and 2.26 W with peak-to-peak fluctuations of ±0.42% and ±0.18%, respectively. These results represent, to the best of our knowledge, the widest continuous MHF tuning bandwidth achieved by a temperature-tuned SRO at high pump power, providing a high-power dual-band coherent source for precision spectroscopy.

Photonics doi: 10.3390/photonics13080789

Authors: Peirui Ji Chenguan Fu Guofeng Zhang Yijun Du Angyang Ma Changsheng Li Dongxu Wu Yibin Tian

Multi-view fringe projection profilometry systems that integrate a telecentric projector with multiple oblique-view cameras offer unique advantages for inspecting high dynamic-range surfaces featuring densely packed, intricate microstructures. Nevertheless, such systems encounter fundamental calibration challenges, namely, sign ambiguity in the rotation matrices and truncated extrinsic parameters inherent to telecentric projector models, as well as difficulties in multi-view point cloud registration. This paper introduces a novel calibration framework with three principal contributions. First, we resolve the sign ambiguity by calibrating the telecentric projector under a quasi pinhole model and directly transferring the extrinsic sign conventions, thereby obviating the need for costly precision displacement stages or elaborate virtual targets. Second, we fix the axial-gauge freedom by constraining the origin of the projector coordinate system to lie on the XY-plane of the camera coordinate system. Third, we establish precise relative poses between all cameras and a designated reference camera, enabling unified multi-view point cloud registration directly within the projector coordinate frame, which substantially reduces alignment errors and accelerates data processing. Experimental results demonstrate marked improvements in accuracy: reprojection root-mean-square errors of 0.084 pixels for the cameras and 0.106 pixels for the projector, corresponding to in-plane spatial resolutions of 0.21 µm and 0.26 µm, respectively. The proposed method offers a robust solution for micron-level inspection in semiconductor packaging and precision manufacturing.

Photonics doi: 10.3390/photonics13080788

Authors: Hai Lin Yuqian Liu Yutong Wu Yidan Zhang Tianyang Deng Yubin Liu

Optical coherence tomography (OCT) and photoacoustic imaging (PAI) provide complementary structural and absorption contrasts but require different coupling conditions: 1310 nm swept-source OCT is attenuated by water, whereas PAI requires acoustic coupling. We developed a large-field dual-modal imaging system combining temporal medium separation with hardware-based coordinate locking. The OCT head, linear-array ultrasound transducer, and photoacoustic excitation fiber bundle were mounted on a rigid common platform, and a one-time calibration established a two-dimensional affine transformation between the modality coordinate systems. OCT was acquired in air and PAI in deionized water within a common large-field coordinate range. In five paired air–water measurements with an approximately 23 mm water path, the displayed OCT peak level decreased from 98.4 ± 1.5 dB in air to 79.4 ± 1.8 dB in water, corresponding to a mean reduction of 19.0 ± 1.4 dB. Quantitative registration was evaluated using a 5 × 5 dual-modal landmark phantom, with nine landmarks used for affine calibration and 16 excluded landmarks reserved for independent validation. The mean two-dimensional validation error was 0.235 ± 0.128 mm, with an RMSE of 0.266 mm and a maximum error of 0.446 mm. Five additional medium-switching cycles performed without recalibration yielded an overall registration error of 0.369 ± 0.163 mm across 80 validation measurements. PA spatial resolution was further characterized using six thin hair targets, yielding lateral and axial FWHM values of 0.342 ± 0.069 mm and 0.394 ± 0.073 mm, respectively. These results demonstrate reproducible two-dimensional en face OCT–PA coordinate mapping under modality-specific coupling conditions and support the proposed workflow as a phantom-based technical validation for large-field multimodal imaging.

Photonics doi: 10.3390/photonics13080787

Authors: Bolin Cai Lin Zhang Shi Qiu

Accurately predicting high-energy laser propagation toward fast-moving aerial targets is not straightforward, since the slant range, elevation angle, atmospheric path, accumulated turbulence, and thermal blooming all change along with target motion. Because of this coupled evolution, fixed-path or quasi-static models can hardly describe, in a complete way, the time-varying loss of beam transmission and the weakening of energy concentration. To address this problem, this paper builds on established wave-optics propagation methods to develop a dynamic multi-physics framework for high-speed moving-target engagements. The central idea is to rebuild the propagation environment at every time step according to the instantaneous target position so that the optical path can be linked with height-dependent extinction, refractive-index structure, and absorption profiles; meanwhile, relative motion is represented as an effective transverse sweeping velocity, which helps reduce the accumulation effect of thermal blooming. Based on a unified split-step Fourier method, the model combines Beer–Lambert path attenuation, Kolmogorov-spectrum multilayer phase screens, and thermal-blooming phase modulation, yielding time-resolved indicators such as transmittance, received power, peak irradiance, Strehl ratio, scintillation index, pointing error, and beam-quality factor. Simulation results show that, in a 500 m static horizontal path, the attenuation module reduces to the Beer–Lambert solution, with the maximum relative error kept below 10−14; the turbulence phase-screen implementation is validated against the absolute Kolmogorov prediction. Over the resolved inertial interval, the ensemble-averaged structure function yields a fitted exponent pfit=1.671 (theoretical: 5/3=1.667) and a relative normalization error of 2.5%, confirming that the generated screens reproduce both the Kolmogorov scaling and the prescribed turbulence strength. For a 1.064 μm, 2 kW Gaussian beam propagating over a 50 s trajectory of about 8 km, when the target speed increases from 0 to 300 m/s, the thermal-blooming-related beam-quality factor decreases from roughly 1.8 to 1.2, which indicates that dynamic geometry, turbulence, attenuation, and thermal blooming need to be considered together when evaluating laser propagation performance.

Photonics doi: 10.3390/photonics13080786

Authors: Oleg V. Ivanov James M. Gilbert

We obtain exact solutions for high-order hybrid cladding modes of standard fibers, paying special attention to the case in which two hybrid cladding modes have very close propagation constants (crossover points). We calculate the mode fields, dispersion, and polarization distribution of cladding modes at crossover. We discuss the applicability of the linearly polarized modes approximation for calculating modes in this case. We show that, at the crossover points, the modes are not standard HE and EH hybrid modes but radial and azimuthal modes with field distributions resembling those of TE and TM modes. We analyze the dispersion of hybrid modes with azimuthal number equal to 1 and find crossover points in the range 0.6–1.7 μm for fibers with various V-numbers. For standard fibers, the first 23 hybrid modes have crossovers at wavelengths below 1.2 μm. Accounting for the new hybrid modes reveals the splitting of resonances in long-period fiber gratings. A linear combination of crossover modes can be used to form approximate HE and EH modes with uniform linear polarization for the HE mode and a magnetic-dipole-like field for the EH mode.

Photonics doi: 10.3390/photonics13080785

Authors: Xingxin Song Bing Zhou Jiale Zhao Jiaju Ying Yudan Chen Lei Deng

Land-based hyperspectral imaging provides high spatial and spectral resolution for detecting camouflaged targets, but practical deployment remains limited by strong target background spectral similarity, scarce annotated hyperspectral samples, and the computational cost of full-band processing. To address these issues, this paper proposes HCTDNet (Hyperspectral Camouflaged Target Detection Network), a land-based hyperspectral image analysis framework. The method first employs band extraction for data dimensionality reduction, compressing multi-channel hyperspectral images into 3-channel virtual RGB representations, which reduces spectral redundancy while preliminarily enhancing camouflaged target saliency. A pre-trained RGB camouflaged target detector is then adopted as the backbone model, with its parameters frozen to maintain stability, while trainable modality-specific prompts are learned to improve training efficiency. Finally, model fine-tuning is performed using a self-constructed camouflaged target dataset to enhance robustness in detecting camouflaged targets within virtual RGB images. During inference, preprocessed hyperspectral images are fed into the model to generate detection results for camouflaged target regions. The experiments performed on our self-collected land-based hyperspectral dataset with camouflaged targets reveal that HCTDNet achieves superior detection performance compared with seven classical hyperspectral target detection methods while maintaining an average inference speed of approximately 16 FPS. The proposed framework provides an efficient and near-real-time applicable solution for land-based hyperspectral camouflaged target detection, showing significant practical potential.

Photonics doi: 10.3390/photonics13080784

Authors: Yiyi Yang Lin Qi Dexian Yan Yi Wang

To meet the growing demand for reliable space–air–sea-integrated communications and underwater information backhaul, this paper proposes and analyzes a three-hop heterogeneous space–air–sea communication system consisting of a satellite, a high-altitude platform (HAP), a sea-surface buoy, and an autonomous underwater vehicle (AUV). Specifically, the satellite-to-HAP link employs free-space optical (FSO) transmission, the HAP-to-sea-surface buoy link adopts mixed FSO/radio-frequency (RF) transmission, and the sea-surface buoy-to-AUV link utilizes underwater wireless optical communication (UWOC). To enhance the reliability of the HAP-to-sea-surface buoy link in complex atmospheric and maritime environments, a threshold-based adaptive combining scheme for mixed FSO/RF transmission is designed. Meanwhile, nonzero-boresight pointing error models are incorporated into the FSO and UWOC links to characterize practical link misalignment. Based on the proposed system model, analytical expressions for the end-to-end bit error rate (BER) are derived and validated through Monte Carlo simulations. The numerical results show that the proposed adaptive combining scheme achieves better BER performance than conventional dual-hop and hard-switching schemes. In addition, the effects of pointing errors, underwater turbulence, underwater transmission distance, shadowed fading, detection techniques, and modulation schemes on the system BER performance are further investigated. This work provides theoretical guidance for reliable cross-domain heterogeneous transmission in future space–air–sea integrated communication systems.

Photonics doi: 10.3390/photonics13080783

Authors: Donghua Zhou Quan Luo Yiyou Fan Wei Jiang Jinshan Su

Manufacturing errors introduce phase deviations in Mach–Zehnder interferometers (MZIs) that degrade the fidelity of optical processors. To address this issue, we employ a Broyden–Fletcher–Goldfarb–Shanno (BFGS) quasi-Newton method for phase calibration of a 4×4 reconfigurable MZI optical processor based on the Reck architecture. By optimizing the mapping from the target matrix to the optical network, the method determines the optimized phase parameters of 12 phase shifters. Thermo-optic simulations are further used to establish the relationship between the applied bias voltage and the induced phase shift, providing a link between the optimized phase parameters and the electrical driving conditions. Compared with Particle Swarm Optimization (PSO), Genetic Algorithms (GA), and Gradient Descent with Momentum (GDM), the BFGS method provides faster convergence and high calibration fidelity. These results demonstrate an efficient approach for phase calibration of programmable MZI optical processors.

Photonics doi: 10.3390/photonics13080782

Authors: Qiuhan Sui Zhichao Feng Rong Xu Chunzhu Zhao Aimin Wang

We report a 1100 nm all-polarization-maintaining (all-PM) fiber laser based on gain-managed nonlinear amplification (GMNA), and demonstrate its capability for in vivo two-photon imaging. The home-built fiber oscillator functioned using a nonlinear amplification loop mirror (NALM), delivering a 38.1 MHz, 13.8 mW, 1024 nm signal laser. The pre-chirp management (PCM) was incorporated with GMNA to enable efficient nonlinear amplification. The system ultimately generated pulses with an energy of 110 nJ and a duration of 56 fs, with the corresponding 10 dB spectral range spanning from 1041 nm to 1117 nm. This 1100 nm ultrafast fiber laser provides a convenient light source for multi-photon microscopy (MPM).

Photonics doi: 10.3390/photonics13080781

Authors: Jinghao Zhang Mengqi Xie Dengyue Zuo Jianhui Qiao Mengying Zhao Zhou Yang Dong Wang Wanli He Hui Cao Yinjie Chen

Cholesteric liquid crystals (CLCs) possess the unique ability to selectively reflect incident circularly polarized light, exhibiting tremendous potential in diverse optical applications. In this study, a trilayer composite architecture of polymer-stabilized cholesteric liquid crystals (PSCLCs) was successfully fabricated. Introducing poly(vinyl alcohol) (PVA) as intervening barrier layers enabled the formation of independent and mutually non-interfering broadband reflection bands within each respective layer. Initially, a single-layer system was evaluated to identify the effects of component concentrations and polymerization conditions on the reflection bandwidth. Under optimal conditions, a maximum reflection bandwidth of 890 nm was achieved. Building upon these parameters, the effective concatenation of two independent reflection bands was accomplished by precisely regulating the concentration of the chiral dopant R5011 in the adjacent layers. Subsequently, the trilayer PSCLC film was constructed, ultimately broadening the total reflection bandwidth to 1650 nm. Characterization via polarized optical microscopy (POM) confirmed that the liquid crystal molecules consistently maintained a well-defined planar texture throughout the fabrication process of the multilayer films. Additionally, the film shows good infrared shielding performance. Its ability to regulate ambient light makes it highly promising as an optical filter and thermal management component in LC smart windows and emerging displays.

Photonics doi: 10.3390/photonics13080780

Authors: Yingying Hu Yan Shi Yuxuan Ye Chunliu Sun Lin Yin Chunlian Zhan

Reflective SLMs are gaining increasing attention for aspherical surface testing due to their dynamic adjustability, fast operation, and high optical efficiency, offering clear advantages over costly CGH methods. This paper presents a wavefront measurement system that avoids the need for dedicated beam-splitting or complex long optical paths typically required when using a reflective SLM in the interference section. The measured results are comparable to those obtained with a commercial lens compensation method, with the overall RMS values agreeing to within approximately 0.01λ, and the maximum RMS deviation from the mean remaining within 0.025λ across all temperature conditions. With its simple structure and short optical path, the system enables fast measurement.

Photonics doi: 10.3390/photonics13080779

Authors: Amjad Ali Syed Raza Mehdi Shulan Lin Ying Xu Pablo Palacios Jativa Waseem Ur Rahman Baseerat Bibi Ameen Alkasem Mehboob Hussain Zeeshan Shafiq

Visible light communication (VLC) has emerged as a transformative optical wireless technology for sixth-generation (6G) networks, offering license-free spectrum access, inherent electromagnetic-interference immunity, high spatial confinement, and the unique ability to combine high-speed wireless connectivity with solid-state lighting infrastructure. However, the transition from conventional VLC links to practical 6G optical wireless systems requires far more than advanced modulation and signal processing. Future VLC performance will be strongly determined by the co-design of photonic front-ends, including high-speed transmitters, spectrally engineered emitters, reconfigurable optical interfaces, intelligent receivers, and energy-autonomous detection units. This article provides a comprehensive, device-centered review of photonic hardware and artificial intelligence (AI) enablers for next-generation 6G VLC systems. Particular attention is given to micro-LEDs, laser diodes, color-conversion materials, including perovskite quantum dots, advanced photodetectors, imaging receivers, wavelength-shifting fiber receivers, solar-cell-based receivers, optical reconfigurable intelligent surfaces (RISs), metasurfaces, beam-steering components, and optical wireless power transfer. This review discusses how AI can support inverse photonic design, transmitter and receiver calibration, nonlinear impairment mitigation, channel-aware beam control, and energy-aware resource management. Unlike broader VLC surveys that mainly emphasize network architecture, this article provides a device-centered perspective on AI-enabled photonic integration for 6G VLC, supported by a comprehensive survey of recent experimental demonstrations. Key challenges related to bandwidth, optical efficiency, receiver field of view, mobility, safety, standardization, and practical deployment are summarized, followed by a research roadmap for 2025–2032.

Photonics doi: 10.3390/photonics13080778

Authors: Yunhan Wu Bo Ni Lifu Wu

Highly saturated structural colors are crucial for the micro-nanophotonic displays. However, achieving Schrödinger’s red pixels remains a challenge due to the difficulty in suppressing higher-order resonances in the blue-green wavelength band. In this paper, we utilize the TuRBO algorithm for global physical parameter optimization of periodic Sb2S3 nanopillar metasurface structures to realize the high-saturation red. Based on the intrinsic dispersion characteristics of Sb2S3 characterized by high extinction coefficients in the blue-green wavelength band, the high-saturation Schrödinger’s red that surpasses the Adobe RGB boundary has been produced successfully. The calculated results show that after 45 iterations, the structure achieves gradient refractive index matching, aligning the intrinsic dispersion of Sb2S3 with the ideal reflection spectrum of Schrödinger’s red pixels, thereby effectively suppressing higher-order resonances in the blue-green wavelength band. Ultimately, an ultra-high saturation red is achieved with CIE coordinates of (0.6507, 0.3043) in the CIE chromaticity space.

Photonics doi: 10.3390/photonics13080777

Authors: Xinyu Cui Xinyue Zhang Da Sun Aoran Dai

Hyperspectral unmixing (HU) requires effective modeling of spectral–spatial information and local–global feature interactions to achieve accurate abundance estimation and endmember extraction. Although Transformer-based HU methods are effective in capturing long-range dependencies, they often neglect the intrinsic spectral consistency of hyperspectral data and do not fully exploit the global spectral–spatial correlations in hyperspectral image cubes. To address these issues, this paper proposes a Spectral Consistency–guided Transformer with Volumetric Linear Self-Attention (SCVolFormer) for hyperspectral unmixing. A Spectral Consistency Block (SCB) is introduced to preserve consistency across adjacent spectral bands and produce physically meaningful feature representations. A spectral grouping strategy is further adopted to partition the high-dimensional spectrum into locally continuous subspaces, reducing computational cost. In addition, a shared-weight Transformer encoder with Volumetric Linear Self-Attention (VolLSA) is designed to model interactions between the spectral and spatial dimensions and capture long-range dependencies within hyperspectral image cubes. A decoder is then used to estimate abundance maps and reconstruct hyperspectral images. Experiments on one synthetic dataset and three real hyperspectral datasets demonstrate that SCVolFormer outperforms state-of-the-art methods in abundance estimation and endmember extraction, confirming the effectiveness of spectral consistency guidance and volumetric attention modeling for hyperspectral unmixing.

Photonics doi: 10.3390/photonics13080776

Authors: Yuxin Xia Zhiguang Li

Nominal launch-power profiles can lose quality-of-transmission (QoT) margin when span and equipment parameters vary. We study this effect using C-band GNPy 2.14.1 simulations that recompute amplified-spontaneous-emission (ASE) noise and Gaussian-noise (GN)-model nonlinear interference under perturbations. Ten runs use 384 training scenarios and 1024 intensified-stress scenarios with scalar and spectral multipliers of 1.25 and 1.50. In paired within-GNPy comparisons, a finite-sample 5% lower-tail-mean selector, defined as the mean of the 20 worst training utilities, improves fifth-percentile minimum-channel generalized signal-to-noise-ratio (GSNR) margin over nominal optimization by 0.247 dB, with a 95% confidence-interval half-width of 0.014 dB. After normalization to the nominal total launch power, the gain remains 0.179 dB (half-width 0.017 dB), suggesting that spectral shape is a major contributor to the paired difference in this comparison. The gain lies between 0.245 and 0.248 dB when the training-tail fraction varies from 1% to 10%; relaxing the per-channel ceiling from 3.0 to 3.5 dBm removes almost all active bounds while retaining a 0.246 dB gain. Selected profiles mainly raise the low-frequency edge, and the benefit appears near the modeled reach boundary rather than on high-margin metro links. Erbium-doped fiber amplifier noise figure, gain ripple, and reconfigurable optical add-drop multiplexer equalization lead the sensitivity ranking. Reduced Manakov checks preserve power ordering while exposing model offsets. The results describe the specified GNPy configuration, finite search, and synthetic perturbation laws; field-calibrated performance remains to be established.

Photonics doi: 10.3390/photonics13080775

Authors: Zhuoshen Shi Sheng Dong Yicheng Huang Junqi Wang Jiakang Shi Jianghai Wo Xudong Wang Jiejun Zhang Jianping Yao

Discrete energy-level dynamics provide a fundamental framework for understanding driven two-state systems. Here, we propose and experimentally demonstrate a classical microwave-photonic analog based on two active fiber loops. Reciprocal electro-optic sidebands coherently couple two independently sustained lasing modes. When the input RF power is increased from 11.7 to 18.7 dBm, the measured oscillation frequency increases from 0.55 to 1.00 MHz, consistent with the modulation-induced coupling predicted by the coupled-mode model. The experiment therefore demonstrates RF-power-controlled Rabi-like oscillation in an active dual-loop system. Because the optical fields and detected intensities are classical, the platform reproduces reduced two-state dynamics rather than quantum-state evolution. This proof-of-concept provides a basis for quantum-inspired microwave-photonic signal processing and future short-cavity implementations.

Photonics doi: 10.3390/photonics13080774

Authors: Junsong Wang Mingxu Piao Xian Zhang Keyan Dong Zhongju Ren Huilin Jiang

Conventional cooled infrared optical systems employ a cold stop, which disrupts optical-path symmetry and constrains exit-pupil matching. Consequently, reducing the refractive lens count increases the residual broadband-aberration burden, motivating the introduction of an ultrathin phase-compensation element near the exit pupil. Conventional solutions therefore tend to use complex optical configurations with large volume and high weight, making it challenging to meet the demands of modern lightweight and compact detection systems. Metalenses offer a new approach for aberration correction through the flexible phase manipulation enabled by their unit cells. However, severe chromatic dispersion of metalenses under broadband conditions remains a major obstacle to their practical application. To address this issue, a hybrid refractive–metalens design method for cooled infrared optical systems is proposed. Based on the distinctive phase distribution characteristics of metalenses, an achromatic theoretical formulation applicable to broadband infrared wavelengths is derived. Guided by this theory, a cooled mid-wave infrared refractive–metalens hybrid optical system is designed, featuring a full field of view of 126°, an F-number of 2, and an operating wavelength band of 3.3–5 μm. In comparison with a conventional eight-element refractive system of identical specifications, the proposed hybrid system reduces the total optical-element count from eight to four and achieves reductions of 21% in total track length and 79% in system weight, while maintaining a full-field polychromatic MTF above 0.4 at 33 lp/mm. In addition, the narcissus effect is effectively mitigated under the modeled conditions. This approach enables high-performance aberration correction using metalenses while offering a new design paradigm for simplified infrared optical systems.

Photonics doi: 10.3390/photonics13080773

Authors: Yuelai Chen Ruoshi Gu Aleksandra Panajotović Jun Zhang Jun Huang Liang Zhang Xiaolin Zhou

Photon-counting constitute a promising technology for ultra-weak signal satellite communications. Considering the Poisson shot noise impairment, atmospheric turbulence fading, and multi-user interference, in this paper, a meta-learning-driven photon-counting multi-user single-input multiple-output (MU-SIMO) scheme is developed and analyzed. Referred to as meta-learning-driven signal detection (Meta-SD), this scheme can achieve rapid convergence with limited samples and significantly improve system detection performance. Simulation results demonstrate that the proposed meta-learning scheme outperforms the mean square error based signal detection (MSE-SD) baseline, in terms of detection accuracy, robustness to signal-dependent Poisson shot noise, convergence speed, and generalization to few-shot detection tasks with previously untrained signal classes. Specifically, Meta-SD achieves nearly a tenfold reduction in BER, compared with the derived MSE-SD benchmark, in a 4×8 MU-SIMO scenario at Es=−140 dBJ.

Photonics doi: 10.3390/photonics13080772

Authors: Jiyeon Baek Yuna Lee Hyunchae Chun

Compact mobile optical wireless communication (OWC) terminals are constrained not only by link budget but also by mobile-side size, weight, power consumption, and cost (SWaP-C). This paper proposes and models a remodulation-based bidirectional OWC transceiver module that integrates a fluorescent concentrator (FC) receiver and a modulating retroreflector (MRR) transmitter. The FC receives the downlink by absorbing a wavelength-dependent fraction of an interrogation beam, Stokes-shifting the absorbed light, and guiding the emission to an edge photodetector. The transmitted fraction of the same interrogation beam reaches the MRR and is remodulated for low-power uplink transmission without a mobile-side optical source. The central design variable is therefore not the optical power alone, but the pair consisting of the interrogation wavelength and the downlink modulation depth. A fully absorbed wavelength with high modulation depth is used for downlink-only operation, a pass-through wavelength with zero modulation depth is used for uplink-only operation, and an absorption-shoulder wavelength with intermediate modulation depth is used for simultaneous downlink and uplink remodulation. A spectral photon-transfer model, a direct-detection communication model, a self-interference model, and a weighted rate-optimization framework are developed. Simulation results show that the optimized wavelength shifts from the FC absorption peak in downlink-dominant operation to the FC pass-through window in uplink-dominant operation, while the optimal downlink modulation depth decreases to preserve uplink carrier margin. The proposed architecture is particularly well-suited for drones, robots, vehicles, and distributed sensors requiring robust optical downlink reception and low-SWaP-C uplink signaling.

Photonics doi: 10.3390/photonics13080771

Authors: Ju Wang Manyun Liu Hao Luo Xingmiao Li Xuemin Su Chuang Ma Jinlong Yu

A high-stability actively mode-locked fiber laser (AMLFL) is proposed and experimentally demonstrated. This AMLFL is based on a distributed feedback laser diode (DFB-LD) injection locking with Fabry-Perot (F-P) etalon frequency stabilization. In this system, a wavelength modulation method is employed to generate the feedback signal for frequency stabilization. The stabilization mechanism utilizes the linear response characteristic of the first-order derivative of the F-P etalon transmission peak. This achieves wavelength stabilization of the DFB-LD. Subsequently, the stabilized light source is injected into the ring cavity of the AMLFL. The proposed system does not require modification to the existing AMLFL cavity. It also features a simple structure and low implementation cost. Experimental results show that, with frequency stabilization, the wavelength drift of a selected spectral line is reduced to within the 10 pm resolution of the OSA. Meanwhile, the standard deviations of the 5 GHz spectral component power fluctuation and the average output optical pulse power are 0.01 dB and 0.01 dB, respectively.

Photonics doi: 10.3390/photonics13080770

Authors: Shichao Chang Mengqi Suo Chaowei Sun Anbo Hu Kang Li Jichao Yang Danyi Zhang Fazhan Tao Tianqing Jia Hongxing Xu

High-quality and high-efficiency machining of film cooling holes (FCHs) is critical for enhancing the performance of gas turbines and aero-engines. Nanosecond fiber lasers offer advantages such as high stability, good beam quality, and ease of integration. However, when machining FCHs, issues including low drilling efficiency and significant thermal effects severely limit their industrial applications. In this study, a dual-rotary trepanning system was developed based on a nanosecond fiber laser, a galvanometer, and a five-axis cradle machine. High-quality FCHs with a diameter of 0.6 mm were efficiently machined in a 3-mm-thick DD6 superalloy plate within only 6.5 s. Compared with the method using machine tool rotation alone, the average recast layer thickness on the inner wall was reduced by 62.1% to 6.7 μm, and the average surface roughness was reduced by 61.1% to 0.35 μm. These improvements are primarily attributed to the galvanometer speed being two orders of magnitude higher than that of the machine tool, which significantly reduces the laser pulse overlap rate and the thermal accumulation effect. Moreover, the kerf widened by the galvanometer rotation allows the ablation products to expand more fully and be expelled efficiently, thereby reducing impact, scratching, and debris adhesion on the inner wall and improving the drilling efficiency. Furthermore, 10 × 10 FCH arrays were machined on both vertical and inclined plates, demonstrating high consistency and stability, indicating the potential for industrial applications in the field of FCH machining.

Photonics doi: 10.3390/photonics13080769

Authors: Zhen Wang Junjie Xu Lifei Wang Zhen Zhang

Film cooling holes are critical cooling structures that enhance the temperature-bearing capacity of turbine blades. Irregularly shaped holes, such as laidback fan-shaped holes (LFSHs), have been demonstrated to exhibit superior cooling performance compared to traditional circular holes. However, the complex shapes and structures pose significant challenges for femtosecond laser processing. Due to the extremely limited reports available, the dependency of femtosecond laser processing on the formation of irregularly shaped holes has not yet been well understood. In this paper, the fabrication process of the LFSH expansion sections and the influence of defocus distance are discussed in detail. By systematically characterizing the surface micro- and nanostructures, three-dimensional topography, and roughness of the expansion sections, the processing differences under positive defocus, zero defocus, and negative defocus are compared. The formation mechanisms of the micro-hole structures on the expansion sections under positive defocus and zero defocus are elaborated by incrementally increasing the number of processing cycles. Under negative defocus, only a smooth honeycomb-like structure forms on the surface of the expansion section, yielding the highest surface quality. Under different processing times, the roughness under negative defocus can be reduced by up to 84.8% and 95.8% compared to zero defocus and positive defocus, respectively. Furthermore, a correlation between the surface micro-hole structure and the edge structure of the hole walls is established.

Photonics doi: 10.3390/photonics13080768

Authors: Li Yuan Jun Xing Bowen Shen Yuancheng Du Wenchi Wei Xicheng Rao

Underground cables are critical infrastructure for electrical power and communication transmission, and their reliable operation is of paramount importance to urban public safety. Although Distributed Acoustic Sensing (DAS) enables wide-range, continuous, and real-time monitoring, traditional DAS signal processing methods suffer from poor intrusion discrimination and weak anti-interference capability. To address these limitations, we propose a dual-branch network based on multi-domain feature fusion, integrating a Multilayer Perceptron, a Long Short-Term Memory network (LSTM), and an attention mechanism. Vibration signals corresponding to four representative high-risk intrusion events were acquired through controlled field experiments, and a standardized, category-balanced dataset was constructed accordingly. Time-domain, frequency-domain and joint time-frequency features were extracted and mapped through a time-frequency weighting transformation to form one branch of the network, while the parallel branch employed an LSTM to capture long-range temporal dependencies. A multi-head attention mechanism enables deep adaptive fusion of two types of modal information and overcomes the limitations of conventional simple feature concatenation. Comparative experiments against KNN, 1D-CNN and LSTM baselines demonstrate that the proposed model achieves a test accuracy of 98.89%, outperforming all reference methods. Ablation studies further validate the necessity and effectiveness of each constituent module within the proposed architecture. The results indicate that this approach provides reliable support for DAS-based online monitoring of power cables against external damage.

Photonics doi: 10.3390/photonics13080767

Authors: Donglin Wang Tianhao Feng Ziyan Li Xuezhen Zhai Dewei Liu Jimin Shang Lamei Zhang

The increasing demand for broadband and self-powered optoelectronic devices has stimulated the development of two-dimensional (2D) material-based photodetectors. Here, we construct a self-powered photodetector based on a MoSe2/SnS2 van der Waals (vdW) heterostructure. The unique combination of MoSe2 and SnS2 provides complementary light absorption and a type-II band alignment, which generates an interfacial built-in electric field to facilitate photogenerated carrier separation and transport, enabling efficient self-powered operation without external bias. The device exhibits a broad spectral photoresponse from ultraviolet (UV) to infrared (IR) wavelengths (350–1050 nm). Under zero bias, the photodetector achieves a responsivity of 0.6 mA/W and a rapid response time of 5.8/6.1 ms (rise/fall) under 405 nm illumination, demonstrating effective self-driven photoresponse. With an applied reverse bias of −2 V, the device further reaches a responsivity of 161 mA/W and a detectivity of 8.6 × 1010 Jones under 405 nm illumination. In addition, the device exhibits a low dark current of 5 pA and a high on/off ratio of 1180. These results demonstrate the potential of the MoSe2/SnS2 vdW heterostructure for broadband and self-powered optoelectronic applications.

Photonics doi: 10.3390/photonics13080766

Authors: Gabit Nazymbekov Gyula Polónyi Luis Nasi György Tóth

GaP and ZnTe semiconductor crystals are numerically investigated for terahertz (THz) generation driven by ultrafast 10.6 µm CO2 laser pumping. At this wavelength, low-order multiphoton absorption is effectively suppressed, enabling the study of intrinsic performance limits governed by nonlinear optical effects, material dispersion, and THz absorption. A one-plus-one-dimensional (1+1D) frequency domain propagation model is employed, including optical rectification, cascaded nonlinear interactions, self-phase modulation, second-harmonic generation, and pulse-front-tilt-related dispersion. The simulations reveal markedly different behavior in the two crystals. In ZnTe, the large effective nonlinear coefficient enables conversion efficiencies exceeding 1% while maintaining good THz pulse quality. In GaP, strong self-phase modulation and nonlinear pulse compression can enhance the predicted efficiency but at the cost of waveform distortion and increased damage risk. MV/cm-level THz electric fields are predicted in both materials; however, ZnTe provides a more favorable compromise between conversion efficiency and pulse quality. The results provide practical guidelines for optimizing CO2-laser-driven semiconductor THz sources.

Photonics doi: 10.3390/photonics13080765

Authors: Xiaohua Zeng Hui Pang Cheng Xu Axiu Cao Yongqi Fu Qiling Deng

Diffractive optical elements (DOEs) have become core components for converting Gaussian beams into flat-top beams due to their advantages of flexible design, compact size, and precise control over light field distribution. However, during the fabrication of multi-step DOEs, some processes such as ion beam etching tend to produce etching depth errors, which cause deviations of the surface micro–nano phase structures from the designed values and thus severely degrade the beam shaping performance. This paper proposes a focusing DOE for flat-top beam shaping, which combines the focusing phase with the phase optimized by the weighted constraint iterative algorithm to establish a phase distribution resilient to etching depth errors. Thus, the proposed focusing DOE exhibits significantly improved robustness to etching depth errors and effectively reduces the structural complexity of laser optical systems. Our experimental results show that the designed DOE can stably convert the incident Gaussian beam into flat-top beams within the etching depth error range of ±30 nm, with both the flat-top beam uniformity and diffraction efficiency above 95%, and the maximum tolerable etching depth error reaches ±90 nm.

Photonics doi: 10.3390/photonics13080764

Authors: Ou Liao Zhao Li

This work investigates the optical soliton solutions and their dynamical behaviors of the fractional stochastic Biswas–Arshed equation (FSBAE). By employing the definition of truncated M-fractional derivative and a wave transformation, we reduce the FSBAE to an integrable ordinary differential equation. Applying the qualitative and bifurcation theory of planar dynamical systems, we analyze its bifurcations and construct five explicit types of exact traveling wave solutions, namely solitary wave solutions, kink-type exponential function solutions, Jacobi elliptic sn-type periodic solutions, Jacobi elliptic cn-type periodic solutions, and exponential-form solitary wave solutions. Furthermore, to explore the model’s sensitivity to perturbations, we introduce a time-periodic perturbation term, then the emergence of chaotic behavior is demonstrated by providing 2D and 3D phase portraits.

Photonics doi: 10.3390/photonics13080763

Authors: Joseph R. Hamilton Summer K. Levis Guy M. Hagen

Correlative microscopy techniques are used for many different applications in the biological sciences because the comparison of different imaging methods allows researchers to gain more and often complementary information about their samples. Correlative light and electron microscopy (CLEM) methods have been developed to preserve biological samples to withstand the harsh environments necessary for electron microscopy. After first being imaged using widefield (WF) and super-resolution structured illumination fluorescence microscopy (SIM), a NanoSuit chemical treatment was applied to a mammalian tissue sample before imaging with scanning electron microscopy (SEM). This was done to compare the image quality and resolution of each technique. SEM yields higher resolution and offers validation of results from SIM.

Photonics doi: 10.3390/photonics13080762

Authors: Fangjie Li Jintian Bian Hui Kong Zhonghe Wang Haiping Xu Yuntao Xie Ke Sun

Existing long-wave infrared (LWIR) BaGa4Se7 optical parametric oscillators (BGSe-OPOs) adopt linear-cavity configurations yet struggle to balance high beam quality and high output energy. To overcome this limitation, we report a compact Type I phase-matched ring cavity BGSe-OPO pumped by a 1.06 μm laser. Operating at 8.5 μm, the OPO generates 1.2 mJ single pulses with a peak power of 0.25 MW and an optical-to-optical conversion efficiency of 2%. The estimated beam quality factor M2 is 9, representing a threefold enhancement relative to linear-cavity under identical pump conditions. The system features a compact footprint of 400 × 200 mm2 and a far-field divergence angle of 4 mrad after 6× beam expansion, enabling practical applications in far-field monitoring.

Photonics doi: 10.3390/photonics13080761

Authors: Yaling Yang Yuanqing Wang Lei Guo Haiping Xu Hao Zhang Hui Kong Jintian Bian

A high-efficiency extra-cavity Raman generator is demonstrated using a potassium yttrium tungstate (KYW) crystal for the first time, achieving switchable yellow-green laser output at 556 nm and 560 nm. The KYW Raman crystal is single-pass pumped by 532 nm green light, which is obtained by the second-harmonic generation of a 1064 nm flash-lamp-pumped Nd:YAG electro-optic (EO) Q-switched laser. When the incident pump energy at 1064 nm is 156 mJ under a repetition rate of 1 Hz, the maximum output energy reaches 51.3 mJ at 532 nm with a slope efficiency of 41.5%. By adjusting the polarization direction of the 532 nm light, two characteristic Raman shifts of the KYW crystal can be selectively excited at 765 cm−1 and 905 cm−1. Furthermore, the 556 nm and 560 nm visible lasers are generated with the maximum output energies of 11.6 mJ and 16.2 mJ, corresponding to the slope efficiencies of 37.6% and 48.2%, respectively. Compared with the 532 nm pulse width (18.2 ns), an obvious Raman pulse width compression effect occurred (8.2 ns).

Photonics doi: 10.3390/photonics13080760

Authors: Yadong Han Jie Yu Qi Zhu Liping Wang

The illumination system in extreme ultraviolet lithography (EUVL) achieves pupil shaping by adjusting the facet-matching relationship of fly’s eyes. We propose an algorithm based on integer programming and energy equalization for solving the facet-matching relationship, which constrains the energy consistency across different pupil regions, enhancing pupil performance and achieving high uniformity at the mask plane. We model an illumination system to validate the proposed algorithm. Simulation results demonstrate that the proposed algorithm successfully realizes various illumination modes, achieving a maximum pupil ellipticity of 1.94%, maximum X/Y pupil pole balance of 1.45%, and maximum quadrant pole balance of 1.82%. Furthermore, it achieved an illumination field at the mask plane with a pre-correction nonuniformity of 0.45–0.59%.

Photonics doi: 10.3390/photonics13080759

Authors: Yunhe Zhao Jin Lu Siyu Chen Chengbo Mou Yunqi Liu Kaiming Zhou Lin Zhang

Few-mode fiber (FMF) grating sensing exploits the mode-dependent responses of multiple spatial modes to external perturbations, enabling vector and dual-parameter measurements. This paper reviews the research progress of FMF grating sensors from the perspectives of fundamental mechanisms, fabrication techniques, signal demodulation, and representative sensing applications. First, the mode propagation characteristics, coupled mode theory, and phase matching conditions of FMF gratings are discussed. Then, the fabrication strategies of FMF gratings are summarized, followed by an analysis of signal demodulation methods. Particular attention is given to vector sensing and dual-parameter sensing. Finally, the remaining challenges related to mode excitation stability, spectral resonance overlap, fabrication repeatability, packaging reliability, and long-term calibration are discussed, and future opportunities in mode engineering, multidimensional demodulation, and intelligent sensing systems are outlined.

Photonics doi: 10.3390/photonics13080758

Authors: Pierre Jeunesse Yanis Abdedou Mirza Barlas Aloïs Baudry Sylvie Lebrun

Optical microfibers are fabricated by pulling classical silica fibers until reaching diameters of a few micrometers or less. These devices are significantly exploited in many science and engineering fields, ranging from fundamental research to practical applications. Despite their many attractive advantages, a major technological challenge remains: heating caused by laser absorption from surface defects and contaminants. In the present study, we propose, for the first time to our knowledge, a novel method to measure the temperature evolution of laser self-heated microfibers in air at a wavelength of 1.48 µm. This method, simple and fast, enables us to investigate the influence of the diameters and lengths of the microfibers. We found that the temperature of the microfibers increases linearly with the power and measured a rise of 70 °C for a 1 µm diameter and 20 mm length microfiber at a moderate power of 160 mW. A numerical model considering the microscale and the heat exchange with air is proposed and is adjusted with experimental data, providing values for the thermal transfer coefficient. By investigating power scaling, this work enables the prediction of temperature increases in self-heated microfibers in air, paving the way for new insights into the self-cleaning of microfiber-based devices and for optimized control of light propagation at high power levels.

Photonics doi: 10.3390/photonics13080757

Authors: Maryam Azizi Mohammad Soroosh Mohammad Javad Maleki Sandip Swarnakar

Accurate and non-invasive monitoring of glucose levels remains a critical challenge in diabetes management, motivating the development of highly sensitive optical biosensors. In this work, a surface plasmon resonance-based biosensor operating in the Kretschmann configuration is proposed and numerically investigated for glucose detection. The sensor architecture consists of a BK7 prism/TiO2/Ag/graphene multilayer, and the effect of incorporating a hexagonal boron nitride (h-BN) interlayer with varying thicknesses is systematically analyzed to enhance sensing performance. Electromagnetic simulations were performed using the finite-difference time-domain method in Lumerical FDTD Solutions at a wavelength of 633 nm. Key performance parameters, including angular sensitivity, full width at half maximum, detection accuracy, figure of merit, signal-to-noise ratio, and limit of detection, were evaluated for glucose concentrations corresponding to refractive indices ranging from 1.3282 to 1.3767 RIU. The conventional BK7/TiO2/Ag/TiO2/Graphene/Sensing Medium (SM) configuration achieved a sensitivity of 167.48 deg/RIU. By introducing an h-BN layer, significant performance enhancement was observed. The optimized structure with an 8 nm h-BN layer exhibited a maximum angular sensitivity of 205.35 deg/RIU, representing an improvement of approximately 22.6% over the reference design, while maintaining a low detection limit of 2.43 × 10−4 RIU. The results further reveal that h-BN thickness plays a crucial role in balancing sensitivity and resonance quality, where excessive thickness broadens the resonance curve and degrades detection accuracy. The proposed graphene-h-BN-assisted SPR platform demonstrates high potential for high-performance, non-invasive glucose monitoring and provides practical design guidelines for next-generation plasmonic biosensors.

Photonics doi: 10.3390/photonics13080756

Authors: Xinyu Wang Liang Deng Kun Gong Shuhao You Ziyi Yang Zhongkai Huang Haolin Lu Guankui Long

The time-resolved fluorescence spectrum of a driven two-level system is investigated using a time-dependent variational method. We first establish the transient build-up of the Mollow triplet via the Lindblad master equation under the rotating-wave approximation, providing a complete visualization of the spectral evolution from initial turn-on to steady state. To go beyond the perturbative regime, we employ the multiple Davydov D2 ansatz (multi-D2), which uses the σz eigenstates as the basis and naturally accommodates arbitrary system–bath coupling types and spectral densities. The multi-D2 method converges with M = 4 multiplicities in studied cases, outperforming the multi-D1 ansatz (M = 8) for the σx coupling benchmark. For pure σz (dephasing) coupling under resonant driving, we find that the time-resolved spectrum reveals a distinct fluorescence peak at the Rabi frequency—a signature of dressed-state transitions induced by the dephasing channel that remains hidden in population dynamics. Under mixed σx–σz coupling, the spectrum exhibits combined features of both Mollow triplet and σz-mediated emission. The effects of sub-Ohmic, Ohmic, and super-Ohmic spectral densities are systematically compared. While the resonant spectral weight J(ω0) governs the overall dissipation rate, a controlled comparison at fixed J(ω0) reveals that the super-Ohmic regime exhibits an intrinsic shape-dependent suppression of sideband emission under resonant driving, highlighting an asymmetric role of the spectral density exponent in engineering transient fluorescence. Our work establishes the multi-D2 ansatz as a versatile tool for simulating time-resolved fluorescence in complex bosonic environments.

Photonics doi: 10.3390/photonics13080755

Authors: Ziming Zhang Qiancheng Zhao

Integrated waveguide photonics has experienced tremendous growth, evolving from discrete components to multi-functional devices and complex architectures for optical power routing and information processing [...]

Photonics doi: 10.3390/photonics13080754

Authors: Manjun Yan Qiuju Wu Long Ma

Haze conditions degrade image quality via atmospheric scattering and absorption, posing challenges for optical imaging applications. In recent years, deep learning has emerged as an effective method for dehazing images. However, data-driven deep learning dehazing methods typically require large amounts of labeled training data and offer limited interpretability. In this paper, we propose a physics-guided dehazing method based on polarization imaging. By integrating polarization imaging with the physical model, the proposed method enables neural network training using only a set of hazy images captured at different polarization angles, thereby reducing the reliance on labeled training data. Experimental results show that the proposed method significantly outperforms commonly used deep learning dehazing methods in terms of contrast and mean gradient, while exhibiting strong generalization and physical interpretability. By integrating physical model and polarization imaging into deep learning, this method overcomes the limitations of traditional deep learning dehazing methods and paves the way for optical imaging in haze conditions.

Photonics doi: 10.3390/photonics13080753

Authors: Liujiang Wang Chenxiao Wang Fan Yang Liaoxin Sun Bimu Yao

For scalable quantum networks and distributed quantum sensing, remote NV color-center spin ensembles require controllable long-range coherent connections. However, the magnetic dipole coupling between NV centers decays rapidly with distance, making direct stable coherent exchange difficult to achieve. Here, we propose an active-microwave-cavity-assisted scheme for long-range synchronization, in which traveling microwave photons establish a remote cavity–spin coupling channel, while saturable gain compensates cavity loss and propagation attenuation. The results show that the system exhibits collective superradiant emission under gain-off and forms a hysteretic synchronized state controlled by the propagation phase, separation distance, and optical cooling rate under gain-on. Moreover, the active cavity can mediate long-range phase synchronization between two remote NV ensembles, with local optical cooling enabling switching between in-phase and out-of-phase synchronized states. This scheme provides a theoretical route toward room-temperature, optically reconfigurable spin networks.

Photonics doi: 10.3390/photonics13080752

Authors: Zbigniew Bielecki Janusz Mikołajczyk

Underwater wireless optical communication (UWOC) has emerged as a key technology for high-speed, low-latency data transmission in aquatic environments, enabling applications in autonomous underwater vehicles (AUVs), remotely operated vehicles (ROVs), subsea sensor networks, and the Internet of Underwater Things (IoUT). This paper reviews photodetector technologies that shape UWOC system performance, covering both mature and emerging detector classes. We discuss the operating principles, key parameters, and practical trade-offs of photomultiplier tubes (PMTs), p-i-n photodiodes (PINs), avalanche photodiodes (APDs), single-photon avalanche diodes (SPADs), and silicon photomultipliers (SiPMs/MPPCs). We also present emerging photodetector technologies, including perovskite-based structures, SiC photoelectrochemical devices, scintillating optical fibers, and photovoltaic solar cells. A comparative analysis of reported UWOC experiments reveals a clear sensitivity–bandwidth trade-off among detector technologies: PIN-based receivers achieve the highest data rates (up to 25 Gbps) but are generally restricted to short-range links, whereas SPAD- and SiPM-based receivers provide sensitivities below −80 dBm and support transmission distances exceeding 200 m, at the cost of moderate data rates. The findings indicate that SiPM/MPPC arrays currently offer the most promising compromise between sensitivity and data rate for long-range UWOC applications.

Photonics doi: 10.3390/photonics13080751

Authors: Mansoor Qadir Muhammad Umar Muhammad Ismail Mohmand Waqas A. Imtiaz Sajjad Aleem

The study examines the application of machine learning algorithms (MLAs) to enhance free space optical (FSO) communication performance by predicting quality-factor (QF) from key system parameters. FSO technology has developed as a promising solution for front-haul links in 5G, beyond the 5G (B5G), and 6G transmission networks. Nevertheless, performance of an FSO communication link is limited by environmental challenges like weather conditions, attenuation and turbulence that can degrade signal quality and affect QF at the receiving end. Using the data collected through simulation analysis in OptiSystem, we trained several MLAs in order to analyze performance of this system through accurate prediction of the QF. Analysis shows that received power serves as an important parameter in translating the overall QF of the received signal. Furthermore, it is shown that the prediction accuracy of the tree-based approach such as random forest and gradient boosting ranges above 97% are reliant on channel conditions and the predicted parameter type.