Photonics doi: 10.3390/photonics13050481

Authors: Hengrui Guan Xuefeng Lei Yuheng Chu Xinxin Zhao Dapeng Kuang Maoxin Song Mingchun Ling Jin Hong

Thin-film interference in photoresist stacks can become a significant source of uncertainty in lithographic focus metrology, particularly when high measurement stability is required. To evaluate this effect, a Fresnel-based multilayer reflection model is used to analyze the optical response of the resist stack and to guide the selection of dual-wavelength illumination. On this basis, a dual-wavelength optical triangulation system is developed for focus metrology in 350 nm lithography, with signal acquisition performed by a linear charge-coupled device (LCCD). Rather than improving precision by reducing detector pitch, the system employs a two-stage sub-pixel localization strategy in which template matching provides coarse spot localization and weighted centroid interpolation refines the final position within localized calculation windows, keeping the computational cost manageable. A covariance-based uncertainty analysis predicts a total root-mean-square uncertainty of 27.23 nm. Prototype experiments were performed on a bare silicon wafer to establish the intrinsic performance of the instrument before introducing process-dependent optical effects. Under these conditions, the system achieved a vertical resolution of 10 nm, a repeatability of 35 nm, and a stability of 13.16 nm. The additional uncertainty expected under resist-coated-wafer conditions was assessed separately through the thin-film model. These results verify the baseline capability of the proposed system and support the feasibility of the dual-wavelength strategy for focus metrology in 350 nm lithography.

Photonics doi: 10.3390/photonics13050480

Authors: Xinhua Zhang Yuchun Liu Xinyue Zhang Lingchen Kong Cuihong Jin Yajuan Han Mengqing Jiang Shiying Qiao Xinyan Gong

To explore the strong coupling between surface phonon polaritons (SPhPs) and quantum dots in one-dimensional periodic microstructures for quantum information processing, we establish a comprehensive theoretical model for SPhPs at air–polar dielectric interfaces. By rigorously deriving the dispersion relations, we reveal the decisive role of scale effects on bandgap formation: continuous spectra without bandgaps emerge at the nanoscale (d∼10–100 nm), whereas periodic modulation induces significant Bloch mode folding and tunable bandgaps (0.5–5 μm width) at the microscale (d∼1–10 μm). Based on Fourier bandwidth limitations, we determine optimal channel widths (Ly≥10 μm) for maintaining low-loss modes with energy deviations below 1%. Through electromagnetic field quantization, we obtain analytical expressions for SPhP mode amplitudes and quantum dot transition rates. Calculations demonstrate that in micrometer-scale CsI structures, spontaneous emission rates can be modulated significantly: suppressed to <0.1 times the free-space values within bandgaps (excited-state lifetimes extended to ∼10 ns) and enhanced 5–8 times at conduction band edges. Leveraging these characteristics, we propose a scheme for batch quantum state manipulation of 102–103 arrayed quantum dots via selective excitation of specific Bloch modes using controlled laser frequency and angle, enabling parallel single-qubit gates with theoretical fidelity > 99%. Compared with surface plasmon polariton schemes, our approach utilizes the low-loss infrared characteristics of SPhPs (Q∼100–1000, 1–2 orders higher) to reduce decoherence rates, offering a new pathway for room-temperature solid-state quantum computing and on-chip multi-node entanglement distribution.

Photonics doi: 10.3390/photonics13050479

Authors: Siyuan Cheng Guangyi Zhang Tao Ju

We propose a terahertz achromatic polarization-multiplexed structured light metasurface based on the particle swarm optimization (PSO) algorithm, operating from 0.8 to 0.95 THz. A dielectric silicon meta-atom array combined with propagation phase modulation is employed to achieve broadband wavefront control under two orthogonal linear polarizations. By constructing a phase-response database and using PSO for global optimization of phase compensation factors at multiple frequencies, the metasurface simultaneously satisfies different target phase profiles while suppressing chromatic aberration. Two multifunctional devices are designed. The first generates a conventional focused spot under x-polarized incidence and a first-order Bessel beam under y-polarized incidence. The second produces a focused vortex beam with topological charge l = 1 under x polarization and a focused vortex beam with l = 2 under y polarization. Full-wave simulations demonstrate stable focal positions, low inter-channel crosstalk, and good achromatic performance across the operating band. The Bessel beam preserves its nondiffracting core, while both vortex channels exhibit clear phase singularities and well-defined orbital angular momentum states. Most operating frequencies maintain relatively high focusing efficiency. Compared with conventional cascaded optical components, our design provides a compact and stable platform for terahertz structured light generation, orbital angular momentum multiplexing, nondiffracting imaging, and multidimensional polarization information processing.

Photonics doi: 10.3390/photonics13050478

Authors: Ningjing Xiang

In this paper, we investigate the propagation properties of a partially coherent Gaussian Schell-model (GSM) beam by effective beam parameters in oceanic turbulence. We provide detailed analytical derivations based on the extended Huygens–Fresnel integral and the cross-spectral density function. It is found that the angle-of-arrival fluctuation, spreading, and wander of the partially coherent GSM beam decrease with increasing source coherence parameter and turbulent kinetic energy dissipation rate, and with decreasing temperature fluctuations and mean-square temperature dissipation rate. At 200 m propagation distance, the relative mean-squared width under salinity-dominated conditions (ω = −2) is approximately 0.02% larger than that under temperature-dominated conditions (ω = −5), indicating that salinity fluctuations cause more obvious beam spreading.

Photonics doi: 10.3390/photonics13050477

Authors: Bing Han Jian Kang Meng Zhang Qian Wu

This study proposes a novel hybrid prediction model (QGCN-LSTM) that combines Quantum Graph Convolutional Networks (QGCN) with classical Long Short-Term Memory (LSTM). The model takes space-time data as input and employs a hierarchical graph-based quantum encoding strategy. Specifically, classical spatial features are first aggregated into critical regional hubs, which are then mapped into the Hilbert space through a dense quantum encoding layer. Multi-scale features are extracted through the collaborative computation of QGCN and quantum gated recurrent units, and a quantum attention module is introduced to dynamically screen key information. Finally, the prediction results are generated through quantum measurement and a classical output layer. In the space-time data prediction task of urban traffic flow, a benchmark model system covering classical, cutting-edge, and traditional architectures was constructed. The experimental results show that QGCN-LSTM utilizes quantum entanglement gates to establish non-local road network associations, dynamically allocate feature weights to enhance the impact of critical time steps, and achieves deep compression of lines through quantum line pruning technology, effectively alleviating the common problem of “poor plateau” in quantum neural network training. In terms of prediction accuracy, the mean absolute error (MAE) of its key hub nodes is reduced by 34.1% compared to the graph convolution LSTM (GCN-LSTM) model, and the Spatial Correlation Index (SCI) is improved to 0.89. In addition, it also shows excellent performance in dynamic response, edge computing efficiency, and other aspects, meeting the real-time requirements of the traffic signal control system. This study provides an effective paradigm for the application of quantum collaborative architecture in complex spatiotemporal prediction tasks.

Photonics doi: 10.3390/photonics13050476

Authors: Bingyan Fu Wanqi Zhang Taiming Hu Guangqing Liu Yuxuan Li Xiang Yi

Accurate modeling of irradiance scintillation is important for evaluating underwater wireless optical communication (UWOC) systems operating in oceanic turbulence. Existing studies have mainly focused on weak oceanic turbulence conditions, while irradiance scintillation modeling under arbitrary oceanic turbulence strength remains insufficiently developed. In this work, the Gaussian beam is adopted as the representative model of practical laser beams, whereas the plane-wave and spherical-wave cases are introduced as limiting cases to support the derivation and theoretical completeness of the Gaussian-beam formulation. A unified theoretical framework is developed based on the general oceanic turbulence optical power spectrum (OTOPS). Building upon previously reported weak-turbulence results, the scintillation index (SI) under saturated strong turbulence is first derived using asymptotic theory. Then, within the extended Rytov approximation, an effective-scale treatment is introduced to characterize the contributions of large- and small-scale eddies to irradiance fluctuations. By connecting the weak- and saturated-turbulence limits through asymptotic matching, a closed-form SI expression valid over a wide range of oceanic turbulence strengths is obtained. Numerical results show that the proposed model agrees well with the corresponding boundary cases and reproduces the characteristic “bump” behavior of oceanic turbulence, while highlighting the influence of ocean-specific cutoff spatial frequencies on the predicted scintillation peaks. These results provide a physically consistent analytical framework for UWOC channel modeling and performance evaluation under arbitrary oceanic turbulence strength.

Photonics doi: 10.3390/photonics13050475

Authors: Hongyan Meng Hengli Feng Yang Liu Wenqiang Shi Jue Wang Yang Jia Jijuan Jiang Guan Wang Jia Liu Junguo Lu Jingyi Liu Yachen Gao

In this work, we propose a novel transformer-based deep learning model for the design of electromagnetically induced transparency (EIT) metasurfaces, which consists of a forward network to predict transmission spectra from structural parameters and an inverse network to retrieve structural parameters from target spectra. To train the model, we generated a dataset of 23,500 samples by automating CST simulations. The well-trained model can predict all seven structural parameters of an EIT metasurface from a given target spectrum within milliseconds, achieving a mean square error (MSE) of 8.49 × 10−4 at convergence. The mean errors between the predicted data and target parameters remain below 0.25 μm. The relative spectral error (RSE) is employed to evaluate the discrepancy between the spectra from predicted structures and the targets, with a maximum RSE of 0.57%. Benchmarking against two other neural networks confirms the superior predictive capability and accuracy of our model. Furthermore, the method not only streamlines EIT metasurface design but is readily adaptable to diverse metasurface devices across the electromagnetic spectrum, establishing a versatile platform for metasurface inverse design.

Photonics doi: 10.3390/photonics13050474

Authors: Yuanze Hong Zhixiang Xie Yuhang Hu Zhipeng Wei Xiaohua Wang Lin Pan

The design of photodetectors tailored to specific wavelengths in the mid-infrared (MIR) band serves as a foundational enabler for advancements in scientific research, industrial inspection, and environmental monitoring. Metasurfaces, composed of artificially engineered subwavelength unit cells, enable precise tailoring of light–matter interactions, achieving near-unity absorption at target wavelengths and thereby significantly boosting the sensitivity and spectral selectivity of MIR photodetectors. In this study, we developed a double-C open-loop metasurface and optimized its geometric parameters to realize high-efficiency absorption at 4 μm and 6 μm. Utilizing Te thin films fabricated via magnetron sputtering, we constructed a metasurface-enhanced mid-infrared photodetector based on Te thin films. The optimized metasurface structure enhances the light absorption of the Te thin film by a factor of eight within the target wavelength band. Ultimately, the metasurface-enhanced Te-based device achieved responsivities of 10.5 A/W and 13.7 A/W at 4 μm and 6 μm, respectively, representing enhancements of 3.6-fold and 3-fold compared to the initial Te thin-film device. This work provides a critical reference for enhancing the detection performance of infrared photodetectors at specific wavelengths through precise nanophotonic design.

Photonics doi: 10.3390/photonics13050473

Authors: Zixuan Wang Qinghua Yu

Due to the lack of rotational symmetry, off-axis two-mirror freeform optical systems usually exhibit coupled alignment degrees of freedom and poor sensitivity-matrix conditioning, which increase the difficulty of alignment. To address this issue, a geometrically constrained alignment method based on a multi-zone computer-generated hologram (CGH) is proposed. A multi-zone CGH integrating null compensation and mark projection on a single substrate was designed to provide both wavefront and spatial references. In combination with a staged alignment procedure, the projected optical marks were used to assist in establishing the relative positional relationship between the interferometer–CGH subsystem and the PM-SM system, while also providing geometric references for secondary-mirror pose adjustment during wavefront-guided iterative alignment. Results from multiple alignment experiments show that the mean wavefront RMS was 0.0577λ at 632.8 nm, with a standard deviation of 0.004λ. These results suggest that, under the present experimental conditions, the proposed method exhibits good repeatability and convergence stability and can provide a reference for the alignment of freeform optical payloads.

Photonics doi: 10.3390/photonics13050472

Authors: Bakhe Nleya Beverly Pule

The rapid advancement of beyond-5G and 6G services is creating computational challenges that classical optimisation methods for Elastic Optical Networks (EONs) cannot effectively handle. Specifically, the multi-objective Routing and Spectrum Assignment (RSA) problem—aimed at minimising blocking probability, maximising spectral efficiency, and reducing fragmentation—poses significant challenges and is NP-hard, particularly in dynamic traffic. This paper introduces a hybrid framework that combines quantum and classical computing, dividing the optimisation tasks into classical pre-processing, a quantum optimisation core, and classical post-processing with Pareto frontier management. The RSA problem is modelled using a Quadratic Unconstrained Binary Optimisation (QUBO) formulation that accounts for blocking, efficiency, and a quadratic fragmentation metric. Simulations conducted on NSFNET and UBN topologies under Poisson traffic conditions revealed that even in realistic, noisy quantum environments, this hybrid method reduces the blocking probability by 14% and improves fragmentation by 7.3% compared to the top classical heuristics. A scaling analysis indicates a key point of around 220 variables where this hybrid strategy surpasses traditional meta-heuristics in both solution quality and execution time, emphasising its significant potential in the current NISQ era.

Photonics doi: 10.3390/photonics13050471

Authors: Maximino R. Tapia-Garcia Juan C. Hernandez-Garcia Roberto Rojas-Laguna Julian M. Estudillo-Ayala Stephanie G. Hernandez-Garcia Olivier Pottiez Jose D. Filoteo-Razo Jesus P. Lauterio-Cruz Daniel Jauregui-Vazquez

We present the integration of an automated polarization control system into a figure-eight fiber laser with the aim of self-tuning noise-like pulses (NLPs). The system optimizes polarization adjustments by using adaptive control and predictive neural networks (NNs), enhancing temporal and spectral behavior. This approach enables precise control over pulse characteristics, achieving an average output power of 275.25 mW (302.8 nJ) for signal emission at ~1567 nm; adjustable NLP envelope durations from 13 ns to 48 ns, corresponding to spectral widths from 50 to more than 200 nm; and the ability to increase in a controllable way the repetition frequency up to 100 times the fundamental frequency, which corresponds to 909 kHz, through cavity harmonic pulse generation. A multiparameter pulsed regime-seeking algorithm stabilizes high-energy NLPs in the fundamental or harmonic regime while predictive networks optimize the cavity response, and the process is completed in an average time of 11.5 s. The automated polarization control system enables high cavity harmonic pulse generation, as well as broadband supercontinuum (SC) spectrum with high flatness.

Photonics doi: 10.3390/photonics13050470

Authors: Guohua Yan Mei Huang Zhaohui Yu Chuanqian Peng

Continuous-spectrum illumination induces severe channel crosstalk and resolution degradation in color imaging systems. To address this issue, this paper introduces near-monochromatic LED illumination matched to the lens design wavelength and proposes a linear crosstalk correction method based on CCD intensity superposition. Experiments under ISO 12233 evaluation reveal that mixed-color LED illumination only enhances the blue channel, while separate monochromatic illumination and image synthesis effectively eliminate crosstalk and boost overall resolution. The proposed linear method calibrates crosstalk coefficients via a standard reference area to correct single-shot mixed-color images, bringing R/B channels close to pure monochromatic performance, with minor over-sharpening in the green channel. This approach provides a feasible linear correction solution to suppress crosstalk and improve resolution for single-shot industrial color imaging with near-monochromatic LED illumination.

Photonics doi: 10.3390/photonics13050469

Authors: Xiaohong Duan Mohammad Syuhaimi Ab-Rahman

To address the demand for low-cost, low-loss, and environmentally friendly optical power dividers in short-range visible light communication (VLC) systems, a low-loss 1 × 2 Y-branch optical splitter based on the integration of a planar optical waveguide (POW) and plastic optical fiber (POF) is proposed and experimentally demonstrated. The device employs a large-core step-index POF with a core diameter of 1 mm, enabling efficient coupling of multimode optical signals. The design and structural optimization of the 1 × 2 POF splitter are simulated by the beam propagation method (BPM). We fabricated the device through a low-cost manual assembly process, followed by packaging and experimental characterization. Measurements at 650 nm on ten samples show a minimum insertion loss of 3.4 dB and a lowest excess loss of 0.8 dB. The splitting ratio ranges from 49.6%:50.4% to 37%:63%, with a minimum uniformity of 0.06, indicating stable power distribution performance. These results confirm that a simple, low-cost fabrication approach can achieve practical optical performance, offering a feasible route toward scalable polymer-based photonic integration.

Photonics doi: 10.3390/photonics13050468

Authors: Hanping Zhang Xinyi Zhu Yurong Wang E Wu Guang Wu

In single-photon detection, dark count represents a critical limitation, particularly for high-sensitivity applications. Conventional estimators based on the binary per-gate observable become ill-conditioned when the dark count per-gate probability approaches unity, a situation common in first-photon-gated detectors with extended gate width. This work proposes a complementary characterization method based on the statistical expectation of dark count arrival time. This approach captures the cumulative temporal behavior of dark count across multiple gating cycles, providing a more accurate estimation of the dark count rate. Both numerical simulations and experimental results demonstrate that our method yields significantly more stable and precise measurements compared to the conventional approach. Specifically, while the conventional method introduces errors up to ±4% at larger gate widths, the proposed timing-based method converges to a significantly lower residual error of approximately −0.17%. These findings offer a promising route to enhance the characterization and performance of first-photon-gated single-photon detectors in practical applications.

Photonics doi: 10.3390/photonics13050467

Authors: Yaohui Zhang Handing Xia Zefeng Yao Xiaocheng Tian Junwen Zheng Jianbin Li Fan Zhang Rui Zhang

Single-frequency fiber lasers (SFFLs) are essential for applications such as gravitational wave detection, high-precision spectroscopy, and inertial confinement fusion, requiring narrow linewidth, low noise, and high output power. Here, we present a comparative study of 1 μm waveband distributed Bragg reflector (DBR) SFFLs with varying cavity parameters. Numerically, we investigate the effects of key cavity parameters on laser performance by plotting contour maps of output power versus grating reflectivity and lasing wavelength. We also simulate intensity noise transfer functions from pump fluctuations. Increasing pump power shifts the relaxation oscillation peak to higher frequency and reduces its amplitude, which originates from the higher intracavity photon density that speeds up the damping of perturbations. Experimentally, we construct two lasers using 6.5 mm and 10.5 mm YDFs spliced between FBG pairs. These lasers employ low-reflectivity FBGs centered at 1053 nm and 1064 nm, with reflectivities of 74% and 55%, respectively. The corresponding maximum output powers are 29.7 mW and 197 mW. The 1053 nm SFFL exhibits a relative intensity noise (RIN) of −102 dBc/Hz at 2.07 MHz, a linewidth of 12.52 kHz, and a mode-hop-free tuning range of 0.64 nm. Although increasing the pump power suppresses the relaxation oscillation peak, it broadens the linewidth due to laser phase noise degradation caused by pump noise-induced temperature fluctuations in the gain fiber. For SFFLs, the output powers should be selected according to the specific application, as a higher output power inherently leads to a broader linewidth. These insights are essential for optimizing such lasers and underscore their strong potential for future applications.

Photonics doi: 10.3390/photonics13050466

Authors: Mohan Wang Lu Zhang Yachen Gao Zhiguo Zhang

This study investigates the 2.0 μm luminescence efficiency of Tm3+-doped crystals under 793 nm excitation. An analytical model decomposing laser slope efficiency into the quantum defect, fluorescence quantum efficiency (ηq), and a mode matching factor was established, highlighting  ηq optimization as key. Using a high-precision spectral system, the comparative study of Tm:YAG and Tm:YAP crystals revealed unprecedented ηq  values of 184.8% and 190.6%, respectively. This breakthrough, corroborated by double-exponential decay kinetics, verifies the cross-relaxation-dominated quantum cutting mechanism. Superior performance of Tm:YAP crystal is attributed to its lower phonon energy, effectively suppressing non-radiative losses, providing a foundation for high-performance 2.0 μm lasers.

Photonics doi: 10.3390/photonics13050465

Authors: Peiqi Li Ming Hou Jiahong Zhang Di Ma Yingna Li

A reconfigurable broadband signal channelized reception technique based on parallel Mach–Zehnder modulators (MZMs) is proposed. In the upper branch, the unknown broadband signal is modulated onto the ±1st-order sidebands of a frequency-shifted optical carrier. In the lower branch, N parallel MZMs are employed, with each MZM generating two local oscillator (LO) comb lines, which beat with the broadband signal from the upper branch to produce 4N sub-channels. By adjusting the frequency shift of the acousto-optic frequency shifter (AOFS) and the frequency of the LO signals, the system achieves tunability over an operating frequency band of 8 to 40 GHz, enabling simultaneous tuning of the sub-channel bandwidth, the number of sub-channels, and their center frequencies. Simulation experiments show that this technique can down-convert a wideband signal with a frequency band of 12–16 GHz to eight intermediate frequency (IF) signals with eight center frequencies of 0.5 GHz and a bandwidth of 0.5 GHz and down-convert a wideband signal with a frequency band of 32–40 GHz to eight IF signals with eight center frequencies of 1 GHz and a bandwidth of 1 GHz, and the image rejection ratio (IRR) is greater than 26 dB, the passband power fluctuation is less than 0.5 dB, and the spurious-free dynamic range (SFDR) is 95.17 dB·Hz2/3.

Photonics doi: 10.3390/photonics13050464

Authors: Yaqi Wu Bowen Liu Genyu Bi Minglie Hu

We extended self-similar amplification to a large-mode-area tapered Yb-doped fiber (LMA T-YDF) with longitudinally decreasing nonlinearity. The theoretical analysis and numerical simulation demonstrate that T-YDFs with different nonlinearity profiles can achieve self-similar evolution, which is confirmed by a self-similar amplifier that employs two kinds of T-YDFs. Further experimental study indicates that the T-YDF with a large core diameter at the thin end can achieve self-similar evolution across a wide range of pump powers and generate 51 W average power, 34 fs nearly transform-limited (TL) pulses with 32 dB gain. To the best of our knowledge, this is the first theoretical and experimental demonstration of self-similar amplification in T-YDFs. The high-gain feature of the T-YDF simplifies the laser system and can be used to build a compact all-fiber high-power femtosecond laser source.

Photonics doi: 10.3390/photonics13050463

Authors: Liang Zhu Jin Liu Haima Yang Jingru Zhang Damin Ding Wenyao Xia

A reflective plasmonic humidity sensor based on an Au–PVA–Au nanohole sandwich structure is investigated. The device consists of a periodic gold nanohole array, a poly(vinyl alcohol) (PVA) spacer, and a continuous gold film. A humidity-dependent model considering both the refractive-index decrease and thickness swelling of PVA is established to analyze the optical response and resonance-modulation mechanism. Within the relative humidity range of 20–98%RH, the reflection resonance dip exhibits a continuous blueshift with a total wavelength shift of approximately 135 nm. Piecewise linear fitting shows sensitivities of 1.3857 nm/%RH in the 20–74%RH range and 2.5000 nm/%RH in the 74–98%RH range. At approximately 74%RH, the resonance wavelength, full width at half maximum, and quality factor are about 830 nm, 19 nm, and 43.7, respectively. Decoupling analysis confirms that both PVA refractive-index reduction and thickness swelling contribute to the blueshift, while their combined effect produces the largest response. These results demonstrate that the proposed structure converts humidity-induced optical and geometric variations in PVA into a pronounced wavelength response, providing a mechanism-guided design route for reflective nanoplasmonic humidity sensors based on polymer-assisted cavity modulation.

Photonics doi: 10.3390/photonics13050462

Authors: Xiangyu Wang Can Wang Xi Chen Tun Cao Donghan Ma

Biplane single-molecule localization microscopy (SMLM) enables three-dimensional (3D) super-resolution imaging by extracting the axial position of fluorophores from a pair of emission patterns detected at two axially separated planes. The separation between these two imaging planes, termed the biplane distance, is a key parameter that determines axial localization precision, yet a systematic investigation of its optimal selection remains lacking. Here, we calculate the theoretical localization precision across a range of biplane distances and identify an optimal value, construct a tunable biplane detection module and experimentally evaluate axial localization precision at three different biplane distances using both fluorescent beads and biological specimens. Experimental results confirm the theoretical predictions and provide a practical framework for optimizing the biplane distance in 3D-SMLM systems.

Photonics doi: 10.3390/photonics13050461

Authors: Dongze Wei Haonan Zheng Hao Chi

In this paper, to reduce system complexity and improve performance, we propose a novel compact photonic quantization scheme based on dual-output Mach–Zehnder modulators (DOMZMs). By exploiting the complementary outputs of DOMZMs and introducing a cross-channel differential combination strategy, multiple effective quantization channels are generated without increasing the number of modulators. Furthermore, an adaptive thresholding mechanism based on intrinsic signal intersections enables direct Gray code output with improved noise tolerance. Proof-of-concept experimental results fully confirm the correctness of the principle, and 4-bit quantization is successfully demonstrated. Experimental and numerical results both demonstrate good linearity over the full-scale input range, and confirm the feasibility of the proposed scheme. More performance evaluations are provided through simulations. We also discuss challenges relating to practical deployment of the proposed approach. The presented approach provides a promising solution for compact photonic analog-to-digital conversion systems.

Photonics doi: 10.3390/photonics13050460

Authors: Ruixue Zhu Shuxia Wan Xinyang Su

Optical tweezers (OTs) serve as a core contactless manipulation tool at the micro- and nano-scale, with wide applications in physics, biology, colloid science and other fields. However, conventional single-beam gradient force OTs are limited by diffraction, optical damage, low throughput, and system complexity. To meet the demand for large-scale particle manipulation in complex environments, speckle optical tweezers (SOTs) based on random optical fields have emerged as a promising alternative to conventional OTs that transform random speckle patterns into a controllable manipulation resource. Since their formal establishment, SOTs have developed a solid theoretical foundation and diverse implementation platforms with key breakthroughs in micro- and nanoparticle manipulation. This paper systematically reviews the origin and development of SOTs, elaborates their core principles, summarizes the statistical properties of speckle fields, and introduces typical configurations based on random media, multimode fibers, and spatial light modulators. It also highlights the unique value of SOTs in micro- and nanoparticle manipulation, active particle dynamics, and cold atom physics, with advantages of high throughput, low cost, and environmental adaptability. Finally, future development trends are discussed, including intelligent regulation of optical fields, interdisciplinary applications, system miniaturization and multi-technology integration. This review provides a comprehensive reference for the theoretical development, system optimization, and practical application of SOTs in fields such as statistical physics, biomedicine, microfluidics, and quantum science.

Photonics doi: 10.3390/photonics13050459

Authors: Zishan Zhang Weihua Song Jintao Deng Cong Xia Bin Wu Xinyi Zhao Jianhua Luo

To address the need for high electrical insulation, strong immunity to electromagnetic interference, and miniaturized AC electrical-signal detection in complex electromagnetic environments, we propose and experimentally demonstrate a fiber-optic sensor based on a piezoelectric ceramic (PZT)-driven ring tapered-fiber resonator. The applied AC excitation is converted into periodic mechanical deformation through the inverse piezoelectric effect of the PZT, and the resulting strain modulates the resonator response, enabling optical demodulation of the input frequency and amplitude. A comprehensive figure of merit was introduced to optimize the tapered-fiber geometry, yielding an optimal waist diameter of approximately 10 μm. The sensor can effectively distinguish both single- and dual-frequency AC signals. Over the range of 50–500 Hz, the demodulated frequency agrees closely with the input frequency, with a linear fitting coefficient of 0.9999. At a fixed driving frequency of 250 Hz, the amplitude of the characteristic spectral peak increases nearly linearly with the input voltage amplitude, with a fitting coefficient of 0.9945. The device also exhibits good stability over 30–150 °C and during 70 h of continuous operation. With its simple structure, low cost, and strong immunity to electromagnetic interference, this sensor provides a practical solution for AC electrical-signal detection in complex environments.

Photonics doi: 10.3390/photonics13050458

Authors: Tao Yuan Ji Liu Boyang Zhang Jinhui Wu Yiman Zhang

This paper proposes a Misalignment Differential Vortex Beam Interferometer (MD-VBI) integrated with DenseNet-169 for high-precision asymmetric shaft alignment. The system employs a polarization-multiplexed differential configuration to linearly map nanometric displacement to interference-fringe rotations. Building upon the optical architecture’s intrinsic suppression of common-mode thermal drift, the DenseNet-169 model serves as a robust demodulation backend, further mitigating inherent system-level optical noise to precisely decode misalignment signatures in interferograms. Experimental results demonstrate a Mean Absolute Error (MAE) of 0.382 nm within a 0–500 nm range. By decoupling true asymmetric shaft misalignment from intrinsic system noise and common-mode drift, the system provides a robust, non-contact solution for nanometric metrology under realistic conditions.

Photonics doi: 10.3390/photonics13050457

Authors: Siddhant Vernekar Jolly Xavier

An ever-increasing demand for higher photon generation rates in quantum light sources often leads to the generation of multiple photon pairs, making quantum secure imaging, sensing, and communication vulnerable to photon number splitting (PNS) attacks. Here, we investigate the use of weak coherent sources (WCS) and heralded single-photon sources (HSPS) in conjunction with quantum key distribution protocols to mitigate these risks. Our initial observation shows that the BB84 protocol using HSPS has an advantage in secured information transfer over the WCS. We then extend our comparative study between WCS and HSPS to high dimensional protocols and conduct a rigorous analysis to estimate a benchmark for quantum advantage in secure bit rate thresholds for secure information transfer. When combined with high-dimensional states (hybrid encoding), the two-state non-orthogonal encoding protocol offers an increased resistance to PNS and unambiguous state discrimination attacks. These findings suggest that integrating high dimensional encoding would strengthen the security and performance of quantum secure imaging, sensing, and communication systems for practical and resilient implementations at shorter distances.

Photonics doi: 10.3390/photonics13050456

Authors: Yujie Ji Peiying Li Yanhong Xiao Yuzhuo Wang Junlei Duan

Spin squeezing can suppress quantum projection noise via interparticle entanglement, therefore enabling measurement sensitivities beyond the standard quantum limit. In practice, however, the Gaussian and finite intensity profiles of the optical probe beam induce spatially inhomogeneous atom-light interactions. As polarized atoms move within a vapor cell, they experience position-dependent optical intensities, generating transit noise that limits spin squeezing performance. Here, we investigate the transit noise in a coated rubidium vapor cell through combined theoretical analysis and experimental measurements. By varying the probe beam diameter, we quantify the dependence of transit noise on beam size and atomic Larmor frequency. Our results show that, for a vapor cell with fixed dimensions, the transit noise increases as the probe beam spot area decreases. Moreover, when the Larmor frequency is below the characteristic linewidth of the transit noise, the noise contribution becomes larger. We further calculated and measured spin squeezing for different beam sizes and found an experimental difference of 2.7±0.2 dB between 2 mm and 0.6 mm, similar to the theoretical prediction of 3.0±0.3 dB. Theoretical analysis under conditions of stronger squeezing shows that transit noise becomes an even more critical limiting factor. These results provide practical guidance for optimizing probe beam parameters and suppressing transit noise in spin squeezing experiments.

Photonics doi: 10.3390/photonics13050455

Authors: Wei Wang Yudong Su Tong Wu Pan Tao Kai Zheng Zheng Zhang Jun Zhou Shixun Dai Peiqing Zhang

Exhaled breath analysis offers a non-invasive route for metabolic monitoring and disease screening, but its practical implementation requires sensing platforms that combine high sensitivity, robustness, and simplicity. Here, we report an evanescent wave-excited fiber-optic surface-enhanced Raman scattering (SERS) sensor based on silver nanocubes (Ag NCs) assembled onto a fiber taper waist (FTW), and the design is further extended to an Ag/graphene oxide (GO) hybrid interface for enhanced gas detection. Finite element and finite-difference time-domain simulations were employed to optimize the FTW geometry and Ag NC dimensions for efficient evanescent-field excitation and plasmonic enhancement. The fabricated FTW-SERS probe achieved a minimum detectable concentration of 10−9 M for crystal violet, together with good linearity and a relative standard deviation below 5%. For gas sensing, ethanol and acetone vapors were detected down to 50 ppm using the Ag NC-based FTW-SERS probe. After introducing a 0.3 mg/mL GO functional layer, the minimum detectable concentrations of both analytes were further reduced to 25 ppm. In addition, proof-of-concept monitoring of exhaled ethanol after alcohol consumption revealed dynamic spectral changes consistent with ethanol metabolism. These results demonstrate the potential of evanescent wave-excited FTW-SERS probes for compact and sensitive breath-analysis applications.

Photonics doi: 10.3390/photonics13050454

Authors: He Li Enbing Qi Jun Liu Shuo Jin Wenqi Ma Junjie Zhang

While fused silica microlens arrays (MLAs) act as crucial components in the fields of infrared optics and laser systems, direct laser writing has been proposed for the fabrication of MLAs. However, the layer-by-layer slicing strategy generally leads to stepped surface textures formed on the microlens surface, resulting in high surface roughness and limited transmittance. This work proposes a temperature-controlled CO2 laser polishing method for the fabrication and subsequent smoothing of fused silica microlens arrays. Specifically, an infrared temperature measurement system is integrated into a CO2 laser direct writing platform. Correspondingly, a proportional-integral-derivative algorithm is used to adjust the laser power in real time based on the temperature deviation at the processing spot, thus maintaining the polishing zone in a molten rather than vaporizing state. Furthermore, a finite element model of laser polishing of fused silica coupled with laser heating and fluid flow is developed, which is used to analyze the spatiotemporal evolution of the temperature field, as well as its correlation with the response of the processed surface. Experimental results show that temperature-controlled laser polishing reduces the surface roughness of the fabricated MLAs by 86.8%, while the transmittance in the visible band remains above 90%. This work provides a feasible closed-loop polishing method and a mechanistic analysis model for the laser polishing of fused silica MLAs.

Photonics doi: 10.3390/photonics13050453

Authors: Yi Shen TienYang Lo Taiki Takashima Shunsuke Murai Katsuhisa Tanaka

Metasurfaces, planar structures made on a subwavelength scale, enable state-of-the-art manipulation of light and have become a promising solution for compact optical devices. However, fabrication of these nanoscale structures relies on demanding processes, limiting their integration into diverse structures, including three-dimensional ones. In this study, we develop a manufacturing and transfer technique that renders the manipulation and deposition of metasurfaces achievable with high freedom by embedding the nanostructure into a flexible polymer matrix. A metasurface consisting of a TiO2 nanoparticle array fabricated by nanoimprint lithography was encapsulated within a poly(methyl methacrylate) (PMMA) layer through spin-coating. The layer containing the metasurface was then detached from the original SiO2 substrate using wet-etching, becoming a free-standing soft sheet carrying nanostructures that can be transferred onto various surfaces. After the transfer, the layer thickness was further tuned through reactive ion etching to modulate the optical response. Incident-angle-resolved transmittance exhibited no significant change in optical bands before and after transfer, confirming that the nanostructure, as well as the photonic band, was well preserved. Thickness reduction of the PMMA cladding induced a clear optical resonance shift, demonstrating controllability of the optical response. This approach provides a versatile route for the installation of metasurfaces and expands the design possibilities for nanophotonic devices.

Photonics doi: 10.3390/photonics13050452

Authors: Bader Alhasson Faroq Razzaz Muhammad Arfan Naila Khaleel

We explore the interaction of a plane electromagnetic wave with a topological insulator (TI) cylinder that is coated with homogeneous magnetized ferrite. TIs display exotic electromagnetic responses due to topological magnetoelectric (TME) phenomena. An analytic theory for the electromagnetic scattering from a TI scatterer is developed. The analytical expressions of the polarized electromagnetic fields for the transverse magnetic (TM) case are formulated. The so-called unknown scattering coefficients are derived by implementing the boundary conditions (BCs) on the surface of a TI. The scattering characteristics of plane waves by a TI scatterer are numerically simulated and discussed. The numerical results demonstrate that the scattering characteristics are strongly influenced by the external magnetic field, axion angle, thickness of coating layer, and incident operating wave frequency. This work could provide valuable theoretical insights into the scattering phenomena of optical waves and find promising applications in optical manipulation, particle radiation force and torque, optical diagnosis, metamaterial structures, and wave optics in random media.

Photonics doi: 10.3390/photonics13050451

Authors: Kyohei Yamashita Ayaka Mori Eiji Tokunaga

The integrating sphere with sample inside (ISSI) method is useful for absorption spectroscopy of scattering samples, but the measured absorbance (Ameas) becomes nonlinear with dye concentration (c) because the sample is placed inside the sphere. This study modeled the Ameas−c relationship for ISSI using a cylindrical cell (CC) and a Brewster cell (BC) with simple analytical expressions based on the fraction of light not passing through the sample and the effective weights of light passing through it multiple times. Four aqueous dye solutions—Trypan Blue, Brilliant Blue FCF, Tartrazine, and New Coccine—were used as non-scattering samples. For CC, a single-pass model reproduced the measured relationship well for all dyes, and linearity was maintained in the low-absorbance region (up to approximately half of the saturation absorbance, Amax/2≈0.67 Abs). For BC, the same low-absorbance region (up to approximately Amax/2≈1.21 Abs) also exhibited practical linearity, but the full relationship including saturation required a multiple-pass model. Model selection based on adjusted RMSE and AICc identified the 3-pass model as the minimum sufficient model for BC. The saturation absorbance Amax was on average 1.81 times higher for BC than for CC (corresponding to an approximately 12-fold expansion in linear intensity ratio), and the upper concentration limit of the linear approximation was on average 1.85 times higher. These results demonstrate that BC extends the measurable concentration range while preserving practical low-absorbance linearity. In addition, the wavelength dependence of Amax observed at short wavelengths is attributed primarily to the reduced reflectance of the BaSO4 integrating-sphere wall rather than to the refractive-index dispersion of the quartz cell.

Photonics doi: 10.3390/photonics13050450

Authors: Xiaoru Li Ning Cui Baolu Guan

Vertical-cavity surface-emitting lasers (VCSELs) have emerged as essential light sources for atomic-precision measurement, quantum-secure communication, high-speed optical transmission, and laser coherent scanning detection, owing to their low power consumption, high-quality beam characteristics, and ease of two-dimensional integration. However, the fundamental limitation on linewidth narrowing in VCSELs arises from their inherently short resonator, resulting in a natural linewidth on the order of 50–100 MHz. This limitation prevents conventional VCSELs from meeting the stringent requirements of advanced applications, making the ultra-narrow linewidth a key focus in optoelectronics research. This review analyzes representative achievements and application scenarios of narrow-linewidth VCSELs, evaluates the merits and limitations of industrial-grade devices, and envisions future directions in next-generation optoelectronic systems. Distinct from existing reviews, it integrates key single-mode fabrication techniques, quantitative linewidth requirements across applications, silicon photonic integration, and scalable manufacturing trends, establishing a complete mechanism–technology–application–industry analytical framework.

Photonics doi: 10.3390/photonics13050449

Authors: Xiaochuan Zheng Yanhua Lu Xuguang Zhang Xingwang Luo Junzhi Ye Peng Huang Haoyue Shen Tianxiang Xie Lei Zhang Jianli Shang Qingsong Gao Weimin Wang

A 1319 nm, single-frequency, two-stage partially end-pumped slab (Innoslab) amplifier with high output power and excellent beam quality was reported. A 3 W, quasi-continuous wave pulsed, single-frequency all-fiber seed laser was amplified to a maximum average power of 154.0 W with a magnification of ~51.3 and overall optical-to-optical efficiency up to 12.0%. The output pulse width was 132.6 μs at a repetition rate of 500 Hz. The beam quality factors of M2 were 1.4 and 1.3 in the horizontal and vertical directions, respectively. The power stability at the maximum output power was 0.43% (RMS) in 10 min. Higher output power and optical-to-optical efficiency could be achieved through optimizing mode matching between the pump beam and the seed laser beam.

Photonics doi: 10.3390/photonics13050448

Authors: Julian Zettl Christian Lutz Ralf Hellmann

We report on a novel approach for the monitoring of tangential laser turning with ultrashort laser pulses. By using an ultra-sonic sensor consisting of a membrane-free optical microphone, the current state of the ablation process can be analyzed, potentially enabling a real-time automated regulation. With its high sensitivity, bandwidth, and sampling rate, it is an ideal tool for process monitoring. The material ablation caused by focused femtosecond laser pulses produces distinct sound waves, which can be detected by the optical microphone. The diameter reduction of a rotating cylindrical workpiece during the laser turning process with ultrashort laser pulses results in a variation in the acoustic emissions. From this, properties like the state of the machining progress can be inferred.

Photonics doi: 10.3390/photonics13050447

Authors: Zijie Wan Bo Ye Yangkun Zou Honggui Cao Shaoda Yin

Chromatic aberration correction remains a major challenge in dual-band infrared continuous zoom optical systems. To address this issue, an achromatic design method based on the equivalent refractive index and equivalent dispersion rate is proposed. Starting from a four-component continuous zoom model, chromatic compensation is introduced into the initial structural parameter calculation, and the initial structural parameters are obtained through an iterative procedure. To validate the proposed method, a MWIR/LWIR dual-band continuous zoom optical system is designed. The final system covers the MWIR (3.7–4.8 μm) and LWIR (8–10 μm) bands with a focal length range of 10–120 mm, and the chromatic focal shift is controlled within the depth of focus. Clear imaging is achieved in both bands over the entire zoom range. These results demonstrate the effectiveness of the proposed achromatic strategy and provide a practical approach for the design of wide-band achromatic zoom optical systems.

Photonics doi: 10.3390/photonics13050446

Authors: Wei Feng Bincheng Wang Xiaoyuan Pan Zhenmin Zhu Shan Lou Dawei Tang Feng Gao Fumin Zhang

Conventional optical imaging struggles to acquire clear images of underwater scenes in turbid water. In this paper, a new dual-DMD single-pixel 3D imaging (DSP3DI) system is designed and constructed to realize the 3D shape reconstruction in highly turbid water conditions. Leveraging the spectral dependence of the scattering coefficient of water on wavelength, the designed system uses a 532 nm laser as the illumination source to minimize scattering and absorption losses during light propagation, and two digital micromirror devices (DMDs) are used to generate phase-shifting fringe patterns and sampling patterns, respectively, and then uses a single-pixel detector to sequentially collect the spatial light field reflected from the surface of the object. A single-pixel imaging (SPI) method based on a cake-cutting strategy for Hadamard encoding reconstructs the deformed fringe images, from which phase information is recovered to calculate the 3D shape of objects. The experimental results show that the system not only achieves millimeter-level measurement accuracy but also successfully reconstructs the 3D shape of complex objects at a sampling rate of 10% and in turbidities as high as 40 NTU. The proposed system, characterized by its compact structure, high measurement accuracy, and strong scattering resistance, offers a novel solution for high-precision 3D imaging in highly turbid water.

Photonics doi: 10.3390/photonics13050444

Authors: Heng Du Lei Zhu Xiangyu Wang Zhouyang He Shiyang Shen Xiaodong Wang

Integrated optical phased array (OPA) chips enable high-speed beam steering via electronic phase control, providing a promising solution for compact pointing, acquisition, and tracking (PAT) systems. However, OPA-PAT systems must simultaneously achieve wide-field-of-view (FOV) coverage and high-precision angle-of-arrival (AOA) detection. To address this challenge, a cascaded AOA detection method based on a multi-sensor collaborative architecture is proposed. This approach utilizes a distributed detector array (DA) for coarse incident angle estimation over a wide-FOV, which then guides a two-dimensional (2D) galvanometer to steer the beam into a quadrant detector (QD) for fine measurement within a narrow-FOV. A prototype system is developed to validate the proposed cascaded algorithm. Experimental results show that within a ±20° FOV, the proposed system achieves root-mean-square errors (RMSEs) of 0.007° in azimuth and 0.01° in elevation. When integrated into an OPA-PAT terminal, static 2D closed-loop tracking is maintained with an overall tracking error better than 0.016° (RMSE). These results demonstrate that the proposed cascaded detection method can simultaneously provide wide-FOV coverage and high-precision AOA measurement, offering a practical solution for wide-FOV OPA-PAT systems.

Photonics doi: 10.3390/photonics13050445

Authors: Zefu Liu Xuqian Qiu Alexander A. Romanov Vasily A. Kostin Alexander A. Silaev Chenhui Lu Yi Liu

We report on the generation and spectral shaping of ultrabroadband terahertz-to-infrared radiation (>119 THz) from air plasma excited by a conventional tightly focused femtosecond Ti:Sa laser pulse with a duration of 35 fs assisted by its second harmonic (SH). A controllable and large frequency detuning between the SH and blueshifted component of the fundamental spectrum was achieved by utilizing spectral broadening of the fundamental pulse under filamentation and adjusting the longitudinal separation of the two cascaded filaments. For convenience, the resulting ultrabroadband emission is divided into a low-frequency part (<30 THz), an intermediate-frequency part (~50 THz), and a high-frequency part (~100 THz) that can be optimized with the filaments’ longitudinal separation. We attribute such ultrabroadband THz radiation generation to the excitation of photocurrent from the nonlinear interaction of SH with both the field at the fundamental frequency and its blueshifted component acquired during filamentation. Theoretical calculations based on time-dependent Schrödinger equation, as well as the Maxwell–Schrödinger equation for spectral broadening dynamics, reproduced the spectral features as well as the distinct dependence of the low- and high-frequency THz components.

Photonics doi: 10.3390/photonics13050443

Authors: Zijian Feng Jun Wang Huarui Wang Qianyu Ren Jia Liu Haiyang Wang Pinggang Jia

To address the demand for underwater acoustic detection with hydrostatic pressure resistance, this paper proposes a fiber-optic Fabry–Perot (F-P) underwater acoustic sensor based on micro-electromechanical system (MEMS) technology. According to the F-P interference principle, the diaphragm deforms under acoustic pressure, inducing variations in the F-P cavity length which modulate the interference spectrum and enable the measurement of underwater acoustic signals. A sensing diaphragm with a composite structure consisting of a central boss and a micro-hole array is designed, which improves the optical signal quality while reducing the influence of the pressure difference between the inner and outer surfaces of the diaphragm on sensor operation. MEMS fabrication, computer numerical control (CNC) machining, and laser fusion splicing technologies are employed to achieve batch fabrication of the sensing units and adhesive-free integration of the sensor. Experimental results show that the proposed sensor exhibits a flat frequency response within ±1.5 dB over the range of 1 kHz to 10 kHz, with an average signal-to-noise ratio (SNR) of 86.35 dB. The sensitivity reaches −181.79 dB re 1 rad/μPa at 10 kHz, with a maximum nonlinearity of 0.48% F.S., a repeatability error of 0.15% F.S. and a dynamic range of 100.83 dB. The proposed sensor features miniaturization, high consistency, hydrostatic pressure self-balancing capability, and immunity to electromagnetic interference, providing a solid foundation for hydrostatic-pressure-resistant underwater acoustic measurements in deep-sea environments.

Photonics doi: 10.3390/photonics13050442

Authors: Li-Ping Li Jin-Xu Du Lei Zhang Zhi-Hong Jiao Song-Feng Zhao Guo-Li Wang

We present a method to control broadband terahertz generation rapidly during the interaction of a strong laser field with a gas. To achieve it, we utilize a few-cycle double-pulse, which is a combination of two identically colored femtosecond fields with a time delay, as a driving laser field. By varying the laser delay, the magnitude of the amplitude of generated terahertz field changes drastically, making it suitable for use as a terahertz optical ultrafast switch, with an optical period of only a few femtoseconds from ON-OFF-ON and an enhancement ratio of 100. Furthermore, a change in time delay can alter the terahertz field waveform, easily generating terahertz electric fields with positive and negative polarity or any phase in the range of [0, 1.0π]. The strength of such terahertz source can be boosted by raising the laser wavelength. Our study will provide an effective approach for ultrafast terahertz modulation.

Photonics doi: 10.3390/photonics13050441

Authors: Yukun Liu Xisheng Li Dabao Lao Zhengyang Zhang Xiaojian Wang Tianqi Chen

To address the multi-sensor collaborative calibration challenges in laser 3D projection systems, a pose calibration method integrating binocular vision and laser ranging is proposed. A multi-coordinate system fusion framework encompassing the camera coordinate system, galvanometer coordinate system, and workpiece coordinate system is established. Through the calculation of reference pose matrices and real-time transformations, adaptive calibration under arbitrary workpiece placements is achieved. Experimental results demonstrate that within a working range of 1.5–2.5 m, the calibration error is 45.5 μm, meeting the high-precision requirements of aerospace precision machining and assembly.

Photonics doi: 10.3390/photonics13050440

Authors: Kun Chen Shunjun Wei Mou Wang Juran Chen Bingyu Han Jin Li Zhe Liu Xiaoling Zhang Yi Liao Pengcheng Gao Xiaolin Mi

Non-Line-Of-Sight (NLOS) Terahertz (THz) radar 3D imaging leverages electromagnetic wave propagation characteristics such as reflection, diffraction, scattering, and penetration to detect, locate, and image hidden targets in occluded environments. It holds significant potential for applications in autonomous driving, disaster rescue, and urban warfare. However, uncertainties introduced by reflecting surfaces and occluding objects in practical NLOS scenarios, such as phase errors, aperture shadowing, and multipath effects, lead to issues like blurred imaging and increased artifacts in radar imaging. To address these challenges, this study proposes a 3D learning imaging method for NLOS THz radar based on a holographic imaging operator, leveraging the adaptive optimization properties of deep unfolding networks and prior environmental perception. First, a 3D imaging model for NLOS THz radar in the Looking Around Corner (LAC) scenario is established. A holographic imaging operator is introduced to enhance imaging efficiency and reduce computational complexity. Second, a high-precision NLOS 3D imaging network is constructed based on the Fast Iterative Shrinkage/Thresholding Algorithm (FISTA) framework. Utilizing features specific to NLOS scenes and designing algorithm parameters as functions of network weights, the method achieves high-precision and high-efficiency in the 3D reconstruction of NLOS targets. Finally, a near-field NLOS radar imaging platform operating at 121 GHz (within the sub-THz regime) is developed. Experimental validations in the LAC scenario are performed on targets, including metal letters “E”, a metal resolution chart, and a pair of scissors. The results demonstrate that the proposed method significantly improves 3D imaging precision, achieving a two-orders-of-magnitude increase in computational speed over traditional imaging algorithms.

Photonics doi: 10.3390/photonics13050439

Authors: Hsin You Hou Cheng-Huan Chen

Light field displays offer promising autostereoscopic 3D visualization with continuous parallax, full-color reproduction, and natural depth cues. In practice, ray-tracing simulation is often employed to evaluate the effective viewing zone of light field systems. However, the actual viewing zone in conventional light field systems is significantly smaller than the simulated value, severely limited by narrow viewing angles due to crosstalk from adjacent elemental images. This study proposes an isolated microlens array (i-MLA) architecture incorporating a custom directional mesh (DM)—a 3D-printed light-blocking barrier grid with tapered pitch—to physically isolate each lenslet and completely suppress crosstalk. Combined with optimized extended coding pitch for a target viewing distance, ray-tracing simulations and experiments on a 13.3-inch 4K display with a 7 mm and 5.4 mm pitch MLA demonstrate dramatic improvement. The conventional light field system provides only a 3.4° margin, which is below the minimum angular separation required for binocular viewing, whereas the i-MLA system achieves a 7.4° margin—twice that of the conventional system. Compared with conventional systems, the i-MLA architecture does not increase overall volume; it simply replaces the glass gap with a single, simple optical element to achieve a wider viewing angle while preserving the compact form factor.

Photonics doi: 10.3390/photonics13050438

Authors: Ruying Xiong Lin Peng Xu Zheng Junhui Wu Hongjie Liu Xiaodi Tan

A collinear holographic data storage system stores two-dimensional information in the three-dimensional spatial domain of the medium, offering features such as high speed, high density, and long lifespan, making it a promising technology for the future of data storage. However, a collinear holographic data storage system is limited by the alignment error of the optical system and is also sensitive to environmental noise and external interference, which increases the reading error. When recording and reading holographic storage materials, synchronous marks are used for positioning to correct data misalignment. Therefore, optimizing synchronous mark design of data pages is crucial for improving storage stability and reading accuracy. In this paper, we propose a star-shaped synchronous mark to replace the square-shaped synchronous mark, which improves the holographic grating coupling efficiency. Experimental results show that this method enhances reconstruction strength and reduces reading errors caused by external factors. The star-shaped synchronous mark achieves a better spectral match with the reference pattern, yielding a stronger diffracted signal. Experimental results show that this method reduces the bit error rate by approximately 25% compared to square-shaped synchronous marks under displacement multiplexing.

Photonics doi: 10.3390/photonics13050436

Authors: Jozsef Seres Enikoe Seres Thorsten Schumm

We simulate the generation of multiple harmonics up to the sixth order extending into vacuum ultraviolet. The harmonics are generated by χ(n):χ(m) cascades, containing second- or third-order perturbative nonlinear processes. We identify three additional phase-matching conditions beyond standard phase matching, namely when only the first step or only the second step of the cascades are phase-matched and when the non-phase-matched second or third harmonic produces quasi-phase matching for higher-order harmonics, causing an essential enhancement of the harmonic signals.

Photonics doi: 10.3390/photonics13050437

Authors: Haisong Tang Shuang Liu Huan Zhan Guanghua Cheng Wei Zhang

Direct welding of fused silica to pure titanium (Ti) foil using conventional methods faces significant challenges, such as poor interfacial wettability, insufficient joint strength, and the need for interlayers or surface pretreatments. Existing femtosecond (fs) laser welding techniques for these materials often require high-energy millijoule (mJ)-level pulses or alloy interlayers. Moreover, reports on direct microjoule (μJ)-level fs laser welding of Ti foil to fused silica remain scarce. This study successfully demonstrates a direct welding process for pure Ti foil and fused silica using μJ-level fs laser pulses under ambient conditions, achieving joints with a maximum shear strength of 9.19 MPa. Microstructural analysis revealed an elemental interdiffusion region at the weld interface, supported by mechanical interlocking effects. X-ray photoelectron spectroscopy (XPS) confirmed the occurrence of interfacial chemical reactions, forming titanium silicide (TiSi2) and titanium oxide (TiO2). Additionally, a 24 h water immersion test of a square sealed cavity revealed outstanding hermeticity, with no water ingress. This work provides a simple, efficient, and robust solution for high-strength, additive-free bonding of fused silica to Ti foil under low-energy processing conditions.

Photonics doi: 10.3390/photonics13050435

Authors: Levin Stolz Alwin Kienle Florian Foschum

Accurate determination of the optical absorption coefficient, μa, in turbid media is fundamental to biomedical optics and material characterization. Integrating sphere techniques, which measure total transmittance and reflectance, are a standard method for this purpose. However, the inverse models typically employed rely on the assumption of isotropic light propagation. In fiber-structured materials—a common geometry in biological tissue–this assumption often breaks down, leading to significant quantification errors. In this study, we investigated this effect using Monte Carlo simulations and proof-of-concept experiments on mechanically stretched PTFE tape. The medium was modeled as a slab of aligned dielectric cylinders embedded in an isotropic matrix, and the performance of an isotropic inverse model was compared with that of an anisotropic inverse model. The isotropic model showed substantial systematic errors in μa, with a mean absolute error (MAE) of 19.3%, typical errors between approximately −40% and 50%, and outliers reaching up to 300%. In contrast, the matched anisotropic model achieved a MAE of 1.2%. Even when the structural parameters of the anisotropic model were perturbed, the MAE remained low at 1.8% for moderate perturbations and 3.9% for severe perturbations. The simulation results therefore indicate that, for the integrating sphere framework considered here, incorporating anisotropic light propagation can improve absorption retrieval more strongly than precise knowledge of all geometric details. Measurements on stretched PTFE tape showed the same qualitative trend and provide proof-of-concept experimental support for the simulation-based findings.

Photonics doi: 10.3390/photonics13050434

Authors: Hong Zhang Zhangyi Du Yitian Zuo Yajie Huang Jinkang Wu Zhinuo Chen Yifei Gao Junbao Hu Yu Lei

Precise sculpturing of light empowers light with abundant phenomena across fundamental physics and practical applications. The emergence of metasurfaces provides a pivotal solution to the limitations of traditional optical components, which make it difficult to meet the integration requirements of diverse applications, and they are distinguished by their ultra-thin profiles, low optical losses, and high degree of controllability. In this paper, we elucidate the core physical principles to manipulate phase, amplitude, and polarization with meta-optic architecture, along with nonlocal effects. Specifically, we revisit the research progress and typical applications of meta-waveguides, meta-fibers, meta-lasers, meta-spectrometers, and meta-sensing. Finally, it looks forward to the future development direction of meta-optics in exploring the limits of light field control, chip-scale functional integration, and discovering new physical effects, providing theoretical and technical references for the development of metaphotonic devices.

Photonics doi: 10.3390/photonics13050433

Authors: Minsi Lin Zhenqi Huang Yue Shen Haobin Xiao Yingying Fu Mingjie Zhang Yuanzhi Chen Yi Zhou Siqi Zhu Zhenqiang Chen

The long-wave infrared band, which at room temperature covers the infrared radiation of humans and objects, has significant applications across various fields including wireless communication, national defense, military, biomedical, and advanced driver assistance systems. Metalens provides a pathway to lightweight, compact, and integrated solutions for infrared imaging and sensing systems, marking an inevitable trend in future development. This study presents a design for a high numerical aperture of 0.89 in a polarization-insensitive all-chalcogenide metalens operating at 10 µm, utilizing the commercially available chalcogenide glass material As2Se3 via a transmission phase approach. Building upon this, we have achieved, for the first time, a high numerical aperture of 0.84 for an all-chalcogenide broadband LWIR achromatic metalens operating in the 9.5–10.5 µm range, with significantly improved focusing performance through the application of particle swarm optimization algorithms. The superior performance of the all-chalcogenide LWIR metalens, combined with the advantages of chalcogenide glass over traditional LWIR materials such as Si or Ge—namely, lower cost, reduced optical loss, and a smaller thermo-optic coefficient—suggests it has significant potential for broader applications.

Photonics doi: 10.3390/photonics13050432

Authors: Seyed Saman Mahjour Fernando M. Araújo-Moreira

Continuous-Variable Quantum Key Distribution (CV-QKD) offers practical advantages for secure communication, but laser linewidth-induced phase noise remains a critical performance limitation. This work presents a comprehensive simulation-based analysis quantifying the impact of laser linewidth on secret key rate (SKR) in Gaussian-modulated coherent-state CV-QKD systems. We develop a detailed noise model incorporating detector electronics, Raman scattering, phase recovery, ADC quantization, and laser relative intensity noise. Through systematic parameter sweeps spanning linewidths from 10 Hz to 250 kHz, modulation variances from 1 to 20 SNU, and fiber distances up to 100 km, we identify three distinct operational regimes and optimization strategies for both transmitted local oscillator (TLO) and local–local oscillator (LLO) configurations under homodyne and heterodyne detection. Results show that metropolitan-scale links (50 km) require linewidths below 5 kHz to maintain secure operation, with performance decreasing beyond 25 kHz. We demonstrate that modulation variance must be jointly optimized with laser quality, with optimal values decreasing from 3–4 SNU at narrow linewidths to 2–2.5 SNU at moderate linewidths. The analysis reveals asymmetric sensitivity in LLO systems where local oscillator linewidth degrades performance more strongly than signal laser linewidth. These quantitative findings provide practical design guidelines for achieving secure CV-QKD operation over metropolitan distances with realistic hardware constraints, supporting deployment of defense-grade quantum communication networks.

Photonics doi: 10.3390/photonics13050431

Authors: Kerui Fu Tianling Qin

Memristors, as transformative electronic devices designed to transcend the von Neumann architecture, enable the physical unification of information storage and computation, thereby offering a foundational hardware pathway toward energy-efficient, brain-inspired computing. Their intrinsic analog resistive switching, non-volatility, and history-dependent learning capabilities allow them to natively implement in-memory computing and emulate synaptic plasticity, addressing the critical bottlenecks of energy and speed in conventional systems. Notably, the evolution from electrically controlled memristors to optoelectronic memristors marks a paradigm shift from pure computing to integrated sensing-processing, opening new dimensions for high-speed, parallel, and adaptive signal processing. In recent years, significant progress has been made in the development of memristor-based neuromorphic vision and tactile systems, on-chip signal processors, and dynamic trajectory trackers, demonstrating their potential in edge intelligence, adaptive robotics, and real-time perceptual tasks. This review systematically summarizes the latest advances in memristor technology, providing a comprehensive analysis of their operating mechanisms, material and structural innovations, and cutting-edge applications in neuromorphic perception and computing. Furthermore, it discusses the key challenges and future directions for the development and integration of memristor-based systems in the post-von Neumann era.

Photonics doi: 10.3390/photonics13050430

Authors: Na Xie Jingyi Fu Yunan Wu Bingqing Xie Ning Ma Jun Chang

Field-deflection optical elements play a significant role in various high-resolution and field-of-view (FOV) expandable optical systems. In our previous work, we proposed a self-achromatic double-faced grism (DFG) configuration capable of field deflection. In this paper, to effectively guide the design and fabrication of DFG diffractive microstructures, we analyze the diffraction process of light within the DFG based on scalar diffraction theory, establish a diffraction efficiency model for the DFG, and propose an optimization method to improve diffraction efficiency over all FOVs while minimizing diffraction efficiency variations across the waveband. Following this design approach, two DFGs for 3–5 μm were specifically designed: one for a 4.4° FOV with 1.6° deflection, and the other for a 40° FOV with 15° deflection. Further optimization increased the bandwidth-integrated average diffraction efficiency (BIADE) to above 0.93 and reduced the diffraction efficiency variations across the waveband by 23.5% and 48%. And based on a tolerance analysis model, we performed tolerance analysis of the designed DFGs. The BIADE of both elements across all FOVs can exceed 0.86 after adding fabrication errors, maintaining a high diffraction efficiency. The results demonstrate that this method can effectively guide the design and manufacturing of diffractive microstructures.

Photonics doi: 10.3390/photonics13050429

Authors: Yichen Wang Hongjun Tian Yuhan Zhou Yang Xiong Yichen Li Manlin Wang Yijie Yin Xiaoyin Guo Jiani Wu Jiesen Zhang Ying Tang Shuai Huang

Accurate target perception in unstructured outdoor environments remains a fundamental challenge in computational imaging and machine vision, primarily due to severe optical degradation caused by variable illumination, specular highlights, and dense foliage occlusion. Existing optical sensing systems often struggle to maintain robustness under these physical constraints, especially when deployed on edge devices with strict computational limits. To address these challenges, this paper proposes Orchard-YOLO, a lightweight, computationally efficient object detection network designed to maintain robustness against environmental and optical noise in complex orchard environments. Unlike generic architectures, Orchard-YOLO introduces three architectural enhancements for robust detection: (1) a High-Resolution P2 Detection Head to preserve high-frequency optical details and fine-grained texture cues often lost during digital downsampling; (2) Coordinate Attention (CA) mechanisms integrated into the feature fusion pathway to filter out background optical interference and enhance spatial discrimination for heavily occluded targets; and (3) a Ghost-convolution-based backbone to optimize the inference pipeline for real-time edge processing. Evaluated on a comprehensive multi-fruit dataset under simulated optical stress (including ±50% illumination variation and up to 70% occlusion), Orchard-YOLO achieves 94.8% mAP@0.5. It shows improved robustness under illumination variation and occlusion compared to baseline models, while achieving up to 25 FPS on an NVIDIA Jetson Nano edge device. These results suggest that Orchard-YOLO offers a detection framework suitable for resource-constrained orchard perception.

Photonics doi: 10.3390/photonics13050428

Authors: Pengfei Huang Tao Wang

Mixed-scene holographic 3D display for film and television visual content presentation remains challenging because recorded digital holograms and computer-generated holograms (CGHs) are produced under different numerical and hardware constraints. Direct hologram superposition typically causes strong zero-order interference, diffraction efficiency degradation, and sampling pitch mismatch between the recording sensor and the replay panel, while conventional resizing reduces the effective replay aperture and narrows the available parallax. To address these issues, this paper proposes a zero-order-suppressed single-hologram fusion framework with parallax-preserving digital resizing. A recorded digital hologram is first processed by Gaussian high-pass filtering to suppress the dominant zero-order component, then resampled to match the LCOS replay pitch, and finally normalized and fused with a CGH generated through bipolar intensity encoding. On this basis, two resizing routes are developed: a spatial-domain method for aperture-preserving whole-scene scaling and a frequency-domain method for object-selective scaling and translation. Optical validation on a three-channel LCOS prototype shows that the quantitative diffraction efficiency analysis predicts an increase from approximately 10.1% to 20.05% per reconstructed object for the two-hologram fusion case, and the revised experimental results are consistent with this improvement trend. The experiments further verify replay scaling at multiple factors, the selective manipulation of physical and virtual objects, mixed-scene color replay, and occlusion-consistent depth ordering. Together with the distortion analysis, these results demonstrate improved replay visibility after fusion while maintaining geometric controllability and effective replay aperture. By relying on hologram-domain preprocessing and resizing rather than full mixed-scene recomputation, the proposed method also reduces computational burden. The study therefore provides an efficient and controllable mixed-scene holographic replay framework for visually enriched film and television content presentation, although its depth applicability remains bounded and dedicated real-time timing benchmarks are left for future work.

Photonics doi: 10.3390/photonics13050427

Authors: Zunhui Wang Yicheng Wang Qingli Ma Yanhua Wu

Underwater gated time-correlated single-photon-counting (TCSPC) LiDAR is advantageous when weak target echoes coexist with strong backscatter. However, under the first-photon-triggering and SPAD dead-time mechanism, the estimated time of flight becomes dependent on the return strength, thereby producing a range walk error (RWE). This paper develops a condition-calibrated correction framework for accumulated-histogram underwater ranging in the low-photon regime. A non-homogeneous Poisson first-arrival model that jointly includes gate-limited signal photons and in-gate background triggering yields a computable expression for the total trigger probability and the conditional first-arrival time. A first-order expansion around Npe≈0 leads to an approximately linear RWE–Npe relation under the present system–water condition. A density-based signal-window localization method and a noise-occlusion-compensated estimator of Npe are combined with reference-plane differential calibration. Experiments in a 10 m clear-freshwater tank at 9.11 m show that the mean absolute error is reduced from 39.205 mm to 2.130 mm, corresponding to a 94.57% improvement. Compared with a quadratic model used under higher-photon conditions, the proposed linear model yields an order-of-magnitude smaller residual error in the low-photon region (Npe<1.6).

Photonics doi: 10.3390/photonics13050426

Authors: Zhuxiong Ye Shu Liu Mingkang Han Jia Feng Mustafa Shah Yongze Zhao Pengjun Wang Liangchao Chen Wei Han Zengming Meng Lianghui Huang

We demonstrate a sideband Pound–Drever–Hall (SPDH) locking scheme that enables the simultaneous narrow-linewidth stabilization and continuous broadband frequency tuning of a laser referenced to an ultra-stable cavity. The method employs dual-frequency modulation applied to a fiber electro-optic modulator, where high-frequency modulation generates tunable sidebands and low-frequency modulation provides the error signal. We experimentally stabilize a 922 nm seed laser to the cavity and achieve a laser linewidth of 85(1) kHz with frequency noise suppression of up to 25 dB. The residual amplitude modulation (RAM) remains below 0.08% across the full tuning range. In addition, we demonstrate a continuous frequency tuning range of 1.4 GHz for a frequency-doubled 461 nm laser, with scan rates up to 317 MHz/s, while preserving stable locking to the cavity. This approach avoids complex waveform generation and provides a simple and robust solution for broadband laser frequency control.

Photonics doi: 10.3390/photonics13050425

Authors: Sabai Phuchortham Hakilo Sabit

Free-space optical (FSO) communication is an attractive high-capacity wireless technology for terrestrial, aerial, and satellite links. However, FSO performance is strongly affected by multiple impairments, including path loss, turbulence attenuation, pointing errors, and equipment loss. Therefore, accurate performance evaluation requires channel modelling that accounts for both deterministic power losses and stochastic channel effects. This paper presents a comprehensive and structured review of FSO channel modelling, covering the transmission, propagation medium, and receiver sections. The composite channel response is represented using a mathematical formulation. Commonly used FSO models are reviewed and organised, including Beer–Lambert and geometrical loss, Kim and Kruse path loss models, Lognormal, Gamma–Gamma, K, and Málaga distributions, along with pointing-error and angle-of-arrival models. Each model is explained in terms of its physical meaning, assumptions, and applicable operating conditions. Lastly, a numerical example is presented to demonstrate how deterministic losses and stochastic channel effects can be combined in FSO performance evaluation.

Photonics doi: 10.3390/photonics13050423

Authors: Zhichao Chen Zhaoyan Liu Shi Qiu Huijing Zhang Yuwei Chen Weiyuan Yao Tong Zhang Yu Zhang Hongjia Cheng Feihong Wang Zhan Shu

Multispectral LiDAR can simultaneously obtain distance and spectral information and shows great potential for underwater detection. However, absorption and scattering caused by suspended particles in water lead to energy attenuation and multiple scattering, which affect echo intensity and ranging accuracy, while the propagation characteristics under multi-wavelength conditions remain insufficiently studied. In this study, a simplified underwater propagation simulation model for multispectral LiDAR is established based on the equivalent spherical-particle assumption, combining Mie scattering theory with a semi-analytical Monte Carlo method. The effects of particle size on echo intensity and ranging error are analyzed under fixed concentration conditions. Based on this model, a detection-threshold-constrained optimal wavelength selection criterion is formulated. Multi-distance analysis (3, 5, 8, and 15 m) confirms that the preferred wavelength is primarily governed by particle size and remains stable across depths. The results show that the optimal detection wavelength shifts with particle size, being about 560 nm for fine particles and gradually moving toward the 400–480 nm blue–green band for larger particles. Experimental validation shows that the simulation-based ranging correction reduces RMSE by 9.4–25.9% (average 18.1%) and MAE by 11.8–29.7% (average 22.0%) across five experimental distances. The results provide a preliminary reference for wavelength selection in multispectral LiDAR systems under simplified conditions.

Photonics doi: 10.3390/photonics13050424

Authors: Jianming Liu Wenbin Lin Wei Liu Jinjuan Cheng Chengcheng He Wei Xu Jia Guo

Amplified Spontaneous Emission (ASE) sources are widely employed as highly stable broadband sources in fields such as high-precision navigation and optical detection. Erbium-doped fiber (EDF), as the core active component in ASE sources, has long been a key subject of thermal stability research. We fabricated a low-doped EDF with an 80 μm-cladding using the vapor phase doping (VPD) technique. This EDF was compared with a commercial 125 μm-cladding EDF using a double-pass forward (DPF) optical path configuration with a narrowband filter. We investigated the temperature-dependent characteristics of the ASE spectra generated by the two EDFs with different parameters. The temperature drift performance of the two EDFs was analyzed based on three critical indicators of the spectrum: mean wavelength, spectral bandwidth, and output power. In comparison with the commonly used EDF, the results show that a properly designed small-cladding EDF with an appropriate length can deliver higher ASE output power and exhibit a lower mean-wavelength temperature drift. This study provides an important guideline for promoting the miniaturization of high-precision fiber-optic sensing devices.

Photonics doi: 10.3390/photonics13050422

Authors: Ruyue Xiao Ming Hao Shuang Liang Weigang Hou Jianming Tang

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

Photonics doi: 10.3390/photonics13050421

Authors: Ruotong Mei Weidong Bai Xinming Zhang Junhong Wang Yu Wang Baoquan Jin

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

Photonics doi: 10.3390/photonics13050420

Authors: Silun Du Tianshu Wang Bo Zhang Shimeng Tan Tuo Chen

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

Photonics doi: 10.3390/photonics13050419

Authors: Yan Xu Shuai Wang

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

Photonics doi: 10.3390/photonics13050418

Authors: Haoxuan Li Xuhao Fan Qirui Sun Shian Mi Changyi Pan Huiyong Deng Ning Dai Yufeng Shan

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

Photonics doi: 10.3390/photonics13050417

Authors: Ting Zeng Chunyang Bi Zhichao Bi Jun Zhou Sen Gong

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

Photonics doi: 10.3390/photonics13050416

Authors: Yan-Yi Li Chuen-Lin Tien Hsi-Fu Shih Han-Yen Tu Chih-Cheng Chen

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

Photonics doi: 10.3390/photonics13050415

Authors: Dacai Liu Bin Li

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

Photonics doi: 10.3390/photonics13050414

Authors: Zhaohui Wang Jiameng Dong Shangsu Ding Song Yu Bin Luo

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

Photonics doi: 10.3390/photonics13050413

Authors: Dongjing Li Chenyang Cui Fan Guo Pingping Min

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

Photonics doi: 10.3390/photonics13050412

Authors: Jialing Chen Zixi Yu Jianglong Lei Yuanxiang Wang Qingli Jing

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

Photonics doi: 10.3390/photonics13050411

Authors: Wenning Xu Rongqing Tan Zhiyong Li

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

Photonics doi: 10.3390/photonics13050410

Authors: Haichen Wang Jiahao Si Jingxuan Chen Zhaozheng Yi Shuyuan Shi Mingjin Wang Wanhua Zheng

We propose a monolithically integrated optical frequency comb (OFC) generation platform on thin-film lithium niobate (TFLN), featuring cascaded dual-drive Mach–Zehnder modulators (DDMZM) and a Si3N4-assisted spot size converter (SSC). To capture microscopic mode mismatches and spatial phase accumulation often overlooked in idealized scalar simulations, we establish a multi-physics co-simulation framework integrating finite-difference time-domain (FDTD) analysis with macroscopic transmission modeling. Based on this framework, the cascaded modulator architecture generates 25 highly stable comb lines with a dense 2 GHz spacing and an envelope flatness within 2 dB. Tolerance analysis indicates that the comb generation is highly resilient to typical manufacturing and environmental variations, including thermal bias drift, RF phase mismatch, and half-wave voltage (Vπ) dispersion. Furthermore, physical-layer modeling shows that the integrated SSC reduces fiber-to-chip coupling loss to 0.55 dB per facet, preserving the necessary optical power budget. To validate the platform’s viability as a multi-wavelength continuous-wave source for spatial-division multiplexed (SDM) interconnects, a parallel transmission over a 20 km standard single-mode fiber is modeled. Using a digital signal processing (DSP)-free 10 Gb/s non-return-to-zero (NRZ) scheme, the 25-channel system maintains a worst-case bit error rate strictly below the forward error correction (FEC) threshold. This work offers a practical, physics-based evaluation framework for high-density co-packaged optics (CPO).

Photonics doi: 10.3390/photonics13050409

Authors: Dongdong Ye Lipeng Hu Jianfei Xu Yadong Yang Zeping Liu Sitong Li Jiabao Li Longhai Liu Changpeng Li

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

Photonics doi: 10.3390/photonics13050408

Authors: Ming-Jun Li

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

Photonics doi: 10.3390/photonics13050407

Authors: Xiaoming Chen Xiaoyu Jiang Yingqing Huang Xi Wang Chaoqun Ma

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

Photonics doi: 10.3390/photonics13050406

Authors: Xiaohan Mei Yi Tan Ce Wang Jiayao Wu Ping Yang Shuai Wang

Laser beam combining is essential for achieving high-power and high-radiance output. However, atmospheric turbulence induces independent tip–tilt aberrations across discrete sub-beams in laser array systems, which severely degrades the concentration of far-field energy. Traditional wavefront sensing techniques are primarily designed for the continuous wavefront of a single laser and are not directly applicable to laser array, whereas indirect optimization-based methods often suffer from slow convergence and limited real-time performance. To address these limitations, this study introduces a tip–tilt aberration compensation system for laser array propagation based on cooperative beacons with a shared-aperture transmit–receive configuration. The primary innovation consists of a modified Shack–Hartmann wavefront sensor (SHWFS) tailored to a discrete multi-beam layout, which facilitates the direct, independent, and simultaneous measurement of tip–tilt aberrations for each sub-beam. In conjunction with a segmented deformable mirror (SDM), the architecture can facilitate real-time closed-loop correction with high bandwidth and high precision. Numerical simulations of a 7-, 19-, and 37-beam laser array, together with validation experiments utilizing a 30-beam configuration, demonstrate that the proposed approach effectively suppresses tip–tilt error induced by turbulence. After closed-loop correction, the Strehl ratio (SR) increases above 0.92 (r0=5 cm), while the beam quality factor β reduces below 1.37 (r0=5 cm). Furthermore, the system retains performance stability as the number of sub-beams increases, demonstrating the scalability of the proposed method. In contrast to conventional approaches designed for a continuous wavefront, the proposed method offers a feasible approach for a discrete laser array system, providing robust and scalable tip–tilt correction under varying atmospheric conditions.

Photonics doi: 10.3390/photonics13050405

Authors: Zhenjie Zhou Qingtao Wu Xiaoping Wang

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

Photonics doi: 10.3390/photonics13050404

Authors: Léa Chaccour

Vertical-External-Cavity Surface-Emitting Lasers (VECSELs) have gained significant attention over the past two decades due to their versatility in a wide range of photonic applications. This review focuses on VECSEL configurations for dual-wavelength emission, highlighting their use in high-resolution spectroscopy, terahertz (THz) generation, and advanced optical communication. We explore recent developments in VECSEL designs, including systems utilizing birefringent crystals for polarization-based frequency separation and configurations with dual-VECSEL chips or dual-gain regions within a single cavity. These two-wavelength VECSELs enable diverse operation modes, including narrow-linewidth, pulsed, multimode, and frequency-converted emission, with high-brightness output, excellent beam quality, and tunable wavelengths. Additionally, the review discusses advancements in dual-frequency VECSELs, with applications in LIDAR systems for environmental monitoring, highly stable optical clocks, and fiber sensors. We examine improvements in cavity design, semiconductor structures, and power stabilization, which have enhanced frequency stability and spectral purity, making VECSELs suitable for precision metrology and sensing applications.

Photonics doi: 10.3390/photonics13050403

Authors: Shuo Qiao Yixuan Liang Zhangfu Huang Ziqiang Hu Wenjie Tao

Sapphire, with excellent optical properties and high hardness, has become a key hard and brittle material component in extreme environments like aviation equipment and infrared detection systems. Its processing quality directly determines the performance of various equipment systems. To address processing defects, technologies such as multi-wire cutting, magnetorheological polishing, chemical mechanical polishing, femtosecond laser processing, and ion beam etching have been developed and studied to improve the surface quality of sapphire components. This paper focuses on key technologies, including sapphire’s nano-scale surface morphology control, intrinsic nano-surface atomic-level defect control, and combined process systems for precision and shape control. These technologies lay the foundation for sapphire components’ process chain manufacturing to achieve high-precision shape and surface quality control.

Photonics doi: 10.3390/photonics13050402

Authors: Meryem Filiz Ömer Galip Saraçoğlu

In this study, a simulation-based investigation of the variations of the bit error rate (BER) and the maximum quality factor are presented for short- (S-), conventional- (C-), and long- (L-) band wavelengths in a photonic ultra-wideband (UWB) circuit using a semiconductor optical amplifier (SOA) with different bias currents and a bandpass filter (BPF). Gaussian quadruplet UWB pulses are generated at the S-, C-, and L-band wavelengths, which are commonly used in fiber transmission lines. An analysis of the temporal and spectral features of the generated pulses is carried out. The highest maximum quality factor and the lowest minimum BER are obtained in the C-band at an SOA bias current of 150 mA. This study simultaneously investigates both UWB pulse generation and transmission performance. The proposed circuit has a simple design and high applicability, as it employs a SOA, a Gaussian optical filter, a low-pass filter (LPF) and a single BPF.

Photonics doi: 10.3390/photonics13050401

Authors: Gordon R. M. Robb Kelsey O’Donnell Gian-Luca Oppo Thorsten Ackemann

We study a 1D model of a diffraction-mediated self-structuring instability which can occur when a Bose–Einstein condensate is illuminated by a pump laser and its reflection from a single feedback mirror. We carry out a linear stability analysis and, using numerical simulations, investigate the dynamics of the self-structuring process. Two dynamical regimes are identified: one in which the system behaves as a continuous space-time crystal oscillating between two states (one spatially uniform and one spatially periodic) and another where many condensate momentum states are involved and the condensate density develops chevrons which form and disperse quasi-periodically. We show the dependence of the pattern modulation depth and pattern formation time on pump saturation parameter and compare the simulation results with analytical expressions derived from a quantum Hamiltonian Mean Field model. The results show that this system offers a route to the first experimental realisation of the quantum Hamiltonian Mean Field model and of a continuous space-time crystal with a tunable spatial period.

Photonics doi: 10.3390/photonics13040400

Authors: Yiting Wang Xi He Zongqiang Fu Rui Duan Xiubin Yang

Compared with the conventional push-broom imaging mode, conical scanning extends the imaging swath through rotational scanning and is suitable for high-resolution, wide-swath remote sensing. To achieve continuous full-coverage imaging, the camera must be mounted at a certain tilt angle and employ an off-axis optical system with a sufficiently large field of view (FOV). However, the tilted installation causes nonuniform irradiance and increased off-axis distortion, while wide-field off-axis imaging also introduces radiometric consistency problems in focal-plane multi-detector stitching. To address these issues, this study investigates the optical design of a tilted off-axis camera for conical-scan imaging. Under the constraints of full coverage and swath requirements, key optical parameters were jointly determined, and a lightweight wide-coverage off-axis three-mirror system was designed, optimized, and evaluated. The final system has a focal length of 1545 mm, an F-number of 8.4, and a full FOV of 23.4° × 11.7°. The modulation transfer function is greater than 0.41 at the Nyquist frequency, and the maximum distortion is less than 2.5446%. In addition, for the focal-plane optical stitching structure, the coupled effects of local structural vignetting and global geometric vignetting induced by the tilted installation were analyzed. The results show that the gray-level difference in the adjacent detector overlap regions is only 0.31–0.53 digital numbers (DN), and the full focal plane shows a smooth gray-level attenuation rate of 5.39–6.77%. These results indicate that vignetting has no significant effect on focal-plane stitching. The proposed camera is well suited for conical-scan imaging.

Photonics doi: 10.3390/photonics13040399

Authors: Chen-Rui Yang Huan-Huan Cheng Shao-Xiong Wu Cheng-Hua Bai

We propose a proposal to achieve strong mechanical squeezing in an optomechanical system through the joint effect of a weak squeezed vacuum field and Duffing nonlinearity. The squeezing of the cavity field induced by the squeezed vacuum field is transferred to the mechanical oscillator, which has already been squeezed via Duffing nonlinearity. This joint effect significantly enhances the degree of mechanical squeezing, enabling it to exceed the 3 dB strong mechanical squeezing limit. Moreover, the resulting mechanical squeezing exhibits remarkable robustness against thermal noise. The joint effect proposed in this scheme can be directly observed through homodyne detection of the cavity output field. This novel approach opens up a new avenue for generating a strong mechanical squeezed state and provides a promising pathway for the applications of macroscopic quantum control in quantum sensing and quantum information processing.

Photonics doi: 10.3390/photonics13040398

Authors: Peiyi Lu Weiwei Liu Silin Yang

Realizing high-quality-factor (high-Q) plasmonic resonances in the visible regime is critical for enhancing light-matter interactions and advancing biochemical sensing. However, traditional localized surface plasmon resonances (LSPRs) typically suffer from broad spectral linewidths due to severe radiative damping. In this work, we propose a simple two-dimensional symmetric gold nanohole-array metasurface that supports a symmetry-protected bound state in the continuum (SP-BIC) at normal incidence. By introducing extrinsic symmetry breaking via oblique incidence, this non-radiative dark state is successfully transformed into an observable high-Q quasi-BIC Fano resonance. Cartesian multipole decomposition reveals that this sharp mode (λ≈688 nm) is predominantly driven by a tightly confined Magnetic Dipole (MD) excitation, which drastically suppresses radiative leakage compared to the highly damped Electric Dipole (ED)-dominated LSPR. Consequently, the quasi-BIC mode exhibits an ultra-narrow spectral linewidth (FWHM≈17.4 nm). While its bulk sensitivity (236.9 nm/RIU) is slightly lower than that of the LSPR mode, the exceptionally sharp resonance yields a remarkably low Limit of Detection (LOD) of 7.35×10−3 RIU, achieving a nearly five-fold improvement over the traditional LSPR. Furthermore, the quasi-BIC mode maintains an outstanding Figure of Merit (FOM up to ∼19.7 RIU−1) across the entire sensing range. By eliminating the need for complex asymmetric nanofabrication, this robust angle-tuned design strategy provides a highly promising platform for the development of high-resolution, low-cost optical biosensors.

Photonics doi: 10.3390/photonics13040397

Authors: Hua Yan Yi Yang Liyuan Song

The rapid development of 6G wireless networks requires ultra-high data rates that traditional microwave frequencies cannot support. Photonics-assisted terahertz (THz) technologies offer a promising solution by combining high-capacity optical fibers with wideband wireless transmission. However, as bandwidth expands, sampling frequency offset (SFO) becomes a critical issue that degrades signal quality in single-carrier systems. This paper evaluates the performance of two main compensation methods within a photonics-assisted THz system operating at 320 GHz. We compare the Gardner clock recovery algorithm and the Digital Interpolation Compensation Algorithm (DICA) across various modulation formats and offset levels. Our findings indicate that the Gardner algorithm is effective for low-order modulation when the SFO is below 100 ppm, but its performance fails outside this range. Conversely, the DICA provides robust compensation up to 1000 ppm regardless of the modulation format, provided that the exact offset value is known. Without proper compensation, the system BER increases significantly as the SFO grows. These results demonstrate the complementary nature of these two algorithms and provide a practical guide for selecting compensation strategies in future high-speed THz communication links.

Photonics doi: 10.3390/photonics13040396

Authors: Jiangyu Tian Libing Jin Jun Chang

Infrared focal plane arrays (IRFPAs) often suffer from spatiotemporal nonuniformity that persists after conventional two-point nonuniformity correction (NUC), especially under temperature drift and time-varying readout conditions. These residuals are typically structured, including column-group striping caused by shared column-end circuits and row-wise baseline/common-mode drift induced by row-scanning paths. We propose a structured, digital-twin-inspired detector-side refinement of two-point NUC that augments the bias term with interpretable low-dimensional components: a static column bias vector capturing group-correlated residuals and a row-related structured term consisting of a static row baseline and a frame-synchronous common-mode component with row-dependent sensitivity, while keeping the two-point gain/offset backbone unchanged. Rather than representing a full system-level digital twin of the infrared payload, the proposed framework serves as a detector-side virtual representation of dominant readout-induced structured residual states that can be estimated and updated from calibration data. Experiments on blackbody calibration data across multiple temperature points demonstrate that the column-related structured component significantly reduces group-wise column residuals, the row-related structured component suppresses time-varying row striping, and the combined method improves both column- and row-direction metrics consistently across temperatures.

Photonics doi: 10.3390/photonics13040395

Authors: Zhaoyang Shi Shuai Dong Zhixing Qiao Chaofan Feng Yafang Xu Jianyong Hu Hongpeng Wu Ruiyun Chen Guofeng Zhang Suotang Jia Liantuan Xiao Chengbing Qin

Detection of trace gases with high sensitivity and weak excitation power is highly desired for long-range remote sensing. Here, we report the detection of the greenhouse gas nitrous oxide (N2O) with the power of excitation light down to picowatts, by converting the mid-infrared laser to near-infrared photons through an intra-cavity-enhanced sum-frequency upconversion system. The intra-cavity-enhanced pumping power of 1064.0 nm reaches about 200.0 W, resulting in the conversion of the 4514.6 nm mid-infrared laser to 861.1 nm with an efficiency up to 73.4% under optimal conditions. The upconverted light is then detected by a single-photon avalanche detector, followed by a time-correlated single-photon counting module, which can measure the arrival time of each upconverted photon. By performing discrete Fourier transformations of the arrival time of the detected photons, the frequency spectrum can be determined. By using frequency modulation, this method can suppress background noise significantly. Consequently, the excitation power can be brought down to about 100 pW with the concentration of N2O being 10 ppm. As a demonstration of application, the presented system is also used for N2O sensing in an open-path geometry, highlighting the potential for stand-off leak detection. Our proposal offers promising applications to monitor trace gases over long distances with weak excitation powers.

Photonics doi: 10.3390/photonics13040394

Authors: Yifan Deng Jingya Sun Manlou Ye Xiaokang Dong Xiang Li Yang Yang

Femtosecond laser processing has become a powerful approach for high-precision micro- and nanofabrication in transparent materials, owing to its ultrashort pulse duration and minimized thermal effects. However, the limited predictability of processing depth remains a major obstacle to practical applications. Here, we present a morphology prediction framework for femtosecond Bessel laser processing of ZnS that integrates plasma physics modeling with deep learning. Through combined experimental measurements and plasma physics simulations, the influence of laser pulse energy on electron density evolution and material removal depth is systematically investigated. The results reveal the dominant roles of multiphoton ionization, avalanche ionization, and free-electron dynamics in deep-volume processing, and demonstrate the strong sensitivity of the processing morphology to the plasma distribution. Conventional plasma models can accurately reproduce the ablation diameter, yet exhibit significant limitations in predicting the processing depth. We propose a physics data-based framework for femtosecond Bessel beam processing, which integrates a depth-residual regression network conditioned on the peak electron density distribution to effectively learn and compensate for systematic modeling errors in plasma-based simulations. This strategy leads to excellent agreement between predicted and experimental processing depths and three-dimensional morphologies under various energy conditions. The model achieves a mean absolute error (MAE) of 4.9 nm at the pixel level for 3D crater reconstruction. Under rigorous crater-grouped cross-validation with Leave-One-Group-Out evaluation, the model achieves a mean R2 of 0.74 across 8 independent craters, demonstrating reliable generalization to unseen energy conditions. These results demonstrate that incorporating physical priors into data-driven learning provides an effective pathway to overcoming accuracy limitations in modeling complex laser–matter interactions. This approach offers a reliable tool for quantitative prediction and parameter optimization in deep femtosecond laser processing of transparent materials and enabling highly controllable and reproducible micro- and nanofabrication for advanced photonic and three-dimensional optical applications.

Photonics doi: 10.3390/photonics13040393

Authors: Xulin Luo Yollanda Bella Christy Yahui Li Yuan Ou Hongli Chen Jiaxuan Shi Wenyun Luo Qiang Guo

The transverse beam size is a key parameter for characterizing the performance of synchrotron radiation sources. Accurate measurement of the transverse beam size is crucial for assessing beam quality. In this study, a fiber array-photomultiplier tube (PMT) beam measurement system was developed to enable high-precision sampling of beam profile information for beam-size measurement. Furthermore, a hybrid method integrating nonlinear least squares (NLLS) fitting and Gaussian basis-function fitting was proposed to reconstruct the beam intensity profile from discrete sampling data. Before performing NLLS fitting, a median absolute deviation (MAD)-based threshold filter is employed to remove outliers and suppress random noise, thereby improving the stability and robustness of the parameter estimation. The filtered data are then fitted using NLLS to obtain the reconstructed distribution. To capture potential high-order modal features in the beam profile, a Gaussian basis-function fitting model was also introduced for comparison, and its performance was evaluated under complex intensity distributions. Additionally, the relationship between the full width at half maximum (FWHM) and beam intensity was experimentally verified while accounting for measurement effects in the system. The results demonstrate that the proposed hybrid algorithm improves reconstruction accuracy and robustness, enabling precise recovery of the beam-intensity profile in the fiber-array PMT system.

Photonics doi: 10.3390/photonics13040392

Authors: Meet Kumari Jyoteesh Malhotra Satyendra K. Mishra

Mode division multiplexing (MDM) is an emerging optical communication solution for high-capacity wired–wireless applications. Due to the presence of modal crosstalk and link impairments in MDM, this work aims to design a system that provides low complexity, an improved Shannon Capacity limit, and high spectral efficiency. In this work, a quad modal MDM system using an integrated parabolic index multimode fiber and free-space optics (PIMMF-FSO) link is presented. Four linearly polarized (LP) modes, LP01, LP22, LP03, and LP13 based on a 16 × 10 Gbps MDM system offering different sixteen channels, are realized. Results show that the system can sustain a 7.5 dB insertion loss over 100 m FSO and a 100 m fiber range for different LP modes under the impact of clear air, moderate haze, heavy rain and wet snow climates with weak turbulence. A faithful fiber range of 3000 m can be obtained successfully in the proposed system with a −10 dB link loss, −7.62 dBm received power and 10 dB noise. Compared to existing designs, the proposed design offers optimum performance in terms of high channel capacity and a high traffic rate with low complexity and high spectral efficiency. Additionally, high received power, with acceptable noise, link loss, FSO misalignments and fiber nonlinearities, is successfully obtained.

Photonics doi: 10.3390/photonics13040391

Authors: David Levy Robert E. Camley

Attenuated total reflection (ATR) has long relied on a familiar trio—prism, metal film, and substrate—to unlock insights into material properties. In this work, we demonstrate the possibility and limitations of a prismless ATR system that utilizes the relative permittivity of air in place of a prism, followed by a plasmonic test material, which is in turn followed by a novel material characterized by low real relative permittivity and near-zero imaginary relative permittivity. We show that such measurements are possible in the visible IR and UV frequency ranges. In addition, we illustrate the advantages of prismless ATR.

Photonics doi: 10.3390/photonics13040390

Authors: Houzhi Cai Zhaoyang Xie Zhiying Deng Youlin Ma Lijuan Xiang

Microchannel plate gated framing camera is commonly used in inertial confinement fusion diagnostics. However, it is a vacuum electronic device with bulkiness and non-single-line-of-sight imaging. To reduce the size of the camera and achieve a single line of sight image, a CMOS image sensor composed of a pixel unit and a readout circuit is presented to form the framing camera. The CMOS image sensor has a 32 × 32 × 4 pixel array with ultrashort shutter-time and four-frame imaging. The pixel array and analog to digital converter (ADC) readout circuit are designed using a standard 0.18 μm CMOS process. The pixel array includes 5T structured pixel units, a voltage-controlled delay, a clock tree and the row decoding scan circuits. A temporal resolution of 65 ps for the pixel circuit is achieved. The ADC readout circuit is composed of a counter, a comparator, a ramp generator and a register, which operates at a sampling frequency of 24.41 kS/s. An effective number of bits of 11.3, a spurious free dynamic range (SFDR) of 73.4 dB, and a signal-to-noise ratio (SNR) of 70.0 dB for the ADC are achieved. The CMOS image sensor will provide a novel and important imaging method for the field of ultrafast science.

Photonics doi: 10.3390/photonics13040389

Authors: Xinyu Chen Jinquan Yu Tingting Li Huan Gao Xin Gui Min Zhu Jiaqi Wang Zhengying Li

Reliable in situ temperature monitoring is essential for the safe operation of liquid-nitrogen-cooled superconducting cables, yet conventional electrical sensors are often difficult to scale to multi-point deployment in cryogenic, high-current environments. This work presents a fiber Bragg grating (FBG) sensing solution for in situ temperature monitoring of superconducting cables in liquid nitrogen. An FBG array packaged with a polyimide-coated fiber inside a 3 mm stainless-steel tube is deployed along the cable centerline to provide multi-point temperature measurements of the cable core. The system is validated under liquid-nitrogen immersion with a 2000 A current turn-on/turn-off test, with a 1 Hz update rate and a steady-state temperature fluctuation within ±0.1 °C. Experimental results show a continuous temperature decrease during liquid-nitrogen cooling, followed by a cryogenic plateau, during which a spatially consistent 0.6–0.7 °C current-induced temperature rise is observed across multiple sensing points in the present 2000 A turn-on/turn-off test, followed by recovery after current shutoff. Small-amplitude fluctuations during the plateau are attributed to packaging-dependent thermal coupling between the centerline-deployed sensor and the cable core. These results indicate that the proposed FBG-based approach enables reliable cryogenic thermometry for superconducting cables in liquid nitrogen and provides a practical tool for in situ operational condition assessment.

Photonics doi: 10.3390/photonics13040388

Authors: Sofiane Haddadi Abdelhalim Bencheikh Michael Fromager Kamel Aït-Ameur

Research into the spatial reshaping of monochromatic laser beams grew significantly in the late 1990s due to improvements in the fabrication of diffractive optical elements. Nowadays, some applications, such as optical coherence tomography, necessitate the use of broadband light beams with a spectral width of hundreds of nanometers. The difficulty in reshaping such spectrally broadened beams lies in the wavelength dependence of the beam shaping process. This paper presents a numerical study of the wavelength dependence of two beam shaping techniques that allow a Gaussian beam to be transformed into a flat-top or doughnut intensity profile in the focal plane of a focusing lens. The first technique is based on the diffraction of an incident Gaussian beam passing through a simple binary diffractive optical element. The second technique can be described as an interferometric method, as it involves the coaxial superposition of two Gaussian beams emerging from a Michelson interferometer. We compared the stability of these two techniques’ ability to reshape the beam versus the spectral bandwidth of the incident Gaussian beam. We showed that the interferometric method is more resilient than the diffractive method to changes in the spectral bandwidth of the Gaussian beam. We also considered the case of a quasi-monochromatic beam delivered by a widely tunable laser and reshaped using the interferometric method, where the dispersion of beam reshaping could be mitigated by two programmable liquid lenses that enable control of the curvature of the Michelson interferometer mirrors.

Photonics doi: 10.3390/photonics13040387

Authors: Filip Fuňák Rastislav Róka

To ensure high-quality and reliable service provision for customers, advanced optical networks without active elements have been developed to increase operating reliability, network scalability, and resource efficiency. To this end, wavelength division multiplexing-based passive optical networks (WDM-PON) now have a markedly enhanced role. An important aspect of the WDM-PON design is represented by traffic protection schemes, which play a key role in network reliability. Managing the power budget for optical links allows us to achieve a practically sustainable and realizable infrastructure of advanced passive optical networks. In this work, we focused on simulation model development for the power budget calculation for the WDM-PON optical link and the subsequent optical power budget evaluation of presumptive WDM-PON traffic protection schemes.

Photonics doi: 10.3390/photonics13040386

Authors: Haoyue Liu Zexiao Li Xiaodong Zhang

In fringe projection profilometry, phase accuracy is a key factor in determining the ultimate measurement precision. However, errors stemming from the nonlinear response of the projector and camera are introduced into the phase map, manifesting as periodic artifacts that seriously compromise measurement fidelity. Although traditional phase filtering can effectively mitigate these artifacts, it often introduces edge blurring and detail loss. To address this, we first establish models for both the nonlinear error and its propagation and then propose a novel phase filtering algorithm based on low-pass guided filtering. This method effectively suppresses nonlinear artifacts while preserving edges, thereby improving calibration and measurement accuracy without requiring additional hardware. Our algorithm enhances the traditional four-step phase-shifting method: in simulations, it reduces calibration error by 52.2% (from 0.1490 mm to 0.0712 mm), and measurement error by over 36.8% (from 0.0855 mm to 0.0559 mm); in real experiments, these reductions are 54.1% (from 0.1180 mm to 0.0875 mm) and more than 36.7% (from 0.0954 mm to 0.0604 mm), respectively. Experimental results show that our method achieves accuracy comparable to the eight-step phase-shifting method while preserving the efficiency of the four-step method, highlighting its significant practical value.

Photonics doi: 10.3390/photonics13040385

Authors: Jingrui Yang Qinglei Zhao Shuai Liu Meihua Xia Jing Guo Yinghong Yu Chao Li Xiao Tang Shuxin Wang Qinglong Hu Fengwei Guan Qiang Liu Mingdong Zhu Qi Song

Three-dimensional seed phenotyping requires imaging systems capable of achieving micron-level resolution across a centimeter-level field of view (FOV), a goal constrained by the resolution–FOV trade-off in conventional light field architectures. This paper presents a hardware–software co-optimized framework that integrates a reconfigurable optical system with computational imaging pipelines to address this limitation. At the hardware level, we develop a tunable-focus lens module that enables flexible adjustment of the effective focal length, combined with a custom-designed microlens array (MLA). A mathematical model is established to analyze the interdependencies among FOV, lateral resolution, depth of field (DOF), and system configuration, guiding the design of individual optical components. On the computational side, we propose a hybrid aberration correction strategy: first, a co-calibration of lens and MLA aberrations based on line-feature detection; second, a conditional generative adversarial network (cGAN) with attention-guided residual learning to enhance sub-aperture images, achieving a PSNR of 34.63 dB and an SSIM of 0.9570 on seed datasets. Experimentally, the system achieves a resolution of 6.2 lp/mm at MTF50 over a 2–3 cm FOV, representing a 307% improvement over the initial configuration (1.52 lp/mm). The reconstruction pipeline combines epipolar plane image (EPI) analysis with multi-view consistency constraints to generate dense 3D point clouds at a density of approximately 1.5 × 104 points/cm2 while preserving spectral and textural features. Validation on bitter melon and rice seeds demonstrates accurate 3D reconstruction and accurate extraction of morphological parameters across a large area. By integrating optical and computational design, this work establishes a reconfigurable imaging framework that overcomes the resolution–FOV limitations of conventional light field systems. The proposed architecture is also applicable to robotic vision and biomedical imaging.

Photonics doi: 10.3390/photonics13040384

Authors: Xinjie Zhu Zijian Zhang Samuel Lawman Xingyu Yang Yalin Zheng Yaochun Shen

Ultrahigh lateral resolution (UHLR) optical coherence tomography (OCT) technology, also called optical coherence microscopy (OCM), has gained popularity, especially in the field of biomedical imaging. In these systems, high numerical aperture (NA) Microscope objectives (MO) are employed in OCM systems to offer better than 3 µm lateral resolution. However, in the implemented broadband OCM configuration, the use of complex multi-element microscope objectives can reduce the detected returned signal compared with a simpler imaging lens configuration. This reduction in detected returned signals can become an important practical limitation in many OCM applications, particularly for biomedical imaging when high imaging speed is crucial. This study investigates whether a single off-the-shelf lens can provide a practical alternative to conventional MOs, achieving higher throughput while maintaining reasonable spatial resolution. We systematically evaluated 14 commercial lenses using Zemax OpticStudio simulations, identifying an aspherized achromatic lens (Edmund Optics #85302) that best met these key criteria. To validate its feasibility for OCM, performance was tested in both Full-Field Time-Domain OCM (FF-TD-OCM) and Line-Field Spectral-Domain OCM (LF-SD-OCM) configurations. Using a broadband composite Superluminescent Diode (SLD) source (750–920 nm), we quantified the resolvable features, axial resolution, and overall light transmission. The validated system demonstrated near-diffraction-limited performance. In the LF-SD-OCM setup, it successfully resolved features as fine as Group 8, Element 6, corresponding to a 2.2 µm line pair pitch (~1.1 µm line width) and achieved a 2.86 µm axial resolution in air. A through-focus comparison further showed practically useful contrast retention around focus. Additional imaging of onion epidermal tissue and ex vivo porcine corneal tissue demonstrated that the proposed lens could provide interpretable structural images on representative biological samples. Under the tested LF-SD-OCM detection configuration, the selected lens delivered approximately 2.0 dB higher returned signal than the Mitutoyo MY10X-823 objective according to 1.59× larger received signal.

Photonics doi: 10.3390/photonics13040383

Authors: Tong Cheng Wen-Quan Jing Jin-Xu Du Zeng-Qiang Yang Zhi-Hong Jiao Guo-Li Wang Song-Feng Zhao

Achieving selective cleavage of specific chemical bonds using ultrafast laser pulses remains a central challenge in ultrafast strong-field molecular physics. Here, we theoretically investigate the coherent control of strong-field dissociation of the heteronuclear molecular ion HD+ initially prepared in vibrationally excited states driven by an ultrashort pulse with a quadratic spectral phase. Our results reveal a pronounced sensitivity of both the total dissociation probability and the branching ratio (H+ + D vs. H + D+) to the chirp rate of the laser pulse. To uncover the underlying physical mechanism, we analyze the population dynamics in the coupled 1sσ and 2pσ electronic states and identify pronounced Rabi oscillations arising from the coherent interplay between multiphoton excitation and field-induced stimulated emission. By tuning the laser chirp rate, these oscillations can be suppressed via quantum interference, thereby reshaping the dissociation dynamics and significantly enhancing the dissociation probability of the H + D+ channel. These findings demonstrate that spectral-phase engineering provides a robust and versatile strategy for selective control of branching ratios in strong-field molecular dissociation.

Photonics doi: 10.3390/photonics13040382

Authors: Zhichong Wang Junhui Hu Zhen Yang Anna Kafar Piotr Perlin Shuiqing Li Heqing Deng Jiangyong Zhang Sha Shiong Ng Mundzir Abdullah Junwen Zhang Nan Chi Chao Shen

With the advent of 6G mobile communication, the demand for ultra-high bandwidth wireless communication has increased rapidly, drawing significant attention to visible light communication (VLC) as a promising emerging technology. GaN-based laser diodes (LDs) are regarded as high-speed light sources for VLC owing to their high modulation bandwidth and high optical power density. Apart from the active region design, the LD’s structure also plays a crucial role in determining their dynamic properties, which have yet to be thoroughly studied in III-nitride LDs. In this work, we systematically investigate InGaN/GaN laser diodes with three ridge waveguide configurations: a conventional single-ridge structure, a dual-ridge large-mesa structure, and a dual-ridge small-mesa structure. The threshold current, small-signal modulation bandwidth of devices with different structures are comparatively analyzed. Experimental results reveal that the double-ridge small mesa laser diode achieves a modulation bandwidth of −3 dB at 6.02 GHz. These results provide valuable insights into the structural optimization of GaN-based high-speed laser diodes and offer practical guidance for the development of high-performance, energy-efficient VLC transmitters.