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Photonics
Volume 12
Issue 5
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Photonics, Volume 12, Issue 5 (May 2025) – 29 articles

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14 pages, 1794 KiB  
Article
Feasibility of Photoplethysmography in Detecting Arterial Stiffness in Hypertension
by Parmis Karimpour, James M. May and Panicos A. Kyriacou
Photonics 2025, 12(5), 430; https://doi.org/10.3390/photonics12050430 (registering DOI) - 29 Apr 2025
Abstract
Asymptomatic peripheral artery disease (PAD) poses a silent risk, potentially leading to severe conditions if undetected. Integrating new screening tools into routine general practitioner (GP) visits could enable early detection. This study investigates the feasibility of photoplethysmography (PPG) monitoring for assessing vascular health [...] Read more.
Asymptomatic peripheral artery disease (PAD) poses a silent risk, potentially leading to severe conditions if undetected. Integrating new screening tools into routine general practitioner (GP) visits could enable early detection. This study investigates the feasibility of photoplethysmography (PPG) monitoring for assessing vascular health across different blood pressure (BP) conditions. Custom femoral artery phantoms representing healthy (0.82 MPa), intermediate (1.48 MPa), and atherosclerotic (2.06 MPa) vessels were tested under hypertensive, normotensive, and hypotensive conditions to evaluate PPG’s ability to distinguish between vascular states. Extracted features from the PPG signal, including amplitude, area under the curve (AUC), median upslope–downslope ratio, and median end datum difference, were analysed. Kruskal–Wallis tests revealed significant differences between healthy and unhealthy vessels across BP states, supporting PPG as a screening tool. The fiducial points from the second derivative of the photoplethysmography signal (SDPPG) were analysed. The ratio was most pronounced between healthy and unhealthy phantoms under hypertensive conditions (ranging from –2.13 to –2.06), suggesting a change in vascular wall distensibility. Under normotensive conditions, the difference in ratios between healthy and unhealthy phantoms was smaller (0.01), and no meaningful difference was observed under hypotensive conditions, suggesting the reduced sensitivity of this metric at lower perfusion pressures. Intermediate states were challenging to detect, particularly under hypotension, suggesting a need for further research. Nonetheless, this study highlights the promise of PPG monitoring in identifying vascular stiffness. Full article
38 pages, 10775 KiB  
Review
Comparison of Thin-Film Lithium Niobate, SOH, and POH for Silicon Photonic Modulators
by Tai-Cheng Yu, An-Chen Liu, Wei-Ta Huang, Chang-Chin Wu, Chung-Hsun Li, Tsung-Sheng Kao, Shu-Wei Chang, Chin-Wei Sher, Huang-Yu Lin, Chi-Wai Chow and Hao-Chung Kuo
Photonics 2025, 12(5), 429; https://doi.org/10.3390/photonics12050429 - 29 Apr 2025
Abstract
Optical modulators are indispensable components in optical communication systems and must be designed to minimize insertion loss, reduce driving voltage, and enhance linearity. State-of-the-art silicon modulator technology has limitations in terms of power, performance, and spatial size. The addition of materials such as [...] Read more.
Optical modulators are indispensable components in optical communication systems and must be designed to minimize insertion loss, reduce driving voltage, and enhance linearity. State-of-the-art silicon modulator technology has limitations in terms of power, performance, and spatial size. The addition of materials such as thin-film lithium niobate (TFLN), silicon–organic hybrids (SOH), and plasma–organic hybrids (POH) has improved the modulation performance in silicon photonics. An evaluation of the differences among these modulators and their respective performance characteristics is conducted. Full article
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19 pages, 5431 KiB  
Article
Polarization-Insensitive Silicon Grating Couplers via Subwavelength Metamaterials and Metaheuristic Optimization
by Jorge Parra
Photonics 2025, 12(5), 428; https://doi.org/10.3390/photonics12050428 - 29 Apr 2025
Abstract
Silicon photonics is the leading platform in photonic integrated circuits (PICs), enabling dense integration and low-cost manufacturing for applications such as data communications, artificial intelligence, and quantum processing, to name a few. However, efficient and polarization-insensitive fiber-to-PIC coupling for multipoint wafer characterization remains [...] Read more.
Silicon photonics is the leading platform in photonic integrated circuits (PICs), enabling dense integration and low-cost manufacturing for applications such as data communications, artificial intelligence, and quantum processing, to name a few. However, efficient and polarization-insensitive fiber-to-PIC coupling for multipoint wafer characterization remains a challenge due to the birefringence of silicon waveguides. Here, we address this issue by proposing polarization-insensitive grating couplers based on subwavelength dielectric metamaterials and metaheuristic optimization. Subwavelength periodic structures were engineered to act as uniaxial homogeneous linear (UHL) materials, enabling tailored anisotropy. On the other hand, particle swarm optimization (PSO) was employed to optimize the coupling efficiency, bandwidth, and polarization-dependent loss (PDL). Numerical simulations demonstrated that a pitch of 100 nm ensures UHL behavior while minimizing leaky waves. Optimized grating couplers achieved coupling efficiencies higher than −3 dB and a PDL of below 1 dB across the telecom C-band (1530–1565 nm). Three optimization strategies were explored, balancing efficiency, the bandwidth, and the PDL while considering the Pareto front. This work establishes a robust framework combining metamaterial engineering with computational optimization, paving the way for high-performance polarization-insensitive grating couplers with potential uses in advanced photonic applications. Full article
(This article belongs to the Special Issue Photonics Metamaterials: Processing and Applications)
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16 pages, 3126 KiB  
Article
Waveguide Coupled Full-Color Quantum Dot Light-Emitting Diodes Modulated by Microcavities
by Yilan Zhang, Wenhao Wang, Fankai Zheng, Jiajun Zhu, Guanding Mei, Yuxuan Ye, Jieyu Tan, Hechun Zhang, Qiang Jing, Bin He, Kai Wang and Dan Wu
Photonics 2025, 12(5), 427; https://doi.org/10.3390/photonics12050427 - 29 Apr 2025
Abstract
Integrated light-emitting diodes (LEDs) with waveguides play an important role in applications such as augmented reality (AR) displays, particularly regarding coupling efficiency optimization. Quantum dot light-emitting diodes (QLEDs), an emerging high-performance optoelectronic device, demonstrate substantial potential for next-generation display technologies. This study investigates [...] Read more.
Integrated light-emitting diodes (LEDs) with waveguides play an important role in applications such as augmented reality (AR) displays, particularly regarding coupling efficiency optimization. Quantum dot light-emitting diodes (QLEDs), an emerging high-performance optoelectronic device, demonstrate substantial potential for next-generation display technologies. This study investigates the influence of microcavity modulation on the output of QLEDs coupled with a silicon nitride (SiNx) waveguide by simulating a white light QLED (W-QLED) with a broad spectrum and mixed RGB QDs (RGB-QLED) with a comparatively narrower spectrum. The microcavity converts both W-QLED and RGB-QLED emissions from broadband white-light emissions into narrowband single-wavelength outputs. Specifically, both of them have demonstrated wavelength tuning and full-width at half-maximum (FWHM) narrowing across the visible spectrum from 400 nm to 750 nm due to the microcavity modulation. The resulting RGB-QLED achieves a FWHM of 11.24 nm and reaches 110.76% of the National Television System Committee 1953 (NTSC 1953) standard color gamut, which is a 20.95% improvement over W-QLED. Meanwhile, due to the Purcell effect of the microcavity, the output efficiency of the QLED coupled with a SiNx waveguide is also significantly improved by optimizing the thickness of the Ag anode and introducing a tilted reflective mirror into the SiNx waveguide. Moreover, the optimal output efficiency of RGB-QLED with the tilted Ag mirror is 10.13%, representing a tenfold increase compared to the sample without the tilted Ag mirror. This design demonstrates an efficient and compact approach for the near-eye full-color display technology. Full article
(This article belongs to the Special Issue Quantum Dot Light-Emitting Diodes: Innovations and Applications)
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16 pages, 3746 KiB  
Article
Theoretical Research on Large Field-of-View Polarization Imaging Based on Dynamic Vision Sensors
by Xiaotian Lu, Kunpeng Xing, Siran Li, Ziyu Gu and Lei Xin
Photonics 2025, 12(5), 426; https://doi.org/10.3390/photonics12050426 - 29 Apr 2025
Abstract
The combination of dynamic vision sensors (DVSs) and polarization can overcome the limitation of DVSs whereby they can only detect dynamic scenes, and it also has the ability to detect artificial targets and camouflaged targets, and is thus expected to become a new [...] Read more.
The combination of dynamic vision sensors (DVSs) and polarization can overcome the limitation of DVSs whereby they can only detect dynamic scenes, and it also has the ability to detect artificial targets and camouflaged targets, and is thus expected to become a new means of remote sensing detection. Remote sensing detection often requires the field-of-view (FOV) and width to be large enough to improve detection efficiency, but when large FOV polarization imaging is performed, the polarization state in the edge FOV and the center FOV will not be consistent, which does not meet the paraxial approximation condition, and the inconsistency increases as the angle between the incident light and the optical axis increases. This affects the accuracy of target detection, so in this paper, based on the characteristics of polarization imaging using a DVS, factors such as the polarizer rotation step, incident light polarization state, and incident angle are considered to establish a theoretical model of large FOV polarization imaging using DVSs. And the influence of the detection ability is analyzed for three types of incident conditions, namely linearly polarized light, natural light, and partially polarized light. The results show that when the rotation step is 5°, the highest false alarm rate for natural light incident in the edge FOV will be nearly 53%, and the highest false alarm rate for linearly polarized light incident will be nearly 32%. Full article
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15 pages, 8047 KiB  
Article
Compact Four-Channel Optical Emission Module with High Gain
by Xiying Dang, Linyi Li, Man Chen, Zijian Hu, Tianyu Yang, Zeping Zhao and Zhike Zhang
Photonics 2025, 12(5), 425; https://doi.org/10.3390/photonics12050425 - 28 Apr 2025
Viewed by 8
Abstract
In this paper, a four-channel optical emission module is developed using hybrid integration technology that integrates directly modulated laser (DML) chips, low-noise amplifier (LNA) chips, and control circuits, with dimensions of 24.4 mm × 21 mm × 5.9 mm. This module enables high-gain [...] Read more.
In this paper, a four-channel optical emission module is developed using hybrid integration technology that integrates directly modulated laser (DML) chips, low-noise amplifier (LNA) chips, and control circuits, with dimensions of 24.4 mm × 21 mm × 5.9 mm. This module enables high-gain signal output and minimizes crosstalk between neighboring channels while improving integration. An equivalent circuit model of radio frequency (RF) signal transmission is established, and the accuracy of the model and the effectiveness of the approach to improve signal gain are verified using simulations and experiments. With optimized thermal management, the module has the ability to operate at stable temperatures across an ambient range of −55 °C to 75 °C. The module has a channel wavelength spacing of approximately 1 nm, and the −3 dB bandwidth of each channel exceeds 20 GHz. The crosstalk between neighboring channels is less than −65 dB. In the range of 0.8~25 GHz, the four-channel gain is approximately 15 dB through the integration of the LNA chip. The module achieves a noise figure (NF) of less than 30 dB. Full article
(This article belongs to the Special Issue Microwave Photonics: Science and Applications)
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20 pages, 2763 KiB  
Review
Recent Advances of Guided Mode Resonant Sensors Applied to Cancer Biomarker Detection
by Pankaj K. Sahoo, Arshad Ahmad Bhat, Mandeep Singh and Kezheng Li
Photonics 2025, 12(5), 424; https://doi.org/10.3390/photonics12050424 - 28 Apr 2025
Viewed by 38
Abstract
Guided mode resonance (GMR)-based sensors have emerged as a promising technology for the early screening of cancer, offering advantages such as sensitivity, specificity, low cost, non-invasiveness, and portability. This review article provides a comprehensive overview of the latest advancements in GMR technology and [...] Read more.
Guided mode resonance (GMR)-based sensors have emerged as a promising technology for the early screening of cancer, offering advantages such as sensitivity, specificity, low cost, non-invasiveness, and portability. This review article provides a comprehensive overview of the latest advancements in GMR technology and its applications in biosensing, with a specific focus on cancer. The current state of cancer diagnosis and the critical need for point-of-care (POC) devices to address these challenges are discussed in detail. Furthermore, the review systematically explores various strategies employed in GMR-based cancer detection including design principles and the integration of advanced technologies. Additionally, it aims to provide researchers valuable insights for developing GMR sensors capable of detecting cancer biomarkers outside the laboratory environment. Full article
(This article belongs to the Section Biophotonics and Biomedical Optics)
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10 pages, 3231 KiB  
Article
A Flexible Photonic Method for Angle-of-Arrival and Frequency Measurements
by Yunkun Luo, Yang Jiang, Jing Xu, Xiaohong Lan, Jinjian Feng, Jiancheng Yu, Qianyou Long, Tingyi Jiang, Hui Zhang and Yu Wu
Photonics 2025, 12(5), 423; https://doi.org/10.3390/photonics12050423 - 28 Apr 2025
Viewed by 41
Abstract
A microwave photonic approach for measuring the angle of arrival (AOA) and frequency is proposed and experimentally demonstrated. The AOA-dependent phase difference and frequency of two received signals were mapped to intensity information through subtractive and differential operations, which were achieved by a [...] Read more.
A microwave photonic approach for measuring the angle of arrival (AOA) and frequency is proposed and experimentally demonstrated. The AOA-dependent phase difference and frequency of two received signals were mapped to intensity information through subtractive and differential operations, which were achieved by a delayed superposition structure with phase inversion. By measuring the output signal powers, both the phase difference and frequency of the two signals could be determined. The theoretical analysis results are given in detail. In this proof-of-concept experiment, the system had a phase difference measurement range of 340 degrees, with a maximum error of 2.9 degrees. The frequency measurement covered 1–10 GHz, with a maximum error of 2.2%. The proposed approach offers a straightforward method for measuring the AOA and frequency under the same configuration, which provides new insight into AOA- and frequency-measurement techniques. Full article
(This article belongs to the Section New Applications Enabled by Photonics Technologies and Systems)
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17 pages, 4106 KiB  
Review
Molecular Alignment Under Strong Laser Pulses: Progress and Applications
by Ming Wang, Enliang Zhang, Qingqing Liang and Yi Liu
Photonics 2025, 12(5), 422; https://doi.org/10.3390/photonics12050422 - 28 Apr 2025
Viewed by 68
Abstract
Molecular alignment under strong laser pulses is an important tool for manipulating quantum states and investigating ultrafast phenomena. This review summarizes two decades of advancement in laser-driven alignment techniques, such as cross-polarized double pulses, optical centrifuges, and elliptically truncated fields. Given the prominent [...] Read more.
Molecular alignment under strong laser pulses is an important tool for manipulating quantum states and investigating ultrafast phenomena. This review summarizes two decades of advancement in laser-driven alignment techniques, such as cross-polarized double pulses, optical centrifuges, and elliptically truncated fields. Given the prominent emphasis on transformational applications in current alignment research, we outline its importance in cutting-edge applications under strong laser pulses, such as chiral discrimination, high-harmonic generation (HHG), photoelectron angular distributions (PADs) and ionization yields in photoionization, and Terahertz (THz) manipulation. These interdisciplinary developments provide fundamental insights into ultrafast molecular dynamics. They also establish frameworks for advanced light–matter interaction control. Full article
(This article belongs to the Special Issue Advances in Ultrafast Laser Science and Applications)
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16 pages, 3858 KiB  
Article
Feasibility and Safety of Endoscopic Balloon-Assisted Laser Treatment (EBLT) for Gastroesophageal Reflux Disease: Functional, Structural, and Gene Expression Analysis in Preclinical Model
by Boram Cha, Hyejin Kim, Van Gia Truong, Sun-Ju Oh, Seok Jeong and Hyun Wook Kang
Photonics 2025, 12(5), 421; https://doi.org/10.3390/photonics12050421 - 28 Apr 2025
Viewed by 81
Abstract
Gastroesophageal reflux disease (GERD) is a prevalent disorder caused by lower esophageal sphincter (LES) dysfunction, often requiring long-term treatment. This study assessed the feasibility of endoscopic balloon-assisted laser treatment (EBLT) using a porcine GERD model. One week after GERD induction, EBLT was performed [...] Read more.
Gastroesophageal reflux disease (GERD) is a prevalent disorder caused by lower esophageal sphincter (LES) dysfunction, often requiring long-term treatment. This study assessed the feasibility of endoscopic balloon-assisted laser treatment (EBLT) using a porcine GERD model. One week after GERD induction, EBLT was performed on three animals, while one served as a control. A 980 nm continuous-wave laser was delivered at 30 W for 90 s (energy = 2700 J and power density = 2.17 W/cm2) in a circumferential, non-contact manner using a balloon-assisted catheter. Real-time mucosal temperature monitoring was achieved using a fiber Bragg grating (FBG) sensor integrated with the balloon, maintaining temperatures below 40 °C. Endoscopic ultrasound and manometry were used to evaluate LES thickness and pressure before and after treatment. After a 12-week observation period, esophageal tissues were harvested for histological and gene expression analysis. Compared to the control, the treated group showed an increase in LES thickness (3.6 ± 0.2 mm vs. 1.5 mm) and relative LES pressure changes (2.9 ± 1.6 vs. 0.6). Upregulation of fibrosis- and hypertrophy-related genes suggested structural remodeling of the LES. No adverse effects or mucosal injury were observed. These findings support EBLT as a promising and minimally invasive strategy for GERD treatment. Full article
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10 pages, 5727 KiB  
Article
Dual-Band Topological Valley Cavity in Mid-Infrared Range
by Chen Kang, Jinling Yu, Can Chen, Yunfeng Lai, Shuying Cheng, Yonghai Chen, Yuan Li, Shuman Liu, Jinchuan Zhang and Fengqi Liu
Photonics 2025, 12(5), 420; https://doi.org/10.3390/photonics12050420 - 28 Apr 2025
Viewed by 135
Abstract
Topological edge states, emerging at boundaries between regions with distinct topological properties, enable unidirectional transmission with robustness against defects and disorder. However, achieving dual-band operation with high performance remains challenging. Here, we integrate dual-band topological edge states into a valley photonic crystal cavity [...] Read more.
Topological edge states, emerging at boundaries between regions with distinct topological properties, enable unidirectional transmission with robustness against defects and disorder. However, achieving dual-band operation with high performance remains challenging. Here, we integrate dual-band topological edge states into a valley photonic crystal cavity operating in the mid-infrared region, leveraging triangular scatterers. A key contribution of this work is the simultaneous realization of ultra-high Q-factors (up to 6.1593 × 109) and uniform mode distribution (inverse participation ratio < 2) across both bands. Moreover, the dual-band cavity exhibits exceptional defect tolerance. These findings provide a promising platform for mid-infrared photonic integration, paving the way for high-performance optical cavities in multifunctional photonic systems. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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18 pages, 5158 KiB  
Article
Research on Maximum Likelihood Decoding Algorithm and Channel Characteristics Optimization for 4FSK Ultraviolet Communication System Based on Poisson Distribution
by Li Kuang, Yingkai Zhao, Kangjian Li, Xingfa Wang, Linyi Li, Huishi Zhu, Weijie Zhang and Jianguo Liu
Photonics 2025, 12(5), 419; https://doi.org/10.3390/photonics12050419 - 27 Apr 2025
Viewed by 111
Abstract
This study focuses on a 4FSK-modulated ultraviolet (UV) communication system, introducing an innovative symbol-level maximum likelihood decoding approach based on Poisson statistics. A forward error correction (FEC) coding mechanism is integrated to enhance system robustness. Through Monte Carlo simulations, the proposed decoding scheme [...] Read more.
This study focuses on a 4FSK-modulated ultraviolet (UV) communication system, introducing an innovative symbol-level maximum likelihood decoding approach based on Poisson statistics. A forward error correction (FEC) coding mechanism is integrated to enhance system robustness. Through Monte Carlo simulations, the proposed decoding scheme and the error correction performances of Reed–Solomon (RS) and Low-Density Parity-Check (LDPC) codes are evaluated in UV channels. Both RS and LDPC codes significantly improve the Bit Error Rate (BER), with LDPC codes achieving superior gains under low SNR conditions. Hardware implementation and field tests validate the decoding algorithm and LDPC-optimized 4FSK system. Under non-line-of-sight (NLOS) conditions (10–45° transmit elevation angle), stable 60 m communication with BER < 10−3 is achieved. In line-of-sight (LOS) scenarios, the system demonstrates 900 m range with BER < 10−3, highlighting practical applicability in challenging atmospheric environments. Full article
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14 pages, 2088 KiB  
Review
Optical Link Design for Quantum Key Distribution-Integrated Optical Access Networks
by Sunghyun Bae and Seok-Tae Koh
Photonics 2025, 12(5), 418; https://doi.org/10.3390/photonics12050418 - 27 Apr 2025
Viewed by 157
Abstract
To achieve commercial scalability, fiber-based quantum key distribution (QKD) systems must be integrated into existing optical communication infrastructures, rather than deployed exclusively on dedicated dark fibers. Integrating QKD into optical access networks (OANs) would be particularly advantageous, as these networks provide direct connectivity [...] Read more.
To achieve commercial scalability, fiber-based quantum key distribution (QKD) systems must be integrated into existing optical communication infrastructures, rather than deployed exclusively on dedicated dark fibers. Integrating QKD into optical access networks (OANs) would be particularly advantageous, as these networks provide direct connectivity to end users for whom security is critical. Such integration can address the inherent security vulnerabilities in current OANs, which are primarily based on time-division multiplexing passive optical networks (TDM-PONs). However, integrating QKD into PONs poses significant challenges due to Raman noise and other detrimental effects induced by PON signals, which intensify as the launched power of PONs increases to support higher transmission speeds. In this study, we review recent advancements in both QKD and access network technologies, evaluate the technical feasibility of QKD-OAN integration, and propose cost-effective strategies to facilitate the widespread deployment of QKD in future access networks. Full article
(This article belongs to the Special Issue Optical Signal Processing for Advanced Communication Systems)
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15 pages, 371 KiB  
Article
Circuit-QED for Multi-Loop Fluxonium-Type Qubits
by Larisa-Milena Pioraş-Ţimbolmaş, Levente Máthé and Liviu P. Zârbo
Photonics 2025, 12(5), 417; https://doi.org/10.3390/photonics12050417 - 25 Apr 2025
Viewed by 83
Abstract
Fluxonium qubits, designed to mitigate charge noise and enhance anharmonicity, are among the most promising superconducting platforms for quantum computing. To understand and exploit their quantum properties and design novel fluxonium-based architectures with improved functionalities, these systems require an accurate Hamiltonian formulation to [...] Read more.
Fluxonium qubits, designed to mitigate charge noise and enhance anharmonicity, are among the most promising superconducting platforms for quantum computing. To understand and exploit their quantum properties and design novel fluxonium-based architectures with improved functionalities, these systems require an accurate Hamiltonian formulation to capture their energy level structure and quantum dynamics. This work presents a systematic method for constructing the Hamiltonian for multi-loop circuits that partitions the system into a set of uncoupled harmonic oscillators and a coupled anharmonic part originating from the Josephson circuit elements, allowing clear identification of independent modes and isolating the nonlinearity in the Josephson terms. While demonstrated for fluxonium-type multi-loop circuits, this method can be generalized to other superconducting qubit architectures within the broader context of circuit QED, making it a versatile tool for exploring different circuit configurations. Our systematic and flexible modeling approach forms the theoretical basis for the qubit measurement and control experiments validating multi-loop fluxonium architectures. Full article
(This article belongs to the Special Issue Quantum Dot Light-Emitting Diodes: Innovations and Applications)
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11 pages, 4986 KiB  
Article
Improved Optical Signal Processing with On-Chip Programmable Filter
by Tiantian Li, Yumeng Liu, Luwen Xing, Shuo Lang, Zhangfeng Ge, Dongdong Han, Zhanqiang Hui, Huimin Du and Haowen Shu
Photonics 2025, 12(5), 416; https://doi.org/10.3390/photonics12050416 - 25 Apr 2025
Viewed by 80
Abstract
Bandwidth-limited transmitters have become a severe issue with the rapid growth of bandwidth-hungry services. We investigate the impact of an on-chip optical pre-emphasizer on a bandwidth-limited transmitter and quantitatively analyze the results of bandwidth extension. Improvements in eye diagram performance are discussed. The [...] Read more.
Bandwidth-limited transmitters have become a severe issue with the rapid growth of bandwidth-hungry services. We investigate the impact of an on-chip optical pre-emphasizer on a bandwidth-limited transmitter and quantitatively analyze the results of bandwidth extension. Improvements in eye diagram performance are discussed. The 3 dB electro-optical bandwidth of the transmission system is effectively extended from 18 GHz to 40 GHz. The extinction ratio of the on–off keying (OOK) signal at data rates of 20 to 50 Gbps is improved by 0.64–3.2 dB. Additionally, the Q factor of the eye diagram increases by 0.78–4.36 at data rates ranging from 20 to 50 Gbps. Full article
(This article belongs to the Special Issue Advanced Fiber Laser Technology and Its Application)
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15 pages, 2584 KiB  
Article
Measurement of Coherence Time in Cold Atom-Generated Tunable Photon Wave Packets Using an Unbalanced Fiber Interferometer
by Ya Li, Wanru Wang, Qizhou Wu, Youxing Chen, Can Sun, Hai Wang and Weizhe Qiao
Photonics 2025, 12(5), 415; https://doi.org/10.3390/photonics12050415 - 25 Apr 2025
Viewed by 95
Abstract
In the realm of quantum communication and photonic technologies, the extension of coherence time for photon wave packets is essential for improving system efficacy. This research introduces a methodology for measuring coherence time utilizing an unbalanced fiber interferometer, specifically designed for tunable pulse-width [...] Read more.
In the realm of quantum communication and photonic technologies, the extension of coherence time for photon wave packets is essential for improving system efficacy. This research introduces a methodology for measuring coherence time utilizing an unbalanced fiber interferometer, specifically designed for tunable pulse-width photon wave packets produced by cold atoms. By synchronously generating write pulses, signal light, and frequency-locking light from a single laser source, the study effectively mitigated frequency discrepancies that typically arise from the use of multiple light sources. The implementation of frequency-resolved photon counting under phase-locked conditions was accomplished through the application of polarization filtering and cascaded filtering techniques. The experimental results indicated that the periodicity of frequency shifts in interference fringe patterns diminishes as the differences in delay arm lengths increase, while fluctuations in fiber length and high-frequency laser jitter adversely affect interference visibility. Through an analysis of the correlation between delay and photon counts, the coherence time of the write laser was determined to be 2.56 µs, whereas the Stokes photons produced through interactions with cold atoms exhibited a reduced coherence time of 1.23 µs. The findings suggest that enhancements in laser bandwidth compression and fiber phase stability could further prolong the coherence time of photon wave packets generated by cold atoms, thereby providing valuable technical support for high-fidelity quantum information processing. Full article
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21 pages, 10397 KiB  
Article
Design and Analysis of High-Precision Workbench with Large Stroke and Heavy Load for Fabricating Large-Area Grating
by Guangdong Yu, Heshig Bayan, Qi Chen, Hao Chen, Xin He and Xuefeng Yao
Photonics 2025, 12(5), 414; https://doi.org/10.3390/photonics12050414 - 24 Apr 2025
Viewed by 156
Abstract
When scanning beam interference lithography (SBIL) technology is used for grating fabrication, the stroke, bearing capacity, and accuracy of the workbench determine the size and accuracy of the grating. For large-area gratings with dimensions exceeding the meter level, the existing workbench cannot fully [...] Read more.
When scanning beam interference lithography (SBIL) technology is used for grating fabrication, the stroke, bearing capacity, and accuracy of the workbench determine the size and accuracy of the grating. For large-area gratings with dimensions exceeding the meter level, the existing workbench cannot fully meet the requirements. Therefore, the structure design, drive type, and assembly technology of the workbench were studied in this research, and a two-dimensional workbench with a large stroke, heavy load, and high precision was developed. The performance of this workbench was tested. The stroke of the workbench can reach 1800 mm × 700 mm; the straightness is better than 1.5 μm for the whole stroke range. The load can be up to 2.5 t and the positioning accuracy can achieve the nanometer level. A scanning exposure experiment was carried out with this workbench and a grating of 1400 mm × 420 mm was made. The performance index of the grating was outstanding, achieving the intended goals of the experiment. Full article
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23 pages, 4596 KiB  
Review
Multimodal Imaging in Stem Cell Therapy for Retinal Disease
by Mi Zheng and Yannis M. Paulus
Photonics 2025, 12(5), 413; https://doi.org/10.3390/photonics12050413 - 24 Apr 2025
Viewed by 195
Abstract
Stem cell therapy has emerged as a promising approach for treating various retinal diseases, particularly degenerative retinal diseases such as geographic atrophy in age-related macular degeneration (AMD), retinitis pigmentosa (RP), and Stargardt disease. A wide variety of imaging techniques have been employed in [...] Read more.
Stem cell therapy has emerged as a promising approach for treating various retinal diseases, particularly degenerative retinal diseases such as geographic atrophy in age-related macular degeneration (AMD), retinitis pigmentosa (RP), and Stargardt disease. A wide variety of imaging techniques have been employed in both preclinical and clinical settings to assess the efficacy and safety of stem cell therapy for retinal diseases. These techniques can be classified into two categories: methods for imaging stem cells and those for the overall morphology and function of the retina. The techniques employed for stem cell imaging include optical imaging, magnetic resonance imaging (MRI), and radionuclide imaging. Additional imaging techniques include fundus photography, fluorescein angiography, and fundus autofluorescence. Each technique has its own advantages and disadvantages, and thus, the use of multimodal imaging can help to overcome the shortcomings and achieve a more comprehensive evaluation of stem cell therapy in retinal disease. This review discusses the characteristics of the main techniques and cell-labeling techniques applied in stem cell therapy, with a particular focus on the applications of multimodal imaging. Furthermore, this review discusses the challenges and prospects of multimodal imaging in stem cell therapy for retinal disease. Full article
(This article belongs to the Special Issue Exploring Cutting-Edge Technologies and Applications of Optics and Photonics)
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9 pages, 8350 KiB  
Communication
Asymmetry Analysis of the Autler–Townes Doublet in the Trap-Loss Fluorescence Spectroscopy of Cesium MOT with Single-Step Rydberg Excitation
by Xiaokai Hou, Yuewei Wang, Jun He and Junmin Wang
Photonics 2025, 12(5), 412; https://doi.org/10.3390/photonics12050412 - 24 Apr 2025
Viewed by 109
Abstract
The Autler–Townes (AT) doublet, a fundamental manifestation of quantum interference effects, serves as a critical tool for studying the dynamic behavior of Rydberg atoms. Here, we investigate the asymmetry of the Autler–Townes (AT) doublet in the trap-loss fluorescence spectroscopy (TLFS) of cesium (Cs) [...] Read more.
The Autler–Townes (AT) doublet, a fundamental manifestation of quantum interference effects, serves as a critical tool for studying the dynamic behavior of Rydberg atoms. Here, we investigate the asymmetry of the Autler–Townes (AT) doublet in the trap-loss fluorescence spectroscopy (TLFS) of cesium (Cs) atoms confined in a magneto-optical trap (MOT) with single-step Rydberg excitation using a 319-nm ultraviolet (UV) laser. A V-type three-level system involving the ground state 6S1/2 (F = 4), excited state 6P3/2 (F′ = 5), and Rydberg state (nP3/2 (mJ = +3/2)) is theoretically modeled to analyze the nonlinear dependence of the AT doublet’s asymmetry and interval on the cooling laser’s detuning. Experiments reveal that as the cooling laser detuning Δ1 decreases from −15 MHz to −10 MHz, the AT doublet exhibits increasing symmetry, while its interval shows a nonlinear decrease. Theoretical simulations based on the density matrix equation and Lindblad master equation align closely with experimental data, confirming the model’s validity. This study provides insights into quantum interference dynamics in multi-level systems and offers a systematic approach for optimizing precision measurements in cold atom spectroscopy. Full article
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13 pages, 3166 KiB  
Article
Dynamic Measurement of Flowing Microparticles in Microfluidics Using Pulsed Modulated Digital Holographic Microscopy
by Yunze Lei, Yuge Li, Xiaofang Wang, Kequn Zhuo, Ying Ma, Sha An, Juanjuan Zheng, Kai Wen, Lihe Yan and Peng Gao
Photonics 2025, 12(5), 411; https://doi.org/10.3390/photonics12050411 - 24 Apr 2025
Viewed by 132
Abstract
We propose a pulsed modulated digital holographic microscopy (PM-DHM) technique for the dynamic measurement of flowing microparticles in microfluidic systems. By digitally tuning the pulse width and the repetition rate of a laser source within a single-frame exposure, this method enables the recording [...] Read more.
We propose a pulsed modulated digital holographic microscopy (PM-DHM) technique for the dynamic measurement of flowing microparticles in microfluidic systems. By digitally tuning the pulse width and the repetition rate of a laser source within a single-frame exposure, this method enables the recording of multiple images of flowing microparticles at different time points within a single hologram, allowing the quantification of velocity and acceleration. We demonstrate the feasibility of PM-DHM by measuring the velocity, acceleration, and forces exerted on PMMA microspheres and red blood cells flowing in microfluidic chips. Compared to traditional frame-sampling-based imaging methods, this technique has a much higher time resolution (in a range of microseconds) that is limited only by the pulse duration. This method demonstrates significant potential for high-throughput label-free flow cytometry detection and offers promising applications in drug development and cell analysis. Full article
(This article belongs to the Special Issue Advanced Quantitative Phase Microscopy: Techniques and Applications)
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10 pages, 7014 KiB  
Communication
Impact of Non-Vertical Sidewalls on Bandgap Properties of Lithium Niobate Photonic Crystals
by Peyman Bagheri, Xiaoyan Zhou and Lin Zhang
Photonics 2025, 12(5), 410; https://doi.org/10.3390/photonics12050410 - 24 Apr 2025
Viewed by 138
Abstract
We investigate the influence of non-vertical sidewall angles on the band structure characteristics of thin-film lithium niobate (LN) photonic crystals (PhCs), considering both suspended LN membranes and LN on insulator (LNOI) configurations. Utilizing the gap-to-midgap ratio as a figure-of-merit, we observe a 34% [...] Read more.
We investigate the influence of non-vertical sidewall angles on the band structure characteristics of thin-film lithium niobate (LN) photonic crystals (PhCs), considering both suspended LN membranes and LN on insulator (LNOI) configurations. Utilizing the gap-to-midgap ratio as a figure-of-merit, we observe a 34% reduction for a suspended LN PhC with 60° sidewall angles compared to the one with vertical sidewalls and a more substantial 73% reduction for LNOI PhCs with 70° sidewall angles. We address this challenge through the optimization of geometrical parameters of PhC unit cells with non-vertical sidewalls, taking fabrication feasibility into account. Our work provides a design guideline for the development of realistic LN PhC devices for future large-scale LN photonic circuits. Full article
(This article belongs to the Special Issue Recent Progress in Integrated Photonics)
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12 pages, 1136 KiB  
Article
Hybrid Method for Solving the Radiative Transport Equation
by André Liemert, Dominik Reitzle and Alwin Kienle
Photonics 2025, 12(5), 409; https://doi.org/10.3390/photonics12050409 - 24 Apr 2025
Viewed by 134
Abstract
The spherical harmonics method (PN method) is often used for solving the radiative transport equation in terms of analytical functions. A severe and unsolved problem in this context was the evaluation of the angle-resolved radiance near sources and boundaries, which is [...] Read more.
The spherical harmonics method (PN method) is often used for solving the radiative transport equation in terms of analytical functions. A severe and unsolved problem in this context was the evaluation of the angle-resolved radiance near sources and boundaries, which is a serious limitation of this method in view of concrete applications, e.g., in biomedical optics for investigating the different types of optical microscopy, within NIR spectroscopy, such as for the determination of ingredients in foods or in pharmaceuticals, and within physics-based rendering. In this article, we report on a hybrid method that enables accurate evaluation of the angle-resolved radiance directly at the boundary of an anisotropically scattering medium, avoiding the problems of the traditional PN methods. The derived integral equation needed for the realization of the hybrid PN method is formally valid for an arbitrary convex bounded medium. The proposed approach can be evaluated with practically the same computational effort as the traditional PN method while being far more accurate. Full article
(This article belongs to the Special Issue Biomedical Photonics)
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12 pages, 2674 KiB  
Article
Effect of Lower-Level Relaxation on the Pulse Generation Performance of Q-Switched Nd:YAG Laser
by Fuqiang Ma, Shiyu Wang, Bingbin Li, Peijin Shang, Jinyou Li and Zheyuan Li
Photonics 2025, 12(5), 408; https://doi.org/10.3390/photonics12050408 - 24 Apr 2025
Viewed by 121
Abstract
When analyzing and designing Q-switched Nd:YAG lasers, the impact of lower-energy-level relaxation on the pulse waveform is often ignored. This approximation typically does not result in significant deviations when the laser pulse duration is much longer than the relaxation time of the lower [...] Read more.
When analyzing and designing Q-switched Nd:YAG lasers, the impact of lower-energy-level relaxation on the pulse waveform is often ignored. This approximation typically does not result in significant deviations when the laser pulse duration is much longer than the relaxation time of the lower energy level. However, when the pulse duration approaches the nanosecond range, the spontaneous emission time of lower energy level in the Nd:YAG crystal, which is approximately 30 ns, can severely affect the pulse waveform. In this study, a theoretical model is proposed to investigate the influence of lower-energy-level relaxation on the output pulse waveform of an Nd:YAG laser. Specifically, the output waveform of a narrow-pulse-width Q-switched Nd:YAG laser is simulated. The results indicate that for narrow-pulse-width laser output, lower-energy-level relaxation causes a secondary peak to appear after the main peak of the Q-switched pulse. The energy of this secondary peak is more than two times higher than that of the main peak. An experimental system for acousto-optic Q-switched Nd:YAG lasers has also been established, and the Q-switched pulse waveforms are measured under conditions similar to those in the simulations. The tail peak phenomenon observed in the experiments is consistent with the simulation results, verifying the accuracy of the theoretical model. These findings provide a crucial theoretical foundation for understanding and optimizing Nd:YAG lasers and have significant implications for the development of similar technologies. In laser technology, particularly for applications requiring high precision and performance, considering such factors is essential for optimizing the design and functionality of laser systems. Full article
(This article belongs to the Special Issue Photodetectors for Next-Generation Imaging and Sensing Systems)
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10 pages, 7380 KiB  
Communication
Far-Field Topological Structure of the Second Harmonic from Higher-Order Poincaré Sphere Beam
by Yangyang Li, Ziping Zhu, Yuanxiang Wang, Jiantai Dou, Li Fan, Bo Li and Youyou Hu
Photonics 2025, 12(5), 407; https://doi.org/10.3390/photonics12050407 - 24 Apr 2025
Viewed by 159
Abstract
In this paper, the far-field topological structures (FFTSs) of the second harmonic (SH) from higher-order Poincaré sphere (HOPS) beams, including circularly polarized vortex beams (VBs), cylindrically vector beams (CVBs) and elliptically polarized CVBs (EPCVBs), were demonstrated and reported. To begin with, the hidden [...] Read more.
In this paper, the far-field topological structures (FFTSs) of the second harmonic (SH) from higher-order Poincaré sphere (HOPS) beams, including circularly polarized vortex beams (VBs), cylindrically vector beams (CVBs) and elliptically polarized CVBs (EPCVBs), were demonstrated and reported. To begin with, the hidden FFTSs of the SH after propagating the twice Rayleigh range were simulated based on the vectorial coupled wave equations and the Collins formula. Then, the experimental setup was established to achieve the SH from the HOPS by applying two orthogonal 5% MgO: PPLN crystals, the FFTSs of which were demonstrated. The theoretical and experimental results indicate that for the circularly polarized VBs, the FFTSs of the SH still exhibit the 135°-linearly polarized VBs, which is similar to that of the SH in-source plane, because the SH is the eigen-mode of free space, while for the CVBs, the FFTSs of the SH generally show the disappearance of the central dark core, replaced by the maximum light intensity at the center due to the topological phase transition during propagation. Especially of note, for the EPCVBs, the FFTSs of the SH display the maximum light intensity at the center, but the FFTSs in the horizontal and vertical directions reveal rotational symmetry related to the chirality of the EPCVBs. The results reveal the evolution mechanisms of the SH from the HOPS in the far field, which may facilitate the applications of the SH from HOPS beam. Full article
(This article belongs to the Special Issue Fundamentals and Applications of Vortex Beams)
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15 pages, 2658 KiB  
Article
55% Efficient High-Power Multijunction Photovoltaic Laser Power Converters for 1070 nm
by Simon Fafard and Denis Masson
Photonics 2025, 12(5), 406; https://doi.org/10.3390/photonics12050406 - 23 Apr 2025
Viewed by 87
Abstract
High-efficiency multijunction laser power converters are demonstrated for the first time at high power for optical inputs around 1070 nm. The InP-based photovoltaic power-converting III–V heterostructures are designed with eight lattice-matched InGaAsP subcells (PT8-1070 nm). Conversion efficiencies of 55% were obtained at 18 [...] Read more.
High-efficiency multijunction laser power converters are demonstrated for the first time at high power for optical inputs around 1070 nm. The InP-based photovoltaic power-converting III–V heterostructures are designed with eight lattice-matched InGaAsP subcells (PT8-1070 nm). Conversion efficiencies of 55% were obtained at 18 W of output power. Endurance testing was performed for over 1000 h of continuous operation with an average output power of 13.2 W at an input wavelength of 1064 nm. An average steady-state efficiency of 54.4% at an ambient temperature of ~20 °C was obtained for that duration. The results demonstrate that 1 cm2 optical power converter devices can produce electrical outputs of 20 W at maximum power voltages around Vmpp ~6 V, thus retaining an optimal load near Rmpp at ~2 ohms. Efficiencies between 57.9% and 59.0% were also obtained for smaller 0.029 cm2 chips for input intensities between 35 and 69 W/cm2. This is an important development for power beaming applications: the unprecedented combination of power and conversion efficiency capabilities is expected to enable deployments for key wavelengths between 1040 and 1080 nm. Full article
(This article belongs to the Special Issue High-Performance Semiconductor Optoelectronic Devices)
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15 pages, 7802 KiB  
Article
Adaptive Generation Method for Small Volume Easy Fabrication Freeform Unobscured Three-Mirror Systems Based on Machine Learning
by Yiwei Sun, Yangjie Wei and Ji Zhao
Photonics 2025, 12(5), 405; https://doi.org/10.3390/photonics12050405 - 22 Apr 2025
Viewed by 99
Abstract
Freeform unobscured multiple-mirror systems have been widely applied in high-precision optical fields due to their high imaging quality and no chromatic aberration and central obstruction. However, how to design a freeform unobscured multiple-mirror system with small system volume, imaging quality, and low manufacturing [...] Read more.
Freeform unobscured multiple-mirror systems have been widely applied in high-precision optical fields due to their high imaging quality and no chromatic aberration and central obstruction. However, how to design a freeform unobscured multiple-mirror system with small system volume, imaging quality, and low manufacturing difficulty is challenging. This study proposes an adaptive generation method for freeform unobscured three-mirror systems with small volume and ease of fabrication based on machine learning, considering the fabrication constraints, volume limitations, imaging quality, and design efficiency. First, an error function based on volume, fabrication, and imaging quality functions is constructed, and a dataset is generated using this error function. Then, a machine learning model is trained using this dataset, enabling efficient prediction of the parameters for small-volume, easy-to-fabricate freeform unobscured three-mirror systems. Finally, the parameters of the freeform unobscured three-mirror system are predicted using the trained model, and combined with the freeform surface generation method, a freeform unobscured three-mirror imaging system is automatically obtained. Experimental results demonstrate that our method can effectively generate freeform unobscured three-mirror systems that meet the requirements for small volume and easy fabrication, providing a new approach for optical design. Full article
(This article belongs to the Special Issue Emerging Topics in Freeform Optics)
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15 pages, 7143 KiB  
Article
Orthogonal Frequency Division Multiplexing for Visible Light Communication Based on Minimum Shift Keying Modulation
by Ying Zhang, Kexin Li and Yufeng Yang
Photonics 2025, 12(5), 404; https://doi.org/10.3390/photonics12050404 - 22 Apr 2025
Viewed by 138
Abstract
With the rapid development of visible light communication (VLC) technology, traditional modulation schemes can no longer meet the high demands for bandwidth efficiency and signal stability in complex application scenarios. In particular, in orthogonal frequency division multiplexing (OFDM) systems, issues such as the [...] Read more.
With the rapid development of visible light communication (VLC) technology, traditional modulation schemes can no longer meet the high demands for bandwidth efficiency and signal stability in complex application scenarios. In particular, in orthogonal frequency division multiplexing (OFDM) systems, issues such as the nonlinearity of Light-Emitting Diodes (LEDs) and carrier frequency offset have worsened system performance. To address these challenges, this paper proposes an N-order Minimum Shift Keying (NMSK) OFDM system with Fast Hartley Transform (FHT) for signal mapping. Monte Carlo simulations systematically compare the performance of low-order and high-order NMSK modulations under various conditions. The results indicate that low-order NMSK exhibits superior robustness against bit errors and interference, while high-order NMSK can maintain a stable PAPR and provide higher spectral efficiency in high-bandwidth demand scenarios. Further experiments validate the stability of high-order NMSK in high-density multi-user and Industrial Internet of Things (IIoT) environments, proving its adaptability and effectiveness in such scenarios. The high-order NMSK modulation scheme provides strong support for the reliability and bandwidth efficiency of future 6G VLC networks, offering significant application prospects. Full article
(This article belongs to the Special Issue Photonics: 10th Anniversary)
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11 pages, 3727 KiB  
Article
Dynamically Tunable Singular States Through Air-Slit Control in Asymmetric Resonant Metamaterials
by Yeong Hwan Ko and Robert Magnusson
Photonics 2025, 12(5), 403; https://doi.org/10.3390/photonics12050403 - 22 Apr 2025
Viewed by 130
Abstract
This study presents a novel method for dynamically tuning singular states in one-dimensional (1D) photonic lattices (PLs) using air-slit-based structural modifications. Singular states, arising from symmetry-breaking-induced resonance radiation, generate diverse spectral features through interactions between resonance modes and background radiation. By strategically incorporating [...] Read more.
This study presents a novel method for dynamically tuning singular states in one-dimensional (1D) photonic lattices (PLs) using air-slit-based structural modifications. Singular states, arising from symmetry-breaking-induced resonance radiation, generate diverse spectral features through interactions between resonance modes and background radiation. By strategically incorporating air slits to break symmetry in 1D PLs, we demonstrated effective control of resonance positions, enabling dual functionalities including narrowband band pass and notch filtering. These singular states originate from asymmetric guided-mode resonances (aGMRs), which can be interpreted by analytical modeling of the equivalent slab waveguide. Moreover, the introduction of multiple air slits significantly enhances spectral tunability by inducing multiple folding behaviors in the resonance bands. This approach allows for effective manipulation of optical properties through simple adjustments of air-slit displacements. This work provides great potential for designing multifunctional photonic devices with advanced metamaterial technologies. Full article
(This article belongs to the Special Issue Optical Metasurfaces: Applications and Trends)
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16 pages, 5371 KiB  
Article
Series/Parallel Boost/Buck DC/DC Converter as a Visible Light Communication HB-LED Driver Based on Split Power
by Daniel G. Aller, Diego G. Lamar, Juan R. Garcia-Mere, Marta M. Hernando, Juan Rodriguez and Javier Sebastian
Photonics 2025, 12(5), 402; https://doi.org/10.3390/photonics12050402 - 22 Apr 2025
Viewed by 166
Abstract
This paper presents a high-brightness LED (HB-LED) driver for visible light communication (VLC) based on two converters. The first is a high-frequency buck DC/DC converter and the second is a low-frequency boost DC/DC converter, connected in series with respect to the LED load [...] Read more.
This paper presents a high-brightness LED (HB-LED) driver for visible light communication (VLC) based on two converters. The first is a high-frequency buck DC/DC converter and the second is a low-frequency boost DC/DC converter, connected in series with respect to the LED load and connected in parallel at the input, forming a series/parallel boost/buck DC/DC converter. It is well known that a VLC system needs to perform two different tasks: biasing the HB-LED load and transmitting the communication signal. These typically have different power requirements; the bias power is 75%, while the communication power is 25% of the total power. The requirements of each converter are also different; the communication signal requires a fast output response and, therefore, a high switching frequency, while the biasing control does not require a converter with a high output voltage response. The proposed architecture in this paper takes advantage of the differences between the two tasks and achieves high efficiency and high communication performance by means of splitting power between the two DC/DC converters. The high-frequency buck DC/DC converter tracks the communication signal, while the low-frequency boost DC/DC converter is responsible for lighting tasks. This technique enables high efficiency because most of the power is processed by the low-frequency converter, while a minor part of the power is processed by the high-frequency converter, achieving high communication performance. To provide experimental results, the proposed VLC HB-LED driver was built and validated by reproducing a 64-QAM with a bit rate up to 1.5 Mbps, reaching 91.5% overall efficiency. Full article
(This article belongs to the Special Issue Advanced Technologies in Optical Wireless Communications)
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