Photonics

20 pages, 2217 KiB

Open AccessArticle

AI-Based Prediction of Visual Performance in Rhythmic Gymnasts Using Eye-Tracking Data and Decision Tree Models

by Ricardo Bernardez-Vilaboa, F. Javier Povedano-Montero, José Ramon Trillo, Alicia Ruiz-Pomeda, Gema Martínez-Florentín and Juan E. Cedrún-Sánchez

Photonics 2025, 12(7), 711; https://doi.org/10.3390/photonics12070711 (registering DOI) - 14 Jul 2025

Abstract

Background/Objective: This study aims to evaluate the predictive performance of three supervised machine learning algorithms—decision tree (DT), support vector machine (SVM), and k-nearest neighbors (KNN) in forecasting key visual skills relevant to rhythmic gymnastics. Methods: A total of 383 rhythmic gymnasts aged 4 [...] Read more.

Background/Objective: This study aims to evaluate the predictive performance of three supervised machine learning algorithms—decision tree (DT), support vector machine (SVM), and k-nearest neighbors (KNN) in forecasting key visual skills relevant to rhythmic gymnastics. Methods: A total of 383 rhythmic gymnasts aged 4 to 27 years were evaluated in various sports centers across Madrid, Spain. Visual assessments included clinical tests (near convergence point accommodative facility, reaction time, and hand–eye coordination) and eye-tracking tasks (fixation stability, saccades, smooth pursuits, and visual acuity) using the DIVE (Devices for an Integral Visual Examination) system. The dataset was split into training (70%) and testing (30%) subsets. Each algorithm was trained to classify visual performance, and predictive performance was assessed using accuracy and macro F1-score metrics. Results: The decision tree model demonstrated the highest performance, achieving an average accuracy of 92.79% and a macro F1-score of 0.9276. In comparison, the SVM and KNN models showed lower accuracies (71.17% and 78.38%, respectively) and greater difficulty in correctly classifying positive cases. Notably, the DT model outperformed the others in predicting fixation stability and accommodative facility, particularly in short-duration fixation tasks. Conclusion: The decision tree algorithm achieved the highest performance in predicting short-term fixation stability, but its effectiveness was limited in tasks involving accommodative facility, where other models such as SVM and KNN outperformed it in specific metrics. These findings support the integration of machine learning in sports vision screening and suggest that predictive modeling can inform individualized training and performance optimization in visually demanding sports such as rhythmic gymnastics. Full article

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19 pages, 1609 KiB

Open AccessArticle

Research on Networking Protocols for Large-Scale Mobile Ultraviolet Communication Networks

by Leitao Wang, Zhiyong Xu, Jingyuan Wang, Jiyong Zhao, Yang Su, Cheng Li and Jianhua Li

Photonics 2025, 12(7), 710; https://doi.org/10.3390/photonics12070710 (registering DOI) - 14 Jul 2025

Abstract

Ultraviolet (UV) communication, characterized by non-line-of-sight (NLOS) scattering, holds substantial potential for enabling communication networking in unmanned aerial vehicle (UAV) formations within strong electromagnetic interference environments. This paper proposes a networking protocol for large-scale mobile ultraviolet communication networks (LSM-UVCN). In large-scale networks, the [...] Read more.

Ultraviolet (UV) communication, characterized by non-line-of-sight (NLOS) scattering, holds substantial potential for enabling communication networking in unmanned aerial vehicle (UAV) formations within strong electromagnetic interference environments. This paper proposes a networking protocol for large-scale mobile ultraviolet communication networks (LSM-UVCN). In large-scale networks, the proposed protocol establishes multiple non-interfering transmission paths based on a connection matrix simultaneously, ensuring reliable space division multiplexing (SDM) and optimizing the utilization of network channel resources. To address frequent network topology changes in mobile scenarios, the protocol employs periodic maintenance of the connection matrix, significantly reducing the adverse impacts of node mobility on network performance. Simulation results demonstrate that the proposed protocol achieves superior performance in large-scale mobile UV communication networks. By dynamically adjusting the connection matrix update frequency, it adapts to varying node mobility intensities, effectively minimizing control overhead and data loss rates while enhancing network throughput. This work underscores the protocol’s adaptability to dynamic network environments, providing a robust solution for high-reliability communication requirements in complex electromagnetic scenarios, particularly for UAV swarm applications. The integration of SDM and adaptive matrix maintenance highlights its scalability and efficiency, positioning it as a viable technology for next-generation wireless communication systems in challenging operational conditions. Full article

(This article belongs to the Special Issue Free-Space Optical Communication and Networking Technology)

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14 pages, 26034 KiB

Open AccessArticle

High-Performance Self-Powered Broadband Photodetectors Based on a Bi₂Se₃ Topological Insulator/ReSe₂ Heterojunction for Signal Transmission

by Yun Wei, Peng Wan, Lijian Li, Tao He, Wanyu Ma, Tong Xu, Bingwang Yang, Shulin Sha, Caixia Kan and Mingming Jiang

Photonics 2025, 12(7), 709; https://doi.org/10.3390/photonics12070709 (registering DOI) - 14 Jul 2025

Abstract

Topological insulators (TIs) hold considerable promise for the advancement of optoelectronic technologies, including spectroscopy, imaging, and communication, owing to their remarkable optical and electrical characteristics. This study proposes a novel combination of Bi

_{2}

Se

_{3}

TIs and ReSe

_{2}

[...] Read more.

Topological insulators (TIs) hold considerable promise for the advancement of optoelectronic technologies, including spectroscopy, imaging, and communication, owing to their remarkable optical and electrical characteristics. This study proposes a novel combination of Bi

_{2}

Se

_{3}

TIs and ReSe

_{2}

for self-powered broadband photodetectors with high sensitivity and fast response time. The Bi

_{2}

Se

_{3}

/ReSe

_{2}

heterojunction photodetector achieves broadband response spectra ranging for 375 nm to 1

μ

m. It demonstrates a significant responsivity of 64 mA/W at a wavelength of 600 nm (1 mW/cm

^{2}

), exhibits a rapid response speed of 345

μ

s rise/336

μ

s fall time, and has a 3 dB bandwidth of 1.4 kHz under zero-bias conditions. The high performance can be attributed to the suitable energy band structure of Bi

_{2}

Se

_{3}

/ReSe

_{2}

and high carrier mobility in surface states of Bi

_{2}

Se

_{3}

. Excitingly, self-powered TIs photodetectors allow for high-quality signal transmission. The TIs employed in photodetectors can stimulate the production of new optoelectronic features, but they could also be used for highly integrated photonic circuits in the future. Full article

(This article belongs to the Special Issue New Perspectives in Photodetectors)

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13 pages, 2828 KiB

Open AccessArticle

Efficient Single-Exposure Holographic Imaging via a Lightweight Distilled Strategy

by Jiaosheng Li, Haoran Liu, Zeyu Lai, Yifei Chen, Chun Shan, Shuting Zhang, Youyou Liu, Tude Huang, Qilin Ma and Qinnan Zhang

Photonics 2025, 12(7), 708; https://doi.org/10.3390/photonics12070708 (registering DOI) - 14 Jul 2025

Abstract

Digital holography can capture and reconstruct 3D object information, making it valuable for biomedical imaging and materials science. However, traditional holographic reconstruction methods require the use of phase shift operation in the time or space domain combined with complex computational processes, which, to [...] Read more.

Digital holography can capture and reconstruct 3D object information, making it valuable for biomedical imaging and materials science. However, traditional holographic reconstruction methods require the use of phase shift operation in the time or space domain combined with complex computational processes, which, to some extent, limits the range of application areas. The integration of deep learning (DL) advancements with physics-informed methodologies has opened new avenues for tackling this challenge. However, most of the existing DL-based holographic reconstruction methods have high model complexity. In this study, we first design a lightweight model with fewer parameters through the synergy of deep separable convolution and Swish activation function and then employ it as a teacher to distill a smaller student model. By reducing the number of network layers and utilizing knowledge distillation to improve the performance of a simple model, high-quality holographic reconstruction is achieved with only one hologram, greatly reducing the number of parameters in the network model. This distilled lightweight method cuts computational expenses dramatically, with its parameter count representing just 5.4% of the conventional Unet-based method, thereby facilitating efficient holographic reconstruction in settings with limited resources. Full article

(This article belongs to the Special Issue Advancements in Optical Metrology and Imaging)

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11 pages, 2054 KiB

Open AccessArticle

Polarization-Enhanced Multi-Target Underwater Salient Object Detection

by Jiayi Song, Peikai Zhao, Jiangtao Li, Liming Zhu, Khian-Hooi Chew and Rui-Pin Chen

Photonics 2025, 12(7), 707; https://doi.org/10.3390/photonics12070707 (registering DOI) - 12 Jul 2025

Abstract

Salient object detection (SOD) plays a critical role in underwater exploration systems. Traditional SOD approaches encounter notable constraints in underwater image analysis, primarily stemming from light scattering and absorption effects induced by suspended particulate matter in complex underwater environments. In this work, we [...] Read more.

Salient object detection (SOD) plays a critical role in underwater exploration systems. Traditional SOD approaches encounter notable constraints in underwater image analysis, primarily stemming from light scattering and absorption effects induced by suspended particulate matter in complex underwater environments. In this work, we propose a deep learning-based multimodal method guided by multi-polarization parameters that integrates polarization de-scattering mechanisms with the powerful feature learning capability of neural networks to achieve adaptive multi-target SOD in an underwater turbid scattering environment. The proposed polarization-enhanced salient object detection network (PESODNet) employs a multi-polarization-parameter-guided, material-aware attention mechanism and a contrastive feature calibration unit, significantly enhancing its multi-material, multi-target detection capabilities in underwater scattering environments. The experimental results confirm that the proposed method achieves substantial performance improvements in multi-target underwater SOD tasks, outperforming state-of-the-art models of salient object detection in detection accuracy. Full article

(This article belongs to the Section New Applications Enabled by Photonics Technologies and Systems)

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17 pages, 2877 KiB

Open AccessArticle

Research on High-Precision Initial Pointing for Near-Earth Laser Communication

by Yuang Li, Xuan Wang, Junfeng Han and Xinxin Quan

Photonics 2025, 12(7), 706; https://doi.org/10.3390/photonics12070706 (registering DOI) - 12 Jul 2025

Abstract

This paper proposes a systematic ground experimental method to address the insufficient initial pointing accuracy of optical terminals in free space optical communication (FSO). By utilizing a multi-coordinate system transformation model combined with geodetic coordinates obtained from a Global Navigation Satellite System (GNSS), [...] Read more.

This paper proposes a systematic ground experimental method to address the insufficient initial pointing accuracy of optical terminals in free space optical communication (FSO). By utilizing a multi-coordinate system transformation model combined with geodetic coordinates obtained from a Global Navigation Satellite System (GNSS), the elevation and azimuth angles of the optical terminal are calculated to achieve initial pointing. High-precision horizontal installation and true north direction calibration are accomplished using a GNSS dual-antenna system and a digital inclinometer to suppress mechanical installation errors. Furthermore, an iterative stellar calibration method is proposed, leveraging ephemeris to precompute stellar positions and optimize correction values through multiple observations, significantly improving pointing accuracy. In a 104.68 km span experiment conducted in the Qinghai Lake area, the azimuth and elevation angle errors of the optical terminal were reduced to −0.0293° and −0.0068°, respectively, with the uncertainty region narrowed to 0.0586°. These results validate the effectiveness of the proposed method in high-precision rapid link establishment, providing technical support for the engineering application of satellite-to-ground laser communication. Full article

(This article belongs to the Special Issue Laser Communication Systems and Related Technologies)

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15 pages, 11880 KiB

Open AccessArticle

A Low-Cost Portable SDIMM for Daytime Atmospheric Optical Turbulence Measurement in Observatory Site Testing: Primary Results from Ali Site

by Jingxing Wang, Jing Feng, Xuan Qian, Yongqiang Yao, Mingyu Zhao, Kaifeng Kang and Tengfei Song

Photonics 2025, 12(7), 705; https://doi.org/10.3390/photonics12070705 - 11 Jul 2025

Abstract

Atmospheric optical turbulence intensity, quantified by the Fried parameter (

r_{0}

), serves as a critical metric for astronomical site testing and selection. The Solar Differential Image Motion Monitor (SDIMM), adapted from the methodology of the Differential Image Motion Monitor (DIMM), is [...] Read more.

Atmospheric optical turbulence intensity, quantified by the Fried parameter (

r_{0}

), serves as a critical metric for astronomical site testing and selection. The Solar Differential Image Motion Monitor (SDIMM), adapted from the methodology of the Differential Image Motion Monitor (DIMM), is a dedicated instrument for daytime

r_{0}

measurements. Conventional SDIMM systems typically employ telescopes with apertures ≥30 cm and reconstruct wavefront segmentation at the exit pupil, resulting in bulky configurations that impede portability. To address the demands of multi-site surveys, we developed a low-cost, portable SDIMM system that directly adopts the DIMM optical path without backend wavefront reconstruction, instead deriving

r_{0}

through image processing algorithms. Integrated with a 20 cm aperture telescope, the system achieves a total weight of <20 kg, significantly enhancing field portability. This paper details the instrument’s architecture, measurement principles, and comparative tests with a traditional SDIMM, demonstrating strong consistency between the two systems. Field measurements conducted at the Ali Observatory (elevation: 5050 m) from 16 August to 10 December 2024 yielded the site’s first continuous daytime

r_{0}

dataset, with values ranging from 1.5 cm to 12 cm and a mean of 4.09 cm. The compact SDIMM provides a cost-effective and easily deployable solution for comparative daytime

r_{0}

assessments across multiple candidate astronomical sites. Full article

(This article belongs to the Special Issue Recent Advances in Optical Turbulence)

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17 pages, 8874 KiB

Open AccessArticle

Adaptive DBP System with Long-Term Memory for Low-Complexity and High-Robustness Fiber Nonlinearity Mitigation

by Mingqing Zuo, Huitong Yang, Yi Liu, Zhengyang Xie, Dong Wang, Shan Cao, Zheng Zheng and Han Li

Photonics 2025, 12(7), 704; https://doi.org/10.3390/photonics12070704 - 11 Jul 2025

Abstract

Adaptive digital back-propagation (A-DBP) is a promising candidate for mitigating Kerr nonlinearity due to its ability to estimate the optimal nonlinear scaling factor adaptively. However, the adaptive process relying on the gradient-dependent algorithm is prone to fluctuation, leading to extra iterations or even [...] Read more.

Adaptive digital back-propagation (A-DBP) is a promising candidate for mitigating Kerr nonlinearity due to its ability to estimate the optimal nonlinear scaling factor adaptively. However, the adaptive process relying on the gradient-dependent algorithm is prone to fluctuation, leading to extra iterations or even divergence and resulting in huge computational efforts in A-DBP. In this paper, an improved A-DBP algorithm with long-term memory (LTM) is proposed, employing root mean square propagation (RMSProp) to achieve low-complexity and high-robustness compensation performances. The A-DBP-LTM algorithm based on RMSProp was numerically validated through the simulated transmission of 69 Gbaud DP-16QAM over 2000 km and further verified through an experiment involving 26-λ 63 Gbaud DP-16QAM transmission over 1200 km. Compared with conventional digital back-propagation and A-DBP based on a gradient-descent algorithm, our proposed method allows substantial complexity reductions of 31.35% and 58.47%, respectively. Furthermore, high robustness in only a few iterations and a 0.33 dB improvement in the optical signal–noise ratio penalty were also experimentally demonstrated. Full article

(This article belongs to the Special Issue Next-Generation Optical Networks Communication)

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14 pages, 3008 KiB

Open AccessArticle

Characterization of Chirp Properties of an 850 nm Single-Mode Multi-Aperture Vertical-Cavity Surface-Emitting Laser and Analysis of Transmission Performance over Multimode and Single-Mode Fibers

by Xin Chen, Nikolay Ledentsov, Jr., Abdullah S. Karar, Jason E. Hurley, Oleg Yu. Makarov, Hao Dong, Ahmad Atieh, Ming-Jun Li and Nikolay Ledentsov

Photonics 2025, 12(7), 703; https://doi.org/10.3390/photonics12070703 (registering DOI) - 11 Jul 2025

Abstract

By measuring the transfer function of the single-mode multi-aperture vertical-cavity surface-emitting laser (SM MA VCSEL) transmitting over a long single-mode fiber at 850 nm, we confirm that the chirp of the SM MA VCSEL under study is dominated by transient chirp with an [...] Read more.

By measuring the transfer function of the single-mode multi-aperture vertical-cavity surface-emitting laser (SM MA VCSEL) transmitting over a long single-mode fiber at 850 nm, we confirm that the chirp of the SM MA VCSEL under study is dominated by transient chirp with an alpha value of −3.81 enabling a 19 GHz bandwidth over 10 km of single-mode fiber. The detailed measurement of the VCSEL with different bias currents also allows us to recover other key characteristics of the VCSEL, thereby enabling us to practically construct the optical eye diagrams that closely match the experimentally measured ones. The link-level transfer function can be obtained using an analytical equation including effects of modal dispersion and laser chirp–chromatic dispersion (CD) interaction for an MMF of a given length and bandwidth grade. The narrow linewidth and chirp characteristics of the SM MA VCSEL enable transmission performance that surpasses that of conventional MM VCSELs, achieving comparable transmission distances at moderate modal bandwidths for OM3 and OM4 fibers and significantly longer reaches when the modal bandwidth is higher. The transmission performance was also confirmed with the modeled eye diagrams using extracted VCSEL parameters. The chirp properties also provide sufficient bandwidth for SM MA VCSEL transmission over kilometer-scale lengths of single-mode fibers at a high data rate of 100G or above with sufficient optical power coupled into the fibers. Advanced transmission distances are possible over multimode and single-mode fibers versus chirp-free devices. Full article

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

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17 pages, 5754 KiB

Open AccessArticle

Simulation-Driven End-to-End Deep Learning Method for White-Light Interference Topography Reconstruction

by Xuan Qi, Yudong Lian, Yulei Wang and Zhiwei Lu

Photonics 2025, 12(7), 702; https://doi.org/10.3390/photonics12070702 - 11 Jul 2025

Abstract

White-light interferometry is essential for surface topography measurement in precision manufacturing, yet existing algorithms face challenges in accuracy, speed, and robustness. Motivated by the application of deep learning in optical metrology, this study presents a novel simulation-driven, end-to-end deep learning approach that significantly [...] Read more.

White-light interferometry is essential for surface topography measurement in precision manufacturing, yet existing algorithms face challenges in accuracy, speed, and robustness. Motivated by the application of deep learning in optical metrology, this study presents a novel simulation-driven, end-to-end deep learning approach that significantly advances white-light interference topography reconstruction. Validation with 200 simulated interferograms shows strong agreement with reference measurements. The neural network processes interferograms in <0.4 s with <0.3% calculation error, demonstrating real-time capability and noise robustness. Using simulated and experimental data from trapezoidal gratings, the method achieves a reconstruction error of 47.12 nm (<λ/8, λ ≈ 550 nm), outperforming traditional techniques by 9.0%. These results confirm the method’s superior accuracy, speed, and reliability for industrial metrology applications. Full article

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

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12 pages, 1896 KiB

Open AccessArticle

A 6 kW Level Linearly Polarized Near-Diffraction-Limited Monolithic Fiber Laser with a 0.43 nm Linewidth

by Zixiang Gao, Qiang Shu, Fang Li, Chun Zhang, Fengyun Li, Xingchen Jiang, Yu Wen, Cheng Chen, Li Li, Qiuhui Chu, Rumao Tao, Honghuan Lin, Zhitao Peng and Jianjun Wang

Photonics 2025, 12(7), 701; https://doi.org/10.3390/photonics12070701 - 11 Jul 2025

Abstract

A high-power, narrow-linewidth, all-fiber polarization-maintaining (PM) amplifier has been demonstrated. A lasing power of 5870 W has been delivered in master oscillator power amplifier architecture with cascaded white noise source (WNS) phase modulation and bidirectional pumping schemes. The maximal power was limited by [...] Read more.

A high-power, narrow-linewidth, all-fiber polarization-maintaining (PM) amplifier has been demonstrated. A lasing power of 5870 W has been delivered in master oscillator power amplifier architecture with cascaded white noise source (WNS) phase modulation and bidirectional pumping schemes. The maximal power was limited by the onset of stimulated Brillouin scattering. At the maximum power operation, the amplifier exhibited a 3 dB spectral linewidth of 0.43 nm with beam quality being M² < 1.33 and polarization extinction ratio (PER) being 16.3 dB. To the best of our knowledge, this represents the highest spectral brightness and PER achieved by PM fiber laser systems around 6 kW-level operation. Full article

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

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16 pages, 3084 KiB

Open AccessArticle

Generating Large Time–Bandwidth Product RF-Chirped Waveforms Using Vernier Dual-Optical Frequency Combs

by Mohammed S. Alshaykh

Photonics 2025, 12(7), 700; https://doi.org/10.3390/photonics12070700 - 11 Jul 2025

Abstract

Chirped radio-frequency signals are essential waveforms in radar systems. To enhance resolution and improve the signal-to-noise ratio through higher energy transmission, chirps with high time–bandwidth products are highly desirable. Photonic technologies, with their ability to handle broad electrical bandwidths, have been widely employed [...] Read more.

Chirped radio-frequency signals are essential waveforms in radar systems. To enhance resolution and improve the signal-to-noise ratio through higher energy transmission, chirps with high time–bandwidth products are highly desirable. Photonic technologies, with their ability to handle broad electrical bandwidths, have been widely employed in the generation, filtering, processing, and detection of broadband electrical waveforms. In this work, we propose a photonics-based large-TBWP RF chirp generator utilizing dual optical frequency combs with a small difference in the repetition rate. By employing dispersion modules for frequency-to-time mapping, we convert the spectral interferometric patterns into a temporal RF sinusoidal carrier signal whose frequency is swept through the optical shot-to-shot delay. We derive analytical expressions to quantify the system’s performance under various design parameters, including the comb repetition rate and its offset, the second-order dispersion, the transform-limited optical pulse width, and the photodetector’s bandwidth limitations. We benchmark the expected system performance in terms of RF bandwidth, chirp duration, chirp rate, frequency step size, and TBWP. Using realistic dual-comb source parameters, we demonstrate the feasibility of generating RF chirps with a duration of 284.44

μ

s and a bandwidth of 234.05 GHz, corresponding to a TBWP of

3.3 \times 10^{7}

. Full article

(This article belongs to the Section New Applications Enabled by Photonics Technologies and Systems)

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16 pages, 5752 KiB

Open AccessArticle

Hybrid-Integrated Multi-Lines Optical-Phased-Array Chip

by Shengmin Zhou, Mingjin Wang, Jingxuan Chen, Zhaozheng Yi, Jiahao Si and Wanhua Zheng

Photonics 2025, 12(7), 699; https://doi.org/10.3390/photonics12070699 - 10 Jul 2025

Abstract

We propose a hybrid-integrated III–V-silicon optical-phased-array (OPA) based on passive alignment flip–chip bonding technology and provide new solutions for LiDAR. To achieve a large range of vertical beam steering in a hybrid-integrated OPA, a multi-lines OPA in a single chip is introduced. The [...] Read more.

We propose a hybrid-integrated III–V-silicon optical-phased-array (OPA) based on passive alignment flip–chip bonding technology and provide new solutions for LiDAR. To achieve a large range of vertical beam steering in a hybrid-integrated OPA, a multi-lines OPA in a single chip is introduced. The system allows parallel hybrid integration of multiple dies onto a single wafer, achieving a multi-fold improvement in tuning efficiency. In order to increase the range of horizontal beam steering, we propose a half-wavelength pitch waveguide emitter with non-uniform width to reduce the crosstalk, which can remove the higher-order grating lobes in free space. In this work, we simulate OPA individually for four-lines and eight-lines. As a result, we simultaneously achieved a beam steering with nearly ±90° (horizontal) × 17.2° (vertical, when four-line OPA) or 39.6° (vertical, when eight-line OPA) field of view (FOV) and a high tuning efficiency with 1.13°/nm (when eight-line OPA). Full article

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18 pages, 1900 KiB

Open AccessArticle

Recovery of Optical Transport Coefficients Using Diffusion Approximation in Bilayered Tissues: A Theoretical Analysis

by Suraj Rajasekhar and Karthik Vishwanath

Photonics 2025, 12(7), 698; https://doi.org/10.3390/photonics12070698 - 10 Jul 2025

Abstract

Time-domain (TD) diffuse reflectance can be modeled using diffusion theory (DT) to non-invasively estimate optical transport coefficients of biological media, which serve as markers of tissue physiology. We employ an optimized N-layer DT solver in cylindrical geometry to reconstruct optical coefficients of bilayered [...] Read more.

Time-domain (TD) diffuse reflectance can be modeled using diffusion theory (DT) to non-invasively estimate optical transport coefficients of biological media, which serve as markers of tissue physiology. We employ an optimized N-layer DT solver in cylindrical geometry to reconstruct optical coefficients of bilayered media from TD reflectance generated via Monte Carlo (MC) simulations. Optical properties for 384 bilayered tissue models representing human head or limb tissues were obtained from the literature at three near-infrared wavelengths. MC data were fit using the layered DT model to simultaneously recover transport coefficients in both layers. Bottom-layer absorption was recovered with errors under 0.02 cm⁻¹, and top-layer scattering was retrieved within 3 cm⁻¹ of input values. In contrast, recovered bottom-layer scattering had mean errors exceeding 50%. Total hemoglobin concentration and oxygen saturation were reconstructed for the bottom layer to within 10 μM and 5%, respectively. Extracted transport coefficients were significantly more accurate when obtained using layered DT compared to the conventional, semi-infinite DT model. Our results suggest using improved theoretical modeling to analyze TD reflectance analysis significantly improves recovery of deep-layer absorption. Full article

(This article belongs to the Special Issue Optical Technologies for Biomedical Science)

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23 pages, 3781 KiB

Open AccessArticle

Influence of Uncertainties in Optode Positions on Self-Calibrating or Dual-Slope Diffuse Optical Measurements

by Giles Blaney, Angelo Sassaroli, Tapan Das and Sergio Fantini

Photonics 2025, 12(7), 697; https://doi.org/10.3390/photonics12070697 (registering DOI) - 10 Jul 2025

Abstract

Self-calibrating and dual-slope measurements have been used in the field of diffuse optics for robust assessment of absolute values or temporal changes in the optical properties of highly scattering media and biological tissue. These measurements employ optical probes with a minimum of two [...] Read more.

Self-calibrating and dual-slope measurements have been used in the field of diffuse optics for robust assessment of absolute values or temporal changes in the optical properties of highly scattering media and biological tissue. These measurements employ optical probes with a minimum of two source positions and a minimum of two detector positions. This work focuses on a quantitative analysis of the impact of errors in these source and detector positions on the assessment of optical properties. We considered linear, trapezoidal, and rectangular optode arrangements and theoretical computations based on diffusion theory for semi-infinite homogeneous media. We found that uncertainties in optodes’ positions may have a greater impact on measurements of absolute scattering versus absorption coefficients. For example, a 4.1% and 19% average error in absolute absorption and scattering, respectively, can be expected by displacing every optode in a linear arrangement by 1 mm in any direction. The impact of optode position errors is typically smaller for measurements of absorption changes. In each geometrical arrangement (linear, trapezoid, rectangular), we identify the direction of the position uncertainty for each optode that has minimal impact on the optical measurements. These results can guide the optimal design of optical probes for self-calibrating and dual-slope measurements. Full article

(This article belongs to the Special Issue Photonics: 10th Anniversary)

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10 pages, 294 KiB

Open AccessArticle

Delayed Choice for Entangled Photons

by Rolando Velázquez, Linda López-Díaz, Leonardo López-Hernández, Eduardo Hernández, L. M. Arévalo-Aguilar and V. Velázquez

Photonics 2025, 12(7), 696; https://doi.org/10.3390/photonics12070696 - 10 Jul 2025

Abstract

The wave–particle duality is the quintessence of quantum mechanics. This duality gives rise to distinct behaviors depending on the experimental setup, with the system exhibiting either wave-like or particle-like properties, depending on whether the focus is on interference (wave) or trajectory (particle). In [...] Read more.

The wave–particle duality is the quintessence of quantum mechanics. This duality gives rise to distinct behaviors depending on the experimental setup, with the system exhibiting either wave-like or particle-like properties, depending on whether the focus is on interference (wave) or trajectory (particle). In the interaction with a beam splitter, photons with particle behavior can transform into a wave behavior and vice versa. In Wheeler’s delayed-choice gedanken experiment, this interaction is delayed so that the wave that initially travels through the interferometer can become a particle, avoiding the interaction. We show that this contradiction can be resolved using polarized entangled photon pairs. An analysis of Shannon’s entropy supports this proposal. Full article

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

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28 pages, 7517 KiB

Open AccessReview

A Review of the Research Progress on Optical Fiber Sensors Based on C-Type Structures

by Zhijun Gao, Zhenbo Li and Yu Ying

Photonics 2025, 12(7), 695; https://doi.org/10.3390/photonics12070695 - 10 Jul 2025

Abstract

With the continuous advancement of optical fiber micromachining technology, C-type optical fibers have demonstrated significant potential in the field of optical fiber sensing. By partially or completely removing specific regions of the cladding, a “leakage window” is created, enabling interaction between the optical [...] Read more.

With the continuous advancement of optical fiber micromachining technology, C-type optical fibers have demonstrated significant potential in the field of optical fiber sensing. By partially or completely removing specific regions of the cladding, a “leakage window” is created, enabling interaction between the optical field and external substances. This structure has facilitated the development of various sensors. This paper reviews recent progress in the research and applications of C-type optical fibers in optical sensing. Based on sensing principles and application scenarios, C-type optical fiber sensors can be categorized into two main types: interferometric and photonic crystal types. This article discusses the fundamental operating principles and structural characteristics of each type, and provides a detailed comparison of their respective advantages and disadvantages. Studies have shown that sensors based on C-type fiber structures offer notable benefits such as simple fabrication, excellent mechanical performance, strong anti-interference capability, and high sensitivity. Therefore, they hold great promise for applications in intelligent monitoring, environmental detection, and healthcare. Finally, this review outlines future research directions for C-type fiber sensors. As technology continues to evolve, future studies are expected to focus on improving sensor stability, expanding application scenarios, and addressing challenges in current fabrication techniques. Full article

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

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12 pages, 1199 KiB

Open AccessArticle

The Potential of Tunable Femtosecond Laser Light to Prevent Melanoma A375 Cell Growth: An In Vitro Investigation

by Safaa Taha, Khalid T. Nawaf, Hala M. Rifaat, Ahmed O. El-Gendy and Tarek Mohamed

Photonics 2025, 12(7), 694; https://doi.org/10.3390/photonics12070694 - 10 Jul 2025

Abstract

The incidence and mortality rates associated with melanoma are increasing. Due to their high proliferation rate, ability to self-renew, and resistance mechanisms, cancer cells often withstand conventional therapies such as radiation and chemotherapy. Therefore, further research is required to develop novel melanoma therapies [...] Read more.

The incidence and mortality rates associated with melanoma are increasing. Due to their high proliferation rate, ability to self-renew, and resistance mechanisms, cancer cells often withstand conventional therapies such as radiation and chemotherapy. Therefore, further research is required to develop novel melanoma therapies with fewer adverse effects, but effective therapeutic impacts. This study aims to investigate how femtosecond laser treatment affects melanoma cells using the A375 cell line as an in vitro model. A375 melanoma cells were plated at a concentration of 10⁴ cells per well in 96-well plates and incubated overnight; then, they were subjected to femtosecond laser irradiation for durations of 3, 5, or 10 min, maintaining a steady power of 100 mW. The laser operated across different wavelengths in the ultraviolet, visible, and infrared ranges. Cell viability was evaluated 24 h after irradiation using the MTT assay. The results showed the significant inhibition of melanoma cell growth with various femtosecond laser parameters, particularly at 380 and 400 nm. At 380 nm, the cell viability was reduced by approximately 90%, and at 400 nm by 73%, after 10 min of exposure. Additional reductions were observed at 420 nm (47%) and 440 nm (18%), while no significant effects were found at 700–780 nm. The most effective exposure time was 10 min. Femtosecond laser radiation exerts a noteworthy anticancer effect on A375 cells, particularly at specific wavelengths and exposure durations, underscoring the potential of femtosecond laser therapy for treating melanoma. Exploring the underlying mechanisms of these effects and evaluating the clinical potential of this treatment modality requires further research. Full article

(This article belongs to the Section Biophotonics and Biomedical Optics)

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27 pages, 1665 KiB

Open AccessArticle

A Heuristic Optical Flow Scheduling Algorithm for Low-Delay Vehicular Visible Light Communication

by Zhengying Cai, Shumeng Lei, Jingyi Li, Chen Yu, Junyu Liu and Guoqiang Gong

Photonics 2025, 12(7), 693; https://doi.org/10.3390/photonics12070693 - 9 Jul 2025

Abstract

Vehicular visible light communication (VVLC) with ultralow electromagnetic interference has great potential to propel the growth of the Internet of Vehicles (IoV). However, ensuring quick response times and minimal delays in VVLC is a significant challenge brought on by fast-moving vehicles. In response [...] Read more.

Vehicular visible light communication (VVLC) with ultralow electromagnetic interference has great potential to propel the growth of the Internet of Vehicles (IoV). However, ensuring quick response times and minimal delays in VVLC is a significant challenge brought on by fast-moving vehicles. In response to this problem, we propose a heuristic optical flow scheduling algorithm. First, the optical flow scheduling problem of VVLC is built as a multi-objective optimization model considering the makespan, delay, schedulable ratio, and bandwidth utilization with non-conflict constraints. Second, an improved artificial plant community (APC) algorithm with enhanced global and local search capabilities is proposed to achieve low-delay communication for time-sensitive optical flows. Finally, a series of benchmark experiments are conducted to show that the proposed algorithm can efficiently schedule optical flows with minimal delay. The cost of this algorithm is very low, and it is suitable for deployment on edge computing platforms such as vehicles. Full article

(This article belongs to the Special Issue New Advances in Optical Wireless Communication)

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