Photonics

44 pages, 2486 KiB

Open AccessReview

A Comprehensive Review of Rubidium Two-Photon Vapor Cell Optical Clock: Long-Term Performance Limitations and Potential Improvements

by Asagwegbe C. Obaze-Adeleke, Bryan Semon and Thejesh N. Bandi

Photonics 2025, 12(5), 513; https://doi.org/10.3390/photonics12050513 (registering DOI) - 20 May 2025

Abstract

Two-photon vapor cell-based optical clocks are strong candidates for next-generation portable atomic standards, offering simplicity, compactness, and high performance. Their narrow clock transitions with counter-propagating beams enable first-order Doppler-free operation. However, systematic perturbations such as the AC Stark shift, temperature-induced shift, and drifts [...] Read more.

Two-photon vapor cell-based optical clocks are strong candidates for next-generation portable atomic standards, offering simplicity, compactness, and high performance. Their narrow clock transitions with counter-propagating beams enable first-order Doppler-free operation. However, systematic perturbations such as the AC Stark shift, temperature-induced shift, and drifts resulting from the laser system pose challenges cause instabilities to medium- to long-term performance. This paper provides a comprehensive overview of Rb two-photon vapor cell optical standards, focusing on the long-term performance-limiting effects and potential mitigation strategies, aiming for clock stabilities better than 1 × 10

^{- 15}

over the averaging time of a day and beyond. Full article

33 pages, 2544 KiB

Open AccessReview

Research Progress on Modulation Format Recognition Technology for Visible Light Communication

by Shengbang Zhou, Weichang Du, Chuanqi Li, Shutian Liu and Ruiqi Li

Photonics 2025, 12(5), 512; https://doi.org/10.3390/photonics12050512 - 19 May 2025

Abstract

As sixth-generation mobile communication (6G) advances towards ultra-high speed and global coverage, visible light communication (VLC) has emerged as a crucial complementary technology due to its ultra-high bandwidth, low power consumption, and immunity to electromagnetic interference. Modulation format recognition (MFR) plays a vital [...] Read more.

As sixth-generation mobile communication (6G) advances towards ultra-high speed and global coverage, visible light communication (VLC) has emerged as a crucial complementary technology due to its ultra-high bandwidth, low power consumption, and immunity to electromagnetic interference. Modulation format recognition (MFR) plays a vital role in the dynamic optimization and adaptive transmission of VLC systems, significantly influencing communication performance in complex channel environments. This paper systematically reviews the research progress in MFR for VLC, comparing the theoretical frameworks and limitations of traditional likelihood-based (LB) and feature-based (FB) methods. It also explores the advancements brought by deep learning (DL) technology, particularly in enhancing noise robustness, classification accuracy, and cross-scenario adaptability through automatic feature extraction and nonlinear mapping. The findings indicate that DL-based MFR substantially enhances recognition performance in intricate channels via multi-dimensional feature fusion, lightweight architectures, and meta-learning paradigms. Nonetheless, challenges remain, including high model complexity and a strong reliance on labeled data. Future research should prioritize multi-domain feature fusion, interdisciplinary collaboration, and hardware–algorithm co-optimization to develop lightweight, high-precision, and real-time MFR technologies that align with the 6G vision of space–air–ground–sea integrated networks. Full article

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10 pages, 1738 KiB

Open AccessArticle

A Preliminary Study on the Principle of Linear Effect Scaling Laws for Laser Atmospheric Transmission

by Xin Ye, Chengyu Fan, Wenyue Zhu, Pengfei Zhang, Xianmei Qian, Jinghui Zhang and Tao Jiang

Photonics 2025, 12(5), 511; https://doi.org/10.3390/photonics12050511 - 19 May 2025

Abstract

Numerical simulations were performed to rapidly predict and evaluate laser beam expansion caused by linear atmospheric transmission effects, such as turbulence and jitter, thereby enhancing the accuracy of the scaling law. Simulation results indicate that the turbulence term coefficient in the beam expansion [...] Read more.

Numerical simulations were performed to rapidly predict and evaluate laser beam expansion caused by linear atmospheric transmission effects, such as turbulence and jitter, thereby enhancing the accuracy of the scaling law. Simulation results indicate that the turbulence term coefficient in the beam expansion calibration expression correlates linearly with the initial beam mass and inversely with the transmission distance. By fitting a nonlinear surface, the relationship between the turbulence term coefficient, initial beam mass, and transmission distance was established. Additionally, under turbulence-free conditions, a calibration expression relating initial beam mass to transmission distance was derived. The tracking jitter-term coefficient was determined to be 3.69, effectively characterizing beam expansion due to system jitter error. Based on simulation outcomes, a scaling law model for beam expansion induced by linear atmospheric transmission effects was clearly established. The model closely matched the simulation data, with a root mean square error (RMSE) of 3.88. Compared with existing scaling law simulations, the proposed calibration expression significantly enhances the accuracy in predicting and evaluating beam expansion caused by linear atmospheric transmission effects. It also provides a more precise characterization of variations in beam expansion during laser transmission. Full article

(This article belongs to the Special Issue Laser as a Detection: From Spectral Imaging to LiDAR for Remote Sensing Applications)

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8 pages, 1334 KiB

Open AccessCommunication

Electrically Stimulated and Frequency-Tunable Photonic Tonic Spiking Neuron Based on a DFB-LD Under Optical Feedback

by Zhiqiang Lei, Chaotao He, Qiupin Wang, Pu Ou, Zhengmao Wu and Guangqiong Xia

Photonics 2025, 12(5), 510; https://doi.org/10.3390/photonics12050510 - 19 May 2025

Abstract

Based on a distributed feedback laser diode (DFB-LD) under optical feedback, a novel scheme for generating neuron-like tonic spiking is proposed, and the characteristics of the generated neuron-like tonic spiking are numerically investigated. Firstly, through adopting the Lang–Kobayashi model to analyze the nonlinear [...] Read more.

Based on a distributed feedback laser diode (DFB-LD) under optical feedback, a novel scheme for generating neuron-like tonic spiking is proposed, and the characteristics of the generated neuron-like tonic spiking are numerically investigated. Firstly, through adopting the Lang–Kobayashi model to analyze the nonlinear dynamics of the DFB-LD under optical feedback, the switching between different dynamic states is observed by continuously increasing the biased current of the DFB-LD, and the current regions required for driving the DFB-LD into the stable states and period one (P1) states are determined. Next, a rectangular electrical pulse is introduced as a stimulus signal to modulate the DFB-LD, and the lower and upper current values of the rectangular electrical pulse are set at the regions in which the DFB-LD operates at the stable state and P1 state, respectively. Under suitable operation parameters, sub-nanosecond tonic spiking can be generated. Finally, through adjusting the delayed time of optical feedback and selecting the matched rectangular electrical pulse, the frequency of tonic spiking can be detuned within a range exceeding 5 GHz. Full article

(This article belongs to the Special Issue Neuromorphic Photonics)

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19 pages, 4268 KiB

Open AccessArticle

A μrad Accuracy and nW Detection Sensitivity Four-Quadrant Heterodyne Coherent Angular Measurement System

by Ziqi Zhang, Shoufeng Tong, Peng Lin, Dixiang Zeng and Xiaonan Yu

Photonics 2025, 12(5), 509; https://doi.org/10.3390/photonics12050509 - 19 May 2025

Abstract

In gravitational wave measurement and inter-satellite laser communication systems, the relative rotation and motion between the transmitter and receiver terminals introduces small angular deviations over a link distance of thousands of kilometers, which need to be measured with high accuracy and sensitivity. The [...] Read more.

In gravitational wave measurement and inter-satellite laser communication systems, the relative rotation and motion between the transmitter and receiver terminals introduces small angular deviations over a link distance of thousands of kilometers, which need to be measured with high accuracy and sensitivity. The heterodyne coherent angle measurement has a higher measurement accuracy and detection sensitivity compared with the traditional direct detection technique, which performs angle measurement through the phase of a beat frequency signal. In this paper, we propose a four-quadrant heterodyne coherent angle measurement technique with μrad accuracy and nW-level detection sensitivity. A mathematical model of a differential wavefront sensing (DWS) angle solution was formulated, and a Monte Carlo simulation system was built for performance testing. An experimental system was devised to assess the accuracy and sensitivity of the heterodyne coherent measurement method and to compare the performance with that of the direct detection method. The experimental results showed that for azimuth and pitch axes, the accuracy of the heterodyne coherent angle measurement was 2.54 μrad and 2.85 μrad under the same signal power of −16 dBm, which had a 5-fold improvement compared with direct detection. The sensitivity of the heterodyne coherent detection was −50 dBm at the 20 μrad accuracy threshold, which was a 1000-fold improvement compared with direct detection. This research is of great significance for the phase measurement and tracking system in the field of gravitational wave detection and has a guiding role in system design work in the field of inter-satellite laser communication. Full article

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

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13 pages, 12842 KiB

Open AccessArticle

Strong Coupling Based on Quasibound States in the Continuum of Nanograting Metasurfaces in Near-Infrared Region

by Yulun Zhao, Junqiang Li, Yuchang Liu, Yadong Yue, Yongchuan Dang, Yilin Wang, Kun Liang and Li Yu

Photonics 2025, 12(5), 508; https://doi.org/10.3390/photonics12050508 - 19 May 2025

Abstract

Quasibound states in the continuum (qBICs) have aroused much attention as a feasible stage to investigate optical strong coupling due to their extremely high-quality factors (Q-factors) and extraordinary electromagnetic field enhancement. However, current demonstrations of strong coupling based on qBICs have primarily focused [...] Read more.

Quasibound states in the continuum (qBICs) have aroused much attention as a feasible stage to investigate optical strong coupling due to their extremely high-quality factors (Q-factors) and extraordinary electromagnetic field enhancement. However, current demonstrations of strong coupling based on qBICs have primarily focused on the visible spectral range, while research in the near-infrared (NIR) regime remains scarce. In this work, we design a nanograting metasurface supporting Friedrich–Wintgen bound states in the continuum (FW BICs). We demonstrate that FW BIC formation stems from destructive interference between Fabry–Pérot cavity modes and metal–dielectric hybrid guided-mode resonances. To investigate the qBIC–exciton coupling system, we simulated the interaction between MoTe₂ excitons and nanograting metasurfaces. A Rabi splitting of 55.4 meV was observed, which satisfies the strong coupling criterion. Furthermore, a chiral medium layer is modeled inside the nanograting metasurface by rewriting the weak expression and boundary conditions. A mode splitting of the qBIC–chiral medium system in the circular dichroism (CD) spectrum demonstrates that the chiral response successfully transferred from the chiral medium layer to the exciton–polaritons systems through strong coupling. In comparison to the existing studies, our work demonstrates a significantly larger CD signal under the same Pascal parameters and with a thinner chiral dielectric layer. Our work provides a new ideal platform for investigating the strong coupling based on quasibound states in the continuum, which exhibits promising applications in near-infrared chiral biomedical detection. Full article

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8 pages, 1732 KiB

Open AccessCommunication

Nonlinear-Optical-Loop-Mirror-Based Mode-Locked Fiber Laser Sensor for Low-Temperature Measurement

by Xian-An Dou, Linchan Li, Chang Liang, Haiping Xu, Qing Ye, Hui Kong, Jintian Bian and Lei Guo

Photonics 2025, 12(5), 507; https://doi.org/10.3390/photonics12050507 - 19 May 2025

Abstract

A temperature-sensing scheme is realized by a passively mode-locked Yb-doped fiber laser based on the nonlinear optical loop mirror (NOLM). The ambient temperature can be measured by detecting the pulse repetition frequency of the mode-locked fiber laser by an oscilloscope. When the ambient [...] Read more.

A temperature-sensing scheme is realized by a passively mode-locked Yb-doped fiber laser based on the nonlinear optical loop mirror (NOLM). The ambient temperature can be measured by detecting the pulse repetition frequency of the mode-locked fiber laser by an oscilloscope. When the ambient temperature increases from −40 °C to 6 °C, the pulse repetition frequency decreases linearly with a temperature sensitivity of 72.548 Hz/°C. The experimental results prove the feasibility of the mode-locked laser sensor operating in a low-temperature environment. Full article

(This article belongs to the Special Issue Advances in Solid-State Laser Technology and Applications)

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8 pages, 2358 KiB

Open AccessArticle

Passive Time-Division Multiplexing Fiber Optic Sensor for Magnetic Field Detection Applications in Current Introduction

by Yong Liu, Junjun Xiong, Junchang Huang, Fubin Pang, Yi Zhao and Li Xia

Photonics 2025, 12(5), 506; https://doi.org/10.3390/photonics12050506 - 19 May 2025

Abstract

Under the dual impetus of the “Dual Carbon” goals and the construction of smart grids, the development of new energy power infrastructure has been fully realized. The All-Fiber Optical Current Transformer (FOCT), leveraging its unique advantages, is in the process of supplanting traditional [...] Read more.

Under the dual impetus of the “Dual Carbon” goals and the construction of smart grids, the development of new energy power infrastructure has been fully realized. The All-Fiber Optical Current Transformer (FOCT), leveraging its unique advantages, is in the process of supplanting traditional current transformers to become the core component of power system monitoring equipment. Currently, to achieve higher precision and stability in magnetic field or current detection, FOCT structures frequently incorporate active components such as Y-waveguides and phase modulators, and closed-loop feedback systems are often used in demodulation. This has led to issues of high cost, complex demodulation, and difficult maintenance, significantly hindering the further advancement of FOCTs. Addressing the problems of high cost and complex demodulation, this paper proposes a passive multiplexing structure that achieves time-domain multiplexing of pulsed sensing signals, designs a corresponding intensity demodulation algorithm, and applies this structure to FOCTs. This enables low-cost, simple-demodulation current sensing, which can also be utilized for magnetic field detection, showcasing vast application potential. Full article

(This article belongs to the Special Issue Optical Fiber Sensors: Design and Application)

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9 pages, 4016 KiB

Open AccessCommunication

Longitudinal Polarization Vortices Generated via Terahertz Ring Resonator

by Mingyu Ji, Tengjiao Wang and Jingya Xie

Photonics 2025, 12(5), 505; https://doi.org/10.3390/photonics12050505 - 18 May 2025

Viewed by 94

Abstract

Vortex beams characterized by helical phase wavefronts enable innovative explorations of optical and physical interactions. This work experimentally realizes longitudinally polarized vortices with arbitrary topological charges in terahertz (THz) frequencies using a silicon ring resonator integrated with a second-order diffraction grating. The implemented [...] Read more.

Vortex beams characterized by helical phase wavefronts enable innovative explorations of optical and physical interactions. This work experimentally realizes longitudinally polarized vortices with arbitrary topological charges in terahertz (THz) frequencies using a silicon ring resonator integrated with a second-order diffraction grating. The implemented configuration enables flexible topological charge manipulation in longitudinally polarized electric fields through the excitation of quasi-transverse-magnetic (TM) waveguide modes with different frequencies. By employing a terahertz near-field measurement system, the spatial intensity patterns and phase characteristics of emitted waves are quantitatively analyzed via a precision probe. This strategy shows promising potential for applications in particle manipulation techniques and advanced imaging technologies. Full article

(This article belongs to the Special Issue Recent Progress in Integrated Photonics)

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13 pages, 2099 KiB

Open AccessArticle

Image-Based Laser-Beam Diagnostics Using Statistical Analysis and Machine Learning Regression

by Tayyab Imran and Muddasir Naeem

Photonics 2025, 12(5), 504; https://doi.org/10.3390/photonics12050504 - 18 May 2025

Viewed by 96

Abstract

This study is a comprehensive experimental and computational investigation into high-resolution laser beam diagnostics, combining classical statistical techniques, numerical image processing, and machine learning-based predictive modeling. A dataset of 50 sequential beam profile images was collected from a femtosecond fiber laser operating at [...] Read more.

This study is a comprehensive experimental and computational investigation into high-resolution laser beam diagnostics, combining classical statistical techniques, numerical image processing, and machine learning-based predictive modeling. A dataset of 50 sequential beam profile images was collected from a femtosecond fiber laser operating at a central wavelength of 780 nm with a pulse duration of approximately 125 fs. These images were analyzed to extract spatial and temporal beam characteristics, including centroid displacement, Full Width at Half Maximum (FWHM), ellipticity ratio, and an asymmetry index. All parameters were derived using intensity-weighted algorithms and directional cross-sectional analysis to ensure accurate and consistent quantification of the beam’s dynamic behavior. Linear regression models were applied to horizontal and vertical intensity distributions to assess long-term beam stability. The resulting predictive trends revealed a systematic drift in beam centroid position, most notably along the vertical axis, and a gradual broadening of the horizontal FWHM. The modeling further showed that vertical intensity increased over time while horizontal intensity displayed a slight decline, reinforcing the presence of axis-specific fluctuations. These effects are attributed to minor optical misalignments or thermally induced variations in the beam path. By integrating deterministic analysis with data-driven forecasting, this methodology offers a robust framework for real-time beam quality evaluation. It enhances sensitivity to subtle distortions and supports the future development of automated, self-correcting laser systems. The results underscore the critical role of continuous, high-resolution monitoring in maintaining beam stability and alignment precision in femtosecond laser applications. Full article

(This article belongs to the Special Issue Optical Technologies for Measurement and Metrology)

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20 pages, 6267 KiB

Open AccessArticle

Three-Dimensional Localization of Underwater Nodes Using Airborne Visible Light Beams

by Jaeed Bin Saif, Mohamed Younis and Fow-Sen Choa

Photonics 2025, 12(5), 503; https://doi.org/10.3390/photonics12050503 - 18 May 2025

Viewed by 45

Abstract

Localizing underwater nodes when they cannot be tethered or float on the surface presents significant challenges, primarily due to node mobility and the absence of fixed anchors with known coordinates. This paper advocates a strategy for tackling such a challenge by using visible [...] Read more.

Localizing underwater nodes when they cannot be tethered or float on the surface presents significant challenges, primarily due to node mobility and the absence of fixed anchors with known coordinates. This paper advocates a strategy for tackling such a challenge by using visible light communication (VLC) from an airborne unit. A novel localization method is proposed where VLC transmissions are made towards the water surface; each transmission is encoded with the Global Positioning System (GPS) coordinates with the incident point of the corresponding light beam. Existing techniques deal with the problem in 2D by assuming that the underwater node has a pressure sensor to measure its depth. The proposed method avoids this limitation and utilizes the intensity of VLC signals to estimate the 3D position of the underwater node. The idea is to map the light intensity at the underwater receiver for airborne light beams and devise an error optimization formulation to estimate the 3D coordinates of the underwater node. Extensive simulations validate the effectiveness of the proposed method and capture its performance across various parameters. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Underwater Wireless Optical Communications)

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10 pages, 2638 KiB

Open AccessArticle

Highly Birefringent FBG Based on Femtosecond Laser-Induced Cladding Stress Region for Temperature and Strain Decoupling

by Kuikui Guo, Hao Wu, Yonghao Liang, Mingshen Su, Hongcheng Wang, Rang Chu, Fei Zhou and Ye Liu

Photonics 2025, 12(5), 502; https://doi.org/10.3390/photonics12050502 - 18 May 2025

Viewed by 95

Abstract

We present and demonstrate a highly birefringent fiber Bragg grating (Hi-Bi FBG) that was fabricated using a femtosecond laser to induce a sawtooth stress region near the FBG. The FBG is fabricated with a femtosecond laser point-by-point method, while the sawtooth stress region [...] Read more.

We present and demonstrate a highly birefringent fiber Bragg grating (Hi-Bi FBG) that was fabricated using a femtosecond laser to induce a sawtooth stress region near the FBG. The FBG is fabricated with a femtosecond laser point-by-point method, while the sawtooth stress region is generated in fiber cladding using the femtosecond laser along a sawtooth path. This sawtooth stressor can introduce an anisotropic and asymmetric refractive index profile in the cross-section of the fiber, resulting in additional birefringence up to 2.97 × 10⁻⁴ along the axial direction of the FBG. The central wavelengths of the Hi-Bi FBG at the fast and slow axes exhibit different sensitivities to temperature and strain, allowing simultaneous measurement of the strain and temperature by tracking the resonant wavelength shifts in the two axes. The experimental results show that the temperature sensitivities of the fast and slow axes are 10.32 pm/°C and 10.42 pm/°C, while the strain sensitivities are 0.91 pm/µε and 0.99 pm/µε. The accuracy of this proposed sensor in measuring strain and temperature is estimated to be 2.2 µε and 0.2 °C. This approach addresses the issue of cross-sensitivity between temperature and strain and offers some advantages of low cost, compact size, and significant potential for advancements in practical multi-parameter sensing applications. Full article

(This article belongs to the Special Issue Novel Advances in Optical Fiber Gratings)

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18 pages, 3315 KiB

Open AccessArticle

Fiber Eavesdropping Detection and Location in Optical Communication System

by Yuang Li, Yuyuan Liang, Mingrui Zhang, Shuang Wei, Huatao Zhu, Yajie Li, Yongli Zhao and Jie Zhang

Photonics 2025, 12(5), 501; https://doi.org/10.3390/photonics12050501 - 16 May 2025

Viewed by 53

Abstract

Fiber eavesdropping severely endangers the confidentiality of data transmitted in optical networks. Therefore, it is necessary to explore how to detect and locate fiber eavesdropping in an effective approach. To leverage the advantages of the state of polarization (SOP) in detecting various abnormal [...] Read more.

Fiber eavesdropping severely endangers the confidentiality of data transmitted in optical networks. Therefore, it is necessary to explore how to detect and locate fiber eavesdropping in an effective approach. To leverage the advantages of the state of polarization (SOP) in detecting various abnormal events while addressing its challenges in acquiring the SOP of different fiber links, we propose a multi-channel joint SOP estimation scheme to estimate the SOP of different fiber spans. Based on the proposed scheme, we provide a comprehensive solution for fiber eavesdropping location and detection in optical communication systems. In this solution, the estimated SOP and optical performance monitoring (OPM) data are utilized for rapid fiber eavesdropping detection and coarse location at the span level. The effectiveness of the solution is validated by experiments. In the aspect of detection, we achieve the detection of the start or end of fiber eavesdropping, the overlapping of fiber eavesdropping and abnormal events, and other abnormal events. The overall accuracy is 99.77%. In the aspect of location, we can locate the fiber span that has been eavesdropped. Full article

(This article belongs to the Special Issue Photonics for Emerging Applications in Communication and Sensing II)

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21 pages, 16775 KiB

Open AccessArticle

Non-Iterative Phase-Only Hologram Generation via Stochastic Gradient Descent Optimization

by Alejandro Velez-Zea and John Fredy Barrera-Ramírez

Photonics 2025, 12(5), 500; https://doi.org/10.3390/photonics12050500 - 16 May 2025

Viewed by 29

Abstract

In this work, we explored, for the first time, to the best of our knowledge, the potential of stochastic gradient descent (SGD) to optimize random phase functions for application in non-iterative phase-only hologram generation. We defined and evaluated four loss functions based on [...] Read more.

In this work, we explored, for the first time, to the best of our knowledge, the potential of stochastic gradient descent (SGD) to optimize random phase functions for application in non-iterative phase-only hologram generation. We defined and evaluated four loss functions based on common image quality metrics and compared the performance of SGD-optimized random phases with those generated using Gerchberg–Saxton (GS) optimization. The quality of the reconstructed holograms was assessed through numerical simulations, considering both accuracy and computational efficiency. Our results demonstrate that SGD-based optimization can produce higher-quality phase holograms for low-contrast target scenes and presents nearly identical performance to GS-optimized random phases for high-contrast targets. Experimental validation confirmed the practical feasibility of the proposed method and its potential as a flexible alternative to conventional GS-based optimization. Full article

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

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26 pages, 5185 KiB

Open AccessArticle

Seamless Integration of UOWC/MMF/FSO Systems Using Orbital Angular Momentum Beams for Enhanced Data Transmission

by Mehtab Singh, Somia A. Abd El-Mottaleb, Hassan Yousif Ahmed, Medien Zeghid and Abu Sufian A. Osman

Photonics 2025, 12(5), 499; https://doi.org/10.3390/photonics12050499 - 16 May 2025

Viewed by 39

Abstract

This work presents a high-speed hybrid communication system integrating Underwater Optical Wireless Communication (UOWC), Multimode Fiber (MMF), and Free-Space Optics (FSO) channels, leveraging Orbital Angular Momentum (OAM) beams for enhanced data transmission. A Photodetector, Remodulate, and Forward Relay (PRFR) is employed to enable [...] Read more.

This work presents a high-speed hybrid communication system integrating Underwater Optical Wireless Communication (UOWC), Multimode Fiber (MMF), and Free-Space Optics (FSO) channels, leveraging Orbital Angular Momentum (OAM) beams for enhanced data transmission. A Photodetector, Remodulate, and Forward Relay (PRFR) is employed to enable wavelength conversion from 532 nm for UOWC to 1550 nm for MMF and FSO links. Four distinct OAM beams, each supporting a 5 Gbps data rate, are utilized to evaluate the system’s performance under two scenarios. The first scenario investigates the effects of absorption and scattering in five water types on underwater transmission range, while maintaining fixed MMF length and FSO link. The second scenario examines varying FSO propagation distances under different fog conditions, with a consistent underwater link length. Results demonstrate that water and atmospheric attenuation significantly impact transmission range and received optical power. The proposed hybrid system ensures reliable data transmission with a maximum overall transmission distance of 1125 m (comprising a 25 m UOWC link in Pure Sea (PS) water, a 100 m MMF span, and a 1000 m FSO range in clear weather) in the first scenario. In the second scenario, under Light Fog (LF) conditions, the system achieves a longer reach of up to 2020 m (20 m UOWC link + 100 m MMF span + 1900 m FSO range), maintaining a BER ≤ 10⁻⁴ and a Q-factor around 4. This hybrid design is well suited for applications such as oceanographic research, offshore monitoring, and the Internet of Underwater Things (IoUT), enabling efficient data transfer between underwater nodes and surface stations. Full article

(This article belongs to the Special Issue Optical Wireless Communication in 5G and Beyond)

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12 pages, 701 KiB

Open AccessArticle

Secret Key Agreement for SISO Visible Light Communication Systems

by Dongqin Guo, Jian Zhang, Gang Xin, Keming Tian and Xingyu Xiao

Photonics 2025, 12(5), 498; https://doi.org/10.3390/photonics12050498 - 16 May 2025

Viewed by 19

Abstract

This paper studies the use of secret key agreement for single-input single-output (SISO) visible light communication (VLC) systems. Specifically, we put forward a scheme for secret key generation and analyze the secret key rate in SISO VLC systems. First, we derive the secret [...] Read more.

This paper studies the use of secret key agreement for single-input single-output (SISO) visible light communication (VLC) systems. Specifically, we put forward a scheme for secret key generation and analyze the secret key rate in SISO VLC systems. First, we derive the secret key capacity bounds. Then, we analyze the achievable secret key rate distribution versus the illegal receiver’s location when the legal receiver’s location is fixed. Meanwhile, we deduce the average secret key capacity using random geometry knowledge when all the receivers’ positions are unknown. We also analyze the impact of employing a protected zone on the average secret key capacity and observe that a protected zone can obviously improve the secret performance. Finally, simulations are presented to verify the theoretical analysis. Full article

(This article belongs to the Special Issue Recent Advances in Optical Wireless Communication Systems and Networks)

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14 pages, 4156 KiB

Open AccessArticle

Supercontinuum Generation in Suspended Core Fibers Based on Intelligent Algorithms

by Meiqian Jing and Tigang Ning

Photonics 2025, 12(5), 497; https://doi.org/10.3390/photonics12050497 - 16 May 2025

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Abstract

This study presents a reverse-optimization framework for supercontinuum (SC) generation in Ge₂₀Sb₁₅Se₆₅ suspended-core fibers (SCFs), integrating neural network modeling with the Nutcracker Optimization Algorithm to co-design optimal fiber structures and pump pulse parameters. A high-nonlinearity SCF structure (γ [...] Read more.

This study presents a reverse-optimization framework for supercontinuum (SC) generation in Ge₂₀Sb₁₅Se₆₅ suspended-core fibers (SCFs), integrating neural network modeling with the Nutcracker Optimization Algorithm to co-design optimal fiber structures and pump pulse parameters. A high-nonlinearity SCF structure (γ ≈ 6–7 W⁻¹·m⁻¹) was first designed, and a neural network model was developed to accurately predict effective modal refractive indices and mode-field areas (RMSE < 1%). The generalized nonlinear Schrödinger equation was then used to study spectral broadening influenced by structural and pulse parameters. Global optimization was performed in four-dimensional structural and seven-dimensional combined parameter spaces, significantly enhancing computational efficiency. Simulation results demonstrated that the optimized design achieved a broad and flat SC spectrum extending from 0.7 µm to 25 µm (at –20 dB intensity), with lower peak power requirements compared to previous studies achieving similar coverage. The robustness and manufacturing tolerances of the optimized fiber structure were also analyzed, verifying the reliability of the design. This intelligent reverse-design strategy provides practical guidance and theoretical foundations for mid-infrared SC fiber design. Full article

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

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22 pages, 6192 KiB

Open AccessArticle

Advanced DFE, MLD, and RDE Equalization Techniques for Enhanced 5G mm-Wave A-RoF Performance at 60 GHz

by Umar Farooq and Amalia Miliou

Photonics 2025, 12(5), 496; https://doi.org/10.3390/photonics12050496 - 16 May 2025

Viewed by 26

Abstract

This article presents the decision feedback equalizer (DFE), the maximum likelihood detection (MLD), and the radius-directed equalization (RDE) algorithms designed in MATLAB-R2018a to equalize the received signal in a dispersive optical link up to 120 km. DFE is essential for improving signal quality [...] Read more.

This article presents the decision feedback equalizer (DFE), the maximum likelihood detection (MLD), and the radius-directed equalization (RDE) algorithms designed in MATLAB-R2018a to equalize the received signal in a dispersive optical link up to 120 km. DFE is essential for improving signal quality in several communication systems, including WiFi networks, cable modems, and long-term evolution (LTE) systems. Its capacity to mitigate inter-symbol interference (ISI) and rapidly adjust to channel variations renders it a flexible option for high-speed data transfer and wireless communications. Conversely, MLD is utilized in applications that require great precision and dependability, including multi-input–multi-output (MIMO) systems, satellite communications, and radar technology. The ability of MLD to optimize the probability of accurate symbol detection in complex, high-dimensional environments renders it crucial for systems where signal integrity and precision are critical. Lastly, RDE is implemented as an alternative algorithm to the CMA-based equalizer, utilizing the idea of adjusting the amplitude of the received distorted symbol so that its modulus is closer to the ideal value for that symbol. The algorithms are tested using a converged 5G mm-wave analog radio-over-fiber (A-RoF) system at 60 GHz. Their performance is measured regarding error vector magnitude (EVM) values before and after equalization for different optical fiber lengths and modulation formats (QPSK, 16-QAM, 64-QAM, and 128-QAM) and shows a clear performance improvement of the output signal. Moreover, the performance of the proposed algorithms is compared to three commonly used algorithms: the simple least mean square (LMS) algorithm, the constant modulus algorithm (CMA), and the adaptive median filtering (AMF), demonstrating superior results in both QPSK and 16-QAM and extending the transmission distance up to 120 km. DFE has a significant advantage over LMS and AMF in reducing the inter-symbol interference (ISI) in a dispersive channel by using previous decision feedback, resulting in quicker convergence and more precise equalization. MLD, on the other hand, is highly effective in improving detection accuracy by taking into account the probability of various symbol sequences achieving lower error rates and enhancing performance in advanced modulation schemes. RDE performs best for QPSK and 16-QAM constellations among all the other algorithms. Furthermore, DFE and MLD are particularly suitable for higher-order modulation formats like 64-QAM and 128-QAM, where accurate equalization and error detection are of utmost importance. The enhanced functionalities of DFE, RDE, and MLD in managing greater modulation orders and expanding transmission range highlight their efficacy in improving the performance and dependability of our system. Full article

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

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25 pages, 16145 KiB

Open AccessArticle

Fuzzy Logic-Based Performance Enhancement of FSO Systems Under Adverse Weather Conditions

by Hassan Yousif Ahmed, Medien Zeghid, Akhtar Nawaz Khan and Somia A. Abd El-Mottaleb

Photonics 2025, 12(5), 495; https://doi.org/10.3390/photonics12050495 - 16 May 2025

Viewed by 21

Abstract

In this paper, we propose an application of fuzzy logic control (FLC) to improve the system performance of free-space optics (FSO) networks using the optical code-division multiple-access (OCDMA) technique. The primary objective is to dynamically adjust the bit error rate (BER) threshold at [...] Read more.

In this paper, we propose an application of fuzzy logic control (FLC) to improve the system performance of free-space optics (FSO) networks using the optical code-division multiple-access (OCDMA) technique. The primary objective is to dynamically adjust the bit error rate (BER) threshold at the receiver based on weather conditions (i.e., rain and fog) and the propagation distance (which significantly affects the received power). The FLC module at the receiver integrates and processes these variables to optimize the BER threshold. The FLC module operates through an algorithm comprising eight well-defined steps, ensuring robust and adaptive control of the BER. Simulation results show that the FSO-FLC-based system has significant advantages over traditional approaches. For instance, under heavy rain conditions, the FSO-FLC system supports 12 users compared to a traditional system, which supports 7 users without FLC over a distance of 2.8 km with BER

10^{- 9}

. Similarly, under heavy fog conditions, the FSO-FLC system can support 22 users compared to a traditional system, which supports 18 users without FLC over a distance of 0.5 km with equal BER. These values show that the performance of FSO under weather conditions significantly improves when using the proposed approach. The computational efficiency and real-time feasibility of the FSO-FLC are also analyzed. The complexity of the FLC is O(1), indicating that the execution time remains constant regardless of input size. An Intel Core i7-1165G7 (2.80 GHz) using MATLAB’s fuzzy logic toolbox is used for all experiments. Results show that the proposed FLC executes up to 4 ms per decision cycle, which ensures real-time adaptability for practical FSO communication systems. Full article

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17 pages, 5329 KiB

Open AccessArticle

Stepped Confocal Microlens Array Fabricated by Femtosecond Laser

by Jinchi Wu, Hao Wu, Zheli Lin and Honghao Zhang

Photonics 2025, 12(5), 494; https://doi.org/10.3390/photonics12050494 - 16 May 2025

Viewed by 21

Abstract

Multi-focal microlens arrays provide notable advantages over mono-focal counterparts, such as multi-scale imaging capabilities and optical aberration correction. However, existing multi-focal microlens arrays fabricated on continuous surfaces are incapable of achieving confocal imaging. As a result, multiple focus adjustments are required to acquire [...] Read more.

Multi-focal microlens arrays provide notable advantages over mono-focal counterparts, such as multi-scale imaging capabilities and optical aberration correction. However, existing multi-focal microlens arrays fabricated on continuous surfaces are incapable of achieving confocal imaging. As a result, multiple focus adjustments are required to acquire comprehensive image data, thereby complicating system design and increasing operational duration. To overcome this limitation, a stepped confocal surface microlens array is proposed, capable of simultaneously capturing images with multiple depths of field, various field-of-view scales, and different resolutions—without the need for additional focus adjustments. A combination of femtosecond laser processing and chemical etching was employed to fabricate microlenses with varying curvatures on a stepped fused silica substrate, which was subsequently used as a mold. The final stepped confocal microlens array was replicated via polydimethylsiloxane (PDMS) molding. Preliminary experimental analyses were carried out to determine the relationship between processing parameters and the resulting focal lengths. By precisely controlling these parameters, the fabricated stepped confocal microlens array successfully enabled confocal imaging, allowing for the simultaneous acquisition of diverse image data. This microlens array shows great potential in advancing lightweight, integrated, and highly stable optical systems for applications in optical sensing, spatial positioning, and machine vision. Full article

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18 pages, 4679 KiB

Open AccessArticle

Coal Combustion Warning System Based on TDLAS and Performance Research

by Zhitao Xie, Guanyu Lin, Jianing Wang, Xi Wang, Weijia Li, Pengbo Li and Hengyuan Kong

Photonics 2025, 12(5), 493; https://doi.org/10.3390/photonics12050493 - 15 May 2025

Viewed by 68

Abstract

As the signature gas released before coal combustion, methane’s telemetry accuracy is susceptible to environmental influences, which is a difficult problem that needs to be solved. This article provides a detailed exposition on the application of Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology [...] Read more.

As the signature gas released before coal combustion, methane’s telemetry accuracy is susceptible to environmental influences, which is a difficult problem that needs to be solved. This article provides a detailed exposition on the application of Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology in the field of gas monitoring, with particular emphasis on its advantages in coal safety detection. The coal combustion experiment is conducted to determine the required performance indexes. Through the temperature compensation algorithm and normalized signals, the impact of factors such as ambient temperature and environmental noise is reduced, effectively improving the signal-to-noise ratio and accuracy. The experiments demonstrate that this system effectively lowers the detection limit for methane while improving measurement accuracy, thereby providing robust support for the early warning of coal spontaneous combustion. Full article

(This article belongs to the Special Issue Advances in Laser Spectroscopy: From Fundamentals to Applications)

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27 pages, 6467 KiB

Open AccessArticle

Combining Kronecker-Basis-Representation Tensor Decomposition and Total Variational Constraint for Spectral Computed Tomography Reconstruction

by Xuru Li, Kun Wang, Yan Chang, Yaqin Wu and Jing Liu

Photonics 2025, 12(5), 492; https://doi.org/10.3390/photonics12050492 - 15 May 2025

Viewed by 65

Abstract

Energy spectrum computed tomography (CT) technology based on photon-counting detectors has been widely used in many applications such as lesion detection, material decomposition, and so on. But severe noise in the reconstructed images affects the accuracy of these applications. The method based on [...] Read more.

Energy spectrum computed tomography (CT) technology based on photon-counting detectors has been widely used in many applications such as lesion detection, material decomposition, and so on. But severe noise in the reconstructed images affects the accuracy of these applications. The method based on tensor decomposition can effectively remove noise by exploring the correlation of energy channels, but it is difficult for traditional tensor decomposition methods to describe the problem of tensor sparsity and low-rank properties of all expansion modules simultaneously. To address this issue, an algorithm for spectral CT reconstruction based on photon-counting detectors is proposed, which combines Kronecker-Basis-Representation (KBR) tensor decomposition and total variational (TV) regularization (namely KBR-TV). The proposed algorithm uses KBR tensor decomposition to unify the sparse measurements of traditional tensor spaces, and constructs a third-order tensor cube through non-local image similarity matching. At the same time, the TV regularization term is introduced into the independent energy spectrum image domain to enhance the sparsity constraint of single-channel images, effectively reduce artifacts, and improve the accuracy of image reconstruction. The proposed objective minimization model has been tackled using the split-Bregman algorithm. To evaluate the algorithm’s performance, both numerical simulations and realistic preclinical mouse studies were conducted. The ultimate findings indicate that the KBR-TV method offers superior enhancement in the quality of spectral CT images in comparison to several existing methods. Full article

(This article belongs to the Special Issue Biomedical Optics：Imaging, Sensing and Therapy)

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10 pages, 2234 KiB

Open AccessArticle

Development and Characterization of an Asymmetric MZI Temperature Sensor Using Polymer Waveguides for Extended Temperature Measurement Scopes

by Fumin Liu, Xue Zhang, Tianyue Wang and Guanghao Huang

Photonics 2025, 12(5), 491; https://doi.org/10.3390/photonics12050491 - 15 May 2025

Viewed by 100

Abstract

To meet diverse industrial needs, temperature sensors with a wide measurement range have become a key element. In this paper, we propose an asymmetric Mach–Zehnder interferometer (MZI) temperature sensor based on polymer optical waveguides. Experimental results show that the output interference signal exhibits [...] Read more.

To meet diverse industrial needs, temperature sensors with a wide measurement range have become a key element. In this paper, we propose an asymmetric Mach–Zehnder interferometer (MZI) temperature sensor based on polymer optical waveguides. Experimental results show that the output interference signal exhibits periodic changes with temperature variations. The device exhibits a temperature measurement range of 120 °C and a sensitivity of 0.27 rad/°C. This study provides an effective new approach for developing high-performance, low-cost temperature sensors suitable for an extended temperature measurement range. Full article

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12 pages, 4596 KiB

Open AccessArticle

High-Speed Terahertz Modulation Signal Generation Based on Integrated LN-RMZM and CPPLN

by Hangfeng Zhou, Miao Ma, Chenwei Zhang, Xinlong Zhao, Weichao Ma, Wangzhe Li and Mingjun Xia

Photonics 2025, 12(5), 490; https://doi.org/10.3390/photonics12050490 - 15 May 2025

Viewed by 69

Abstract

With the increasing communication frequencies in 6G networks, high-speed terahertz (THz) modulation signal generation has become a critical research area. This study first proposes an on-chip high-speed THz modulation signal generation system based on lithium niobate (LN), which integrates a pair of racetrack [...] Read more.

With the increasing communication frequencies in 6G networks, high-speed terahertz (THz) modulation signal generation has become a critical research area. This study first proposes an on-chip high-speed THz modulation signal generation system based on lithium niobate (LN), which integrates a pair of racetrack resonator-integrated Mach–Zehnder modulators (RMZMs) with a chirped periodically poled lithium niobate (CPPLN) waveguide. The on-chip system combines near-infrared electro-optic modulation and cascaded difference-frequency generation (CDFG) for high-speed THz modulation signal generation. At 300 K, utilizing two input optical waves at frequencies of 193.55 THz and 193.14 THz, this on-chip system enables high-speed THz modulation signal generation at 0.41 THz, with a 1 Gbit/s modulation rate and a 0.25 V modulation voltage. During the simulation, when the intensity of the input optical waves is 1000 MW/cm², the generated 0.41 THz signal reaches a peak intensity of 21.24 MW/cm². Furthermore, based on theoretical analysis and subsequent simulation, the on-chip system is shown to support a maximum modulation signal generation rate of 7.75 Gbit/s. These results demonstrate the potential of the proposed on-chip system as a compact and efficient solution for high-speed THz modulation signal generation. Full article

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

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13 pages, 6191 KiB

Open AccessArticle

Investigating the Material Composition and Degradation of Wall Paintings at Müstair Monastery Using a Mobile Multi-Spectroscopic System

by Xueshi Bai, Patrick Cassitti, Aude Brebant, Didier Brissaud, Duixiong Sun, Yaopeng Yin and Vincent Detalle

Photonics 2025, 12(5), 489; https://doi.org/10.3390/photonics12050489 - 15 May 2025

Viewed by 143

Abstract

The conservation of cultural heritage requires advanced analytical tools to assess historic materials. In the context of the IPERION-CH project, a mobile multi-spectroscopic characterisation system for the analysis of cultural heritage materials, designated SYSPECTRAL, has been developed. This system integrates Laser-Induced Breakdown Spectroscopy [...] Read more.

The conservation of cultural heritage requires advanced analytical tools to assess historic materials. In the context of the IPERION-CH project, a mobile multi-spectroscopic characterisation system for the analysis of cultural heritage materials, designated SYSPECTRAL, has been developed. This system integrates Laser-Induced Breakdown Spectroscopy (LIBS), Laser-Induced Fluorescence, Raman spectroscopy, and reflectance spectroscopy. The first application of SYSPECTRAL in a real-world setting was carried out at Müstair Monastery (UNESCO World Heritage Site since 1983) for wall paintings. In this study, stratigraphic analysis using LIBS revealed lead- and iron-based pigments in black and red hues, suggesting pigment degradation and restoration interventions. The presence of titanium in white hues indicated possible retouching. Furthermore, the presence of Egyptian blue in blue hues was identified through a combination of elemental and reflectance spectral analysis, underscoring the potential of SYSPECTRAL for heritage conservation. This approach offers comprehensive material characterization with minimal impact, a finding that is of particular significance in the context of heritage conservation. The subsequent phase of research will extend the application of SYSPECTRAL to a wider range of heritage sites, with the objective of enhancing the spectral databases and refining the analytical techniques for the purpose of improving cultural heritage conservation. Full article

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

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11 pages, 4726 KiB

Open AccessArticle

Directional Excitation of Multi-Dimensional Coupled Topological Photonic States Based on Higher-Order Chiral Source

by Jiangle He, Yaxuan Li, Yangyang Fan, Xinwen Zhao, Lin Ding, Xueqi Yuan, Beijia Ji, Junzheng Hu, Lifu Liu, Guangxu Su, Peng Zhan and Fanxin Liu

Photonics 2025, 12(5), 488; https://doi.org/10.3390/photonics12050488 - 15 May 2025

Viewed by 154

Abstract

The topological phase of matter brings extra inspiration for efficient light manipulation. Here, we propose two-parameter tunable topological transitions based on distorted Kagome photonic crystals. By selecting specific splicing boundaries, we successfully visualize several diverse types of robust edge states and corner states. [...] Read more.

The topological phase of matter brings extra inspiration for efficient light manipulation. Here, we propose two-parameter tunable topological transitions based on distorted Kagome photonic crystals. By selecting specific splicing boundaries, we successfully visualize several diverse types of robust edge states and corner states. Through introducing optical vortices with tunable orbital angular momentum, we demonstrate the directional excitation of multi-dimensional topological states as needed. Furthermore, we have studied the coupling effects of multi-dimensional photonic states and the modulation of source in three typical areas. This work provides an instructive avenue for manipulating light in integrated topological photonic devices. Full article

(This article belongs to the Special Issue Polaritons Nanophotonics: Physics, Materials and Applications)

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15 pages, 5461 KiB

Open AccessArticle

Topological Rainbow Trapping with Expanded Bandwidth in Valley Photonic Crystals

by Sayed El. Soliman, Israa Abood, Naglaa Abdel All and Chii-Chang Chen

Photonics 2025, 12(5), 487; https://doi.org/10.3390/photonics12050487 - 14 May 2025

Viewed by 217

Abstract

We introduce a novel approach to achieve broadband rainbow trapping in a 2D photonic crystal (PC) platform. By exploiting the concept of valley PCs, we engineer a structure that supports robust topological edge states. A carefully designed rotational angle gradient along the edge [...] Read more.

We introduce a novel approach to achieve broadband rainbow trapping in a 2D photonic crystal (PC) platform. By exploiting the concept of valley PCs, we engineer a structure that supports robust topological edge states. A carefully designed rotational angle gradient along the edge state path induces frequency-dependent light localization, forming a topological rainbow with a significantly expanded bandwidth. This phenomenon of topological rainbow trapping is attributed to the interplay between valley-dependent topological edge states and the engineered rotational angle gradient. To further enhance light localization and broaden the trapping spectrum, we incorporate a graded radius profile in the bottom row of dielectric columns. Through a combination of rotational angle modulation and radius grading, we successfully realize broadband rainbow trapping with enhanced light localization. Our findings reveal a broad trapping bandwidth spanning from

0.8314 c / a

to

0.9205 c / a

, showcasing the potential of this approach for applications in optical frequency filtering, sensing, and information processing. Full article

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

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18 pages, 4240 KiB

Open AccessArticle

Experimental Validation of Designs for Permeable Diffractive Lenses Based on Photon Sieves for the Sensing of Running Fluids

by Veronica Pastor-Villarrubia, Angela Soria-Garcia, Joaquin Andres-Porras, Jesus del Hoyo, Mahmoud H. Elshorbagy, Luis Miguel Sanchez-Brea and Javier Alda

Photonics 2025, 12(5), 486; https://doi.org/10.3390/photonics12050486 - 14 May 2025

Viewed by 184

Abstract

This study reports the experimental validation of several designs of photon sieves with focusing capabilities. These permeable optical elements were implemented with a spatial light modulator working in pure-amplitude mode. The focal region was scanned using a traveling stage, holding a camera. Using [...] Read more.

This study reports the experimental validation of several designs of photon sieves with focusing capabilities. These permeable optical elements were implemented with a spatial light modulator working in pure-amplitude mode. The focal region was scanned using a traveling stage, holding a camera. Using this experimental setup, we characterized the focal region of the photon sieves and determined some parameters of interest, such as the depth of focus and the transverse extent of the focal region. These parameters and their evolution were measured and analyzed to compare the optical performance of different designs. Moreover, the permeability of the mask was also evaluated and is included in the discussion. When the photon sieve is intended to be used as an optical element for the monitoring of running fluids, one of the designs studied, labeled the Ring-by-Ring method, behaves in a quite balanced manner and thus has become the preferred choice. Through simulations for a refractometric sensor, we obtained the Figure of Merit of the Ring-by-Ring mask, which reached a maximum value of 7860 RIU⁻¹, which is competitive with plasmonic sensing devices. Full article

(This article belongs to the Special Issue Advanced Photonic Integration Technology and Devices)

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11 pages, 3049 KiB

Open AccessCommunication

Efficient Testing Light Path for Aspherical Surfaces Based on Secondary Imaging

by Xinrui Wang, Xin Li, Quan Zhang, Wei Wei and Enchao Liu

Photonics 2025, 12(5), 485; https://doi.org/10.3390/photonics12050485 - 14 May 2025

Viewed by 150

Abstract

At present, off-axis three-mirror optical systems mostly adopt aspherical mirrors with small apertures and small F/# to meet the development requirements of remote sensing payloads towards high precision, small volume, and lightweight design. However, current references rarely provide the derivation, design, and detection [...] Read more.

At present, off-axis three-mirror optical systems mostly adopt aspherical mirrors with small apertures and small F/# to meet the development requirements of remote sensing payloads towards high precision, small volume, and lightweight design. However, current references rarely provide the derivation, design, and detection of the testing light path for aspherical mirrors with small apertures and small F/#. Aiming at the existing gap, this paper proposes a method of decomposing the compensation optical path into two imaging light paths and derives the initial structure of the compensation optical path. Furthermore, specific solutions are proposed from two aspects: the design of the null compensator and the establishment of the testing light path. Finally, the compensation optical path design and detection are carried out for the primary mirror and the tertiary mirror of the self-calibrating real entrance pupil imaging spectrometer, guiding the completion of the system processing, assembly, and adjustment. The detection results show that the RMS of the surface shapes of the primary mirror and the tertiary mirror is 1/40λ (λ = 633 nm). This derivation method and the design method of the initial optical path have the characteristics of simple calculation, rapid optimization, and universal applicability, and are applicable to the detection of all quadratic concave surfaces. Full article

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8 pages, 889 KiB

Open AccessArticle

High-Bandwidth Silicon Strip Waveguide-Based Electro-Optical Modulator in Series Push–Pull Configuration

by Ahmed Shariful Alam, Sherif Nasif and J. Stewart Aitchison

Photonics 2025, 12(5), 484; https://doi.org/10.3390/photonics12050484 - 14 May 2025

Viewed by 199

Abstract

Silicon modulators operating at high speeds are crucial for contemporary optical communication systems; nevertheless, their performance is limited by the resistance–capacitance (RC) time constant. This research introduces a modulator based on a silicon strip waveguide, also known as a capacitively coupled silicon modulator [...] Read more.

Silicon modulators operating at high speeds are crucial for contemporary optical communication systems; nevertheless, their performance is limited by the resistance–capacitance (RC) time constant. This research introduces a modulator based on a silicon strip waveguide, also known as a capacitively coupled silicon modulator (CC-Si), arranged in a series push–pull (SPP) configuration, which effectively addresses the constraints imposed by the RC time constant. The modulator demonstrates a consistent electro-optic (EO) response that extends to 68 GHz. Furthermore, it achieves a phase shift of 0.022 radians for a C-band optical wave when exposed to a 15 GHz radio frequency (RF) modulation signal with an amplitude of 2.45 V. Full article

(This article belongs to the Special Issue Emerging Trends in On-Chip Photonic Integration Technologies)

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