Photonics

14 pages, 5330 KB

Open AccessArticle

Prediction of Shock Wave Velocity Temporal Evolution Induced by Ms-Ns Combined Pulse Laser Based on Attention-LSTM

by Jingyi Li, Rongfan Liang, Junjie Liu and Jingdong Sun

Photonics 2025, 12(10), 1040; https://doi.org/10.3390/photonics12101040 - 21 Oct 2025

Viewed by 215

This study systematically examined shock wave velocity induced by millisecond–nanosecond combined-pulse laser (ms–ns CPL) at a fixed ns laser energy density of 6 J/cm², exploring the effects of varying pulse delays of 0 to 3 ms and ms laser energy densities [...] Read more.

This study systematically examined shock wave velocity induced by millisecond–nanosecond combined-pulse laser (ms–ns CPL) at a fixed ns laser energy density of 6 J/cm², exploring the effects of varying pulse delays of 0 to 3 ms and ms laser energy densities of 226.13 J/cm², 301 J/cm² and 376.89 J/cm². The temporal evolution of shock wave velocity induced by varying laser parameters was predicted by an attention mechanism-based long short-term memory algorithm (Attention-LSTM). The dependence between laser parameters and the evolution of shock wave velocity was captured by the LSTM layer. An attention mechanism was utilized to adaptively increase the weights of important time points during the propagation of the shock wave, thereby improving prediction accuracy. The experimental data corresponding to ms laser energy densities of 226.13 J/cm² and 301 J/cm² were set as the training set. The ms laser energy density of 376.89 J/cm² experimental data was set as test set to evaluate the generalization ability of the model under unknown ms laser energy. The results indicate that when ms laser energy density is 376.8 J/cm², the pulse delay is 2.2 ms. The shock wave velocity induced by the CPL increased by 50.77% compared with that induced by a single ns laser. The proposed Attention-LSTM model effectively predicts the evolutionary characteristics of shock wave velocity. The mean absolute error (MAE), root mean square error (RMSE), mean bias error (MBE) and the correlation coefficient (R²) of the test set are 7.65, 9.01, 1.47 and 0.98, respectively. This study provides a new data-driven approach for predicting the shock wave behavior induced by combined laser parameters and provides valuable guidance for optimizing laser process parameter combinations. Full article

(This article belongs to the Special Issue Lasers and Complex System Dynamics)

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21 pages, 4432 KB

Open AccessArticle

DMSR: Dynamic Multipath Secure Routing Against Eavesdropping in Space-Ground Integrated Optical Networks

by Guan Wang and Xingmei Wang

Photonics 2025, 12(10), 1039; https://doi.org/10.3390/photonics12101039 - 21 Oct 2025

Viewed by 261

Abstract

With the continuous growth of global communication demands, the space-ground integrated optical network (SGION), composed of the satellite optical network (SON) and terrestrial optical network (TON), has gradually become a critical component of global communication systems due to its wide coverage, low latency, [...] Read more.

With the continuous growth of global communication demands, the space-ground integrated optical network (SGION), composed of the satellite optical network (SON) and terrestrial optical network (TON), has gradually become a critical component of global communication systems due to its wide coverage, low latency, and large bandwidth. However, although the high directivity of laser communication can significantly enhance the security of data transmission, it still carries the risk of being eavesdropped on during the process of service routing. To resist eavesdropping attacks during service transmission in the SGION, this paper proposes a secure routing scheme named dynamic multipath secure routing (DMSR). In DMSR, a metric called the service eavesdropping ratio (SER) is defined to quantify the service leakage severity. The objective of DMSR is to reduce each service’s SER by switching its routing path proactively. To realize DMSR, heuristic algorithms are developed to sequentially search for optimal routing paths for service path switching in the TON and SGION. Finally, simulation results demonstrate that DMSR can achieve trade-offs between secure service transmission and network performance at different levels by adjusting its system parameters. Furthermore, the DMSR scheme significantly reduces the SER compared to the baseline schemes, while introducing acceptable increases in computation overhead and service latency. Full article

(This article belongs to the Special Issue Advancements and Future Perspectives in All-Optical Detection and Reliability Improvement Technologies)

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12 pages, 3612 KB

Open AccessArticle

A Broad-Temperature-Range Wavelength Tracking System Employing a Thermistor Monitoring Circuit and a Tunable Optical Filter

by Ju Wang, Manyun Liu, Hao Luo, Xuemin Su, Chuang Ma and Jinlong Yu

Photonics 2025, 12(10), 1038; https://doi.org/10.3390/photonics12101038 - 21 Oct 2025

Viewed by 193

Abstract

A broad-temperature-range wavelength tracking system employing a thermistor monitoring circuit and a tunable optical filter is proposed and experimentally demonstrated. In this scheme, a thermistor monitoring circuit is utilized to acquire the real-time resistance values of a distributed feedback laser diode (DFB-LD). When [...] Read more.

A broad-temperature-range wavelength tracking system employing a thermistor monitoring circuit and a tunable optical filter is proposed and experimentally demonstrated. In this scheme, a thermistor monitoring circuit is utilized to acquire the real-time resistance values of a distributed feedback laser diode (DFB-LD). When the mapping relationship curve among thermistor resistance, temperature, and center wavelength of the DFB-LD is established, the drive voltage of the narrowband tunable optical filter is dynamically adjusted to regulate its filter window. Therefore, wavelength tracking is achieved by matching the filter window and the center wavelength of the DFB-LD. The experimental results show that the proposed system can achieve adaptive wavelength tracking within the operation band of 1539.4 nm to 1548.6 nm across a temperature range from −40 °C to 60 °C. The wavelength detection resolution and the minimum step of wavelength control are better than 0.79 pm and 0.1 nm, respectively. By exploiting the conversion characteristics between the thermistor and the center wavelength of the DFB-LD, this approach transforms laser wavelength detection into a low-cost, real-time electrical measurement, significantly enhancing transmission stability and reliability of laser sources in complex thermal environments. Full article

(This article belongs to the Special Issue Microwave Photonics: Advances and Applications)

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28 pages, 3909 KB

Open AccessArticle

VCSELs: Influence of Design on Performance and Data Transmission over Multi-Mode and Single-Mode Fibers

by Nikolay N. Ledentsov, Nikolay Ledentsov, Jr., Vitaly A. Shchukin, Alexander N. Ledentsov, Oleg Yu. Makarov, Ilya E. Titkov, Markus Lindemann, Thomas de Adelsburg Ettmayer, Nils C. Gerhardt, Martin R. Hofmann, Xin Chen, Jason E. Hurley, Hao Dong and Ming-Jun Li

Photonics 2025, 12(10), 1037; https://doi.org/10.3390/photonics12101037 - 21 Oct 2025

Viewed by 456

Abstract

Substantial improvements in the performance of optical interconnects based on multi-mode fibers are required to support emerging single-channel data transmission rates of 200 Gb/s and 400 Gb/s. Future optical components must combine very high modulation bandwidths—supporting signaling at 100 Gbaud and 200 Gbaud—with [...] Read more.

Substantial improvements in the performance of optical interconnects based on multi-mode fibers are required to support emerging single-channel data transmission rates of 200 Gb/s and 400 Gb/s. Future optical components must combine very high modulation bandwidths—supporting signaling at 100 Gbaud and 200 Gbaud—with reduced spectral width to mitigate chromatic-dispersion-induced pulse broadening and increased brightness to further restrict flux-confining area in multi-mode fibers and thereby increase the effective modal bandwidth (EMB). A particularly promising route to improved performance within standard oxide-confined VCSEL technology is the introduction of multiple isolated or optically coupled oxide-confined apertures, which we refer to collectively as multi-aperture (MA) VCSEL arrays. We show that properly designed MA VCSELs exhibit narrow emission spectra, narrow far-field profiles and extended intrinsic modulation bandwidths, enabling longer-reach data transmission over both multi-mode (MMF) and single-mode fibers (SMF). One approach uses optically isolated apertures with lateral dimensions of approximately 2–3 µm arranged with a pitch of 10–12 µm or less. Such devices demonstrate relaxation oscillation frequencies of around 30 GHz in continuous-wave operation and intrinsic modulation bandwidths approaching 50 GHz. Compared with a conventional single-aperture VCSELs of equivalent oxide-confined area, MA designs can reduce the spectral width (root mean square values < 0.15 nm), lower series resistance (≈50 Ω) and limit junction overheating through more efficient multi-spot heat dissipation at the same total current. As each aperture lases in a single transverse mode, these devices exhibit narrow far-field patterns. In combination with well-defined spacing between emitting spots, they permit tailored restricted launch conditions in MMFs, enhancing effective modal bandwidth. In another MA approach, the apertures are optically coupled such that self-injection locking (SIL) leads to lasing in a single supermode. One may regard one of the supermodes as acting as a master mode controlling the other one. Streak-camera studies reveal post-pulse oscillations in the SIL regime at frequencies up to 100 GHz. MA VCSELs enable a favorable combination of wavelength chirp and chromatic dispersion, extending transmission distances over MMFs beyond those expected for zero-chirp sources and supporting transfer bandwidths up to 60 GHz over kilometer-length SMF links. Full article

(This article belongs to the Special Issue Vertical-Cavity Surface-Emitting Laser Technology: Innovations and Future Trends)

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8 pages, 4614 KB

Open AccessCommunication

A 1908 nm Internal-Cavity Tm-Doped Fiber Laser Pumped by a 1570 nm Er/Yb Fiber Laser

by Yang Li, Yunpeng Wang, Dongming Zhang, Hailin Hu, Wentao Zhou, Xinyu Cai, Weinan Yan, Guanjie Mao, Ming Liu and Pingxue Li

Photonics 2025, 12(10), 1036; https://doi.org/10.3390/photonics12101036 - 20 Oct 2025

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Abstract

An internal-cavity Tm-doped all-fiber laser at 1908 nm in-band-pumped by a 1570 nm Er/Yb co-doped fiber laser is proposed. An external-cavity fiber oscillator composed of a pair of high-reflectivity (HR) fiber Bragg gratings (FBGs) at 1570 nm pumped by 915 nm laser diodes [...] Read more.

An internal-cavity Tm-doped all-fiber laser at 1908 nm in-band-pumped by a 1570 nm Er/Yb co-doped fiber laser is proposed. An external-cavity fiber oscillator composed of a pair of high-reflectivity (HR) fiber Bragg gratings (FBGs) at 1570 nm pumped by 915 nm laser diodes (LDs) serves as the bidirectional pumping source for the 1908 nm internal-cavity fiber oscillator to achieve high-efficiency laser output. Firstly, a maximum output power of 10 W is realized at a 915 nm pump power of 36.8 W in the single 1570 nm Er/Yb fiber oscillator, with a corresponding slope efficiency and a signal-to-noise ratio (SNR) of 28.1% and 62 dB, respectively. The beam quality factor M² of the single 1570 nm Er/Yb fiber oscillator is about 1.2. In the 1908 nm internal-cavity Tm-doped all-fiber laser, the maximum output power is 482 mW when the pump power at 915 nm reaches 12.6 W, with a corresponding slope efficiency of 8.1%. Under the same 915 nm pump power, the slope efficiency of the 1908 nm Tm-doped fiber laser with an external-cavity pump is 5.3%. Full article

(This article belongs to the Special Issue Laser Technology and Applications)

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14 pages, 3001 KB

Open AccessArticle

Investigation of Debris Mitigation in Droplet-Based Terbium Plasma Sources Produced by Laser Ablation Under Varying Buffer Gas Pressures

by Shuaichao Zhou, Tao Wu, Ziyue Wu, Junjie Tian and Peixiang Lu

Photonics 2025, 12(10), 1035; https://doi.org/10.3390/photonics12101035 - 19 Oct 2025

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Abstract

The fragment suppression ability of terbium plasma generated by laser at different environmental pressures is investigated, with a focus on exploring the slowing effect of buffer gas on high-energy particles. Using two-dimensional radiation hydrodynamic simulations with the FLASH code, this study evaluates the [...] Read more.

The fragment suppression ability of terbium plasma generated by laser at different environmental pressures is investigated, with a focus on exploring the slowing effect of buffer gas on high-energy particles. Using two-dimensional radiation hydrodynamic simulations with the FLASH code, this study evaluates the debris mitigation efficiency of terbium plasma across a range of buffer gas pressures (50–1000 Pa). Key findings reveal that helium buffer gas exhibits a nonlinear pressure-dependent response in plasma dynamics and debris suppression. Specifically, at 1000 Pa helium, the plasma shockwave stops within stopping distance

x_{s t}

= 12.13 mm with an attenuation coefficient of b = 0.0013 ns⁻¹, reducing radial expansion by 40% compared to 50 Pa (

x_{s t}

= 23.15 mm, b = 0.0010). This pressure scaling arises from enhanced collisional dissipation, confining over 80% of debris kinetic energy below 200 eV under 1000 Pa conditions. In contrast, argon exhibits superior stopping power within ion energy domains (≤1300 eV), attaining a maximum stopping power of 2000 eV·mm⁻¹ at 1300 eV–a value associated with a 6.4-times-larger scattering cross-section compared to helium under equivalent conditions. The study uncovers a nonlinear relationship between kinetic energy and gas pressure, where the deceleration capability of buffer gases intensifies with increasing kinetic energy. This work demonstrates that by leveraging argon’s broadband stopping efficiency and helium’s confinement capacity, debris and high energy ions can be effectively suppressed, thereby securing mirror integrity and source efficiency at high repetition rates. Full article

(This article belongs to the Special Issue The Principle and Application of Photonic Metasurfaces)

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18 pages, 5635 KB

Open AccessArticle

Multi-Soliton Propagation and Interaction in Λ-Type EIT Media: An Integrable Approach

by Ramesh Kumar Vaduganathan, Prasanta K. Panigrahi and Boris A. Malomed

Photonics 2025, 12(10), 1034; https://doi.org/10.3390/photonics12101034 - 19 Oct 2025

Viewed by 323

Abstract

Electromagnetically induced transparency (EIT) is well known as a quantum optical phenomenon that permits a normally opaque medium to become transparent due to the quantum interference between transition pathways. This work addresses multi-soliton dynamics in an EIT system modeled by the integrable Maxwell–Bloch [...] Read more.

Electromagnetically induced transparency (EIT) is well known as a quantum optical phenomenon that permits a normally opaque medium to become transparent due to the quantum interference between transition pathways. This work addresses multi-soliton dynamics in an EIT system modeled by the integrable Maxwell–Bloch (MB) equations for a three-level

Λ

-type atomic configuration. By employing a generalized gauge transformation, we systematically construct explicit N-soliton solutions from the corresponding Lax pair. Explicit forms of one-, two-, three-, and four-soliton solutions are derived and analyzed. The resulting pulse structures reveal various nonlinear phenomena, such as temporal asymmetry, energy trapping, and soliton interactions. They also highlight coherent propagation, elastic collisions, and partial storage of pulses, which have potential implications for the design of quantum memory, slow light, and photonic data transport in EIT media. In addition, the conservation of fundamental physical quantities, such as the excitation norm and Hamiltonian, is used to provide direct evidence of the integrability and stability of the constructed soliton solutions. Full article

(This article belongs to the Section Quantum Photonics and Technologies)

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9 pages, 2395 KB

Open AccessArticle

A Wide Field of View and Broadband Infrared Imaging System Integrating a Dispersion-Engineered Metasurface

by Bo Liu, Yunqiang Zhang, Zhu Li, Xuetao Gan and Xin Xie

Photonics 2025, 12(10), 1033; https://doi.org/10.3390/photonics12101033 - 19 Oct 2025

Viewed by 332

Abstract

We present a compact hybrid imaging system operating in the 3–5 μm spectral band that combines refractive optics with a dispersion-engineered metasurface to overcome the longstanding trade-off between wide field of view (FOV), system size, and thermal stability. The system achieves an ultra-wide [...] Read more.

We present a compact hybrid imaging system operating in the 3–5 μm spectral band that combines refractive optics with a dispersion-engineered metasurface to overcome the longstanding trade-off between wide field of view (FOV), system size, and thermal stability. The system achieves an ultra-wide 178° FOV within a total track length of only 28.25 mm, employing just three refractive lenses and one metasurface. Through co-optimization of material selection and system architecture, it maintains the modulation transfer function (MTF) exceeding 0.54 at 33 lp/mm and the geometric (GEO) radius below 15 μm across an extended operational temperature range from –40 °C to 60 °C. The metasurface is designed using a propagation phase approach with cylindrical unit cells to ensure polarization-insensitive behavior, and its broadband dispersion-free phase profile is optimized via a particle swarm algorithm. The results indicate that phase-matching errors remain small at all wavelengths, with a mean value of 0.11068. This design provides an environmentally resilient solution for lightweight applications, including automotive infrared night vision and unmanned aerial vehicle remote sensing. Full article

(This article belongs to the Special Issue Optical Metasurfaces: Applications and Trends)

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14 pages, 3213 KB

Open AccessArticle

Beyond Fresnel Wave Surfaces: Theory of Off-Shell Photonic Density of States and Near-Fields in Isotropy-Broken Materials with Loss or Gain

by Maxim Durach and David Keene

Photonics 2025, 12(10), 1032; https://doi.org/10.3390/photonics12101032 - 17 Oct 2025

Viewed by 278

Abstract

Fresnel wave surfaces, or isofrequency light shells, provide a powerful framework for describing electromagnetic wave propagation in anisotropic media, yet their applicability is restricted to reciprocal, lossless materials and far-field radiation. This paper extends the concept by incorporating near-field effects and non-Hermitian responses [...] Read more.

Fresnel wave surfaces, or isofrequency light shells, provide a powerful framework for describing electromagnetic wave propagation in anisotropic media, yet their applicability is restricted to reciprocal, lossless materials and far-field radiation. This paper extends the concept by incorporating near-field effects and non-Hermitian responses arising in media with loss, gain, or non-reciprocity. Using the Om-potential approach to macroscopic electromagnetism, we reinterpret near fields as off-shell electromagnetic modes, in analogy with off-shell states in quantum field theory. Formally, both QFT off-shell states and electromagnetic near-field modes lie away from the dispersion shell; physically, however, wavefunctions of fundamental particles admit no external sources (virtual contributions live only inside propagators), whereas macroscopic electromagnetic near-fields are intrinsically source-generated by charges, currents, and boundaries and are therefore directly measurable—for example via near-field probes and momentum-resolved imaging—making “off-shell” language more natural and operational in our setting. We show that photonic density of states (PDOS) distributions near Fresnel surfaces acquire Lorentzian broadening in non-reciprocal media, directly linking this effect to the Beer–Bouguer–Lambert law of exponential attenuation or amplification. Furthermore, we demonstrate how Abraham and Minkowski momenta, locked to light shells in the far field, naturally shift to characterize source structures in the near-field regime. This unified treatment bridges the gap between sources and radiation, on-shell and off-shell modes, and reciprocal and non-reciprocal responses. The framework provides both fundamental insight into structured light and practical tools for the design of emitters and metamaterial platforms relevant to emerging technologies such as 6G communications, photonic density-of-states engineering, and non-Hermitian photonics. Full article

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12 pages, 2252 KB

Open AccessArticle

Ultra-High Spectral Contrast Nanobeam Photonic Crystal Cavity on Bending Waveguide

by Ping Yu, Peihong Cheng, Zhuoyuan Wang, Jingrui Wang, Fangfang Ge, Huiye Qiu and Daniel Kacik

Photonics 2025, 12(10), 1031; https://doi.org/10.3390/photonics12101031 - 17 Oct 2025

Viewed by 323

Abstract

In this article, one-dimensional photonic crystal cavities on bending waveguides (PCCoBW) used for achieving high-contrast spectra are proposed, analyzed, and experimentally verified on silicon on insulator (SOI). Both air and dielectric modes of the PCCoBW calculated by the finite-difference time-domain (FDTD) method show [...] Read more.

In this article, one-dimensional photonic crystal cavities on bending waveguides (PCCoBW) used for achieving high-contrast spectra are proposed, analyzed, and experimentally verified on silicon on insulator (SOI). Both air and dielectric modes of the PCCoBW calculated by the finite-difference time-domain (FDTD) method show finger-ring-like mode profiles with the achievement of high-quality factors (Q∼10⁶), even when the bending radius is less than 50 times the lattice constant. Straight waveguides side-coupled to the cavity are used to access and measure mode resonances. The measured spectra show a high extinction ratio over 40 dB for dielectric modes and 20 dB for air modes, respectively. Both dielectric and air resonant modes are revealed with Q-factors over 3.3 × 10⁴ and 7.9 × 10⁴, respectively, for the coupled PCCoBWs. The proposed PCCoBW could be implemented as high-contrast notch filtering and would benefit a broad range of applications such as optical filters, modulators, sensors, or switches. Full article

(This article belongs to the Special Issue Recent Advancement in Microwave Photonics)

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58 pages, 3300 KB

Open AccessReview

Roadmap for Exoplanet High-Contrast Imaging: Nulling Interferometry, Coronagraph, and Extreme Adaptive Optics

by Ziming Guo, Qichang An, Canyu Yang, Jincai Hu, Xin Li and Liang Wang

Photonics 2025, 12(10), 1030; https://doi.org/10.3390/photonics12101030 - 17 Oct 2025

Viewed by 510

Abstract

The detection and characterization of exoplanets are central topics in astronomy, and high-contrast imaging techniques such nulling interferometry, coronagraphs, and extreme adaptive optics (ExAO) are key tools for the direct detection of exoplanets. This review synthesizes the pivotal role of these techniques in [...] Read more.

The detection and characterization of exoplanets are central topics in astronomy, and high-contrast imaging techniques such nulling interferometry, coronagraphs, and extreme adaptive optics (ExAO) are key tools for the direct detection of exoplanets. This review synthesizes the pivotal role of these techniques in astronomical research and critically analyzes their role as key drivers of progress in the field. Nulling interferometry suppresses stellar light through the phase control of multiple telescopes, thereby enhancing the detection of faint planetary signals. This technology has evolved from the initial Bracewell concept to the LIFE (Large Interferometer For Exoplanets) technique, which will achieve a contrast ratio of 10⁻⁷ in the mid-infrared wavelength range in the future. Coronagraphs block starlight to create a “dark region” for direct observation of exoplanets. By leveraging innovative mask designs, theoretical contrast ratios of up to 4 × 10⁻⁹ can be achieved. ExAO systems achieve precise wavefront correction to optimize the high-contrast imaging performance and mitigate atmospheric disturbances. By leveraging wavefront sensing, thousand-element deformable mirrors, and real-time control algorithms, these systems suppress the turbulence correction residuals to 80 nm RMS, enabling ground-based telescopes to achieve a Strehl ratio exceeding 0.9. This work provides a comprehensive analysis of the underlying principles, prevailing challenges, and future application prospects of these technologies in astronomy. Full article

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19 pages, 13717 KB

Open AccessArticle

Vector Vortex Beams: Theory, Generation, and Detection of Laguerre–Gaussian and Bessel–Gaussian Types

by Xin Yan, Xin Tao, Minghao Guo, Chunliang Zhou, Jingzhao Chen, Guanyu Shang and Peng Li

Photonics 2025, 12(10), 1029; https://doi.org/10.3390/photonics12101029 - 17 Oct 2025

Viewed by 422

Abstract

A vector vortex beam (VVB) combines the phase singularity of a vortex beam (VB) with the anisotropic polarization of a vector beam, enabling the transmission of complex optical information and offering broad application prospects in optical sensing, high-capacity communication, and high-resolution imaging. In [...] Read more.

A vector vortex beam (VVB) combines the phase singularity of a vortex beam (VB) with the anisotropic polarization of a vector beam, enabling the transmission of complex optical information and offering broad application prospects in optical sensing, high-capacity communication, and high-resolution imaging. In this work, we present a detailed theoretical analysis of the generation and detection of VVBs with Laguerre–Gaussian (LG) and Bessel–Gaussian (BG) forms. Particular emphasis is placed on the polarization characteristics of VVBs, the evolution of beam profiles after passing through polarizers with different orientations, and the interference features arising from the coaxial superposition of a VVB with a circularly polarized divergent spherical wave. To validate the theoretical analysis, LGVVBs were experimentally generated using a Mach–Zehnder interferometer by superposing two vortex beams with opposite topological charges and orthogonal circular polarizations. Furthermore, the introduction of an axicon enabled the direct conversion of LGVVBs into BGVVBs. The excellent agreement between theoretical predictions and experimental observations lays a solid foundation for beginners to systematically understand VVB characteristics and advance future research. Full article

(This article belongs to the Special Issue Fundamentals and Applications of Vortex Beams)

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12 pages, 5562 KB

Open AccessArticle

Random Search Algorithm-Assisted Automatic Mode-Locked Fiber Lasers

by Penghui Yang, Yanrong Song, Lin Mao and Ruyue You

Photonics 2025, 12(10), 1028; https://doi.org/10.3390/photonics12101028 - 16 Oct 2025

Viewed by 274

Abstract

Automatic mode-locking is a crucial approach for achieving ultrashort pulses in fiber lasers. Here, a random search algorithm was developed, and an automatic mode-locked laser was constructed. Numerical simulations of an automatic mode-locked Yb-doped fiber laser were conducted, and both continuous-wave, as well [...] Read more.

Automatic mode-locking is a crucial approach for achieving ultrashort pulses in fiber lasers. Here, a random search algorithm was developed, and an automatic mode-locked laser was constructed. Numerical simulations of an automatic mode-locked Yb-doped fiber laser were conducted, and both continuous-wave, as well as mode-locked pulse states, were successfully obtained. The laser utilized a squeezer-type electrically controlled polarization controller to adjust the mode-locking states and enabled the controllable output of 532.71 fs dissipative solitons and 23.87 ps noise-like pulses, with search times of 14.19 s and 2.37 s, respectively. The center wavelengths were 1034 nm and 1038 nm, with signal-to-noise ratios of 63.1 dBm and 51.2 dBm, respectively. This work effectively addresses the polarization state drift caused by temperature and vibration, enhancing the laser’s environmental adaptability through adaptive monitoring. Full article

(This article belongs to the Special Issue Advanced Fiber Laser Technology and Its Application: 2nd Edition)

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9 pages, 2410 KB

Open AccessArticle

High Quality Factor Unidirectional Guided Resonances in Etchless Lithium Niobate Metagratings for Polarization Modulation

by Zhidong Gu, Jiaxin Peng, Zhiyong Wu, Lei Wang, Jiajun Zhu, Ye Feng, Xinyi Sun, Zhenjuan Zhang and Guoan Zhang

Photonics 2025, 12(10), 1027; https://doi.org/10.3390/photonics12101027 - 16 Oct 2025

Viewed by 271

Abstract

Unidirectional guided resonances (UGRs), as distinctive resonant eigenstates in planar photonic lattices, exhibit unique capability of emitting light in a single direction. In this work, UGRs with high-Q factor and infinite proximity to the

Γ

-point infinitely using etchless lithium niobate (LN) metagratings [...] Read more.

Unidirectional guided resonances (UGRs), as distinctive resonant eigenstates in planar photonic lattices, exhibit unique capability of emitting light in a single direction. In this work, UGRs with high-Q factor and infinite proximity to the

Γ

-point infinitely using etchless lithium niobate (LN) metagratings are proposed and investigated numerically. By adjusting the parameters of metagraings, the Q-factor and asymmetric radiation ratio of UGRs can be flexibly tuned, and the wavelength center of UGRs respect will move with respect to the wave vector along the

Γ

-X direction. Accompanied by the optimizing of asymmetric radiation ratio, the evolution of two dispersion curves from avoided crossing to crossing can be observed. Furthermore, leveraging the polarization sensitivity of UGRs, we achieve a broadband linear-to-circular polarization conversion with a high polarization extinction ratio. This work advances the fundamental understanding of UGRs while potentially offering promising applications in metagratings-based surface-emitting lasers, beam steering, and refractive index sensors. Full article

(This article belongs to the Special Issue Advanced Photonics Metamaterials and Metasurfaces: Science and Applications, 2nd Edition)

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25 pages, 6408 KB

Open AccessReview

Application Prospects of Optical Fiber Sensing Technology in Smart Campus Construction: A Review

by Huanhuan Zhang, Xinli Zhai and Jing Sun

Photonics 2025, 12(10), 1026; https://doi.org/10.3390/photonics12101026 - 16 Oct 2025

Viewed by 426

Abstract

As smart campus construction continues to advance, traditional safety monitoring and environmental sensing systems are increasingly showing limitations in sensitivity, anti-interference capability, and deployment flexibility. Optical fiber sensing (OFS) technology, with its advantages of high sensitivity, passive operation, immunity to electromagnetic interference, and [...] Read more.

As smart campus construction continues to advance, traditional safety monitoring and environmental sensing systems are increasingly showing limitations in sensitivity, anti-interference capability, and deployment flexibility. Optical fiber sensing (OFS) technology, with its advantages of high sensitivity, passive operation, immunity to electromagnetic interference, and long-distance distributed sensing, provides a novel solution for real-time monitoring and early warning of critical campus infrastructure. This review systematically examines representative applications of OFS technology in smart campus scenarios, including structural health monitoring of academic buildings, laboratory environmental sensing, and intelligent campus security. By analyzing the technical characteristics of various types of optical fiber sensors, the paper explores emerging developments and future potential of OFS in supporting intelligent campus construction. Finally, the feasibility of building data acquisition, transmission, and visualization platforms based on OFS systems is discussed, highlighting their promising roles in campus safety operations, the integration of teaching and research, and intelligent equipment management. Full article

(This article belongs to the Special Issue Applications and Development of Optical Fiber Sensors)

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15 pages, 3042 KB

Open AccessArticle

Mathematical Analysis and Freeform Surface Modeling for LED Illumination Systems Incorporating Diffuse Reflection and Total Internal Reflection

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

Photonics 2025, 12(10), 1025; https://doi.org/10.3390/photonics12101025 - 16 Oct 2025

Viewed by 239

Abstract

Indirect lighting systems employing light-emitting diodes (LEDs) and diffuse reflective surfaces are prevalent in applications demanding stringent illumination uniformity. However, conventional diffuse reflection approaches exhibit inherent limitations (inevitable light loss from multiple diffuse reflections and trade-off between uniformity and efficiency). To overcome these [...] Read more.

Indirect lighting systems employing light-emitting diodes (LEDs) and diffuse reflective surfaces are prevalent in applications demanding stringent illumination uniformity. However, conventional diffuse reflection approaches exhibit inherent limitations (inevitable light loss from multiple diffuse reflections and trade-off between uniformity and efficiency). To overcome these constraints, we introduce a novel composite freeform surface illumination system that synergistically integrates total internal reflection (TIR) with diffuse reflection. This design leverages the inherent Lambertian radiation characteristics of LEDs and the properties of ideal diffuse reflectors. A rigorous mathematical model is derived based on the luminous intensity distribution of the LED chip, the prescribed illumination requirements on the target plane, the principle of energy conservation, and Snell’s law. The resulting system of nonlinear equations is solved to generate a series of two-dimensional profile curves, which are subsequently synthesized into an off-axis freeform surface. Simulated results demonstrate that the proposed system achieves higher optical efficiency and superior illumination uniformity compared to traditional diffuse reflector configurations. This universal and feasible methodology broadens the application potential of high-performance diffuse indirect lighting. Full article

(This article belongs to the Special Issue New Perspectives in Micro-Nano Optical Design and Manufacturing)

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14 pages, 3457 KB

Open AccessArticle

Improving the High-Pressure Sensing Characteristics of Y₂MoO₆:Eu³⁺ Using a Machine Learning Approach

by Marko G. Nikolic, Dragutin Sevic and Maja S. Rabasovic

Photonics 2025, 12(10), 1024; https://doi.org/10.3390/photonics12101024 - 16 Oct 2025

Viewed by 243

Abstract

In this study, we explore the potential of applying machine learning (ML) to enhance high-pressure luminescence sensing. We investigate the luminescence behavior of Y₂MoO₆:Eu³⁺, synthesized via a self-initiated, self-sustained reaction. Emission spectra were collected under varying pressures [...] Read more.

In this study, we explore the potential of applying machine learning (ML) to enhance high-pressure luminescence sensing. We investigate the luminescence behavior of Y₂MoO₆:Eu³⁺, synthesized via a self-initiated, self-sustained reaction. Emission spectra were collected under varying pressures using a 405 nm laser diode and an AVANTES AvaSpec 2048TEC USB2 spectrometer. An analysis of the pressure-dependent curve, based on the intensities of two key peaks, indicates a possible crystal phase transition or another underlying physical phenomenon. Moreover, the non-unique relationship between pressure and peak intensity limits its effectiveness for precise sensing. To overcome this challenge, we employ an ML-based approach, combining Uniform Manifold Approximation and Projection (UMAP) for data visualization with a deep neural network to estimate pressure directly from the full luminescence spectrum. This strategy significantly extends the usable pressure range of Y₂MoO₆:Eu³⁺ up to 12 GPa, representing a marked improvement over conventional methods. Full article

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12 pages, 3199 KB

Open AccessArticle

Design Methodology of a VIS Hybrid Refractive–Metalens System with a Wide FOV

by Xingyi Li, Peixuan Wu, Yuanyuan Xing, Peng Shi, Xinjian Yao and Yaoguang Ma

Photonics 2025, 12(10), 1023; https://doi.org/10.3390/photonics12101023 - 16 Oct 2025

Viewed by 261

Abstract

The emergence of metalenses has opened new possibilities for miniaturizing optical systems. However, the limited group delay provided by meta-atoms restricts their aperture size under broadband operation. This challenge has stimulated the development of hybrid refractive–metalens systems, which overcome the performance limitations of [...] Read more.

The emergence of metalenses has opened new possibilities for miniaturizing optical systems. However, the limited group delay provided by meta-atoms restricts their aperture size under broadband operation. This challenge has stimulated the development of hybrid refractive–metalens systems, which overcome the performance limitations of individual metalenses while achieving a more compact form factor than conventional refractive lens assemblies. Here, we propose a design methodology for hybrid lenses that combines ray tracing with full-wave simulation. We analyze key aspects of the metalens within the hybrid system for a wide wavelength band—specifically, dispersion and transmission efficiency. Based on this approach, we designed a high-resolution hybrid lens operating in the 435–656 nm visible band with a 35° field of view. The results demonstrate that the proposed lens achieves imaging performance equivalent to that of conventional refractive systems while reducing the total track length by 29%. This validates the effectiveness of our design method, indicating its strong potential for application in compact and lightweight optical systems. Full article

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

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15 pages, 2817 KB

Open AccessArticle

Universal Phase Correction for Quantum State Transfer in One-Dimensional Topological Spin Chains

by Tian Tian, Yingnan Yan and Shizhen Wang

Photonics 2025, 12(10), 1022; https://doi.org/10.3390/photonics12101022 - 16 Oct 2025

Viewed by 306

Abstract

Gap-protected topological channels are a promising way to realize robust and high-fidelity state transfer in quantum networks. Although various topological transfer protocols based on the Su-Schrieffer-Heeger (SSH) model or its variants have been proposed, the phase accumulation during the evolution, as an essential [...] Read more.

Gap-protected topological channels are a promising way to realize robust and high-fidelity state transfer in quantum networks. Although various topological transfer protocols based on the Su-Schrieffer-Heeger (SSH) model or its variants have been proposed, the phase accumulation during the evolution, as an essential aspect, is underestimated. Here, by numerically studying the phase information of quantum state transfer (QST) in one-dimensional (1D) topological spin chains, we uncover a universal phase correction

ϕ_{0} = (N - 1) π / 2

for both adiabatic and diabatic topological schemes. Interestingly, the site-number-dependent phase correction satisfies

Z_{4}

symmetry and is equally effective for perfect mirror transmission in spin chains. Our work reveals a universal phase correction in 1D topologically protected QST, which will prompt a reevaluation of the topological protection mechanism in quantum systems. Full article

(This article belongs to the Section Quantum Photonics and Technologies)

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16 pages, 923 KB

Open AccessArticle

Photobiomodulation in Complex Female Infertility Profile: A Case Report with 12-Month Follow-Up and Review of Current Mechanism in Reproductive Photomedicine

by Ruth Phypers and Reem Hanna

Photonics 2025, 12(10), 1021; https://doi.org/10.3390/photonics12101021 - 16 Oct 2025

Viewed by 956

Abstract

Female infertility from polycystic ovarian syndrome (PCOS) and endometriosis poses a challenge for both clinicians and women who are trying to conceive. The present clinical single case report aimed to evaluate the efficacy of multiple wavelengths of red and near-infrared (NIR) laser photobiomodulation [...] Read more.

Female infertility from polycystic ovarian syndrome (PCOS) and endometriosis poses a challenge for both clinicians and women who are trying to conceive. The present clinical single case report aimed to evaluate the efficacy of multiple wavelengths of red and near-infrared (NIR) laser photobiomodulation (PBM) for increasing the potential of fertility in a woman with PCOS, endometriosis and low ovarian reserve. The observations helped to inform and establish the following: (1) any adverse effects; (2) the possibility of producing an effective PBM protocol; and (3) a healthy live birth. The case report concerns a female who failed to conceive naturally beyond five years and had experienced one unsuccessful IVF cycle. Methods: Case report of one female subject with infertility issues, which included failure to conceive naturally beyond five years. Previous conditions were recorded and then compared with outcomes from after the patient received a course of PBM treatments. PBM treatments were given at weekly and/or at two-week intervals over a 5-month period during the follicular stage of the menstrual cycle, using IR and NIR wavelengths between 600 and 1000 nm. Results: After five months a spontaneous conception was achieved. The case resulted in a full-term pregnancy and the birth of a healthy baby. Improvements in reproductive health outcomes in this case give reason to suggest that PBM helped to alleviate PCOS and endometriosis which could have been associated with a low ovarian reserve. Conclusions: The case report indicates that a multiwavelength of red and NIR-PBM laser therapy could have positively contributed to a healthy live birth in a female diagnosed with PCOS, endometriosis and a low ovarian reserve. Extensive studies with large data are warranted to validate our PBM dosimetry and treatment protocols to assess the potential impact of PBM for treating endometriosis and PCOS. Subsequently, to understand the genetic and phenotype biomarkers would be an important step further to standardise a range of PBM light dosimetry. Full article

(This article belongs to the Special Issue Shining Light on Healing: Photobiomodulation Therapy)

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20 pages, 4701 KB

Open AccessArticle

FMCW LiDAR Nonlinearity Compensation Based on Deep Reinforcement Learning with Hybrid Prioritized Experience Replay

by Zhiwei Li, Ning Wang, Yao Li, Jiaji He and Yiqiang Zhao

Photonics 2025, 12(10), 1020; https://doi.org/10.3390/photonics12101020 - 15 Oct 2025

Viewed by 252

Abstract

Frequency-modulated continuous-wave (FMCW) LiDAR systems are extensively utilized in industrial metrology, autonomous navigation, and geospatial sensing due to their high precision and resilience to interference. However, the intrinsic nonlinear dynamics of laser systems introduce significant distortion, adversely affecting measurement accuracy. Although conventional iterative [...] Read more.

Frequency-modulated continuous-wave (FMCW) LiDAR systems are extensively utilized in industrial metrology, autonomous navigation, and geospatial sensing due to their high precision and resilience to interference. However, the intrinsic nonlinear dynamics of laser systems introduce significant distortion, adversely affecting measurement accuracy. Although conventional iterative pre-distortion correction methods can effectively mitigate nonlinearities, their long-term reliability is compromised by factors such as temperature-induced drift and component aging, necessitating periodic recalibration. In light of recent advances in artificial intelligence, deep reinforcement learning (DRL) has emerged as a promising approach to adaptive nonlinear compensation. By continuously interacting with the environment, DRL agents can dynamically modify correction strategies to accommodate evolving system behaviors. Nonetheless, existing DRL-based methods often exhibit limited adaptability in rapidly changing nonlinear contexts and are constrained by inefficient uniform experience replay mechanisms that fail to emphasize critical learning samples. To address these limitations, this study proposes an enhanced Soft Actor-Critic (SAC) algorithm incorporating a hybrid prioritized experience replay framework. The prioritization mechanism integrates modulation frequency (MF) error and temporal difference (TD) error, enabling the algorithm to dynamically reconcile short-term nonlinear perturbations with long-term optimization goals. Furthermore, a time-varying delayed experience (TDE) injection strategy is introduced, which adaptively modulates data storage intervals based on the rate of change in modulation frequency error, thereby improving data relevance, enhancing sample diversity, and increasing training efficiency. Experimental validation demonstrates that the proposed method achieves superior convergence speed and stability in nonlinear correction tasks for FMCW LiDAR systems. The residual nonlinearity of the upward and downward frequency sweeps was reduced to

1.869 \times 10^{- 5}

and

1.9411 \times 10^{- 5}

, respectively, with a spatial resolution of 0.0203m. These results underscore the effectiveness of the proposed approach in advancing intelligent calibration methodologies for LiDAR systems and highlight its potential for broad application in high-precision measurement domains. Full article

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

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12 pages, 7227 KB

Open AccessArticle

High-Resolution Interferometric Temperature Sensor Based on Two DFB Fiber Lasers with High-Temperature Monitoring Potential

by Mikhail I. Skvortsov, Kseniya V. Kolosova, Alexander V. Dostovalov, Evgeniy V. Golikov, Alexander A. Vlasov, Sofia R. Abdullina, Andrey A. Rybaltovsky, Denis S. Lipatov, Aleksey S. Lobanov, Mikhail E. Likhachev, Olga N. Egorova and Sergey A. Babin

Photonics 2025, 12(10), 1019; https://doi.org/10.3390/photonics12101019 - 15 Oct 2025

Viewed by 275

Abstract

A high-resolution temperature sensor using the beat frequency measurement between the modes of two DFB fiber lasers is presented. The laser cavities are formed by the femtosecond inscription technique in a highly Er/Yb co-doped phosphosilicate fiber with low optical losses and compact design. [...] Read more.

A high-resolution temperature sensor using the beat frequency measurement between the modes of two DFB fiber lasers is presented. The laser cavities are formed by the femtosecond inscription technique in a highly Er/Yb co-doped phosphosilicate fiber with low optical losses and compact design. The experimental results show a sensitivity of 1 GHz/°C, leading to a temperature resolution of 0.02 °C restricted by the thermistor used in the experiment. The maximum possible resolution determined by the laser linewidth is estimated as 2 × 10⁻⁶ °C. The operation of such a sensor at high temperatures (≈750 °C) with the possibility of further temperature increase is demonstrated. The combination of high resolution and broad temperature range makes the sensor attractive for various applications, especially in high-temperature monitoring. Full article

(This article belongs to the Special Issue Recent Advances in Fiber Laser Technology)

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14 pages, 7151 KB

Open AccessArticle

Design of Hollow-Core Anti-Resonant Fibers Supporting Few Weakly Coupled Polarization-Maintaining Modes

by Linxuan Zong, Jiayao Cheng and Yueyu Xiao

Photonics 2025, 12(10), 1018; https://doi.org/10.3390/photonics12101018 - 15 Oct 2025

Viewed by 358

Abstract

A nested semi-tube hollow-core anti-resonant fiber (HC-ARF) that can support the high-purity transmission of a few polarization-maintaining modes is designed in this paper. By employing bi-thickness hybrid silica/silicon anti-resonant tubes, the birefringence of the orthogonal polarized modes is significantly improved, and the weak [...] Read more.

A nested semi-tube hollow-core anti-resonant fiber (HC-ARF) that can support the high-purity transmission of a few polarization-maintaining modes is designed in this paper. By employing bi-thickness hybrid silica/silicon anti-resonant tubes, the birefringence of the orthogonal polarized modes is significantly improved, and the weak coupling condition of the five lowest-order polarization maintaining modes, including the LP_{01_x}, LP_{01_y}, LP_{11a_x}, LP_{11b_x}, and LP_{11a_y}, can be met. The effective refractive index difference between each pair of the supported adjacent modes is larger than 1.0 × 10⁻⁴. With hybrid multi-layer nested semi-tubes, the confinement losses of the supported modes are all less than 1.50 × 10⁻¹ dB/m within a transmission band from 1.530 to 1.620 μm. The minimum confinement losses of the LP_{01_y}, LP_{01_x}, LP_{11a_y}, LP_{11a_x}, and LP_{11b_x} modes are 3.71 × 10⁻⁴ dB/m, 1.61 × 10⁻³ dB/m, 2.00 × 10⁻² dB/m, 1.30 × 10⁻¹ dB/m, and 4.20 × 10⁻² dB/m, respectively. Meanwhile, the unwanted higher-order modes are filtered out well to guarantee the modal purity. The minimum higher-order-mode extinction ratio of the lowest-loss LP₂₁ mode to the highest-loss LP₁₁ mode remains larger than 139 from 1.545 to 1.615 μm. The numerical results highlight the potential of the proposed polarization-maintaining few-mode hollow-core anti-resonant fibers in many application fields, such as short-range and high-capacity data transmission networks, fiber sensing systems, quantum communication systems, and so on. Full article

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

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10 pages, 3924 KB

Open AccessArticle

1970 W 1030 nm Single-Mode All-Fiber Master Oscillator Power Amplifier with ~3.2 GHz Linewidth Based on Ultra-Low NA Active Fiber

by Yang Liu, Xiaoyong Xu, Mengfan Cui, Wei Li, Chongwei Wang, Yan Peng, Junjie Zheng, Zilun Chen, Yisha Chen, Wei Liu, Hu Xiao, Zefeng Wang and Pengfei Ma

Photonics 2025, 12(10), 1017; https://doi.org/10.3390/photonics12101017 - 15 Oct 2025

Viewed by 301

Abstract

A high-power narrow-linewidth fiber laser with single-mode beam quality is experimentally demonstrated. By employing a cascaded phase modulation strategy, the stimulated Brillouin scattering (SBS) threshold of the laser is effectively increased from 973 W to 1970 W. High-order modes are well suppressed during [...] Read more.

A high-power narrow-linewidth fiber laser with single-mode beam quality is experimentally demonstrated. By employing a cascaded phase modulation strategy, the stimulated Brillouin scattering (SBS) threshold of the laser is effectively increased from 973 W to 1970 W. High-order modes are well suppressed during power scaling, benefiting from the significant bending loss of a low numerical aperture (NA) ytterbium-doped fiber. A maximum output power of 1970 W is achieved, with a linewidth of 3.2 GHz and a beam quality factor M² of 1.14. To the best of our knowledge, this represents the highest reported output power for narrow-linewidth fiber lasers (linewidth < 10 GHz) operating at wavelengths below 1040 nm. Full article

(This article belongs to the Special Issue High-Power Fiber Lasers)

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11 pages, 3558 KB

Open AccessCommunication

Design and Fabrication of Thermopile Infrared Detector Based on Carbon Black Nanoparticle Absorption Layer

by Cheng Lei, Zhenyu Zhang, Boyou Shao, Xiangyang Ren, Tengteng Li, Fengchao Li and Ting Liang

Photonics 2025, 12(10), 1016; https://doi.org/10.3390/photonics12101016 - 14 Oct 2025

Viewed by 313

Abstract

This study demonstrates a high-performance thermopile infrared detector that incorporates a carbon black nanoparticle (CBNP) absorption layer. To overcome the limitations associated with conventional infrared-absorbing materials—including high cost, complex fabrication, and constrained spectral response—a highly porous CBNP thin-film absorption layer was deposited onto [...] Read more.

This study demonstrates a high-performance thermopile infrared detector that incorporates a carbon black nanoparticle (CBNP) absorption layer. To overcome the limitations associated with conventional infrared-absorbing materials—including high cost, complex fabrication, and constrained spectral response—a highly porous CBNP thin-film absorption layer was deposited onto the thermopile sensing area using inkjet printing. Combined with an optimized microcavity design, this approach significantly enhances the photothermal conversion efficiency of the device. Experimental results indicate that the detector equipped with the CBNP absorption layer achieves a responsivity of 47.9 V/W and a detectivity of 1.14 × 10⁸ cm·Hz¹/²·W⁻¹. These values represent improvements of 34.55% in responsivity and 34.28% in detectivity, respectively, compared to a reference device without the CBNP layer. This work provides a promising strategy for the development of low-cost yet high-performance infrared detectors. Full article

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15 pages, 902 KB

Open AccessArticle

Spectral Shaping of an Optical Frequency Comb to Control Atomic Dynamics

by Yichi Zhang, Zhenqi Bai, Hongyan Fan and Ximo Wang

Photonics 2025, 12(10), 1015; https://doi.org/10.3390/photonics12101015 - 14 Oct 2025

Viewed by 286

Abstract

In advanced spectroscopy, the classical symmetric optical frequency comb is limited in temporal flexibility and selection freedom, which constrains the efficiency and stability of quantum manipulation. To overcome this limitation, we propose a method to realize precise energy-level manipulation using a femtosecond non-temporally [...] Read more.

In advanced spectroscopy, the classical symmetric optical frequency comb is limited in temporal flexibility and selection freedom, which constrains the efficiency and stability of quantum manipulation. To overcome this limitation, we propose a method to realize precise energy-level manipulation using a femtosecond non-temporally symmetric optical frequency comb in the semiclassical three-level system. Numerical calculations show that the fall time of the pulse is the key parameter to realize the precise manipulation, and a shorter fall time contributes to the efficient accumulation of population. By optimizing the pulse parameters, 99.15% accumulation of population in the target state can be successfully achieved and stably maintained using an asymmetric slowly turned-on and rapidly turned-off (STRT) pulse train. Our demonstration of the non-temporally symmetric optical frequency comb provides a promising approach to efficient quantum-state preparation using spectral modulation. Full article

(This article belongs to the Special Issue Next-Generation Optical Frequency Combs and Their Photonic Applications)

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21 pages, 18267 KB

Open AccessArticle

Fractional-Order Modeling of a Multistable Erbium-Doped Fiber Laser

by Jorge Eduardo Silva Gómez, José de Jesús Barba Franco, Luís Armando Gallegos Infante, Juan Hugo García López, Rider Jaimes Reátegui and Alexander N. Pisarchik

Photonics 2025, 12(10), 1014; https://doi.org/10.3390/photonics12101014 - 14 Oct 2025

Viewed by 253

Abstract

We propose a novel mathematical model of a multistable erbium-doped fiber laser based on Caputo fractional derivative equations. The model is used to investigate how the laser dynamics evolve as the derivative order is varied. Our results demonstrate that the fractional-order formulation provides [...] Read more.

We propose a novel mathematical model of a multistable erbium-doped fiber laser based on Caputo fractional derivative equations. The model is used to investigate how the laser dynamics evolve as the derivative order is varied. Our results demonstrate that the fractional-order formulation provides a more accurate description of the experimentally observed laser dynamics compared to conventional integer-order models. This study highlights the importance of fractional calculus in modeling complex nonlinear photonic systems and offers new insights into the dynamics of multistable lasers. Full article

(This article belongs to the Special Issue Optical Fiber Lasers and Laser Technology)

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18 pages, 3816 KB

Open AccessArticle

Quantitative Strain Measurements of Kevlar Fibers in Composite Concrete Using Raman Spectroscopy

by Fuyong Qin, Xinmin Fan, Jianxin Zhang, Zaifa Du, Yan Wang, Wenjing Qiu, Jiahui Shi, Xiuhua Zhang, Wendi Wang, Qingju Wu, Yibo Meng and Fengliang Song

Photonics 2025, 12(10), 1013; https://doi.org/10.3390/photonics12101013 - 14 Oct 2025

Viewed by 236

Abstract

This study presents a Raman-spectroscopy-based quantitative analysis technique for measuring strain in Kevlar single fibers embedded in concrete. By irradiating the fibers with a laser, the researchers established a linear relationship between Raman scattering intensity and the fibers’ cross-sectional area, linking spectral parameters [...] Read more.

This study presents a Raman-spectroscopy-based quantitative analysis technique for measuring strain in Kevlar single fibers embedded in concrete. By irradiating the fibers with a laser, the researchers established a linear relationship between Raman scattering intensity and the fibers’ cross-sectional area, linking spectral parameters (e.g., peak position, half-width, intensity, and area) to mechanical strain. Experiments on DuPont Kevlar 49 fibers involved axial tensile loading using a micro-loading device, with Raman spectra (785 nm laser) captured at each displacement step. The results showed that the G’ peak position (1610 cm⁻¹) shifted linearly with strain, while the peak area provided the most reliable correlation. Scanning electron microscopy (SEM) validation confirmed the method’s accuracy for early-stage strain measurements (maximum deviation: 7.31%), although excessive loading caused surface damage and signal distortion. The study demonstrates the feasibility of Raman spectroscopy for micro-scale strain analysis in fiber-reinforced concrete, despite sensitivity to experimental conditions (e.g., laser intensity, optical alignment). Full article

(This article belongs to the Special Issue Advanced Methods in Exploring Light–Matter Interactions and Nonlinear Effects Optics Applications)

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14 pages, 3512 KB

Open AccessArticle

Secure Downlink Transmission with NOMA-Based Mixed FSO/RF Communications in Space–Air–Ground Integrated Networks

by Yu Li, Yongjun Li, Xin Li, Kai Zhang and Shanghong Zhao

Photonics 2025, 12(10), 1012; https://doi.org/10.3390/photonics12101012 - 14 Oct 2025

Viewed by 200

Abstract

Security is paramount in space–air–ground integrated networks (SAGINs) due to their inherent openness and the broadcast characteristics of wireless transmission. In this paper, we propose a secure downlink transmission scheme with NOMA-based mixed FSO/RF communications for SAGINs. Specifically, the satellite communicates with two [...] Read more.

Security is paramount in space–air–ground integrated networks (SAGINs) due to their inherent openness and the broadcast characteristics of wireless transmission. In this paper, we propose a secure downlink transmission scheme with NOMA-based mixed FSO/RF communications for SAGINs. Specifically, the satellite communicates with two ground users through an unmanned aerial vehicle (UAV) relay, where FSO and RF transmissions are adopted for the satellite–relay and relay–user links, respectively. Furthermore, the NOMA technique is integrated to further enhance secrecy performance. Subsequently, exact closed-form expressions for the secrecy outage probability of the downlink transmission link in SAGINs are derived. Finally, Monte Carlo simulations are performed to validate the effectiveness of the proposed secure downlink transmission scheme and the accuracy of the analytical expressions. Full article

(This article belongs to the Special Issue Emerging Technologies for 6G Space Optical Communication Networks)

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21 pages, 1230 KB

Open AccessArticle

Inverse Judd–Ofelt Formalism Based on Radiative Lifetime for Comparative Spectroscopy of RE³⁺ Ions in Glass

by Helena Cristina Vasconcelos, Maria Gabriela Meirelles and Reşit Özmenteş

Photonics 2025, 12(10), 1011; https://doi.org/10.3390/photonics12101011 - 13 Oct 2025

Viewed by 284

Abstract

This work shows that inverse Judd–Ofelt (JO) analysis of relative absorption spectra, anchored by a single lifetime, provides JO parameters and radiative rates without absolute calibration. The method is applied to Er³⁺, Dy³⁺, and Sm³⁺ in a compositionally [...] Read more.

This work shows that inverse Judd–Ofelt (JO) analysis of relative absorption spectra, anchored by a single lifetime, provides JO parameters and radiative rates without absolute calibration. The method is applied to Er³⁺, Dy³⁺, and Sm³⁺ in a compositionally identical oxyfluoride glass. Three well-resolved ground-state 4f–4f absorption bands were selected. After baseline removal and wavenumber-domain integration, their normalized strengths S_rel,k (k = 1, 2, 3; k∈S) define a 3 × 3 system solved by non-negative least squares to obtain the anchor-independent ordering (Ω₂:Ω₄:Ω₆). Absolute scaling uses a single lifetime anchor. We report lifetime-scaled Ω_t and A_rad, and the normalized fractions p_k within the selected triplets; as imposed by the method, the anchor-independent ordering (Ω₂:Ω₄:Ω₆) is analyzed, while absolute A_rad and Ω_t scale with τ_ref. The extracted parameters fall within the expected ranges for oxyfluoride hosts and reveal clear ion-specific trends: Ω₂ follows Dy³⁺ > Er³⁺ > Sm³⁺ (site asymmetry/hypersensitive response), while the ordering Ω₄ > Ω₆ holds across all ions (oxide-rich networks). Er³⁺ exhibits the largest Ω₄ and the smallest Ω₆, indicative of pronounced medium-range “rigidity” with suppressed long-range polarizability; Sm³⁺ shows the lowest Ω₂ (more symmetric/less covalent coordination); and Dy³⁺ the highest Ω₂ (strong hypersensitive behavior). Uncertainty was quantified by Monte Carlo resampling of the preprocessing steps, yielding compact 95% confidence intervals; the resulting JO-parameter trends (Ω₂, Ω₄, Ω₆) and normalized f_k fractions reproduce the characteristic spectroscopic behavior known for each ion. This method enables quantitative JO outputs from uncalibrated spectra, allowing direct spectroscopic comparisons and quick screening when only relative absorption data are available. Full article

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

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