Photonics doi: 10.3390/photonics13070618

Authors: Anna Krivetskaya Tatiana Savelieva Daniil Kustov Igor Romanishkin Walter Blondel Marine Amouroux Kirill Linkov Sergey Kharnas Kanamat Efendiev Polina Alekseeva Vladimir Makarov Victor Loschenov Vladimir Levkin

Gastrointestinal (GI) cancers account for a quarter of all cancer cases worldwide and are responsible for a third of cancer deaths. One of the characteristic features of GI tissue is its multilayered structure, which, in addition to multiple scattering, complicates optical spectral analysis. The use of spectroscopic diagnostics and photodynamic therapy for the detection and treatment of GI cancer is a rapidly developing field. The method proposed in this paper for layer-by-layer optical properties assessment, suitable for real-time clinical application to the walls of hollow organs, allows us to calculate the absorbed dose layer by layer. This paper proposes a method for recording spectral data in two geometries, diffuse reflectance and transmission, using light delivery from both the external and internal surfaces of the gastrointestinal tract wall. Layer-by-layer assessment of optical properties was performed using a developed algorithm based on the inverse adding–doubling method with initial optical properties values determined using the modified two-stream Kubelka–Munk model with the accuracy equal to 86 ± 13%. The method was approved in clinical conditions. Based on the results of the work, the developed method for assessing the optical properties of multilayered biological tissues exhibited sufficient speed and accuracy for in vivo application to personalize laser-induced therapy by correction of the laser dose.

Photonics doi: 10.3390/photonics13070617

Authors: Suiqun Li Yadong Wu Mingpeng Wang Hongying Zhang Xingmao Yan

In this paper, a reconfigurable multi-form radar compound coherent jamming signal generator is proposed based on a dual-polarization quadrature phase shift keying (DP-QPSK) modulator cascaded with an intensity modulator (IM). The radar signal and jamming seed signal are loaded on the upper path and the lower path of the DP-QPSK modulator to achieve carrier-suppressed single-sideband (CS-SSB) modulation and phase modulation, respectively. The periodic rectangular pulse (PRP) signal is fed into the IM to achieve interrupted-sampling repeater jamming in the optical domain. In our proposed scheme, cosine phase modulation and interrupted-sampling repeater jamming (CPMJ-ISRJ) and frequency shift and interrupted-sampling repeater jamming (FSJ-ISRJ) are obtained only by changing the form of the jamming seed signal, without changing the overall structure of the scheme. The jamming effectiveness of the above schemes is evaluated through simulation. Multiple false targets are obtained after cross-correlation with the original radar signal. The number of generated false targets can reach 18. We also conducted a detailed simulation to analyze the impact of different parameters on the jamming effect. Because the scheme is filter-free, it has a large frequency tuning range. Moreover, due to the special CS-SSB modulation, the modulated signals are immune to the chromatic dispersion-induced power fading effect. The proposed scheme has potential application prospects in future electronic countermeasure systems.

Photonics doi: 10.3390/photonics13070616

Authors: Xiaorui Qiao Junhua Zhang Xichen Wang

Distinguishing between human-made interferences and natural background disturbances is of great significance for the safe operation of terrestrial optical cables because human-caused damage can be halted through timely intervention. To address the problem of small-sample disturbance recognition in distributed acoustic sensing (DAS) systems, this paper proposes a fused CNN–SVM classification model based on hybrid features. A convolutional neural network is employed to extract the high-level spatiotemporal features of disturbance signals, which are subsequently fused with statistical features and fed into a support vector machine for classification. Evaluated on open-source data, the proposed model achieves accuracy improvements of 9.1%, 8.7%, and 2.7% over the conventional CNN, the statistical-feature-based SVM, and the conventional CNN-SVM model, respectively. Furthermore, based on field-measured data, a dataset comprising 5664 samples was constructed, covering four typical disturbance-event types: background noise, drilling, knocking, and digging. The field classification results demonstrate that the three-layer convolutional structure of the model achieves a recognition accuracy of 98.5%. Both the ROC curves and multiple evaluation metrics indicate that the proposed three-layer fused CNN–SVM model delivers better classification performance and more balanced category recognition, offering a feasible reference for similar fiber disturbance engineering tasks.

Photonics doi: 10.3390/photonics13070615

Authors: Han Wen Hongyuan Xuan Kong Gao Zhen Yuan Xian Zhao Aimin Wang Yizhou Liu

We demonstrate an 80 MHz, 350 mW, 120 fs, 770 nm femtosecond laser based on a nonlinear compressed 1540 nm femtosecond fiber laser. The home-built 1540 nm fiber laser, delivering 80 MHz, 2.69 W, 269 fs laser pulses, was realized by employing spectral pre-modulation and pre-chirp management inside an Er/Yb co-doped fiber power amplifier. The subsequent nonlinear fiber pulse compression stage was utilized to further nonlinearly compress the pulse duration to 128 fs based on the Gaussian assumption. Detailed numerical simulation was also implemented to investigate the optical dynamics of the nonlinear compression process. Finally, a 0.5 mm thick fan-out periodically poled lithium niobate (PPLN) crystal was utilized to generate the frequency-doubled, 350 mW, 770 nm laser pulses with a 120 fs pulse duration based on the Gaussian assumption.

Photonics doi: 10.3390/photonics13070614

Authors: Tayyab Imran Muddasir Naeem

This article presents a structured spectral-stability assessment of an nJ-class ultrafast fiber laser generating femtosecond pulses with an approximate pulse duration of 115 fs, based on an ensemble of 61 consecutively acquired optical spectra. The study is motivated by the practical need to extract reliable short-sequence stability information from routine compact-spectrometer exports without requiring a separate pulse-diagnostic instrument at the initial assessment stage. For each spectrum, peak wavelength, centroid wavelength, FWHM bandwidth, integrated spectral area, correlation with the ensemble mean spectrum, and RMS deviation were calculated. Principal component analysis (PCA) was then applied to reduce the full spectral ensemble into a compact diagnostic space and to identify the dominant modes of residual spectral variation. The analyzed spectra yielded a peak wavelength of 775.31 ± 0.19 nm, a FWHM bandwidth of 7.95 ± 0.20 nm, an integrated spectral area of 10.43 ± 0.42 a.u.·nm, and a correlation with the mean spectrum of 0.99957 ± 0.00019, confirming a highly repeatable spectral envelope. PCA showed that PC1 and PC2 explained 66.50% and 12.60% of the variance, respectively, while PC3 contributed only 1.90%, indicating that the measured variability was weak and largely low-dimensional. These results demonstrate that consecutively exported optical spectra can provide a defensible and physically interpretable short-sequence stability assessment of ultrafast femtosecond fiber lasers, offering a practical route for routine monitoring, early-stage diagnostics, and future integration with simultaneous temporal and spectral characterization.

Photonics doi: 10.3390/photonics13070613

Authors: Arman Nikraftar Khiabani Nobby Stevens Tom Dhaene Ivo Couckuyt

Visible light positioning (VLP) based on received signal strength (RSS) offers a low-cost solution for indoor localization, being easily implemented in a warehouse based on existing infrastructure. However, RSS-based VLP remains challenging in 3D and yields subpar performance compared to 2D due to the larger localization space, as well as the presence of dark spots where many LEDs are not bright enough. This limits the practical use cases of RSS-based VLP in industrial applications. We study the performance of RSS-based VLP on a 3D simulated environment by training various machine learning models, including Gaussian processes and Kolmogorov–Arnold networks on different representations of RSS data. Our findings show that the use of Gaussian processes for predicting distances to LEDs coupled with a logarithmic transformation and multilateration leads to both high-accuracy and high-precision predictions under thermal noise (p95 localization error of 10 cm under 50 dB SNR). With this technique, RSS-based VLP reaches levels of accuracy in our simulated 3D environment that are comparable to those reported for 2D applications, supporting the extension of RSS-based VLP to height-varying industrial use cases.

Photonics doi: 10.3390/photonics13070612

Authors: Alexey G. Kuznetsov Alexander V. Dostovalov Sergey A. Babin

A Q-switched pulsed laser based on a coupled 7-core Yb-doped fiber with a cavity based on a fiber Bragg grating array has been demonstrated with a maximum energy of microsecond pulses up to 15 μJ at a 1 kHz repetition rate. The lasing spectrum is hybridized so that the laser line maxima of each core are nearly the same, having a negligible spread relative to each other, which is much lower than the wavelength shifts between individual FBGs in the cores. At the same time, the generated power is nearly the same in all the cores. However, when increasing the power beyond the stimulated Raman scattering threshold, the supermodes are destroyed so that the spectra in the cores become increasingly different and less stable, and the output power is mainly concentrated in one of the cores, whereas the pulse shortens significantly to a sub-microsecond duration (300 ns), with damped oscillations appearing at the beginning. The new regimes we demonstrated of the multicore fiber laser are promising for creating powerful pulsed radiation sources with a narrow spectrum.

Photonics doi: 10.3390/photonics13070611

Authors: Yufei Wei Xuehe Zheng Rui Yao Jia Guo Ziyi Tong Zhen Yang Jianlong Zhang Yong Zhang

Single-photon detection is a promising approach for low–slow–small Unmanned Aerial Vehicle (UAV) detection, holding great value in urban air defense and security monitoring. In complex urban environments, intense non-uniform building clutter and multi-echo aliasing easily submerge weak target signals, severely limiting traditional single-photon systems under low signal-to-background ratios. To address this, this paper proposes an urban-oriented detection strategy based on a free-running single-photon array, and designs a dual-optimized pulse position modulation laser detection and range image enhancement algorithm. By establishing temporal correlations via pulse sequence convolution, the algorithm effectively isolates weak UAV echoes from strong background clutter to break through detection limitations. Compared with the popular Markov correction method that often suppresses overlapping weak targets under strong reflections, the proposed method significantly improves small-target feature retention, successfully balancing background elimination and detection sensitivity. Field tests and quantitative evaluations demonstrate that the system reliably eliminates building clutter and achieves stable continuous tracking of weak UAV signals within 1.5 km, providing a highly robust and effective technical solution for urban low-altitude surveillance.

Photonics doi: 10.3390/photonics13070610

Authors: Yanqing Tong Siyuan Huang Jun Li Xiaotian Wang Huanfang Tian Huaixin Yang Shuaishuai Sun Jianqi Li

Time-resolved visualization of local structural dynamics driven by external fields is essential for understanding structure–property relationships in functional materials and devices. Conventional ultrafast methods primarily capture femtosecond-to-picosecond photoinduced dynamics, yet they lack real-space access to spatially inhomogeneous processes occurring at their intrinsic mesoscopic timescales that govern material and device performance—particularly electrically driven processes that closely mimic actual device operating conditions. Here, we report a multifunctional ultrafast transmission electron microscopy (UTEM) platform targeting reversible structural dynamics spanning nanoseconds to microseconds under stroboscopic multi-field excitation. Our system employs photoelectron pulses generated by nanosecond UV laser illumination as the probe, alongside optical and electric pulses as pump excitation. A unified electronic synchronization scheme based on a high-speed photodiode and a digital delay generator enables precise timing control among the optical pump, electrical pump, and photoelectron pulses across the nanosecond-to-microsecond range. Using vanadium dioxide (VO2) as a model system, we demonstrate a combined spatiotemporal resolution with measurable signals on the order of 10 nm–10 ns, allowing real-space mapping of spatially inhomogeneous dynamics. Electrical-pump experiments further reveal Joule-heating-induced non-uniform structural phase transitions and thermal-shock-excited megahertz-range mechanical oscillations. These results establish the developed multi-field UTEM platform as a practical tool for probing local structural dynamics in functional materials under optical and electrical excitation.

Photonics doi: 10.3390/photonics13070609

Authors: Xavier Orlik Aurélien Plyer Elise Colin

Recent studies have proposed improving Laser Speckle Contrast Imaging (LSCI) by using polarimetric filtering to isolate multiply scattered photons from moving red blood cells (RBCs), an approach referred to as Laser Speckle Orthogonal Contrast Imaging (LSOCI). This reliance on multiple scattering enables the development of a calibration method based on a moving reference sample, chosen to generate dynamic circular Gaussian speckle fields that replicate the statistical properties of RBC scattering in both intensity and the distribution of polarization states. Assuming that multiply scattered photons from both RBCs and the reference sample exhibit a homogeneous distribution of polarization states over the Poincaré sphere, the proposed calibration links in vivo speckle contrast reduction bijectively to an equivalent speed of the reference sample. We demonstrate that this equivalent-velocity metric yields consistent in vivo measurements across distinct instruments despite the use of different laser spectral widths, thereby providing a standardized and transferable means to quantify microcirculatory activity.

Photonics doi: 10.3390/photonics13070608

Authors: Bader Alhasson Faroq Razzaz Muhammad Arfan

The interaction of electromagnetic waves with atmospheric aerosols plays a significant role in communication systems and multimedia information transmission. Understanding the interaction of vortex light beams with an aerosol-laden atmosphere is indispensable for establishing a framework of the environmental channel. During the interaction, different optical effects such as absorption and scattering will result in energy attenuation, and this yields the deterioration of the transmission feature of the vortex beam signal. In this study, we present a theoretical analysis of Gaussian vortex beams (GVBs) scattering by diverse aerosol (unformed carbon, dust, sulphate, silicate, soot, and nitrate) particles in the atmosphere on the basis of the well-established generalized Lorenz–Mie theory (GLMT). Combined with the lognormal distribution model for aerosol particles, the attenuation and transmission characteristics of GVBs for different aerosol particles are analyzed. The extinction efficiency (Qext) factor of GVB, caused by the absorption and scattering of various aerosols, becomes smaller compared to that of a basic Gaussian beam (GB). Increasing the OAM mode index, the energy attenuation and transmission caused by aerosol absorption and scattering further decrease. Moreover, this research provides a basis to analyze the optical characteristics of the twisted beams in different atmospheric channels, such as wireless communication networks over aerosol-laden systems and material interactions.

Photonics doi: 10.3390/photonics13070607

Authors: Qifan Zhang Shi Jia Tianhao Zhang Zhuangzhuang Zang Shiqian Jia Lianmeng Wu Hao Luo Jinlong Yu

Optical performance monitoring is essential for adaptive and intelligent coherent optical communication systems. In this paper, a Transformer-based multi-task meta-learning framework is proposed for joint modulation format identification and electrical signal-to-noise ratio (ESNR) estimation from original received waveforms. A simulated coherent optical communication system is established to generate QPSK, 16QAM, and 32QAM signals under different launch-power conditions. The received I/Q waveforms are directly used as model inputs, avoiding handcrafted feature extraction or constellation-image conversion. The proposed model employs a shared one-dimensional Transformer encoder to extract temporal waveform representations. A prototypical classification branch is used for few-shot modulation format identification, while an ESNR regression branch is introduced for continuous signal-quality estimation. The two tasks are jointly optimized under an episodic support-query training mechanism. Experimental results show that the proposed method achieves 99.99% modulation identification accuracy on the test episodes. For ESNR estimation, the model obtains an MAE of 0.1194 dB, an RMSE of 0.1738 dB, and an R2 value of 99.83%. These results demonstrate that the proposed framework can simultaneously provide accurate modulation decisions and reliable ESNR estimation, showing its potential for waveform-based optical performance monitoring.

Photonics doi: 10.3390/photonics13070606

Authors: Weifeng Han Hanchuan Chen Fei Sun Yichao Liu Shuai Zhang

With the expansion of electromagnetic wave communication frequency bands and the improvement of integrated circuit integration, electromagnetic waves emitted by on-chip antennas are easily scattered by electronic components, causing radiation pattern distortion, which limits the improvement of integration and communication stability. Traditional cloaks can reduce electromagnetic scattering, but they cannot achieve broadband and omnidirectional performance simultaneously, and are mostly designed for external sources, making it difficult to protect on-chip antenna radiation patterns. In this work, a broadband air-impedance-matched metamaterial (AIMM) with characteristic impedance matched to free space is proposed in 2–8 GHz, with geometry-tunable phase delay and transmittance higher than 93%. Based on AIMM, a broadband source-surrounded cloak (SSC) is designed, which can guide electromagnetic waves from the surrounded source to bypass obstacles in any direction and restore the original wavefront outside the cloak, so as to protect the radiation pattern from scattering distortion. Numerical simulations show that the SSC works well in the whole bandwidth and remains effective when the source is offset. This work has important potential for improving the integration of integrated circuits and the stability of communication systems.

Photonics doi: 10.3390/photonics13070605

Authors: Andrea Svizzeretto Julia Casanueva Diaz Bas L. Swinkels Mateusz Bawaj

We present a fast time-domain simulator for optical cavities capable of reproducing non-linear dynamical regimes arising from the ring-down effect during resonance crossings at high mirror velocities or from abrupt changes of the input field. The model is based on a recursive formulation of the intracavity electric field as a sum over round trips, preserving the cavity memory while maintaining high computational efficiency. The simulator is designed to achieve three main goals. First, the boundary conditions of the cavity can be modified at each simulation step, allowing arbitrary time-dependent variations of both mirror positions and input electric field during the simulation run. Second, the sampling frequency can be flexibly chosen by the user; however, it is internally adjusted before effectively executing the simulation to remain consistent with the cavity round-trip structure. Finally, high computational efficiency was obtained by avoiding the repeated evaluation of the full electric field history. The framework is validated through comparison with experimental data from the Virgo interferometer during a mechanical excitation experiment, showing good agreement in non-adiabatic regimes. Due to its efficiency and flexibility, the oreonspy simulator provides a versatile tool for time-domain studies of optical resonators and future applications in real-time control and reinforcement-learning-based lock acquisition.

Photonics doi: 10.3390/photonics13070604

Authors: Mohammad Esmaeil Daraei Mehdi Abedi-Varaki Ignas Nevinskas

In this work, plasmonic photoconductive antenna (PCA) structures with different grating-width and gap configurations were numerically investigated to evaluate their influence on transient-current generation and terahertz (THz) emission performance. Two geometrical scaling strategies were considered: a fixed-gap configuration with a constant 100 nm photoconductive gap and a proportional-gap configuration in which the gap size was equal to the grating width. Three-dimensional finite element method (FEM) simulations were performed to analyze transient carrier dynamics, THz pulse electric-field behavior, and frequency-domain spectral response under 800 nm optical excitation. The results demonstrate that reducing the inter-grating gap enhances plasmonic near-field confinement and carrier localization near the metal–semiconductor interface, leading to stronger transient-current responses and enhanced THz characteristics. Spatial field and carrier-distribution analyses further confirmed improved electric-field localization and carrier confinement for the fixed-gap structures. In addition, voltage-dependent investigations showed that increasing the applied bias voltage strengthens carrier acceleration and enhances the simulated THz response within the investigated operating range. The results further demonstrate that the observed enhancement is governed not only by grating periodicity but also by the grating-width/gap-size ratio, highlighting the importance of geometrical fill-factor optimization. Polarization-dependent simulations confirmed the plasmonic origin of the enhanced transient-current generation and THz emission. These findings demonstrate that optimal THz performance arises from a balanced interplay between plasmonic field localization, optical absorption, and carrier-transport dynamics, providing design guidelines for the optimization of plasmonic THz PCAs.

Photonics doi: 10.3390/photonics13060602

Authors: Jiahe Dai Rui Zhou Xueguang Qiao

Precise monitoring of vibration signals is crucial for early fault warning and localization in industrial applications. Traditional electromagnetic accelerometers are often unsuitable for harsh environments characterized by high temperatures, high pressures, and strong electromagnetic fields. Fiber Bragg grating (FBG) accelerometers have become a major research topic in this field due to their unique advantages, including resistance to high temperature and pressure, immunity to electromagnetic interference, and ease of wavelength division multiplexing. This paper provides a systematic review of FBG accelerometers, covering their fundamental principles, classification, performance enhancement strategies, and applications. We focus on reviewing the research progress of FBG accelerometers from two main aspects, single-axis and multi-dimensional vector types, and offer an outlook on future development to provide a reference for the research and application of FBG accelerometers.

Photonics doi: 10.3390/photonics13060603

Authors: Chuang Li Chongxing Liu Changxi Xue Bo Dong

As the prevalence of myopia among adolescents continues to increase, the design and fabrication of myopia control lenses have become an important research direction in modern optics. Existing myopia control lenses mostly adopt purely refractive structures, which suffer from limited design freedom, insufficient chromatic aberration suppression, and relatively large lens thickness, thereby restricting further improvement of optical performance. This paper proposes a refractive–diffractive hybrid design and fabrication method for myopia control lenses. Centered on a harmonic diffractive optical element (HDOE), an optimization model is established to balance achromatization performance and fabrication feasibility. To address the challenges of small period width, tool shadow effect, and sensitivity to machining tolerances in diffractive lenses with large-aperture and high-additional-power, harmonic design is employed to increase the period width, thereby reducing fabrication difficulty and mitigating the influence of shadowing errors on diffraction efficiency. On this basis, two lenses with different phase structures are designed: one adopts a conventional diffractive correction phase to verify the role of HDOE in achromatization and edge-thickness reduction, while the other adopts a high-degree-of-freedom smooth phase to achieve a continuous multifocal visual effect. Both lenses are fabricated by single-point diamond turning (SPDT), and the effects of surface profile and machining parameters on performance are analyzed. Simulations and measurements show that the proposed method provides stable diffraction efficiency and effective chromatic aberration correction across the design band, while reducing the edge thickness by approximately 37.85% without additional thinning of the aspheric substrate. The results indicate that the refractive–diffractive hybrid design provides a feasible design and fabrication approach for functionally more complex myopia control lenses.

Photonics doi: 10.3390/photonics13060601

Authors: Zuqiong Chen Xiaopin Zhong Yibin Tian

Fringe projection profilometry (FPP) is widely used for 3D reconstruction, but conventional single-view FPP systems suffer from inherent occlusions and shadow regions, leading to incomplete surface recovery. In this study, we propose CVA-Net, an end-to-end deep learning framework with cross-view attention (CVA) that directly reconstructs dense depth maps from multi-view fringe patterns. CVA-Net simultaneously processes four fringe images acquired from orthogonal projection directions and leverages a CVA module to explicitly model inter-view dependencies, enabling adaptive fusion of complementary information. A 3D U-Net backbone with attention gates, atrous spatial pyramid pooling (ASPP), and an auxiliary parameter estimation branch further enhances reconstruction accuracy and structural consistency via multitask learning. To support Sim2Real network training, we build a Blender-based digital twin of a multi-view FPP system and generate a large-scale synthetic dataset with perfect ground truth. Extensive experiments on both synthetic and real-world objects demonstrate that CVA-Net significantly outperforms state-of-the-art single-view methods. With a symmetric four-view configuration and fringe period of 8, CVA-Net achieves an MAE of 0.0359 mm, an MSE of 0.0379 mm2 and an RMSE of 0.1947 mm, reducing the MAE, MSE, and RMSE by 32.8%, 54.1%, and 32.2%, respectively, compared to the best single-view competitor. Ablation studies validate the contribution of each architectural component, while real-system experiments demonstrate the feasibility of transferring a network trained purely on synthetic data to practical FPP measurements without domain adaptation. Although further improvements are required to enhance reconstruction accuracy under real imaging conditions, the proposed framework provides an effective initial step toward bridging the gap between digital-twin-based training and real-world multi-view FPP applications. CVA-Net provides a robust, occlusion-aware solution for multi-view FPP reconstruction.

Photonics doi: 10.3390/photonics13060600

Authors: Weile Zhai Jinyuan Ye Ruihao Wang Zhong’ao Yang Jiajun Tan Xiaoyan Pang Wanzhao Cui Yongsheng Gao

In radio-over-fiber (RoF) links, optical single-sideband (OSSB) modulation is an effective method to mitigate power fading caused by chromatic dispersion. However, its low modulation efficiency leads to suboptimal link performance. To address this, we propose a tunable optical carrier-to-sideband ratio (OCSR) OSSB modulation scheme based on a dual-electrode Mach–Zehnder modulator (DEMZM) in a Sagnac loop. Firstly, by adjusting the OCSR, higher radio-frequency (RF) transmission efficiency can be achieved. The experimental results demonstrate that the proposed link provides a 6 dB improvement in received RF power compared to conventional SSB modulation schemes. Furthermore, this approach effectively optimizes nonlinear distortions in the link, achieving a 12.14 dB enhancement in spurious-free dynamic range (SFDR). For tests conducted with a broadband signal featuring a 15 GHz carrier frequency and 500 MHz bandwidth, the optimal error vector magnitude (EVM) reaches 4.88%. Additionally, the link performance can be flexibly improved by adjusting the polarization controller configurations for each channel, making it suitable for multi-user application scenarios.

Photonics doi: 10.3390/photonics13060599

Authors: Jiaqi Ju Pan Qiu Yipeng Tan Zhengguang Shi

In Optical Camera Communication (OCC), precise localization of LED arrays under complex tilt conditions is a core challenge for reliable decoding. This paper proposes an OCC reception scheme for RGB-LED arrays that integrates YOLO-OBB rotated object detection with two-stage geometric correction. The system first employs a YOLOv8n-OBB model to extract a quadrilateral region of interest that tightly encloses the LED array boundary. This effectively suppresses background interference caused by superimposed perspective tilt and in-plane rotation. A coarse-to-fine two-stage correction framework is then applied. The first stage rapidly eliminates the dominant perspective distortion based on the detected bounding-box corners. The second stage performs a refined correction using the actual LED center positions. Two homography matrices are cascaded into a combined transformation, achieving two-stage correction accuracy through a single coordinate mapping. In the corrected image, K-Means clustering constructs a 16 × 16 LED topological grid. A locking strategy is adopted so that subsequent frames skip repeated LED detection and clustering. The steady-state per-frame processing time is reduced to approximately 78.9 ms. Experiments covered 16 cross-combinations of vertical tilt from 0° to 45° (0°, 15°, 30°, 45°) and in-plane rotation from 0° to 40° (0°, 15°, 30°, 40°). The uncorrected scheme and the horizontal-box scheme experienced severe bit errors or complete failure under complicated distortion. The proposed scheme maintained error-free transmission under all 16 tested conditions. The ratios of opposite sides of the corrected LED grid remained stable between 0.997 and 1.004. The system simultaneously achieves high reliability and low-latency real-time processing under complex geometric distortions.

Photonics doi: 10.3390/photonics13060598

Authors: Laura Marinela Ailioaie Constantin Ailioaie Georgiana Diana Ungureanu Cristinel Ionel Stan Anca Sava Dragos Andrei Chiran

Musculoskeletal pain is a major cause of disability and long-term analgesic use, increasing interest in safe non-pharmacological interventions. This focused narrative review examines light-emitting diode (LED)-based photobiomodulation (PBM) for musculoskeletal pain, integrating molecular, mechanistic, clinical, and translational evidence. Red and near-infrared LED-PBM may act through mitochondrial and non-mitochondrial photoacceptors, modulation of ATP production, reactive oxygen species, nitric oxide, calcium signaling, inflammatory pathways, oxidative stress responses, and extracellular matrix repair. Clinical evidence suggests a potential benefit in selected conditions, particularly temporomandibular disorders, fibromyalgia, cervical and myofascial pain, tendon and plantar fascia disorders, knee osteoarthritis, and mild-to-moderate peripheral nerve compression, while findings for non-specific low back pain remain inconsistent. The reviewed literature indicates that therapeutic response depends less on emitter identity alone than on wavelength, irradiance, radiant exposure, treatment geometry, target depth, timing, disease phenotype, and protocol quality. LED-based PBM appears generally well tolerated and clinically promising as an adjunct to rehabilitation, but current evidence is limited by heterogeneous devices, incomplete dosimetry, variable comparators, and short follow-up. Future studies should prioritize standardized reporting, depth-aware dosing, phenotype-based recruitment, biomarker-linked outcomes, and direct laser–LED comparisons under dosimetrically matched conditions.

Photonics doi: 10.3390/photonics13060597

Authors: Haotian Huang Yuzhao Li Nguyentuan Anh Jing Xia Yanfei Lü

We report, for the first time, a continuous-wave (CW) switchable multi-wavelength Nd:Lu2SiO5 (Nd:LSO) laser using two wedge birefringent filters (WBFs) operating on the 4F3/2 → 4I13/2 transition. The threshold equivalence condition was calculated via the two intracavity WBFs to achieve the simultaneous multiple-wavelength operation. Three dual-wavelength pairs (1332/1344 nm, 1344/1359 nm, and 1359/1363 nm), two triple-wavelength combinations (1332/1344/1359 nm and 1344/1359/1363 nm), and a four-wavelength set (1332/1344/1359/1363 nm) were further experimentally demonstrated. These wavelength combinations are mutually switchable via tuning of the WBF. Under an incident pump power of 20 W at 808 nm, the total output powers for the dual-wavelength pairs (1332/1344 nm, 1344/1359 nm, and 1359/1363 nm) were measured to be 1.55 W, 2.17 W, and 3.40 W, respectively. The triple-wavelength outputs at 1332/1344/1359 nm and 1344/1359/1363 nm delivered 1.57 W and 1.91 W, respectively. The four-wavelength emission at 1332/1344/1359/1363 nm reached 913 mW.

Photonics doi: 10.3390/photonics13060596

Authors: Tong Yang Tengfei Hao Yiwen Lu Feifei Yin Kun Xu Ming Li Yitang Dai

In modern radar and electronic countermeasure systems, frequency-agile (FA) signal generators with low phase noise are of vital importance. The optoelectronic oscillator (OEO) is restricted by the periodic boundary condition (PBC), despite its superior performance in phase noise and frequency tunability. This paper proposes a new FA optoelectronic hybrid oscillator scheme, which employs a reconfigurable aperiodic FA filter and a dynamic frequency compensation module to collaboratively break the PBC limitation. It achieves fast switching and fine-grained frequency hopping at the 100 kHz level while maintaining low phase noise. Theoretical and experimental verification show that the system can generate arbitrary FA radio frequency (RF) signals from 1.95 GHz to 2.05 GHz with a tuning range of 103 times the free spectral range (FSR), and the phase noise reaches −120 dBc/Hz at 10 kHz offset. This study provides a novel technical route for generating narrow-step frequency-agile signals and effectively improves target detection accuracy and anti-jamming performance in electronic warfare applications.

Photonics doi: 10.3390/photonics13060595

Authors: Lukasz Pawlik Jacek Lukasz Wilk-Jakubowski

Accurate rainfall monitoring is vital for hydrology and environmental sensing. This study presents a physics-aware deep learning framework using E-band (71–86 GHz) commercial microwave links (CMLs). Using the extensive urban CML dataset and methodology, a bi-directional Long Short-Term Memory (Bi-LSTM) model is developed to classify wet and dry periods under a temporal generalization framework across heterogeneous link configurations. The approach integrates physical signal decomposition, including baseline estimation, gaseous attenuation correction, and wet antenna attenuation (WAA) modeling, with sequence-based learning. Results demonstrate that the temporal deep learning model outperforms classical threshold-based and physical k–R approaches when evaluated over independent temporal validation blocks, effectively reducing sensitivity to path-length-related variability on heterogeneous paths. The model maintains stable performance (loss < 3%) under moderate signal-level noise. SHapley Additive exPlanations (SHAP) confirm the model relies on physical features, such as signal volatility and temporal trends, to reliably differentiate rainfall from WAA. This framework highlights the potential of E-band infrastructure as a distributed sensing network for integrated sensing and communication (ISAC) architectures.

Photonics doi: 10.3390/photonics13060594

Authors: Bing Han Yuanzhang Song Zhijing Fang Hangyu Yue Hongtao Ma Yuegang Fu Jian Song

Accurate spot-centroid localization is fundamental for determining optical metrics such as modulation transfer function (MTF) and effective focal length (EFL). Conventional methods struggle under non-ideal conditions—asymmetric spots, high noise, and vibration—and mid-wave infrared (MWIR) vibration has received little attention. To address these gaps, we propose multi-scale Gaussian fitting with subpixel edge reconstruction (MSGF-SER), combining image pyramid fitting, Zernike-moment edge extraction, and adaptive eccentricity-weighted fusion. Validated on simulated spots with varying SNRs and experimental sequences (visible off-axis aberration, long-wave infrared (LWIR) high-noise, MWIR micro-vibration), MSGF-SER achieved a noise-free RMSE of 0.03 pixel and 0.84 pixel at 5 dB SNR. On real MWIR vibration sequences, the Y-direction standard deviation (STD) dropped to 0.098 pixel, and the trajectory displacement variance was more than an order of magnitude lower than that of conventional methods. MTF deviations remained within 0.01, and the deviation of the measured mean EFL from the nominal focal length was better than 0.05 mm, and the STD was below 0.02 mm. These results demonstrate that MSGF-SER substantially improves centroid localization accuracy, repeatability, and smoothness under challenging conditions, providing reliable support for high-precision optical system parameter measurement.

Photonics doi: 10.3390/photonics13060593

Authors: Nicholas W. Jenkins Wilhelm Eschen Will Hettel John Gallagher Benjamin Shearer Gabriella Seifert Yunzhe Shao Clay Klein Drew Morrill Grzegorz Golba Michaël Hemmer Henry Kapteyn Margaret Murnane

Ptychography implemented with coherent high-harmonic (HHG) sources enables high-resolution, high-fidelity imaging of nanostructures and biosystems. However, when driven by mid-infrared lasers to generate light at higher photon energies, HHG inherently produces a broadband quasi-continuum, which is less suited for coherent imaging compared with a single harmonic order. Consequently, experiments typically select a narrow bandwidth of ≈1%, leaving most of the HHG photons unused, increasing exposure times. In this work, we demonstrate broadband ptychography utilizing an extreme UV (EUV) continuum centered at 92 eV, with a bandwidth of up to 7.9 eV (a relative bandwidth of ~9%). By focusing the HHG beam to a sub-micrometer spot size to relax the temporal coherence constraints, and utilizing a multi-wavelength ptychographic reconstruction algorithm, we achieve a spatial resolution of 42 nm, which is near the diffraction limit of ~30 nm for our setup. To the best of our knowledge, this represents the broadest spectral bandwidth successfully employed to date for EUV ptychography, with the potential to increase the usable photon flux by up to an order of magnitude relative to previous approaches. In the future, broadband soft X-ray ptychography can be used to image hydrated samples around the carbon K-edge and magnetic textures at the L-edges of transition metals.

Photonics doi: 10.3390/photonics13060592

Authors: Xingyu Huang Zhuqiang Zhong Jianjun Chen Yipeng Zhu Jinzhi Xu Haiyang Yang Chuanyi Tao Yanhua Hong

We propose a time-delay signature suppressed broadband chaotic (TSBC) carrier generation scheme and theoretically investigate its performance in a dual-polarization bidirectional chaotic communication system based on synchronized vertical-cavity surface-emitting lasers (VCSELs). The TSBC scheme is implemented by combining fiber Bragg grating (FBG) feedback with an external electro-optic (EO) phase modulation loop to introduce synergistic nonlinear perturbations. The results demonstrate that the proposed TSBC scheme effectively suppresses the time-delay signature (TDS) to less than 0.03 while significantly enhancing the chaotic carrier bandwidth to over 23 GHz for each polarization channel. Meanwhile, high-quality chaotic synchronization can be achieved with laser parameter mismatches of approximately 30%. Finally, an aggregated 46 Gbit/s dual-polarization bidirectional chaotic transmission is demonstrated, which confirms the effectiveness and the potential of the TSBC dual-polarization bidirectional scheme for secure optical communication applications.

Photonics doi: 10.3390/photonics13060591

Authors: Gagik Demirkhanyan Narine Babajanyan Ninel Kokanyan Marco Bazzan Edvard Kokanyan

Based on the analysis of the emission spectra of LiNbO3:Ho3+ and LiNbO3:Yb3+ crystals in the infrared region, the feasibility of radiation-balanced (RB) lasing in the infrared region at room temperature has been investigated. The parameters of RB lasing were calculated, and the optimal pump and laser wavelengths were determined as follows: λOP = 2015.2 nm and λOL = 2072.3 nm for LiNbO3:Ho3+ crystals, and λOP = 1004.7 nm and λOL = 1060.8 nm for LiNbO3:Yb3+ crystals. The dependence of RB lasing intensity on pump intensity, ensuring radiation balance, was established. For representative values of intracavity losses (γi = 0.1%, 0.4%, 0.8%) and output coupler transmission losses (T2 = 2%), the expected output powers of RB lasing were estimated.

Photonics doi: 10.3390/photonics13060590

Authors: Zhaoyi Li Shanmin Gao Rui Liang Zhengyang Zhong Hongtian Zhu Enbo Wang Qi Zhang Zhichun Liu Zhenyin Hai Chenyang Xue

Accurate pressure measurement under high-temperature conditions is challenging for silica-diaphragm-based fiber-optic Fabry–Perot (F-P) sensors because temperature causes both optical cavity length (OCL) baseline drift and pressure-sensitivity variation. In this work, a structurally simple and readily fabricated silica-diaphragm-based fiber-optic F-P pressure sensor was developed, and a neural-network-assisted compensation strategy was proposed to suppress the residual errors of conventional analytical compensation. A temperature-dependent response model was established to describe OCL drift and sensitivity variation. The OCL was demodulated from reflection spectra using an FFT-assisted dual-peak and MMSE refinement method, and static pressure measurements were performed over 25–400 °C and 0–2.4 MPa. Based on the experimentally verified response characteristics, a fitting-based compensation method considering both OCL drift and sensitivity variation was first implemented. A lightweight neural network was then constructed using the OCL variation, ΔOCL, and ambient temperature as physically meaningful input features. Compared with fixed-sensitivity compensation and drift-and-sensitivity fitting compensation, whose maximum full-scale errors were 7.10% F.S. and 2.74% F.S., respectively, the proposed method reduced the maximum error to 0.90% F.S. with an RMSE of 0.0045 MPa. Additional validation at the independent intermediate temperatures of 150, 250, and 350 °C further confirmed the generalization capability of the proposed NNC model between calibrated temperature gradients, achieving an overall RMSE of 0.0055 MPa and a maximum full-scale error below 0.77% F.S. The proposed approach provides a high-accuracy and practical solution for high-temperature pressure monitoring using simple fabricated silica-diaphragm F-P sensors.

Photonics doi: 10.3390/photonics13060588

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

Flat-top beams with uniform intensity distribution and well-defined profiles have broad application prospects. However, current design methods can only achieve circular or square beam shaping. Recently, a method enabling the generation of polygonal flat-top beams has been proposed, yet its energy utilization rate is limited at the 70% level only. To solve this issue, we propose a hybrid iterative algorithm for the purpose of generating polygonal flat-top beams with high diffraction efficiency while maintaining excellent uniformity. The hybrid algorithm combines the advantages of the mixed-region amplitude freedom (MRAF) algorithm and the overcompensation (OC) algorithm. The MRAF is firstly employed to achieve high diffraction efficiency. Subsequently, the OC algorithm is adopted to optimize uniformity. In addition, a more convenient convolution-based method is used to construct the descending edge of the target flat-top beam. A series of polygonal flat-top beams, such as triangular, square, pentagonal, and hexagonal, are obtained, and comparisons with the original methods are also carried out by means of simulations and experiments. Our experimental data demonstrate that an average diffraction efficiency above 97% is achieved.

Photonics doi: 10.3390/photonics13060589

Authors: Martin Weis

Conventional photodetectors and image sensors deliver high-fidelity digital outputs but face a growing data-movement bottleneck: the energy and latency cost of transferring raw pixel streams to off-chip memory and processors increasingly dominates over both sensing and computation in modern machine-vision pipelines. An emerging response is the neuromorphic photodetector, a class of optoelectronic device that converts incident light into an electrical signal while simultaneously storing, modulating, and pre-processing that signal in a manner inspired by biological synapses and retinas. Over the past decade, demonstrations have spanned at least eight material platforms—organic semiconductors, organic–carbon-nanotube hybrids, perovskite and perovskite hybrids, metal oxides (including ultra-wide-bandgap and printable variants), wide-bandgap III-nitrides and 4H-SiC, two-dimensional materials, photo-memristors, and silicon CMOS in-sensor compute architectures—and have been realised through four distinct architectural families: phototransistor synapses, photo-memristors, heterojunction in-sensor compute, and linear photovoltaic neural networks. Here, we provide a quantitative cross-platform benchmark across forty in-scope articles, identify persistent photoconductivity as a near-universal device-physical substrate underlying synaptic functionality, characterise the responsivity–speed–energy trade-off structure observed across platforms, and present a critical assessment of energy-reporting practice in the field. We further identify three best-practice exemplars from three independent material platforms that converge on operating biases of 0.01–0.1 V and energies of 0.07–0.8 fJ per event, and we propose a unified reporting framework to enable meaningful cross-platform benchmarking of next-generation neuromorphic photodetectors.

Photonics doi: 10.3390/photonics13060587

Authors: Thanh Son Pham Xuan Bach Nguyen Bui Xuan Khuyen Vu Dinh Lam Liangyao Chen Youngpak Lee

Reconfigurable photonic metasurfaces enable tunable thermal-emission engineering in the long-wave infrared (LWIR), particularly within the 8–13 μm atmospheric window. This work includes the investigation on a concentric-ring VO2/SiO2/Au metasurface for LWIR spectral-emissivity modulation. Full-wave simulations showed that, in the metallic phase (σ = 2 × 105 S/m where σ is conductivity), the structure exhibited an absorption over 90% across the 9.3–15 μm sub-band, with two near-unity resonances near 10.2 and 13.3 μm. Control structures, gap-dependent spectra, E-field maps, and current-density Cartesian multipole decomposition supported a hybridized-ring mechanism in which both dominant resonances were predominantly electric-dipole-like ring branches whose spectral positions and field localizations were modified by inter-ring coupling. Across the conductivity sweep, the normal-incidence band-averaged 8–13 μm emissivity changed from 0.0184 to 0.8844, corresponding to a switching ratio of 48.06. The four-fold symmetry of unit cell also yielded polarization-insensitive and angularly robust LWIR absorption, while the simplified endpoint thermal-balance estimate indicated a metallic-state net cooling power of 49.3 W m−2 at T = Tamb = 300 K, where Tamb was the ambient temperature, and an estimated equilibrium temperature drop of 4.4 K below the ambient for the metallic-state endpoint, whereas the insulating-state one suppressed this response. These results identify concentric VO2 ring metasurfaces as promising candidates for switchable LWIR thermal-emission control.

Photonics doi: 10.3390/photonics13060586

Authors: Franco Marabelli Giovanni Pellegrini Luca Zagaglia Konstantins Jefimovs Dimitrios Kazazis Francesco Floris

The interaction between light and chiral plasmonic metasurfaces provides a powerful mechanism for controlling polarization states at the nanoscale. Utilizing displacement Talbot lithography for large-area fabrication, we characterized the chiroptical response by measuring the evolution of Stokes parameters to quantify phase retardation between orthogonal polarization components. To elucidate the underlying physical mechanism, we employ a hybrid finite element method and rigorous coupled-wave analysis approach to investigate the behavior of the far-field and local-field configurations. Our results reveal that the phase shift is highly sensitive to symmetry-breaking features, where the interplay between different modes dictates the overall circular dichroism signal. Furthermore, the analysis of local field plots suggests specific contributions of plasmonic modes to the chiroptical response. We conclude that the phase shift effects, characterized via Stokes parameters and modal analysis, provide a robust metric for engineering chiroptical properties in these systems. This work establishes a fundamental framework for developing compact polarization-control elements and enhances the understanding of phase-modulated light-matter interactions in chiral plasmonic metasurfaces.

Photonics doi: 10.3390/photonics13060585

Authors: Hanyi Zhang Rong Fan Yinzhou Zhi Lulu Fang Wenxuan Cheng Yujie Wang Jianfeng Bao Lijing Li

In this study, we present a thermal-aware design of a compact hybrid plasmonic grating (HPG) TE-pass polarizer on X-cut lithium niobate on insulator (LNOI) for fiber-optic gyroscopes (FOGs). In a three-dimensional simulation, the optimization of the trapezoidal sidewall angle (θ = 78°) and the thickness of the Ag grating (13 nm) yield a polarization extinction ratio of 36.2 dB at 1550 nm (with a peak of 41.4 dB at 1548 nm) within a sub-10 μm grating length. This represents a ~3–8 dB improvement over prior LNOI HPG polarizers at the same footprint. A multiphysics thermo-optic analysis over the wide industrial FOG envelope (from −45 to +85 °C) demonstrates that the operating-wavelength polarization extinction ratio remains within the range of 24.7–36.2 dB across the entire 130 K span (worst case 24.7 dB at −25 °C), constrained solely by a modest 10 pm/K Bragg detuning stemming from the pronounced (~5) thermo-optic anisotropy of LN. The insertion loss exhibits a negligible drift of merely 0.73 dB. A fabrication tolerance study identified the Ag thickness as the predominant budgetary constraint (±1 nm tolerance, PER dropping ~10 dB at the resonance edge), while the ridge width and oxide buffer demonstrated comparatively greater flexibility. The device, therefore, fulfills the criteria for FOG-grade polarization suppression across most of the operational temperature range. The −25 °C point is established at the 25 dB threshold, thereby providing concrete design guidelines for ensuring environmentally stable on-chip polarization control on LNOI.

Photonics doi: 10.3390/photonics13060584

Authors: Nannan Xu Mengyu Zong Lianzheng Su Zhe Wang Weiyi Yu Weiyu Fan Linguang Guo Shuai Fu Xinxin Shang Huanian Zhang

Tellurene has a wide bandwidth and low propagation loss at near-infrared wavelengths due to its nonlinear absorption coefficient. Therefore, we prepared tellurene–polyvinyl alcohol (Te-PVA) film as a saturable absorber in an Er-doped fiber laser by liquid phase exfoliation and spin-coating. The modulation depth was 5.25% and the saturation intensity was 17.02 MW/cm. The nonlinear optical properties of the film and its application in high-stability mode-locked operation were studied. A mode-locked pulse with a fundamental frequency of 8.48 MHz and a central wavelength of 1560.10 nm was obtained, with a signal-to-noise ratio which was greater than 75 dB. A traditional soliton mode-locked operation with a pulse width of 1.41 ps was achieved. In addition, eighth- and 19th-harmonic mode-locked operations were obtained by adjusting the pump power and polarization controller. Our results show that Te-PVA film functioned as a saturable absorber which enabled harmonic mode-locking with an SNR of 75 dB in an Er-doped fiber laser. It is thus an excellent ultra-fast photonics material.

Photonics doi: 10.3390/photonics13060583

Authors: Xiaodong Zhao Kaixiang Zhang Chunlei Yu Ning Zhou

We demonstrate a mechanism-oriented Surface-Enhanced Raman Scattering (SERS) platform based on a hybrid structure integrating monolayer molybdenum disulfide (MoS2) and gold nanospheres (AuNSs) on an optical microfiber (MF). The microfiber serves as a whispering-gallery-mode (WGM) microcavity. Monolayer MoS2, grown directly on the microfiber surface via chemical vapor deposition (CVD), provides a chemically active interface for molecular adsorption and charge-transfer-related chemical enhancement. Subsequently deposited AuNSs couple with the microfiber-supported WGM, leading to the formation of hybrid photonic–plasmonic modes. This coupling results in a narrowed scattering resonance and a localized electromagnetic hotspot near the AuNS–microfiber interface. The combined contribution of electromagnetic enhancement from the microfiber–AuNS hybrid cavity and chemical enhancement from the MoS2 layer produces discernible Raman enhancement for Rhodamine 6G (R6G) molecules under proof-of-concept measurement conditions. This work provides a useful platform for studying SERS enhancement mediated by hybrid photonic–plasmonic modes and offers guidance for the future development of optimized fiber-based SERS sensors.

Photonics doi: 10.3390/photonics13060582

Authors: Maxim Durach

Anisotropic, Tellegen, chiral, moving-medium-type, omega, gyrotropic, hyperbolic, and multi-hyperbolic materials form an important class of isotropy-broken photonic media in which wave propagation can no longer be characterized by the Fresnel wave surface alone. Here we show that Fresnel wave surfaces can be converted into propagation maps that organize positive- and negative-phase-velocity propagation together with attenuation and amplification. In Hermitian media, the boundary between forward and backward propagation forms the orthogonal-phase-velocity separatrix. This separatrix is also a continuous locus of orthogonal-phase-velocity exceptional points, where the index-of-refraction operator becomes defective even though the material medium remains Hermitian. In non-Hermitian media, the attenuation–amplification boundary forms the loss–gain separatrix. The associated loss–gain singularities occur where the handedness remains continuous while the gain–loss character changes sign. Their physical importance is revealed by the momentum-resolved density of states: at these points, the Lorentzian linewidth of the non-Hermitian momentum-resolved density of states (DOS) collapses, producing sharp DOS peaks whose sign reverses across the separatrix. Thus, loss–gain singularities are threshold-like singularities of the Fresnel wave-surface propagation map, generated by non-Hermitian linewidth collapse. The result is a compact geometric language for describing how handedness, degeneracy, loss, gain, and momentum-resolved DOS are organized in isotropy-broken electromagnetic materials.

Photonics doi: 10.3390/photonics13060581

Authors: Amr G. AbdElKader Kazutoshi Kato

Graphene–plasmonic electro–optic (EO) modulators have attracted significant interest for compact and energy-efficient integrated photonic systems due to their electrically tunable optical response and strong light–matter interaction. In this work, an ultra-thin silicon-on-insulator (SOI) graphene–plasmonic slot modulator (G-PSM) is investigated using a combined semi-analytical and numerical framework. The analysis integrates finite-temperature Kubo conductivity modeling, perturbation-based effective-index analysis, overlap-factor evaluation, eigenmode analysis, and full-wave simulations to study the influence of silicon thickness on the EO performance of the proposed structure. The obtained results demonstrate that geometry engineering strongly affects modal confinement, overlap enhancement, effective-index perturbation, transmission characteristics, extinction ratio (ER), insertion loss (IL), energy-per-bit consumption, and EO bandwidth. Under optimized operating conditions, the proposed G-PSM achieves an effective refractive-index variation of approximately 3.1×10−3, an ER of approximately 3.5 dB, an IL of 1.5–2 dB, an energy-per-bit consumption of approximately 7.5 fJ/bit, and a 3 dB EO bandwidth approaching 200 GHz. Strong electromagnetic confinement is achieved inside the plasmonic slot region near the graphene-active layer, enabling efficient electro–absorptive and electro–refractive modulation. Excellent agreement between the semi-analytical calculations and numerical simulations validates the developed framework and confirms the suitability of the proposed ultra-thin SOI G-PSM for compact broadband EO modulation in future integrated photonic systems.

Photonics doi: 10.3390/photonics13060580

Authors: Yao Peng Han Ding Lin Miao Qinru Chen Yuan Yao Miao Luo Fang Fang Dong Zhang

A complex curved reflector made from a 40% silicon–aluminum alloy (AlSi40) can meet the requirements of optical systems operating across the infrared, near-infrared, and visible bands. It enables an athermalization design with simplified alignment and assembly, while offering high manufacturing efficiency and low costs. This makes it ideal for widespread use in high-end optical systems. As an enabling technology for the fabrication of AlSi40 freeform mirrors, error compensation in ultra-precision (UP) diamond turning is currently a research hotspot; however, current error compensation methods still have considerable room for improvement in terms of both accuracy and manufacturing efficiency. To address this issue, this study proposes an efficient and highly accurate method: a polar grid is defined in the machining coordinate system, and the corresponding surface point cloud is calculated. Using measured point clouds from reference spheres and freeform form error in the measurement coordinate system, mounting pose errors and form error with measurement error removed are determined via least squares. Machining error at grid points is then calculated via coordinate transformations and bicubic spline interpolation, and applied to correct cutter contact points (CCPs). Cutter location points (CLPs) are finally obtained using piecewise cubic spline fitting and a bisection method. With this method, average form error of four AlSi40 substrates improved from RMS 114.8 nm to 47.9 nm, and for four AlSi40 substrates with nickel–phosphorus (NiP)-plated surfaces, from RMS 71.3 nm to 31.1 nm. The compensated accuracy meets near-infrared stellar tracker requirements without polishing, greatly enhancing freeform mirror manufacturing efficiency.

Photonics doi: 10.3390/photonics13060579

Authors: Haohan Zhou Koustav Pal

Twist-angle engineering in van der Waals bilayers enables excitonic and valley phenomena that are not accessible in naturally stacked crystals. In a dual-gated 3∘ twisted bilayer WSe2 device, low-temperature polarization-resolved photoluminescence spectroscopy reveals a pronounced Stark shift of the interlayer exciton, yielding an effective electron–hole separation of 0.26 nm and indicating a strongly hybridized interlayer excitonic state. The degree of circular polarization (DOCP) is strongly doping-dependent but only weakly affected by the vertical electric field: at zero magnetic field, the DOCP is about 30% in the electron-doped regime and about 18% in the hole-doped regime. An out-of-plane magnetic field of 9 T sharpens this contrast to about 35% and 13%, respectively, suggesting distinct intervalley depolarization dynamics in the two doping regimes. Together, these results show that an electric field primarily tunes exciton energy, whereas doping and a magnetic field control valley polarization, highlighting small-angle twisted bilayer WSe2 as a promising platform for tunable excitonic and valley-optoelectronic functionalities.

Photonics doi: 10.3390/photonics13060578

Authors: Yuyao Pan Xincai Xu Yanfeng Liu Nan Li Xiangtao Yu Wenqiang Li Xingfan Chen Cheng Liu Huizhu Hu

Accurate nanoscale displacement readout is essential for optical inertial sensors, precision positioning, and micro-vibration characterization. In this work, we develop a free-space optical heterodyne interferometric readout system for low-frequency nanoscale displacement sensing and establish an SNR-guided adaptive demodulation framework. Two complementary demodulation strategies are integrated: Bessel-function-based frequency-domain sideband extraction for small-amplitude low-SNR motion and IQ quadrature phase tracking for larger-amplitude displacement. The experimentally demonstrated framework maps the applicability regimes of the two methods and enables wavelength-referenced displacement readout over a range from sub-nanometer narrowband detection to 250 nm under the present experimental conditions. The implemented system achieves a repeated-measurement repeatability of 0.40 nm under a 10 Hz excitation condition, and spectral SNR analysis is consistent with time-domain statistical evaluation. Finally, the readout system is applied to a quartz pendulum inertial structure, demonstrating its potential for photonic displacement sensing and optical inertial sensor characterization.

Photonics doi: 10.3390/photonics13060577

Authors: Alexey V. Shkirin Egor I. Nagaev Dmitry N. Ignatenko Leonid L. Chaikov Andrey N. Lobanov Pavel P. Sverbil Svetlana E. Dimitrieva Maria A. Shermeneva Sergey N. Chirikov Nikolai V. Suyazov

Fluorescence emission-excitation matrices for cow milk samples with different fat contents in the range of 0.05–10% and a constant protein content of 3%, as well as for butter and extracted milk components such as casein and lactose, have been measured using a spectrofluorometer. The influence of the increased fat content on the shape of the fluorescence spectra of milk has been studied. In addition, fluorescence spectra measured for serial dilutions of high-fat milk with water and skim milk, along with aqueous dilutions of skim milk, have shown that the fluorescence diagnostics of fat and protein content in milk can be implemented using excitation at only two wavelengths: 280 and 320 nm. The optimal spectral ranges proposed for detecting the content of milk components via fluorescence measurements can be useful when designing UV LED-based fluorescence analyzers of milk composition.

Photonics doi: 10.3390/photonics13060576

Authors: Xinran Zhao Yuefeng Shen Yi Zhang Ziqiang Zhao Cui Liang Yilan Zhou Tengchao Huang

Fiber optic gyroscopes (FOGs) are core sensors in inertial navigation systems, and their miniaturization and integration are currently hot research topics. This work presents an FOG system driven by a silicon photonics integrated circuit (PIC). The PIC, based on a 90 nm silicon-on-insulator (SOI) process, integrates core components such as polarizers, 3 dB couplers, and phase modulators within a compact footprint of 3 × 0.45 mm2. These components exhibit excellent performance over a wide spectral range and play a crucial role in high-performance FOG systems. Experimental results show that the proposed FOG system can definitively measure the small angular velocity of the Earth’s rotation (±7.5 °/h). Further Allan variance analysis reveals that the FOG system has an angular random walk (ARW) of 0.00358 °/h1/2 and a bias instability (BIS) of 0.1185 °/h. These results demonstrate the application potential of silicon photonics-based FOG systems.

Photonics doi: 10.3390/photonics13060575

Authors: Jihao Dai Hongshuai Qin Guowen Li Jin Liu Xiaoshuai Zhang Huiyu Qi Zhiwen Zheng Xingru Huang

We study whether the semantic category of a hidden display terminal can be inferred from a wall-mediated indirect observation when the display remains outside the camera field of view under a controlled and calibrated scene configuration. This setting provides a security-motivated feasibility test for indirect optical semantic leakage, but it remains challenging for two reasons. First, indirect propagation makes the wall pattern dominated by the occluder contour, while category-bearing evidence survives only as weak radiometric variations, making stable extraction difficult. Second, even after front-end recovery, low-frequency support is relatively stable, whereas the mid- and high-frequency details required for class separation remain weak and distortion-prone; as a result, the classifier may drift toward dominant but weakly informative coarse-grained patterns and fail to consistently accumulate fine-grained discriminative cues. We propose SCISA-Net, which combines scene-constrained inversion with multi-stage Haar-subband attention to reorganize indirect observations, compensate residual feature degradation, and aggregate class-relevant subband evidence. Experiments on a paired 31-class benchmark show stable recognition, robustness to illumination attenuation and ambient background interference, matched scene-operator re-parameterization capability, and clear degradation when key inverse or subband components are disrupted. These results support the feasibility of category-level semantic inference from calibrated wall-mediated indirect observations.

Photonics doi: 10.3390/photonics13060574

Authors: Stefano Longhi

Non-Hermitian photonic lattices offer unconventional control over light evolution owing to modal non-orthogonality and the resulting non-normal dynamical response. In this work, we show that a uniform passive waveguide lattice with dissipation confined to one or a few sites near an edge can exhibit an algebraic(nearly linear) decay of optical power—an absorption law forbidden in orthogonal (normal-mode) dissipative systems, where any superposition of eigenmodes yields purely multi-exponential attenuation. We demonstrate that algebraic absorption arises when the input excitation is appropriately tailored to exploit non-orthogonal modal interference, effectively channeling energy toward the dissipative boundary. In particular, under the condition of coherent perfect absorption (CPA) associated with a spectral singularity of the semi-infinite lattice, nearly complete light absorption accompanied by algebraic decay of the optical power can be achieved. Starting from the minimal configuration of a single lossy edge site, we derive compact analytical expressions for the dynamics and identify the conditions under which linear-like absorption emerges. We then extend the analysis to multiple edge-proximal lossy sites. Our results show that simple dissipative photonic lattices, when driven by suitably prepared input states, enable robust sculpting of absorption laws through non-normal dynamics, providing a new route to programmable attenuation.

Photonics doi: 10.3390/photonics13060573

Authors: Pengtao Zhu Xinlei Shi Zuming Lin Yiwen Xue Yi Liu Yifeng Sun Lei Gao Mingyang Ye Lun Zhang Yuexiang Guo Yin Xu Hualong Bao

We present a family of compact, programmable wavelength demultiplexers enabled by an etchless silicon nitride platform integrated with the low-loss phase-change material Sb2Se3. Using topology optimization (LumOpt) with a p-norm (p = 2) figure-of-merit defined over a 10 nm bandwidth, we design several devices within a common 24 × 24 μm2 design region: single-wavelength routers (1530, 1550, 1570, 1590 nm), two-channel (1550/1570 nm), three-channel (1530/1550/1570 nm), and four-channel (1530–1590 nm) coarse wavelength-division demultiplexers, all sharing the same input/output waveguide configuration. Simulation results show that all devices achieve low insertion loss at target wavelengths (peak transmission better than −1.21 dB across all channels), high average transmission over the respective 10 nm bands (typically within 0.1 dB of the peak), and suppressed crosstalk (worst case below −11.52 dB). Leveraging the reversible amorphous-to-crystalline phase transition of Sb2Se3 via laser pulses, all devices support post-fabrication reconfiguration, overcoming the static functionality of conventional etched photonic circuits. This work establishes a scalable, software-defined platform that combines inverse design and phase-change materials for high-density, reconfigurable wavelength-routing photonic integrated circuits.

Photonics doi: 10.3390/photonics13060572

Authors: Quhao Zhuo Moxuan Luo Yuanqing Li Qiuqi Hu Jianwei Tang Qi Wu Shuai Qu Bang Yang Zhaopeng Xu Yanfu Yang Jinlong Wei Qiaozhi Lei

With the advancement of artificial intelligence (AI) technologies such as large language models and autonomous driving, the data traffic via optical interconnects in data centers has surged significantly. The stability of the optical interconnects relies on intelligent operation and maintenance (O&M). Integrated sensing and communication (ISAC) over fibers enables vibration sensing utilizing existing communication fibers, providing critical support for intelligent O&M in data centers. Compared to sensing in the coherent systems, it is difficult to use phase and state of polarization (SOP) monitoring for vibration detection in intensity-modulation and direct-detection (IM-DD) systems. In this paper, we propose a joint phase-based and SOP-based sensing scheme integrated in IM-DD systems. In the proposed scheme, the received IM-DD communication signals are tapped for sensing with a power ratio of 10%. Then the tapped signals are split for vibration sensing based on SOP and phase, respectively. In the phase-based sensing arm, a circulator, a 3×3 coupler and two Faraday rotating mirrors (FRMs) are used to build an unbalanced Michelson interferometer without phase fading and polarization fading. For the purpose of SOP-based sensing, a polarizer is used to monitor the vibration-induced SOP variations. Experimental results demonstrate that the proposed scheme enables vibration sensing based on both phase and SOP across a frequency range of 200 Hz to 10 kHz. Regarding the communication performance, the integration of the sensing system only induces 0.8 dB received optical power penalty. This vibration-sensing scheme based on both phase and SOP can be integrated into pluggable optical modules, providing an efficient and reliable solution for intelligent optical network O&M.

Photonics doi: 10.3390/photonics13060571

Authors: Xiyu Liu Xin Zhang Junliu Fan Baohua Chen An Xu

Sparse aperture optical systems employ separated subapertures to achieve equivalent large aperture high resolution imaging, but they are also more susceptible to the combined effects of cophase errors and polarization aberrations, which lead to point spread function distortion and image quality degradation. To address this problem, this paper proposes a Polarization Phase Diversity (PPD) method for cophase errors and polarization aberrations in sparse aperture optical systems. Based on the Jones pupil theory, a unified imaging model incorporating both cophase errors and polarization aberrations is established. By combining focused and defocused images acquired under different analyzer directions, a polarization multichannel observation framework is constructed to jointly retrieve piston, tilt, diattenuation, and retardance parameters. Numerical simulations are mainly carried out on a Golay3 sparse aperture optical system, and additional Gaussian-noise and Golay6 simulations are performed to evaluate the robustness and extensibility of the proposed framework. The results show that, except for the phase-equivalent ambiguity of the piston term, the proposed method can accurately recover the cophase and polarization-aberration parameters under noise-free conditions. Compared with conventional methods considering only cophase errors, the proposed method achieves better image restoration performance in terms of image sharpness, detail recovery, PSNR, and SSIM. The noise experiments further demonstrate that the full-parameter restoration maintains clear advantages under noisy observations, and the Golay6 results indicate that the framework can be extended to sparse aperture configurations with more subapertures. The proposed method provides an effective approach for error sensing and image restoration in sparse aperture optical systems.

Photonics doi: 10.3390/photonics13060570

Authors: Jie Yang Yulin Shen Mingzhe Li Yi Zhang Ke Zhang Dehui Pan Jiahui Yao Ming Xin

Thin-film lithium tantalate (LiTaO3) is a promising platform for integrated nonlinear photonics owing to its strong second-order nonlinearity, broad transparency window, and favorable photorefractive resistance. Here, we numerically investigate cascaded fourth-harmonic generation in quasi-phase-matched thin-film LiTaO3 waveguides, targeting ultraviolet generation at 387.5 nm from a 1550 nm pump. Three poling schemes, including square-wave periodic poling, generalized quasi-periodic superlattice (GQPS), and dual-period poling (DPP), are designed to simultaneously compensate the phase mismatch of second-harmonic generation and fourth-harmonic generation. Under a 10 mW average input power and 250 fs pulse duration, the simulated fourth-harmonic conversion efficiencies reach 42.7%, 35.7%, and 57.1%, respectively. The DPP structure provides the highest efficiency by supporting both nonlinear processes with relatively strong low-order reciprocal–vector components. The influence of waveguide geometry errors and temperature tuning is further analyzed, showing that the fourth-harmonic process is more temperature-sensitive than the second-harmonic process. In addition, the feasibility of extending this scheme toward 300 nm deep-ultraviolet generation is discussed. These results provide a design route for compact, efficient, and fabrication-compatible on-chip ultraviolet sources based on thin-film LiTaO3 nonlinear photonics.

Photonics doi: 10.3390/photonics13060569

Authors: Tingting Qin Yang Tu

Indoor visible light communication (VLC) has attracted increasing attention as a promising wireless access technology because of its large unlicensed bandwidth and dual functionality of illumination and data transmission. However, practical VLC systems are vulnerable to line-of-sight (LoS) blockage caused by user mobility, human shadowing, and indoor obstacles, which may degrade link reliability and service continuity. Although hybrid VLC/RF networks can improve robustness by using RF transmission as a backup link, excessive RF fallback under severe optical blockage may overload the bandwidth-limited RF interface and reduce the service quality of RF-associated users. To address this issue, this paper investigates a risk-aware illumination-constrained resource allocation scheme for hybrid VLC/RF indoor networks under random optical blockage. A unified system model is developed by considering Lambertian optical propagation, random optical blockage, RF backup transmission, and working-plane illumination constraints. Based on this model, a joint user association and power allocation problem is formulated under QoS, transmit-power, and illumination requirements. The proposed scheme evaluates VLC service utility under blockage uncertainty, controls RF fallback to avoid excessive backup-link loading, allocates VLC/RF transmission power, and performs illumination feasibility adjustment to preserve the required lighting level. Simulation results show that, under severe blockage conditions, the proposed scheme reduces the outage probability to approximately 0.26, compared with 0.68 for VLC-only transmission and 0.47 for threshold-based VLC/RF switching. For a 20-user network, the proposed scheme achieves an average sum rate of approximately 277 Mbps, maintains a 100% illumination compliance ratio, and achieves higher energy efficiency than the benchmark schemes. Further RF backup analysis shows that the proposed scheme can maintain the service quality of RF-associated users by avoiding excessive RF fallback. These results demonstrate the effectiveness of the proposed framework for reliable and illumination-feasible hybrid VLC/RF indoor communication.

Photonics doi: 10.3390/photonics13060568

Authors: Rui Zhou Han Li Xiaoqin Wei Haitao Zhu Xu Zhou Xiaojie Li Rihui Yao Wei Xu Honglong Ning Junbiao Peng

Accurate Remaining Useful Life (RUL) prediction of TFT-LCD modules is critical for industrial predictive maintenance, yet it remains heavily challenged by complex degradation mechanisms in different climates. Traditional purely data-driven models (SVR, LSTM) often lack physical interpretability, struggling to filter out environmental noise or predict irreversible failures. To address this, we propose a highly reliable prognostic tool based on a Physics-Informed Gaussian Process Regression (PI-GPR) framework, by embedding cumulative thermal load and thermo-mechanical stress into the model’s prior function. Evaluated using one-year field exposure data, the physical constraints empower the model to accurately predict device lifetime under highly variable environments, including luminance fluctuations in tropical hygrothermal conditions and device failures in cold environments. Quantitative results demonstrate that the unified PI-GPR framework achieves an outstanding coefficient of determination (R2 = 0.93) and reduces the RUL prediction error to merely 7.5 days, significantly outperforming conventional shallow learning, deep sequence, and standard probabilistic baselines. Ultimately, this study provides a robust, physically grounded methodology for the health monitoring and life cycle management of display modules in practical industrial applications.

Photonics doi: 10.3390/photonics13060567

Authors: Haiyang Ji Yang Chen Guangliang Sun Ziyuan Liao Yunzhi Zhu Yongtao Wu Yufei Wang Wanhua Zheng

Surface-emitting distributed-feedback (SE-DFB) semiconductor lasers based on second-order gratings face a fundamental triple constraint: the spatial co-location of gain, grating feedback, and vertical radiation functions limits single-mode selectivity, surface extraction efficiency, and far-field beam quality simultaneously. We propose a quasi-parity-time (PT)-symmetric SE-DFB laser with separated gain and radiating-grating sections. In this design, the electrically injected gain section and the passive second-order grating section are placed in different regions along the cavity axis, thereby separating electrical injection from surface emission without epitaxial regrowth. Coupled-mode theory and two-dimensional finite-element simulations demonstrate that the resulting longitudinal non-Hermitian gain–loss asymmetry produces spatial-overlap-dependent threshold discrimination, enabling an isolated low-threshold lasing branch that remains separated from competing cavity modes over the investigated pump-parameter range. Under the HR–AR boundary condition, the proposed design achieves a threshold gain margin of Δg=12.4cm−1, more than six times that of a conventional HR–AR DFB benchmark considered here, together with an upward surface extraction efficiency of 23.4% obtained from 2D FEM simulations. A simplified steady-state rate-equation estimate further suggests that the increased threshold margin can support strong side-mode suppression. The design imposes no regrowth requirement and is fully compatible with standard single-growth InP ridge-waveguide fabrication.

Photonics doi: 10.3390/photonics13060565

Authors: Zeyou Li Han Yang Fei Xu Peng Wang Yihong Qi

Optical nonreciprocity (ONR) plays an important role in laser technique, optical communications, quantum information, etc. Realizing ONR at the quantum level of few photons or single photons is also essential for quantum communications or quantum networks. In this work, we propose a hybrid configuration composed of a quantum dot–metallic nanoparticle (QD-MNP) composite in a ring cavity to achieve ONR at the few-photon level via optical bistability. With the surface plasmon effect of the MNP, the bistable property and regime of photons producing ONR in an asymmetric ring cavity including a QD and an MNP inside can be significantly improved in comparison with the case of a single QD. By using the bistability effect, giant ONR can be achieved in an optimal window of numbers of input photons. The detuning and the coupling strength coefficients of the hybrid system can be adjusted and utilized to optimize the performance of the quantum nonreciprocity. This work may find promising applications in quantum nonreciprocal devices and photonic quantum circuits.

Photonics doi: 10.3390/photonics13060566

Authors: Xinning Huang Zilong Che Junxia Wu

Terrestrial free-space laser communication will play a pivotal role in future 6G networks, owing to its high bandwidth and low latency advantages. However, atmospheric turbulence degrades link performance due to intensity scintillation and phase distortion. This paper proposes a hybrid modulation scheme that combines differential pulse position modulation (DPPM) with differential quadrature phase-shift keying (DQPSK). The scheme exploits the power efficiency of DPPM and the spectral efficiency of DQPSK, while differential phase detection eliminates the need for carrier recovery and enhances robustness to phase noise. A theoretical bit error rate (BER) model is derived and validated under Gamma–Gamma and Log-normal turbulence channels. Results show that the 4DPPM-DQPSK configuration achieves BER up to several orders of magnitude lower than standalone DPPM or DQPSK under weak-to-moderate turbulence, and improves spectral efficiency by 50% relative to 4DPPM. Packet error rate analysis further confirms its reliability for high-capacity terrestrial FSO links.

Photonics doi: 10.3390/photonics13060564

Authors: Tjorven Hilbert Florian Azendorf Hamzeh Beiranvand Johannes Diers Marco Liserre Annika Dochhan Stephan Pachnicke

The concept of Brillouin optical time domain reflectometry as a means of distributed temperature monitoring for battery systems is investigated and deemed feasible. The concept has been investigated regarding measurement speed, temperature accuracy, measurement range, and responsivity of the fiber material. The experiments show fast and accurate measurement results for surfaces with a uniformly distributed temperature, with an average absolute error of 0.51 K, a largest absolute error of 2.07 K, and a measurement time of approximately 60 s for a sampling point after 5 km. Additionally, hotspots only impacting 25% of the sensor fiber have been detected. Furthermore, the established concept provides a reliable and scalable solution for simultaneous temperature monitoring of spatially distributed sampling points without the need for individual cabling of every measurement sensor, like in commonly deployed electrical temperature detectors.

Photonics doi: 10.3390/photonics13060563

Authors: Jilun Zhao Haibo Wang

Ghost imaging, ghost interference, and ghost diffraction retrieve an object’s spatial distribution and interference–diffraction patterns via intensity correlation. Flexibly synthesizing multi-channel optical information within a single correlation architecture is a key challenge in the evolution of optical correlation from fundamental research to information processing platforms. This study proposes the Extended Operational Ghost Correlation Model (EO-GCM), which introduces the four arithmetic operations (addition, subtraction, multiplication, and division) into optical correlation data processing. Within circular complex Gaussian pseudothermal light fields, the study systematically derives the analytical expressions for the second-order intensity fluctuation correlations with these operations. The theory shows that addition and subtraction obey superposition, whereas for multiplication and division, the average intensity of one object path becomes a weighting factor for the information of the other path. When weakly correlated, multiplication yields a weighted sum, whereas division yields a weighted difference, with the two weights having opposite signs. Experiments on ghost interference or diffraction and ghost imaging verify theoretical predictions and confirm the proportionality between the absolute value of the negative weight in division and the average intensity of the numerator path. The proposed model enables basic operations on multipath object signals, endowing optical correlation systems with reconfigurable, weighted correlation fusion-based information modulation capabilities.

Photonics doi: 10.3390/photonics13060562

Authors: Changtao He Kai Liu Zhenyu Liu Yongkang Wu Jinghua Han

Laser-induced damage of sol–gel SiO2 antireflection coatings remains a key reliability issue in high-power laser systems because porous networks, residual hydroxyl groups, and defect-related absorption centers can trigger localized heating and stress concentration under nanosecond irradiation. In this work, continuous-wave CO2 laser conditioning was used as a localized post-treatment method to regulate the microstructure of sol–gel SiO2 coatings on fused silica substrates. The revised manuscript clarifies the processing window, scanning parameters, laser damage testing protocol, and the sample-specific nature of the reported LIDT values. Laser conditioning induces partial densification of the porous coating, dehydration of Si-OH groups, relaxation of the Si-O-Si network, and enhancement of mechanical properties. Under the optimized conditioning condition, the surface roughness decreases from 14.08 nm to 9.76 nm, and the LIDT at 1064 nm increases from 4.8 J/cm2 to 7.0 J/cm2. The LIDT values are discussed as a relative microstructure–property comparison for the present coating system rather than as the upper technological limit of sol–gel silica coatings. Combined FTIR analysis, thermal simulation, morphology observation, and damage probability analysis indicate that the improvement originates from the combined effects of reduced defect absorption, moderated porosity, improved heat dissipation, and enhanced resistance to thermally induced cracking. The results provide a mechanism-guided strategy for using CO2 laser conditioning to tune sol–gel silica coatings while also identifying the need for further validation on higher-LIDT coatings and at application-relevant wavelengths.

Photonics doi: 10.3390/photonics13060561

Authors: Chun-Hao Chen Chun-Hsiung Lin Hao-Chung Kuo Yu-Heng Hong Ching-Yao Liu Kai-En Lin Yueh-Tsung Shieh Shyr-Long Jeng Edward-Yi Chang Wei-Hua Chieng

A quasi-continuous-wave (QCW) laser module based on a half-bridge structure is proposed for the low-voltage silicon photonics application, which forms a continuous-wave (CW) laser output when it equally distributes the heat dissipation into all lasers. Such a QCW laser module is modulated into a CW laser source for the chip-to-chip or board-to-board communication. The source current is alternatively diverted to the high-side and the low-side lasers by turning the corresponding gallium nitride high-electron-mobility transistor (GaN HEMT) on and off. The current redirection modulates multiple QCW laser outputs into a CW laser output; however, an undesirable laser downtime is produced during the transition time of the current redirection. Although for the 10 Gbps data rate transmission, a short laser downtime period may be scheduled for the time to perform either the laser steering task of the free-space optics (FSO) operation or the data pause for the fan-out delay, which is still preferred to be minimized for higher data rate transmission. The power efficiency and the laser downtime are functions of the parameters of the laser diodes, switch parasitic capacitances, input voltage, and the inductor. According to the mathematical derivation of the circuit response, the circuit design rules and the switching control strategy are provided to achieve high efficiency and low laser downtime. In the experiment, we implemented a laser module to achieve an FSO specification with a laser downtime of less than 3 ns, total harmonic distortion (THD) less than 10%, power efficiency greater than 60% and laser power higher than 1 W.

Photonics doi: 10.3390/photonics13060560

Authors: Olga Matveeva Kirill Voronin Maria Titova Sergey Chikalkin Andrey Vyshnevyy Aleksey Arsenin Valentyn Volkov

Miniaturization of photonic integrated circuits is a long-standing problem in optical engineering. Nowadays, the most promising material platform for integrated photonics are anisotropic van der Waals materials due to overcoming the light diffraction limit. Here, we numerically study V-groove channel waveguides formed in a 50 nm-thick slab of the in-plane hyperbolic in visible and near-infrared ranges of van der Waals material, MoOCl2. At the telecom wavelength 1550 nm, a channel supports a guided mode with an effective index 1.0206 and a decay length of 13.7 µm. We also design a Mach–Zehnder-type interferometric layout with a maximum splitter angle of approximately 7° for demonstration of a possible practical application in a telecom range and in-plane angular channel modes’ propagation characteristics. We demonstrate that using MoOCl2 instead of gold leads to a ten-fold reduction in the linear dimensions of the photonic integrated circuit. Therefore, we envision that by combining the extraordinary material properties of MoOCl2 with the V-shaped geometry of waveguides, one can make the integration density of photonic devices close to that of electronics.

Photonics doi: 10.3390/photonics13060559

Authors: Md Ghulam Saber Zhiping Jiang

CO2 gas-line absorption is emerging as a major L-band impairment in low-loss hollow-core fiber (HCF) links. We compare two transponder-side mitigation strategies—spectral pre-emphasis and spectral avoidance—over span lengths of 100–300 km and transmission reach of up to 3000 km. The preferred strategy depends on reach, launch power, span length, and the stability of the live-link absorption comb. Pre-emphasis is favored at short reach and for short spans, whereas spectral avoidance is superior at moderate to long reach, with a peak capacity gain of about 4 Tb/s. Pre-emphasis is also more sensitive to mismatch between the design-time and live-link absorption combs: increasing the live absorption peak from 0.10 to 0.35 dB/km reduces capacity by up to 8.5 Tb/s, while tripling the CO2 absorption linewidth reduces capacity by up to 10.3 Tb/s. We further review implementation options for both methods: DGFF-based pre-emphasis at the WSS sites, and DSP-based avoidance via digital subcarrier multiplexing (DSCM) or entropy-loaded orthogonal frequency-division multiplexing (OFDM). These results provide a concise framework for selecting mitigation strategy under realistic operating conditions.

Photonics doi: 10.3390/photonics13060558

Authors: Zhichao He Yannian Meng Dengke Guo Yuanbo Dai Yachen Liu Xiao Chen

Traditional wireless communication signals are often susceptible to physical obstructions and background noise in complex geographical environments or adverse weather conditions, hindering stable and reliable data transmission. Ultraviolet communication (UVC) offers a compelling solution; its unique scattering mechanism and low background noise characteristics facilitate robust communication under non-line-of-sight (NLOS) conditions. At present, there remains a relative lack of comprehensive reviews spanning UVC, including fundamental theory, physical devices, channel models and networking technologies. This review synthesizes the current state of global research, providing a systematic overview of the background, advantages and application scenarios of UVC. It examines the hardware characteristics of light sources and detectors, evaluates NLOS scattering channel models, analyzes key signal processing techniques, including modulation/demodulation, coding/decoding and multiple-input multiple-output technology. Furthermore, this review conducts an in-depth analysis of multi-user networking protocols and three-dimensional topology control mechanisms. Finally, it identifies the prevailing technical challenges and outlines promising directions for future development.

Photonics doi: 10.3390/photonics13060557

Authors: Anqi Leng Guangmang Cui Yan Chen Jianhua Mo Weize Cui Lize Fang Zhanhong Liu Jufeng Zhao

Recovering high-quality images from low-quality speckle patterns remains a core challenge in scattering imaging, especially under narrowband illumination with ambient light interference and broadband illumination. This paper proposes a dual-input synchronous transmission architecture: after Correction-Smoothing-Phase Optimization (CSPO) preprocessing, two data streams are parallel-fed into Phase-Aligned Coherent Summation (PACS) for efficient and high-precision reconstruction with adaptive fusion, breaking the single-path limitation of traditional methods and balancing imaging efficiency and quality. Additionally, an adaptive enhancement factor feedback mechanism is designed for Median-Unsharp Sharpening Enhancement (MUSE) to dynamically adjust Median Filtering (MF) and Unsharp Masking (USM) parameters, achieving adaptive balance between noise suppression and detail enhancement and improving robustness under extreme lighting. In PACS, a dynamic reference update mechanism is introduced, combined with fixed amplitude to realize iterative phase optimization, effectively suppressing speckle noise and boosting the signal-to-noise ratio of reconstructed images. Experimental results show that the proposed method achieves favorable restoration performance even at a SNR of −8.7 dB under narrowband and broadband illumination with spectral bandwidths of 100 nm, 200 nm, and 280 nm (FWHM), and significantly improves image quality in unknown scattering media, showing great potential for robust speckle reconstruction.

Photonics doi: 10.3390/photonics13060556

Authors: Yang Yang Ke Ma Ruyi Yu Daofu Han

Fiber dispersion causes pulse broadening and signal distortion. Existing dispersion compensation approaches depend on standardized dispersion parameters at specific wavelengths (e.g., 1550 nm), which often mismatch actual fiber dispersion, leading to residual dispersion. We develop a Sagnac ring interferometry and electro-optic modulation system, combined with machine learning, to accurately characterize the C-band dispersion curve of a G.652D fiber, and inversely design a chirped fiber Bragg grating (CFBG) for tailored compensation. However, when attempting to quantify the residual dispersion numerically, conventional differentiation methods yield physically implausible results. Monte Carlo simulations confirm this fundamental unreliability, yielding a 95% confidence interval of 319,605 ps/(nm·km). To circumvent this limitation, we propose a joint evaluation method based on refractive index flatness and group delay uniformity. Within 1545–1555 nm, both indicators fluctuate by no more than 0.015% relative to their means, confirming that residual dispersion has been effectively suppressed. This approach provides a precise, personalized compensation mechanism applicable to optical fibers with individual dispersion characteristics, offering a controllable path for adaptive dispersion compensation in high-speed communication systems.

Photonics doi: 10.3390/photonics13060555

Authors: Yong Du Yi-Jie Li Wei-Min Chi Yu-Chen Tsai Cheng-Fu Yang

A structurally simple three-layer optical absorber is proposed and systematically investigated, consisting of a continuous Ti ground plane, a SiO2 dielectric spacer, and a Ti tetrahedral nanostructure. The absorber is constructed on a periodic square unit cell, where the lateral dimension directly determines the base width and sidewall inclination angle of the tetrahedral structure, thereby enabling effective modulation of the optical response. Full-wave electromagnetic simulations performed using COMSOL Multiphysics (version 6.0) are employed to evaluate the influence of geometric parameters on broadband absorption behavior. The optimized structure achieves a near-unity absorptivity of 0.9999 at 200 nm and maintains an effective absorption bandwidth (absorptivity > 0.9) spanning 200–3000 nm, covering the ultraviolet, visible, and near-infrared spectral regions. Parametric analysis reveals that the tetrahedral height primarily governs long-wavelength extension through enhanced optical path length, graded-index transition, and improved electromagnetic field confinement, while the unit cell width strongly influences impedance matching and localized field localization. In contrast, the Ti ground layer thickness exhibits minimal influence once it exceeds the optical skin depth, confirming its primary role as a transmission-blocking reflective substrate. Impedance retrieval analysis shows that the real part of the normalized impedance remains close to unity and the imaginary part approaches zero over most of the operating range, demonstrating that the ultrabroadband absorption behavior is dominated by effective impedance matching rather than isolated narrowband resonances. Furthermore, electric and magnetic field distribution analyses reveal that electromagnetic energy dissipation is concentrated near the tetrahedral apex and metal–dielectric interfaces, indicating the coexistence of localized plasmonic modes, cavity-assisted absorption, and multi-scale optical confinement.

Photonics doi: 10.3390/photonics13060554

Authors: Jingmin Luan Yifei Xie Xu Zhang Yurui Wu Jian Liu Yao Yu Zehao Wei Zhenhe Ma

Optical coherence tomography angiography (OCTA) is a depth-resolved, label-free optical imaging modality that uses motion contrast from repeated B-scans to reconstruct retinal microvasculature and provide co-registered en face views of the superficial and deep vascular plexuses (SVP and DVP). This capability is valuable for early diabetic retinopathy (DR) assessment, where deep-plexus perfusion deficits may precede clinically evident disease. However, microvascular differences among healthy controls, diabetic eyes without clinically apparent retinopathy, and mild DR are subtle and unevenly distributed across the two vascular slabs, while most deep learning methods prematurely fuse the plexuses and weaken depth-specific evidence provided by OCTA. To address this, we propose Class-Path Specific Representation Distillation and Reasoning (CPS-RDR), an interpretable framework that aligns model reasoning with the layered organization of OCTA. A frozen DINOv2-initialized dual-branch Vision Transformer preserves separate SVP and DVP representations, while class- and path-conditioned diagnostic queries instantiate four reasoning pathways for layer-specific evidence extraction and directional cross-plexus interaction. A lightweight EvidenceFusion head linearly integrates pathway-wise evidence, enabling final predictions to be decomposed into pathway-specific contributions. On 99 eyes from 55 participants, CPS-RDR achieved 97.29% accuracy, 0.9932 macro-AUC, and 0.9829 macro-F1 under five-fold patient-level cross-validation, outperforming seven representative baselines, while producing path-resolved maps that reveal how superficial- and deep-layer optical signals jointly support early DR stratification.

Photonics doi: 10.3390/photonics13060553

Authors: Saad Rustum Usman Habib Muhammad Irfan Muhammad Avais Qureshi Muhammad Ijaz Jayaprasath Elumalai

Radio-over-Fiber (RoF) integrated with Free-Space Optical (FSO) communication as a fronthaul is a promising solution for next-generation wireless systems, but severely suffers from the frequency-selective characteristics of hybrid RoF-FSO channels. This paper presents a measurement-driven, deployment-oriented optimization that jointly performs structure-aware pilot placement and sixth-order polynomial regression channel interpolation to enhance spectral efficiency and signal quality in quasi-static indoor FSO environments. Differential channel analysis across three transmission scenarios—Electrical Back-to-Back (B2B), Fiber B2B, and FSO—identifies critical subcarriers with high frequency-selective variation that require dense pilot allocation. A gradient-based algorithm positions 50 pilots with dense spacing (every 3 subcarriers) in critical regions and sparse spacing (every 9 subcarriers) in stable regions, reducing pilot overhead by 26.5% and increasing data capacity by 5.3% (340 → 358 subcarriers) compared to uniform placement of 68 pilots. Sixth-order polynomial regression models the non-linear channel frequency response, overcoming limitations of conventional linear interpolation. Experimental validation on a 4-QAM RoF-OFDM system over 40.6 MHz bandwidth shows that structure-aware pilot placement alone reduces Error Vector Magnitude (EVM) by 15.9%, while polynomial regression alone improves it by 15.7%. Combined optimization of structure-aware pilot placement with polynomial regression interpolation achieves 23.5% EVM reduction and 460× lower BER, equivalent to 3.2 dB SNR gain at BER = 10−6. Comparative analysis of four system configurations confirms consistent performance advantages across SNRs of 12–30 dB. The proposed measure-once, optimize-forever paradigm requires only one-time channel characterization, making it suitable for short-range controlled quasi-static indoor FSO links in 5G/6G fronthaul, optical wireless networks, and inter-building backhaul applications.

Photonics doi: 10.3390/photonics13060552

Authors: Maryam Dehbashizadeh Chehreghan Ripalta Stabile

We demonstrate a loop-based deep optical convolutional neural network that reuses a single free-space optical hardware to realize network depth through repeated passes. Convolution is implemented with programmable SLM with Fourier plane kernels, nonlinearity is provided by the photorefractive phase-only response of a BSO crystal and converted to an effective intensity activation via spatial filtering, and pooling is performed optically using demagnified imaging with an iris. On MNIST, the BSO-based nonlinearity improves test accuracy from 90.8% (linear) to 95.7%, with optimal operation. We model realistic optical noises (laser fluctuation, aberration, detector misalignment, and dust) and compare them using an SSIM-normalized severity metric. Under noise at (s = 0.35) on Fashion-MNIST, accuracy drops from 88.53% (clean) to 79.5% (noisy inference); a feature-level noise-aware training strategy recovers performance to 86.87%. Together, these advances demonstrate that a compact, loop-based hybrid DOCNN, completed with simple optical nonlinearities, simplified pooling, and noise-aware learning, can improve accuracy under realistic conditions.

Photonics doi: 10.3390/photonics13060551

Authors: Naomi L. Gaggi Xianfeng Shi SaraRose Shannon Ryan Brown Katherine A. Collins Perry Renshaw Ricardo S. Osorio Dan V. Iosifescu

Introduction: Transcranial photobiomodulation (t-PBM) is a non-invasive metabolic neuromodulation technique intended to enhance cerebral bioenergetics by stimulating mitochondrial activity. To characterize both baseline metabolic vulnerability and real-time metabolic engagement during stimulation, this preliminary study integrated phosphorus magnetic resonance spectroscopy (31P-MRS) with resting-state fMRI. Methods: Eleven individuals with mild cognitive impairment (MCI) or early Alzheimer’s disease underwent 31P-MRS to quantify baseline cerebral metabolism (PCr/Pi, pH), followed by MRI sessions during which t-PBM was applied over bilateral frontal sites. Fractional amplitude of low-frequency fluctuations (fALFF), a resting-state index strongly associated with cerebral glucose metabolism, was used as a real-time proxy of metabolic change during stimulation. Results: Linear regression analyses indicated that lower baseline PCr/Pi and lower pH, markers of impaired oxidative metabolism, predicted greater increases in fALFF during t-PBM, most prominently in the right frontal pole (FP2) and, to a lesser extent, right dorsolateral prefrontal cortex (F4). While greater dementia severity also predicted larger fALFF responses in select regions, our findings suggest that t-PBM can boost metabolism in some brain regions where it is compromised, but that this may be independent of cognitive function in early AD/MCI. These findings suggest that t-PBM may preferentially engage brain regions with reduced metabolic capacity to exhibit stronger acute responses. Discussion: Overall, these hypothesis-generating results support the combined use of 31P-MRS and fALFF as complementary biomarkers to quantify baseline metabolic status and real-time target engagement. A single session of t-PBM produced neural activity changes consistent with partial metabolic normalization in vulnerable cortical regions. As these results are preliminary, ongoing longitudinal work with a larger cohort will determine whether baseline metabolic profiles and acute fALFF responses predict clinical outcomes after repeated t-PBM treatment.

Photonics doi: 10.3390/photonics13060550

Authors: Teng Wang Jiahui Hu Xiaofeng Han Jingang Chen Jichao Wang

Conductor on Round Core (CORC) high-temperature superconducting cables are critical for next-generation magnetic confinement fusion devices, yet their performance is highly sensitive to geometric variations, necessitating high-precision non-contact inspection. This study presents a laser triangulation measurement system that integrates an optical structural design scheme and a deep learning-based light-plane calibration method. For optical structural design, we established an imaging model incorporating thin-lens geometry and the Scheimpflug condition, formulated an objective function balancing minimum sensitivity and sensitivity stability, and employed a grid-search strategy under engineering constraints to determine optimal parameters. For calibration, we proposed a deep learning-based light-plane calibration method that predicts point-wise weights for laser point clouds extracted during the calibration process and incorporates them into weighted fitting to improve robustness against noise and outliers. Experimental results demonstrate that the optimized optical structure reduces system measurement error and error variation, while the proposed light-plane calibration method further improves measurement accuracy and repeatability. Together, these enhancements decreased system nonlinearity error from 0.079% to 0.064% compared to conventional settings. Engineering applicability was validated by comparing measured CORC cable diameters with precision micrometer references. The proposed framework may provide a methodological reference for the design and calibration of laser triangulation measurement systems under similar geometric configurations and engineering constraints.

Photonics doi: 10.3390/photonics13060549

Authors: Yuchuan Zhao Zhenhua Su Yiran Zhao Hao Wang Haifeng Zhang Nanxing Yan Chao Mei Haifeng Xiao

Based on vector wave aberration theory, this paper analyzes the relative positions of the third-order coma node and astigmatism nodes under pupil decenter and proposes an initial structure selection criterion in which the coma node coincides with the geometric midpoint of the two astigmatism nodes. Using this criterion, an F/6 off-axis catadioptric telephoto optical system with a focal length of 900 mm and an entrance pupil diameter of 150 mm was designed. The measured on-axis RMS wavefront error was better than 0.025λ at 632.8 nm. The results demonstrate that the system meets the requirements for high-resolution long-focal-length imaging.

Photonics doi: 10.3390/photonics13060548

Authors: Zekun Li Penghui Xin Haoyu Zheng Yu Zheng Leonid F. Chernogor Zhejun Jin Tian Liu

Current terahertz (THz) metasurfaces are often constrained by fixed operational states, lacking the flexibility to switch dynamically between transmission and reflection modes. To address this limitation, we propose a tunable coded metasurface based on the photo-adjustable conductivity of silicon, enabling seamless mode switching and versatile wavefront manipulation. By leveraging the photo-induced dielectric-to-metallic transition, the device functions as a high-efficiency transmission-type polarization converter under zero pump fluence, transforming incident X-polarized waves into Y-polarized waves across a broad frequency range of 0.85–1.5 THz, with a polarization conversion ratio (PCR) exceeding 99%. Upon excitation by 800 nm near-infrared laser pulses, the metasurface transitions to reflection mode, where it simultaneously achieves linear polarization conversion and generates dual-channel orbital angular momentum (OAM) beams through a phase-coding strategy integrated with Fourier convolution. Furthermore, by employing the Gerchberg–Saxton (GS) algorithm to optimize the phase profile, holographic reconstruction is realized in the far field. This design integrates diverse manipulation capabilities into a single, dynamically controllable platform, offering a promising technological approach for THz information processing and integrated photonic systems.

Photonics doi: 10.3390/photonics13060547

Authors: Guoxi Huang Ri Yan Wenjia Li Fan Zhang Tigang Ning Li Pei

High-power 355 nm ultraviolet (UV) lasers, leveraging their short wavelength, high photon energy, and high absorption across a broad range of materials, have become indispensable light sources for precision manufacturing, semiconductor processing, and laser direct imaging (LDI). In this paper, we demonstrate a high-power 355 nm UV laser system based on a narrow-linewidth polarization-maintaining (PM) Yb-doped fiber laser and cascaded frequency conversion. A single-frequency semiconductor laser is employed as the seed source, with its spectral linewidth broadened to 0.32 nm (full width at half maximum, FWHM) via phase modulation to suppress stimulated Brillouin scattering (SBS). Through a PM master oscillator power amplifier (MOPA) architecture, a maximum average output power of 899 W at 1064 nm is achieved with a beam quality factor of M2 = 1.12 (M2x = 1.11, M2y = 1.13). By employing lithium triborate (LiB3O5, LBO) crystals for extracavity cascaded second-harmonic generation (SHG) and sum-frequency generation (SFG), a maximum green output power of 613.7 W at 532 nm is obtained, corresponding to a SHG conversion efficiency of 68.2%, and a maximum UV output power of 227.1 W at 355 nm is achieved, with a total conversion efficiency of 25.2%. At the maximum output power, the UV beam quality factors are M2 = 1.16 (M2x = 1.24 and M2y = 1.09), and the power fluctuation is better than ±1.5% root-mean-square (RMS) over 8 h of continuous operation. These results indicate that the cascaded frequency conversion approach based on narrow-linewidth PM fiber lasers possesses the capability for further scaling to higher-power single-path high-brightness UV output and can provide high-brightness UV sources for applications such as flexible printed circuit (FPC) laser cutting, flat-panel display laser direct imaging, and semiconductor wafer scribing.

Photonics doi: 10.3390/photonics13060546

Authors: Siran Li Xiaotian Lu Yinghui Zhang Yafang Zou Yanning Ma Li Cao

Optically pumped magnetometers (OPM) are core quantum payloads for geomagnetic remote sensing. Among them, the spin-exchange relaxation-free (SERF) OPM with aT-level ultimate sensitivity stands as mainstream. While enlarging the alkali-metal vapor cell of the SERF OPM enhances sensitivity, it triggers complex atomic spin relaxation, notably intensified magnetic field gradient relaxation. To address the dilemma of atomic spin relaxation regulation and the engineering requirements of ultra-high-sensitivity SERF magnetometers, this paper constructs an analytical model of the total relaxation rate that comprehensively considers wall-collision relaxation, spin-destruction collision relaxation, and longitudinal/transverse magnetic field gradient relaxation, etc. The analytical relationship between buffer gas pressure and total relaxation rate for a commonly used spherical vapor cell is derived, revealing the intrinsic correlation among cell size, atomic spin relaxation, and optimal pressure. Based on the theoretical model, the filling parameters of the vapor cell are optimized, and experimental measurements are carried out. The theoretical relaxation results are highly consistent with the experimental ones, realizing the precise optimization of buffer gas pressure. The optimization method proposed in this paper provides a theoretical basis and parameter guidance for the engineering preparation of alkali-metal vapor cells for high-sensitivity SERF magnetometers in remote sensing applications.

Photonics doi: 10.3390/photonics13060545

Authors: Yihua Qian Yaohong Zhao Qing Wang Kun Jia Guobin Zhong Huadan Zheng

Dissolved gas analysis (DGA) is an essential technique for the fault diagnosis and condition monitoring of oil-immersed power transformers. Among various characteristic gases, acetylene (C2H2) is a key indicator of high-energy discharge and arc faults. In this work, a high-sensitivity dissolved acetylene detection system is developed based on clamp-type quartz-enhanced photoacoustic spectroscopy (QEPAS). A specially designed clamp-type quartz tuning fork (Clamp-type QTF) is employed as the acoustic transducer to improve acoustic coupling efficiency and optical alignment tolerance. Compared with conventional standard quartz tuning forks, the clamp-type structure exhibits enlarged acoustic interaction volume, lower damping loss, and higher signal collection capability. A near-infrared distributed feedback (DFB) laser operating at 1531.6 nm is used as the excitation source. The dissolved gas is extracted from transformer oil using a headspace degassing module and introduced into the QEPAS cell for real-time measurement. Experimental results showed that the developed system achieves a 1σ-based SNR-estimated detection limit of 17 ppb at a 50 s integration time, derived from the continuous measurement of 0.75 ppm C2H2, with excellent linearity in the concentration range from 100 ppm to 500 ppm. The measured concentration of dissolved acetylene in transformer oil is in good agreement with gas chromatography (GC), validating the effectiveness and practical applicability of the proposed system.

Photonics doi: 10.3390/photonics13060544

Authors: Qinghe Sun Hui Zhao Teng Yang Shuai Wang Jiale Wang Xuewu Fan

To address limitations in the modulation-quality analysis of high-order Quadrature Amplitude Modulation (QAM) signals, including insufficient timing synchronization accuracy, challenges in carrier recovery, and coupling between synchronization errors and parameter estimation, a cascaded digital baseband processing framework tailored for measurement scenarios is proposed. The proposed framework is designed to integrate synchronization recovery and parameter measurement. In the timing synchronization stage, a feedforward open-loop structure based on the Oerder–Meyr (OM) algorithm is employed to estimate the optimal sampling instants rapidly. In the carrier synchronization stage, a two-stage recovery structure is constructed, comprising coarse frequency offset estimation based on polarity decision and fine synchronization using an improved frequency–phase detector (FPD), thereby achieving both robust acquisition of large frequency offsets and high-precision compensation of residual errors. On this basis, a unified modulation quality evaluation model is established, enabling the joint estimation of the Error Vector Magnitude (EVM) and the Modulation Error Ratio (MER), as well as amplitude, phase, and frequency errors, within a consistent analytical framework. System-level validation of 256 QAM and 1024 QAM signals is conducted using a MATLAB R2021b-based simulation platform. The results demonstrate that stable synchronization recovery can be achieved under timing, frequency, and phase perturbations, yielding well-defined constellation diagrams. In terms of parameter estimation, the relative errors of all evaluated metrics are maintained within 2%, which is significantly below the conventional 5% measurement criterion. Further analysis indicates that the proposed method maintains strong robustness across varying signal-to-noise ratios (SNRs) and sampling rates. The results confirm that the proposed cascaded processing framework effectively unifies synchronization recovery and modulation quality analysis, significantly improving parameter estimation accuracy while maintaining high synchronization precision. This approach provides a practical and efficient solution for high-order QAM signal testing and measurement systems.

Photonics doi: 10.3390/photonics13060543

Authors: Qiheng Wei Yongqiang Zhang Wei Li Fuli Tan Lingyuan Wu Zhaoning Li Yanglong Li Bo Fu

The laser irradiation effect on Charge-Coupled Devices (CCDs) has attracted wide attention in photoelectric countermeasures and imaging system hardening. This review provides a systematic analysis of the phenomena and mechanisms of laser-induced dazzling and damage effects on CCD sensors. It summarizes experimental and theoretical research progress with continuous-wave (CW), pulsed, and composite lasers, revealing distinct interaction mechanisms such as thermal effects, dielectric breakdown, and plasma ablation. The review also covers quantitative evaluation methods for assessing laser irradiation effects. This work provides a comprehensive reference for future studies.

Photonics doi: 10.3390/photonics13060542

Authors: Cheng Qian Shoubo Zhao Yi Liu Yue Yin Wenjie Chen

The unique nature of polarization information can provide a reliable physical prior for underwater multimodal image fusion. Existing methods mainly employ the integration of linearly polarized images from multiple directions, which is essentially an intensity fusion process of images. To solve this problem, we propose an underwater polarization imaging technology based on multi-polarization modality fusion. This method employs the total intensity S0 to provide the basic scene brightness, uses the degree of linear polarization (DoLP) as a physical prior, fully exploits the rich texture features in DoLP to compensate for S0, and integrates color information channels to better preserve the color characteristics of the scene. In addition, we develop a Polarization Feature Enhancement Module (PFEM) tailored for polarization data, which embeds a customized gating mechanism to select features and adaptively fuse feature vectors from different channels. Finally, we construct and publicly release an underwater polarization image dataset with multiple turbidity levels and materials, and systematically verify the robustness of the proposed method.

Photonics doi: 10.3390/photonics13060541

Authors: Zeng-Qiang Yang Tong-Le Wang Bing-Kun Zhan Da-Xin Wang Kai-Wen Zhang Xiao-Fei Zhang

We present a high-precision theoretical study of attosecond transient absorption spectroscopy (ATAS) of atomic hydrogen by numerically solving the time-dependent Schrödinger Equation (TDSE). A broadband extreme ultraviolet (XUV) attosecond pulse creates a wave packet of singly-excited bound states, which is subsequently probed by a time-delayed synthesized optical attosecond pulse (SOAP) with varying bandwidths and durations. When the SOAP has a narrow bandwidth (1.3–1.5 eV) and a long duration (~17 fs), the absorption spectrum exhibits conventional features, namely AC Stark shifts, half-cycle modulations (1.48 fs), and light-induced intermediate states, consistent with previous ATAS studies. In contrast, when the SOAP has a broad bandwidth (0.5–5.5 eV) and an attosecond duration (400 as), the dynamics are completely different. The spectrum reveals transverse wavelike modulations along the absorption lines and, remarkably, quantum beats with distinct frequencies, which are different from previous reports in hydrogen ATAS. To interpret these observations, we employ a dipole-control model. The model quantitatively reproduces the dominant modulation frequencies, identifying resonant couplings via two-photon processes (TPPs, 1.89 eV, period 2.18 fs) and three-photon processes (THPPs, 10.2 eV and 12.1 eV), as well as higher-order couplings. The validity of the δ-like pulse approximation is quantitatively assessed. The model remains accurate for pulse durations shorter than 700 as (bandwidth broader than 3.5 eV) but fails for longer pulses (exceeding 4 fs), where energy level splittings emerge. Our results demonstrate that the dipole-control model provides a reliable and intuitive framework for interpreting complex multiphoton interactions in ATAS, and highlight the unique capability of broadband SOAP probes to resolve attosecond-scale quantum beats inaccessible with conventional few-cycle infrared pulses.

Photonics doi: 10.3390/photonics13060540

Authors: Yunlong Yin Guanlin Li Hongyu Sun Jiayu Wang Jian Zhang Jianan Liu Qi Wang Yingchao Li Haodong Shi Mingce Chen

To meet the demand for high-precision target classification in complex scenes, a hyperspectral–polarimetric–LiDAR multimodal image fusion method tailored for few-shot scenarios is proposed. Feature-mapping functions for polarimetric and LiDAR images are constructed, and a multi-scale hierarchical optimization strategy is employed to jointly enhance low- and high-frequency components across modalities. This approach effectively addresses key challenges under limited training data, such as substantial cross-modal dimensional disparities and the difficulty of robust feature extraction and fusion. The proposed algorithm conducts bimodal image fusion on the NWPUSP spectral-polarization dataset and KAIST spectral-depth dataset. Compared with other fusion methods, it achieves average increases of 7.3% and 4.87% in information entropy, 53.18% and 30.35% in standard deviation, 48% and 108.28% in average gradient, as well as 96.25% and 101.13% in spatial frequency, respectively. Moreover, relying on the self-developed integrated hyperspectral-polarization imaging system and commercial LiDAR, we synchronously and efficiently acquire multimodal images including hyperspectral, polarization and LiDAR images of complex ground object scenes. Comparative experiments are implemented against six other mainstream fusion algorithms. The objective evaluation results show that the average improvements reach 7.19% in information entropy, 46.85% in standard deviation, 76.62% in average gradient and 79.74% in spatial frequency, which notably enhances the feature retention capability of fused images. Under few-shot conditions, the target recognition classification accuracy and Kappa coefficient of the fused image are improved by 9.8% and 11.05%, respectively, compared with those of the unimodal hyperspectral image. This effectively highlights targets under shadow occlusion and compensates for LiDAR’s response deficiencies to surface textures, achieving complementary advantages of multimodal images for ground object targets in complex scenes. This research provides a new solution for future optical multimodal remote sensing and image fusion.

Photonics doi: 10.3390/photonics13060539

Authors: Bin Ni Jia Che Yuanyuan Pan Xinwen Leng Qizhen Zhang Shengbao Wu Jichuan Xiong

As silicon photonics technology advances toward high-density integration scenarios—such as large-scale matrices, optical phased arrays, and optical neural networks—single-layer waveguide routing encounters severe topological challenges, rendering waveguide crossings indispensable fundamental components for constructing complex on-chip interconnect networks. As photonic hubs bridging distinct functional regions, the insertion loss, crosstalk, and bandwidth of these crossings directly dictate the signal integrity and transmission capacity of optical links. This paper systematically reviews recent research progress and key technologies concerning silicon-based waveguide crossings. Initially, the mechanism of scattering loss in direct crossings is elucidated, followed by a detailed examination of three mainstream design paradigms for loss mitigation: multimode interference (MMI) structures based on the self-imaging principle, adiabatic transformation structures relying on mode evolution, and medium engineering structures utilizing sub-wavelength gratings and metamaterials. Furthermore, the application of algorithm-driven inverse design in overcoming the constraints of traditional physical configurations is discussed. Crucially, addressing the urgent demand for ultra-high transmission capacity in the post-Moore era, this review highlights functional crossings capable of polarization division multiplexing (PDM) and mode division multiplexing (MDM), analyzing the design challenges and breakthroughs associated with multi-dimensional light field manipulation. Finally, this paper presents prospects for the future development trends of the waveguide crossing junction.

Photonics doi: 10.3390/photonics13060538

Authors: Adel S. A. Elsharkawi Amany A. Arafa Mohamed A. Swillam

This work presents a refined theoretical and numerical framework for shaping the axial intensity of finite-energy Bessel–Gaussian beams through programmable nonlinear phase modulation. Starting from the scalar Fresnel diffraction integral, we reformulate the propagation of a Gaussian-apodized axicon beam using a dimensionally consistent stationary-phase method. This analysis directly relates the radial phase gradient to the saddle-point trajectory, phase curvature, and on-axis intensity distribution. A Gaussian phase modulation (GPM) serves as a reference design to achieve a flattop axial profile while preserving the characteristic transverse Bessel ring structure. This work is validated against beam propagation simulations and previously reported spatial light modulator (SLM) experiments, confirming its accuracy within the paraxial regime. A parametric study then clarifies the scaling of wavelength, beam waist, axicon angle, and refractive index for extended focusing. Beyond standard GPM, several alternative nonlinear phase functions are systematically compared. High-performing profiles must replicate not only the amplitude scale but, more importantly, the radial phase-gradient structure of the Gaussian reference, which governs energy redistribution from annular regions to the axis. The results identify smooth, localized nonlinear functions as promising candidates for stable flattop Bessel beam generation. The proposed framework offers a flexible optical design for applications such as through-glass via (TGV) micromachining and light-sheet illumination, while prospective high-intensity laser plasma uses remain beyond the present linear model.

Photonics doi: 10.3390/photonics13060537

Authors: Xinyu Shi Chenhao Liu Xinyi Zang Ying Wang Huanjun Lu Can Huang Gaoyuan Chen Chunlan Ma

CsPbI3 exhibits multiple crystal phases, and the kinetic barriers for phase transitions are relatively low, facilitating the formation of abundant interphase boundaries (IBs) during phase transitions. These IB structures significantly influence the optoelectronic performance of the material. In this work, based on three types of CsPbI3 IB structures, we systematically investigate the effects of pressure on the formation feasibility and optoelectronic properties of these IBs by calculating their formation energies, band alignments, optical absorption characteristics, and carrier effective masses. The results show that moderate pressure can increase the formation feasibility of certain IB structures and effectively modulate the band alignment at the CsPbI3 IBs, thereby enabling the switching of different optoelectronic functions within the same material. Meanwhile, the application of pressure can also improve optical absorption and the spectroscopic-limited maximum efficiency, and reduce carrier effective masses in some IB systems, which is beneficial for enhancing carrier transport capabilities. This study demonstrates that pressure serves as an effective means to regulate the IB structures and optoelectronic properties of CsPbI3, providing theoretical support for the design of multifunctional optoelectronic materials based on IB engineering and for expanding photovoltaic applications.

Photonics doi: 10.3390/photonics13060536

Authors: Jinxu Fang Yanyan Qi Yan Liang Heping Zeng

The phenomenon of pulse tailing, primarily caused by relaxation oscillations, presents a significant challenge to increasing the repetition rates of gain-switched semiconductor lasers. This paper proposes a novel approach to mitigate this issue by simultaneously regulating both the magnitude and pulse width of the pump current, enabling stable, tail-free pulse generation across a broad range of repetition frequencies. Numerical solutions to the carrier rate equations are first employed to investigate the origins of optical pulse tailing. By reducing the current injection duration from 200 ps to 50 ps, carrier injection is effectively truncated, suppressing relaxation oscillations. However, this reduction also leads to a decrease in peak optical pulse power, limiting the laser’s applicability. Increasing the injection current’s magnitude provides a solution. Consequently, a high-precision circuit design has been developed to digitally adjust both the magnitude with a precision of ~3 μA and the pulse width with a resolution of 5 ps. This configuration successfully generates 200 ps optical pulses with a single-pulse energy of 0.96 pJ at 1550 nm, over a repetition rate range from 10 kHz to 1 GHz. With this laser as the transmitter, RZ-OOK modulated signal transmission at a slot rate of 250 MHz has been realized. The proposed scheme offers a stable, reliable optical emission source, making it ideal for high-speed, high-capacity optical time-division multiplexing communication, time-resolved spectroscopy, and laser ranging and imaging applications.

Photonics doi: 10.3390/photonics13060535

Authors: Tianyi Guo Yiwei Cheng Xuan Hu Zhengdong Chen Qican Zhang Zhoujie Wu Jie Li

Rotor blades in aero-engines operating in sand-laden environments are highly susceptible to particle-induced erosion. Conventional sand ingestion experiments primarily rely on post-test disassembly, which lacks the capability for real-time surface shape analysis. To overcome this limitation, this study proposes a high-precision three-dimensional (3D) shape measurement method for ultrafast dynamic scenarios, based on pulsed laser illumination and stroboscopic structured light. In the proposed approach, a pulsed laser is employed to illuminate a physical grating, generating stroboscopic structured fringe patterns that are projected onto high-speed rotating blades. The deformed fringe images are synchronously captured by a high-speed camera and processed using Fourier transform profilometry (FTP) to reconstruct fine surface features with high accuracy. Compared with conventional LED-based stroboscopic systems, the pulsed-laser-based scheme effectively suppresses motion blur and significantly improves image intensity under ultra-short exposure conditions. Experimental results demonstrate that stable and high-quality fringe acquisition can be achieved at high rotational speeds. The method enables precise quantification of micro-scale defects, such as scratches and pits, providing a reliable solution for in situ monitoring and performance evaluation in aero-engine sand ingestion tests.

Photonics doi: 10.3390/photonics13060534

Authors: Jiahao Zhao Xizhe Hou Yue Li Xuan Zhang Yongnan Li Chenghou Tu

The topological properties of vector optical fields are traditionally considered strictly conserved during continuous deformations and linear propagation. However, while structured light has been extended into nonlinear regimes, previous studies have predominantly focused on the intensity modulation of specific orbital angular momentum (OAM) components and the pure frequency conversion of structured light. The critical question of whether macroscopic topological invariants remain robust or experience fundamental breakdown during nonlinear light–matter interactions remains largely unexplored. To address this specific gap, we propose and generate multiple fractional vector optical fields (MF-VOFs), establishing their dynamic topological evolution and inherent conservation laws in free space. It should be noted that our experimental results are limited to free-space propagation. Strikingly, we report a significant departure from this paradigm during light–matter interactions: topological nonconservation anomalies manifest when these optical fields interact with nonlinear materials via second- and third-harmonic generation. Through a comprehensive quantitative analysis of the OAM spectrum, we confirm that the asymmetrical reconstruction and spatial transition of the total OAM along the propagation direction serve as the physical origins driving this topological symmetry breaking. These findings provide a fundamentally novel perspective on topological manipulation in nonlinear optical processes, offering advanced strategies for complex structured light generation and high-dimensional optical information processing.

Photonics doi: 10.3390/photonics13060533

Authors: Hengjia Liu Jie Liu Sijiang Wu Shuhua Zhang Jiawei Li Chong Chen Dongsong Sun Yuli Han

Accurate measurements of wind fields in the troposphere and stratosphere are essential for advancing atmospheric dynamics research, improving weather prediction, and supporting aerospace operations. However, a single Doppler lidar technique usually has limited capability to provide vertically extended wind profiles across both aerosol-rich lower altitudes and molecular-dominated higher altitudes. In this paper, we present a hybrid Doppler lidar system that combines a 355 nm scanning incoherent Rayleigh Doppler lidar with a 1550 nm coherent aerosol Doppler lidar for multi-scale wind field detection. The coherent Doppler lidar is used for boundary-layer wind retrievals, while the Rayleigh Doppler lidar, based on the double-edge technique, extends wind profiling from the upper boundary layer to approximately 40 km. Field deployments demonstrate continuous wind profiling from 50 m to 40 km, extending from the boundary layer to the stratosphere. Comparisons with radiosonde measurements show good agreement during the field campaigns, supporting the feasibility of this hybrid configuration for vertically extended wind profiling. The resulting high-resolution wind measurements across multiple atmospheric regions provide valuable data sources for studies of multi-scale circulation research, gravity wave dynamics, and climate-related atmospheric processes.

Photonics doi: 10.3390/photonics13060532

Authors: Jiamei Gu Kun Xie Jiayin Yan Mingyu Li Jian-Jun He

The increasing demand for rapid and multi-target detection in integrated photonic sensing systems necessitates scalable and low-complexity multiplexing strategies. Conventional multiplexing approaches, such as wavelength-division and time-division multiplexing, often suffer from inherent trade-offs between system scalability, performance, and implementation complexity. In this paper, a frequency division multiplexing (FDM) scheme based on cascaded microring resonators (CMRRs) is proposed for multi-channel optical sensing. Distinct sinusoidal voltage signals with different modulation frequencies are applied to the thermal electrodes of reference rings in each sensing channel, enabling the encoding of spectral responses into separable frequency components that can be simultaneously detected by a single photodetector. A numerical demodulation method based on joint least-squares reconstruction of the 2f and 4f modulation frequency components is developed to suppress distortions induced by higher-order spectral terms. Simulations and experiments on a silicon-on-insulator (SOI) platform are conducted to validate the proposed approach. The results show that the system achieves a sensitivity exceeding 300 nm/RIU and a limit of detection on the order of 10−5 RIU, while enabling reliable multi-channel signal separation. These results demonstrate that the proposed FDM scheme provides an effective route to enhance multiplexing scalability without increasing system complexity or hardware cost. The method is compatible with existing multiplexing techniques and offers a practical solution for high-performance multi-channel integrated photonic biosensing applications.

Photonics doi: 10.3390/photonics13060531

Authors: Zhi-Yuan Zheng Ying Yu

We propose a loss-managed BIC-derived GSST metasurface for robust phase-change-tunable mid-infrared transmission suppression. The metasurface consists of a SiO2 substrate, a Si grating layer, and an upper Si/Ge2Sb2Se4Te1 (GSST)/Si trilayer with an off-centered air slot. The slot plays a dual role: it breaks the mirror symmetry of the unit cell to convert a symmetry-protected bound state in the continuum into an externally accessible high-Q resonance, while reducing the effective GSST filling region to limit material-loss participation. Lossless eigenmode analysis confirms the BIC-derived origin of the resonance, with the quality factor following Q∝xdisp−1.993. A crystalline-state loss-channel analysis further identifies xdisp=40nm as a finite-coupling operating point that preserves good up/down radiation balance, a large resonant amplitude factor, and a moderate-high quality factor under the fully crystalline GSST condition. Full-wave simulations show that the transmission-dip wavelength shifts from about 3.9568μm to about 3.9740μm as GSST evolves from the amorphous to the crystalline state, while the extracted quality factor remains in the range of 483–780 and the transmission minimum stays deeply suppressed throughout the phase-change trajectory. A two-port temporal coupled-mode theory analysis reveals that this persistent low-transmission state originates from destructive interference between the resonant and background transmission channels. Fabrication tolerance analysis shows that ±5% variations in GSST thickness and slot width, as well as moderate variations in the slot displacement, preserve the deep transmission suppression across GSST phase states, although the absolute resonance wavelength shifts with geometry. These results provide a practical strategy for balancing radiative coupling and material-loss participation in phase-change high-Q metasurfaces.

Photonics doi: 10.3390/photonics13060530

Authors: Ivan I. Yakovkin Dean R. Evans Victor Yu. Reshetnyak

Spoof surface plasmon polaritons (SSPPs) offer a powerful way to confine and guide light at terahertz and gigahertz frequencies, but their functionality is typically locked by their static geometries. This study demonstrates the active tuning of SSPP resonances using nematic liquid crystals (LCs) integrated into metallic grating structures. Through full-wave numerical simulations, we show that both reorienting the LC director and inducing the nematic–isotropic phase transition enable the efficient modulation of the SSPP resonance frequency. In the terahertz regime, tuning ranges exceeding 150 GHz are achieved while preserving strong resonant absorption. For GHz-scale structures, we identify optimal configurations—such as partially dielectric-filled grooves topped with an LC layer—that overcome field confinement challenges and provide practical frequency shifts of several gigahertz. These results establish LCs as an effective tuning medium for reconfigurable SSPP devices for future applications in emerging THz and GHz photonics.

Photonics doi: 10.3390/photonics13060529

Authors: Eunice Y. Paik George J. de Coster Brandi Wooten Greg Meissner Blair C. Connelly Patrick Taylor

We report a new approach to enhance the photonic response of thin-film topological insulator Bi2Se3 by significantly reducing twin domains and antiphase disorder. The strategy employs closely lattice-matched trigonal substrates combined with surface structuring to preferentially seed a single rotational domain before epitaxy. Characterization using optical second harmonic generation (SHG), nonlinear optical tensor analysis, X-ray diffraction, and atomic force microscopy confirms the near-single crystal Bi2Se3 heteroepitaxial layers. These results show a clear six-fold symmetric sin2(3ϕ) SHG pattern at normal incidence, and a vanishingly small 100-to-1 peak-height ratio from X-ray pole-scans showing negligible twinning. These results show that this approach can yield near perfect single crystal heteroepitaxial Bi2Se3 whose photonic properties converge to those of bulk-grown single crystals.

Photonics doi: 10.3390/photonics13060528

Authors: Xiangqi He Lei Qiao Peigang Xu Kun Chen

To address the issues of traditional high-resolution spatial remote sensing cameras—complex optical systems, heavy weight, long development cycles, and high costs—this study combines the optical design parameters and product characteristics of lightweight remote sensing payloads. Based on the “physical simplification–algorithm enhancement” computational imaging paradigm, an algorithm-side enhancement technical system tailored to these lightweight payloads is constructed. This paper establishes a point-spread function (PSF) model for simplified optical systems and a dedicated imaging degradation model, verifying the compensation mechanism of computational methods against optical degradation effects. It achieves high-performance imaging through “low-precision simplified optics + high-precision algorithms,” providing theoretical support and practical implementation pathways for lightweight, low-cost, and rapid-response spaceborne remote sensing payloads. Experimental results confirm the excellent imaging performance of the camera, validating the effectiveness of the proposed optical design. Compared with the baseline Mask R-CNN (region-convolution neural networks), the AP50 and overall AP (average precision) of the AS Mask R-CNN are improved by 4.0% and 1.0%, respectively. This research offers a robust technical solution for intelligent remote sensing camera modes and serves as valuable reference and technical support for the opto-mechanical co-design of high-resolution remote sensing payloads.

Photonics doi: 10.3390/photonics13060527

Authors: Andrea Volpini Damiano Massella David Alvarez-Outarelo Vahram Voskerchyan Francisco Soares Francisco J. Diaz-Otero Omar Guillan-Lorenzo

We present a monolithically integrated wavelength meter fabricated on an indium phosphide (InP) platform, suitable for seamless integration with active photonic components such as lasers and optical amplifiers. The device architecture incorporates multiple ring resonators and was realized through a commercial multi-project wafer (MPW) process. Experimental characterization over a 1 nm spectral window using a tunable laser demonstrates the feasibility of the approach and validates the operating principle.

Photonics doi: 10.3390/photonics13060525

Authors: Andreas Rittler Jonas Wiedemann Vasilis Papanikolaou Hedieh Ajam Robert Schober Bernhard Schmauss

We present a flexible, centimeter-scale 4 × 4 macro mirror array serving as an intelligent reflecting surface (IRS) for free-space optical communication (FSOC) links in urban environments. The realized array integrates a compact, high-precision tip-tilt mechanism with a minimum scanning increment of 0.87 μrad and achieves a fill factor of 86%, enabling high-power transmission and supporting long-range connectivity. We investigate the multipath interference arising from different optical path lengths between the tiles, which introduce relative time delays at the receiver and can constrain the usable modulation bandwidth. A geometric model is developed to simulate the frequency response of the fabricated mirror array by explicitly accounting for tile-dependent optical path length differences. The simulated responses show excellent agreement with experimental data, validating the theoretical framework. Notably, the measured frequency responses exhibit pronounced minima at specific frequencies, evidencing multipath-induced destructive interference. The results demonstrate that the proposed model can accurately predict the frequency-dependent performance and establish practical bandwidth limits for IRS-assisted FSOC links based on specific array geometries.

Photonics doi: 10.3390/photonics13060526

Authors: Dawid Kucharski

This paper presents a reproducible simulation benchmark for rough surface interferometric profilometry. The benchmark compares three complete reconstruction pipelines under matched detected count assumptions: classical four-step phase-shifting interferometry (PSI), direct coincidence proxy reconstruction, and hybrid coarse-to-fine reconstruction in which a coincidence-derived observable supplies the coarse fringe-order prior. Fifty-nine focus variation (FV) topographies exported as Mountains/DigitalSurf .sur files (Digital Surf, Besancon, France) provide a shared FV prior for simulated optical observations. The coincidence channel is a simulation proxy rather than a validated quantum hardware implementation. The main result is architectural role separation. On the measured surface benchmark, the hybrid branch gives the lowest median detrended height RMSE (314.0 nm) and wins on 32 of 59 surfaces. The same ordering is retained in a rate-based coincidence control, with median hybrid RMSE of 290.9 nm under ideal matched-count rates and 376.3 nm under detector non-idealities. Roughness endpoints define the boundary of this result: hybrid gives the lowest matched bandwidth Sa and Sq errors, whereas direct coincidence proxy reconstruction is selectively strongest for Sz and remains process-dependent. Classical two-colour and classical frontier controls show that following the broad long-wavelength envelope is not sufficient evidence for overall architecture-level superiority within this simulation benchmark. The benchmark identifies coincidence-derived information as most useful when used as a coarse prior inside a hybrid estimator, while final fine texture remains anchored by short-wavelength PSI.

Photonics doi: 10.3390/photonics13060524

Authors: Tatiana Antipova Simona Riurean

Medical lasers are a heterogeneous class of interventional and therapeutic devices. They are differentiated based on their active gain medium, which includes solid-state, gaseous, dye, and semiconductor (diode) formulations. The present article undertakes a systematic evaluation and synthesis of the findings from a reproducible dataset. The present study yields novel scientific results, including a four-level classification of medical lasers that considers the chemical formula for each type of gain medium. In addition, a multisided systemic analysis of the engineering application of medical lasers in clinical practice is conducted, including an analysis of the main engineering challenges as a structured framework. Furthermore, a clustering of engineering applications for medical lasers in 2025 is performed, and a quantitative landscape of medical lasers by variables is presented. The following variables are analyzed: wavelength (nm), power (W)/irradiance (W/cm2), fluence (J/cm2), and exposure time/pulse duration. The objective is to create a year-by-year “trend analysis” for future engineering opportunities (2026–2030). The structure of the article is logical and roughly follows the IMRAD structure, and a thread of argumentation is demonstrated.

Photonics doi: 10.3390/photonics13060523

Authors: Xianzhu Zou Min Li Haifei Lu Xiaoyan Wen Lijie Li Shuo Deng Zhiwen Ming

Monolayer molybdenum disulfide (MoS2) holds great promise for strain-tunable optoelectronic devices. The strain-dependent dielectric function is a core parameter to characterize the tunability of optoelectronic properties. However, due to the extremely short light–matter interaction path length for atomically thin materials, measurements are challenging. In this work, we measured the dielectric function of strained monolayer MoS2 using the surface plasmon resonance (SPR) method with the simulated annealing particle swarm optimization (SAPSO) algorithm. When the applied strain ranged from −0.23% (compressive strain) to +0.20% (tensile strain), the dielectric function at seven characteristic wavelengths around the exciton absorption peaks was extracted. Our results demonstrate that both the real part (ε2r) and the imaginary part (ε2i) of the dielectric function evolved almost linearly with the applied strain from −0.23% to +0.20%. Based on these results, we further obtained the strain-induced variations in the refractive index (n) and the extinction coefficient (k). At exciton absorption peak B (600 nm), the strain-induced change rate for n reached a maximum of about −0.0141%−1. At the rising edge of the B exciton absorption (580 nm), the strain-induced change rate for k reached a maximum of about −0.3261%−1. This work presents a quantitative extraction of strain-dependent dielectric function of monolayer MoS2 over excitonic band-edge wavelengths using phase SPR–SAPSO fitting. The proposed method can be extended to the measurement of other atomically thin materials.

Photonics doi: 10.3390/photonics13060522

Authors: Jia Wei Ning Sun Huishi Zhu Fengrui Liu Jianguo Liu

This paper proposes a rotation-locking alignment scheme and system based on a Gaussian beam addressing the relative displacement between the receiver and the spot center during the fine tracking phase of spaceborne laser communication APT technology caused by platform vibration, temperature variations and other factors. By rotational scanning fitting, the offset angle and offset distance of the receiver relative to the spot center can be derived and achieve high-precision adaptive tracking. The simulated result demonstrates that the fitting distance error of this scheme is less than 1%, and the fitting angle error is less than π/32. At the same time, the system prototype is developed to conduct ground-based validation experiments, including the static test and outdoor link establishment test. The system prototype features simpler structure, lower computational complexity, and easier integration, satisfying the miniaturization and lightweight design requirements for spaceborne laser communication terminals. The test results verify the system can successfully establish stable laser communication links rapidly.

Photonics doi: 10.3390/photonics13060521

Authors: Jennyfer Morales-Marín Walter Torres-Sepúlveda Alejandro Mira-Agudelo

In this work, an adaptive Hartmann–Shack wavefront sensor (AHSS) is proposed, designed, and evaluated. This sensor allows for the modification in the dynamic range of wavefront aberration measurement, defined as the range between the minimum and maximum aberration value that can be measured with the sensor. This capability makes it suitable for studying optical aberrations in both objective systems and the human eye. AHSS consists of sixteen phase profiles corresponding to microlens arrays designed to be projected (one at a time) onto a spatial light modulator (SLM). In each design, the microlens size and focal distance parameters were varied. A calibration process was conducted, and aberration measurements were made in both artificial and real eyes. The results demonstrate good correspondence between the measurements with the AHSS and a conventional Hartmann–Shack sensor, which uses an actual refractive microlens array with fixed size and focal length parameters, proving their feasibility for measuring optical aberrations. The AHSS opens up possibilities for measurements in eyes with special characteristics, such as high aberrations, and enables the implementation of active optics aberration correction systems without the need for an additional refractive (physically lensed) wavefront sensor.

Photonics doi: 10.3390/photonics13060520

Authors: Qiheng Wei Yongqiang Zhang Lingyuan Wu Yuhan Huang Yanglong Li Bo Fu Wei Li Zhao Jiang

Endoscopic imaging plays an important role in minimally invasive surgery, clinical diagnosis, and biomedical research. Conventional endoscopic systems with fixed focal lengths are limited in multi-scale observation, while mechanically driven zoom systems often suffer from increased structural complexity and limited stability. In this work, a dual-degree-of-freedom continuous optical zoom endoscopic system based on liquid lenses is proposed. By employing two independently tunable liquid lenses, the system enables simultaneous modulation of optical power and principal plane position, thereby enhancing the flexibility of continuous focusing and magnification control. A Gaussian-bracket-based model is established to describe optical power redistribution and aberration evolution during the zoom process. The proposed system achieves continuous focusing over a wide range from 10 mm to 1000 mm while maintaining imaging performance close to the diffraction limit. In addition, a 1.2× magnified state is realized at a short focusing distance without significant degradation in image quality. The results demonstrate that the proposed dual-degree-of-freedom design provides a compact and effective solution for high-resolution continuous zoom endoscopic imaging.

Photonics doi: 10.3390/photonics13060519

Authors: Qinpan Bu Yinghui Gu Xiaodan Wang Shiwei Yuan Kai Song Xingyun Zhang

Two-photon microscopy (TPM) has become an indispensable tool in the life sciences, offering exceptional spatial resolution, deep tissue penetration, and low phototoxicity. It provides a revolutionary solution for long-term dynamic observation of living cells and tissues. This article systematically explains the working principles of TPM and reviews its applications in cutting-edge fields such as neuroscience, immunology, oncology, regenerative science and plant biology, highlighting its remarkable multidisciplinary adaptability. Building on this foundation, we further explore the breakthrough directions and future prospects for the development of this technology.