Photonics

14 pages, 2486 KiB

Open AccessArticle

High-Performance O-Band Angled Multimode Interference Splitter with Buried Silicon Nitride Waveguide for Advanced Data Center Optical Networks

by Eduard Ioudashkin and Dror Malka

Photonics 2025, 12(4), 322; https://doi.org/10.3390/photonics12040322 (registering DOI) - 30 Mar 2025

Abstract

Many current 1 × 2 splitter couplers based on multimode interference (MMI) face difficulties such as significant back reflection and limited flexibility in waveguide segmentation at the output, which necessitate the addition of transitional structures like tapered waveguides or S-Bends. These limitations reduce [...] Read more.

Many current 1 × 2 splitter couplers based on multimode interference (MMI) face difficulties such as significant back reflection and limited flexibility in waveguide segmentation at the output, which necessitate the addition of transitional structures like tapered waveguides or S-Bends. These limitations reduce their effectiveness as photonic data-center applications, where precise waveguide configurations are crucial. To address these challenges, we propose a novel nanoscale 1 × 2 angled multimode interference (AMMI) power splitter with silicon nitride (SiN) buried core and silica cladding. The innovative angled light path design improved performance by minimizing back reflections back to the source and by providing greater flexibility of waveguide interconnections, making the splitter more adaptable for data-center applications. The SiN core was selected due to its lower refractive index contrast with silica compared to silicon, which helps further reduce back reflection. The dimensions of the splitter were optimized using full vectorial beam propagation method (FV-BPM), finite-difference time domain (FDTD), and multivariable optimization scanning tool (MOST) simulations to support transmission across the O-band. Our proposed device demonstrated excellent performance, achieving an excess loss of 0.22 dB and an imbalance of <0.01 dB at the output ports at an operational wavelength of 1.31 µm. The total device length is 101 µm with a thickness of 0.4 µm. Across the entire O-band range (1260–1360 nm), the performance of the splitter presented excess loss of up to 1.57 dB and an imbalance of up to 0.05 dB. Additionally, back reflections at the operational wavelength were measured at −40.96 dB and up to −39.67 dB over the O-band. This silicon-on-insulator (SOI) complementary metal-oxide semiconductor (CMOS) compatible AMMI splitter demonstrates high tolerance for manufacturing deviations due to its geometric layout, dimensions, and material selection. Furthermore, the proposed splitter is well-suited for use in O-band transceiver systems and can enhance data-center optical networks by supporting high-speed, low-loss data transmission. The compact design and CMOS compatibility make this device ideal for integrating into dense, high-performance computing environments, ensuring reliable signal distribution and minimal power loss. The splitter can support multiple communication channels, thus enhancing bandwidth and scalability for next-generation data-center infrastructures. Full article

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

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33 pages, 12041 KiB

Open AccessArticle

Experimental Study on Fiber Optic Strain Characterization of Overlying Rock Layer Movement Forms and States Using DFOS

by Tao Hu, Fengjun Wei, Jintao Wang, Yan Wang, Chunhua Song, Kuiliang Han and Kaiqiang Han

Photonics 2025, 12(4), 321; https://doi.org/10.3390/photonics12040321 (registering DOI) - 30 Mar 2025

Abstract

Mastering the movement laws of hard overlying rock layers is the foundation of the development of coal mining technology and plays an important role in improving coal mine safety production. Therefore, an indoor similar simulation experiment was conducted based on an actual coal [...] Read more.

Mastering the movement laws of hard overlying rock layers is the foundation of the development of coal mining technology and plays an important role in improving coal mine safety production. Therefore, an indoor similar simulation experiment was conducted based on an actual coal mining face to test the strain variations of the pre-embedded optical fibers in the model using distributed fiber optic sensing. Finally, the fiber optic strain distribution curve was used to characterize the movement form and state of the overlying rock layer and fractured rock blocks. The experimental results showed the following. (1) The strain distribution of horizontally laid optical fibers is characterized by an upward trapezoidal convex platform, reflecting the evolution law of various horizontal movement forms of overlying rock layers: voussoir beam → cantilever beam → reverse cantilever beam → voussoir beam. The strain curve of vertically laid optical fibers is characterized by two levels of right-handed trapezoidal protrusions above and below, representing the motion state of the upper voussoir beam–lower cantilever beam structure of the overburden. (2) In addition, as excavation progresses, the range and height of the failure deformation of the overlying rock layers develop in a stepped shape. (3) In the end, the final vertical development heights of the cantilever beam structure and the voussoir beam structure in the overburden were 90.27 m and 24.99 m, respectively. The experimental results are highly consistent with the UDEC numerical simulation and mandatory calculation formulas, thus verifying the feasibility of the experiment. These research results provide theoretical and experimental support for safe coal mining in practical working faces. Full article

(This article belongs to the Special Issue Science and Applications of Optical Fiber Sensors: Recent Advances and Future Trends)

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16 pages, 1656 KiB

Open AccessArticle

Electron-Ion Radiative Recombination Assisted by Bicircular Laser Pulses

by Deeksha Kanti, Jerzy Z. Kamiński, Liang-You Peng and Katarzyna Krajewska

Photonics 2025, 12(4), 320; https://doi.org/10.3390/photonics12040320 (registering DOI) - 29 Mar 2025

Abstract

Electron–ion radiative recombination in the presence of a bicircular laser pulse is analyzed beyond the dipole approximation. A bicircular pulse consists of two counter-rotating circularly polarized laser pulses with commensurate carrier frequencies. It is demonstrated that the broad bandwidth radiation can be generated [...] Read more.

Electron–ion radiative recombination in the presence of a bicircular laser pulse is analyzed beyond the dipole approximation. A bicircular pulse consists of two counter-rotating circularly polarized laser pulses with commensurate carrier frequencies. It is demonstrated that the broad bandwidth radiation can be generated in the process and that its spectrum can be significantly enhanced by tailoring the laser field. A special emphasis is put on analyzing temporal properties of generated radiation. Full article

(This article belongs to the Special Issue Ultrashort Laser Pulses)

13 pages, 3649 KiB

Open AccessArticle

Real-Time Unrepeated Long-Span Field Trial over Deployed 4-Core Fiber Cable Using Commercial 130-Gbaud PCS-16QAM 800 Gb/s OTN Transceivers

by Jian Cui, Chao Wu, Zhuo Liu, Yu Deng, Bin Hao, Leimin Zhang, Ting Zhang, Yuxiao Wang, Bin Wu, Chengxing Zhang, Jiabin Wang, Baoluo Yan, Li Zhang, Yong Chen, Xuechuan Chen, Hu Shi, Lei Shen, Lei Zhang, Jie Luo, Yan Sun, Qi Wan, Cheng Chang, Bing Yan and Ninglun Gu Show full author list Hide full author list

Photonics 2025, 12(4), 319; https://doi.org/10.3390/photonics12040319 (registering DOI) - 29 Mar 2025

Abstract

The space-division multiplexed (SDM) transmission technique based on uncoupled multi-core fibers (MCF) shows great implementation potential due to its huge transmission capacity and compatibility with existing transceivers. In this paper, we demonstrate a real-time single-span 106 km field trial over deployed 4-core MCF [...] Read more.

The space-division multiplexed (SDM) transmission technique based on uncoupled multi-core fibers (MCF) shows great implementation potential due to its huge transmission capacity and compatibility with existing transceivers. In this paper, we demonstrate a real-time single-span 106 km field trial over deployed 4-core MCF cable using commercial 800 Gb/s optical transport network (OTN) transceivers. The transceivers achieved a modulation rate of 130 Gbaud with the optoelectronic multiple-chip module (OE-MCM) packaging technique, which enabled the adoption of a highly noise-tolerant probability constellation shaping a 16-array quadrature amplitude modulation (PCS-16QAM) modulation format for 800 Gb/s OTN transceivers, and could realize unrepeated long-span transmission. The 4-core 800 Gb/s transmission systems achieved a real-time transmission capacity of 256 Tb/s with fully loaded 80-wavelength channels over the C+L band. The performance of different kinds of 800 G OTN transceivers with different modulation formats under this long-span unrepeated optical transmission system is also estimated and discussed. This field trial demonstrates the feasibility of applying uncoupled MCF with 800 Gb/s OTN transceivers in unrepeated long-span transmission scenarios and promotes its field implementation in next-generation high-speed optical interconnection systems. Full article

(This article belongs to the Special Issue Optical Networking Technologies for High-Speed Data Transmission)

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19 pages, 2240 KiB

Open AccessArticle

To Stitch or Not to Stitch, That Is the Question: Multi-Gaze Eye Topography Stitching Versus Single-Shot Profilometry

by Wen-Pin Lin, Lo-Yu Wu, Wei-Ren Lin, Lynn White, Richard Wu, Arwa Fathy, Rami Alanazi, Jay Davies and Ahmed Abass

Photonics 2025, 12(4), 318; https://doi.org/10.3390/photonics12040318 (registering DOI) - 28 Mar 2025

Abstract

Purpose: To evaluate whether corneal topography map stitching can fully substitute the traditional single-shot capture methods in clinical settings. Methods: This record review study involved the measurement of corneal surfaces from 38 healthy subjects using two instruments: the Medmont Meridia, which employs a [...] Read more.

Purpose: To evaluate whether corneal topography map stitching can fully substitute the traditional single-shot capture methods in clinical settings. Methods: This record review study involved the measurement of corneal surfaces from 38 healthy subjects using two instruments: the Medmont Meridia, which employs a stitching composite topography method, and the Eye Surface Profiler (ESP), a single-shot measurement device. Data were processed separately for right and left eyes, with multiple gaze directions captured by the Medmont device. Surface registration and geometric transformation estimation, including neighbouring cubic interpolation, were applied to assess the accuracy of stitched maps compared to single-shot measurements. Results: The study evaluated eye rotation angles and surface alignment between Medmont topography across various gaze directions and ESP scans. Close eye rotations were found in the right-gaze, left-gaze and up-gaze directions, with rotation angles of around 8°; however, the down-gaze angle was around 15°, almost twice other gaze rotation angles. Root mean squared error (RMSE) analysis revealed notable discrepancies, particularly in the right-, left-, and up-gaze directions, with errors reaching up to 98 µm compared to ESP scans. Additionally, significance analyses showed that surface area ratios highlighted considerable differences, especially in the up-gaze direction, where discrepancies reached 70% for both right and left eyes. Conclusions: Despite potential benefits, the findings highlight a significant mismatch between stitched and single-shot measured surfaces due to digital processing artefacts. Findings suggest that stitching techniques, in their current form, are not yet ready to substitute single-shot topography measurements fully. Although stitching helps fit large-diameter contact lenses, care should be taken regarding the central area, especially if utilising the stitched data for optimising optics or wavefront analysis. Full article

(This article belongs to the Special Issue Recent Advances in Biomedical Optics and Biophotonics)

15 pages, 1070 KiB

Open AccessArticle

Study of Propagation Characteristics of Light Beam with Orbital Angular Momentum (OAM) Through a Chiral Medium

by Faroq Razzaz and Muhammad Arfan

Photonics 2025, 12(4), 317; https://doi.org/10.3390/photonics12040317 (registering DOI) - 28 Mar 2025

Abstract

The interaction of a Gaussian vortex beam (GVB) with metamaterials during its propagation is of significant interest to the optical community. These GVBs are classified as structured light beams that possess orbital angular momentum (OAM). Understanding the behavior of structured light beams is [...] Read more.

The interaction of a Gaussian vortex beam (GVB) with metamaterials during its propagation is of significant interest to the optical community. These GVBs are classified as structured light beams that possess orbital angular momentum (OAM). Understanding the behavior of structured light beams is essential for clarifying fundamental interaction mechanisms with metamaterial structures. So, this work delves into the investigation of the propagation characteristics of a GVB within a chiral material. The analytical expressions for GVB propagating through a chiral medium are obtained by using the extended Huygens–Fresnel diffraction integral formula and the optical ABCD matrix system. In a chiral medium, GVB exhibits a tendency to fragment into a left circularly polarized (LCP) beam and a right circularly polarized (RCP) beam, each following its unique propagation paths. The beam intensity and gradient force are computed and discussed for OAM mode number, beam waist radius, and chirality parameter. This research will be quite helpful for light manipulation, optical sorting, optical radiation force, the radiative transfer process, and optical guiding. Full article

(This article belongs to the Special Issue Vortex Beams: Transmission, Scattering and Application)

12 pages, 3145 KiB

Open AccessArticle

Multi-Channel Sparse-Frequency-Scanning White-Light Interferometry with Adaptive Mode Locking for Pulse Wave Velocity Measurement

by Yifei Xu, Laiben Gao, Cheng Qian, Yiping Wang, Wenyan Liu, Xiaoyan Cai and Qiang Liu

Photonics 2025, 12(4), 316; https://doi.org/10.3390/photonics12040316 - 28 Mar 2025

Abstract

Fiber-optic Fabry–Pérot (F–P) sensors offer significant potential for non-invasive hemodynamic monitoring, but existing sensing systems face limitations in multi-channel measurement capabilities and dynamic demodulation accuracy. This study introduces a sparse-frequency-scanning white-light interferometry (SFS-WLI) system with an adaptive mode-locked cross-correlation (MLCC) algorithm to address [...] Read more.

Fiber-optic Fabry–Pérot (F–P) sensors offer significant potential for non-invasive hemodynamic monitoring, but existing sensing systems face limitations in multi-channel measurement capabilities and dynamic demodulation accuracy. This study introduces a sparse-frequency-scanning white-light interferometry (SFS-WLI) system with an adaptive mode-locked cross-correlation (MLCC) algorithm to address these challenges. The system leverages telecom-grade semiconductor lasers (191.2–196.15 THz sweep range, 50 GHz step) and a Fibonacci-optimized MLCC algorithm to achieve real-time cavity length demodulation at 5 kHz. Compared to normal MLCC algorithm, the Fibonacci-optimized algorithm reduces the number of computational iterations by 57 times while maintaining sub-nanometer resolution under dynamic perturbations. Experimental validation demonstrated a carotid–radial pulse wave velocity of 5.12 m/s in a healthy male volunteer. This work provides a scalable and cost-effective solution for cardiovascular monitoring with potential applications in point-of-care testing (POCT) and telemedicine. Full article

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10 pages, 1998 KiB

Open AccessArticle

MEMS-Integrated Tunable Fabry–Pérot Microcavity for High-Quality Single-Photon Sources

by Ziyang Zheng, Jiawei Yang, Xuebin Peng and Ying Yu

Photonics 2025, 12(4), 315; https://doi.org/10.3390/photonics12040315 - 28 Mar 2025

Abstract

We propose a micro-electromechanical system (MEMS)-integrated Fabry–Pérot (F–P) microcavity designed for a tunable single-photon source based on a single semiconductor quantum dot (QD). Through theoretical simulations, our design achieved a Purcell factor of 23, a photon extraction efficiency exceeding 88%, and an optical [...] Read more.

We propose a micro-electromechanical system (MEMS)-integrated Fabry–Pérot (F–P) microcavity designed for a tunable single-photon source based on a single semiconductor quantum dot (QD). Through theoretical simulations, our design achieved a Purcell factor of 23, a photon extraction efficiency exceeding 88%, and an optical cavity mode tuning range of more than 30 nm. Experimentally, we fabricated initial device prototypes using a micro-transfer printing process and demonstrated a tuning range exceeding 15 nm. The device exhibits high mechanical stability, full reversibility, and minimal hysteresis, ensuring reliable operation over multiple tuning cycles. Our findings highlight the potential of MEMS-integrated F–P microcavities for scalable, tunable single-photon sources. Furthermore, reaching a strong coupling regime could enable efficient single-photon routing, opening new possibilities for integrated quantum photonic circuits. Full article

(This article belongs to the Special Issue Development and Optimization of High-Power Semiconductor Laser Diodes and Photodetectors)

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13 pages, 7099 KiB

Open AccessArticle

Sizing Single Gold Nanoparticles with Bright-Field Microscopy

by Antonio Sotelo-López, Salvador Mendoza-Acevedo, José Abraham Balderas-López and Braulio Gutiérrez-Medina

Photonics 2025, 12(4), 314; https://doi.org/10.3390/photonics12040314 - 28 Mar 2025

Abstract

We present an experimental procedure for determining the diameter of spherical gold nanoparticles (AuNPs) using a basic bright-field microscopy apparatus. We achieved high-contrast imaging by constructing a bright-field microscopy system with ultrabright LEDs (incorporating low-cost components), employing Köhler illumination near the coherence limit, [...] Read more.

We present an experimental procedure for determining the diameter of spherical gold nanoparticles (AuNPs) using a basic bright-field microscopy apparatus. We achieved high-contrast imaging by constructing a bright-field microscopy system with ultrabright LEDs (incorporating low-cost components), employing Köhler illumination near the coherence limit, and using digital processing to perform image averaging and background subtraction. Our system allows for the detection of 80 nm, 150 nm, and 300 nm diameter AuNPs immobilized on functionalized glass substrates. Through-focus images of the particles show characteristic contrast inversion, from where we find a nearly linear relationship between the minimum intensity contrast and particle diameter (as determined from scanning electron microscopy) for the three sizes studied and for three different illumination wavelengths covering the corresponding AuNP plasmon band (λ = 460 nm, 520 nm, and 620 nm). This behavior was found to be consistent with the corresponding scattering and absorption cross-sections of the AuNPs under the illumination wavelengths considered. Full article

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21 pages, 866 KiB

Open AccessArticle

An Event Recognition Method for a Φ-OTDR System Based on CNN-BiGRU Network Model with Attention

by Changli Li, Xiaoyu Chen and Yi Shi

Photonics 2025, 12(4), 313; https://doi.org/10.3390/photonics12040313 - 28 Mar 2025

Abstract

The phase-sensitive optical time domain reflectometry (Φ-OTDR) technique offers a method for distributed acoustic sensing (DAS) systems to detect external acoustic fluctuations and mechanical vibrations. By accurately identifying vibration events, DAS systems provide a non-invasive solution for security monitoring. However, limitations in temporal [...] Read more.

The phase-sensitive optical time domain reflectometry (Φ-OTDR) technique offers a method for distributed acoustic sensing (DAS) systems to detect external acoustic fluctuations and mechanical vibrations. By accurately identifying vibration events, DAS systems provide a non-invasive solution for security monitoring. However, limitations in temporal signal analysis and the lack of spatial features significantly impact classification accuracy in event recognition. To address these challenges, this paper proposes a network model for vibration-event recognition that integrates convolutional neural networks (CNNs), bidirectional gated recurrent units (BiGRUs), and attention mechanisms, referred to as CNN-BiGRU-Attention (CBA). First, the CBA model processes spatiotemporal matrices converted from raw signals, extracting low-level features through convolution and pooling. Subsequently, features are further extracted and separated along both the temporal and spatial dimensions. In the spatial-dimension branch, horizontal convolution and pooling generate enhanced spatial feature maps. In the temporal-dimension branch, vertical convolution and pooling are followed by BiGRU processing to capture dynamic changes in vibration events from both past and future contexts. Additionally, the attention mechanism focuses on extracted features in both dimensions. The features from the two dimensions are then fused using two cross-attention mechanisms. Finally, classification probabilities are output through a fully connected layer and a softmax activation function. In the experimental simulation section, the model is validated using real-world data. A comparison with four other typical models demonstrates that the proposed CBA model offers significant advantages in both recognition accuracy and robustness. Full article

(This article belongs to the Special Issue Distributed Optical Fiber Sensing Technology)

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11 pages, 4192 KiB

Open AccessArticle

Low-Dark-Current and Wide-Dynamic-Range InGaAs/InAlAs Avalanche Photodiodes with a Dual-Charge Layer

by Guohao Yang, Tianhong Liu, Jinping Li and Cunzhu Tong

Photonics 2025, 12(4), 312; https://doi.org/10.3390/photonics12040312 - 28 Mar 2025

Abstract

This study explores the impact of a dual-charge layer structure on the performance of InGaAs/InAlAs avalanche photodiodes (APDs) with a separate absorption, charge, multiplication, charge, and transit (SACMCT) structure. The dual-charge layer, consisting of p-doped and n-doped charge layers on either side of [...] Read more.

This study explores the impact of a dual-charge layer structure on the performance of InGaAs/InAlAs avalanche photodiodes (APDs) with a separate absorption, charge, multiplication, charge, and transit (SACMCT) structure. The dual-charge layer, consisting of p-doped and n-doped charge layers on either side of the avalanche layer, is designed to precisely control the internal electric field, effectively reduce the dark current, and extend the dynamic range. Simulation results guided the fabrication of a backside-illuminated APD, which achieved a linear operating range of 10–30 V and a dark current as low as 80 nA. The optimized design significantly reduced the dark current and increased the breakdown voltage compared to previously reported APDs. These improvements demonstrate the potential of dual-charge-layer APDs for high-speed optical communications and precision photodetection applications. Full article

(This article belongs to the Special Issue Development and Optimization of High-Power Semiconductor Laser Diodes and Photodetectors)

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8 pages, 1555 KiB

Open AccessCommunication

Tunable All-Fiber Femtosecond Electro-Optic Optical Frequency Comb Operating at 1.5 μm

by Aiguo Zhang, Ke Dai, Lin Huang, Liwen Sheng, Zhiming Liu, Yudong Cui, Xiang Hao and Yusheng Zhang

Photonics 2025, 12(4), 311; https://doi.org/10.3390/photonics12040311 - 28 Mar 2025

Abstract

We propose and demonstrate a tunable femtosecond electro-optic optical frequency comb by shaping a continuous-wave seed laser in an all-fiber configuration. The seed laser, operating at 1.5 μm, is first cascade-phase-modulated and subsequently de-chirped to generate low-contrast pulses of approximately 8 ps at [...] Read more.

We propose and demonstrate a tunable femtosecond electro-optic optical frequency comb by shaping a continuous-wave seed laser in an all-fiber configuration. The seed laser, operating at 1.5 μm, is first cascade-phase-modulated and subsequently de-chirped to generate low-contrast pulses of approximately 8 ps at a repetition rate of 5.95 GHz. These pulses are then refined into clean, high-quality picosecond pulses using a Mamyshev regenerator. The generated source is further amplified using an erbium–ytterbium-doped fiber amplifier operating in a highly nonlinear regime, yielding output pulses compressed to around 470 fs. Tunable continuously across a 5.7~6 GHz range with a 1 MHz resolution, the picosecond pulses undergo nonlinear propagation in the final amplification stage, leading to output pulses that can be further compressed to a few hundred femtoseconds. By using a tunable bandpass filter, the center wavelength and spectral bandwidth can be flexibly tuned. This system eliminates the need for mode-locked cavities, simplifying conventional ultrafast electro-optic combs by relying solely on phase modulation, while delivering femtosecond pulses at multiple-gigahertz repetition rates. Full article

(This article belongs to the Special Issue Advanced Lasers and Their Applications, 2nd Edition )

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12 pages, 4422 KiB

Open AccessCommunication

Machine Learning-Assisted Mitigation of Optical Multipath Interference in PAM4 IM-DD Transmission Systems

by Wenxin Cui, Jiahao Huo, Jin Zhu, Jianlong Tao, Peng Qin, Xiaoying Zhang and Haolin Bai

Photonics 2025, 12(4), 310; https://doi.org/10.3390/photonics12040310 - 28 Mar 2025

Abstract

This paper aims to mitigate multipath interference (MPI) in intensity modulation with direct detection (IM-DD) systems using machine learning techniques, specifically for four-level pulse amplitude modulation (PAM4) systems. We propose a machine learning-assisted MPI mitigation scheme, called KNN-aided SVM+RF-M. In this scheme, KNN-aided [...] Read more.

This paper aims to mitigate multipath interference (MPI) in intensity modulation with direct detection (IM-DD) systems using machine learning techniques, specifically for four-level pulse amplitude modulation (PAM4) systems. We propose a machine learning-assisted MPI mitigation scheme, called KNN-aided SVM+RF-M. In this scheme, KNN-aided SVM serves as a soft decision algorithm that adapts the decision threshold to signal amplitude fluctuations, improving the decision accuracy for MPI-affected PAM4 signals. By replacing the original hard decision in the RF-M algorithm with KNN-aided SVM, we mitigate the error transfer problem inherent in RF-M. MPI mitigation is then achieved through MPI estimation and noise value cancellation methods applied to signals after soft decision processing. Our proposed scheme is validated in a 28 GBaud PAM4-DD transmission system, and the simulation results show that our proposed scheme can improve SIR tolerance by 2 dB and receiver sensitivity by about 1 dB at the 7% HD-FEC threshold compared to the original RF-M scheme. Full article

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

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30 pages, 1959 KiB

Open AccessArticle

Pilot-Assisted Phase Recovery in Coherent Optical Receivers with Robust Locally Weighted Interpolation

by Carlos Carrión Betancourt, Veruska Rodrigues Moreira, Kayol Soares Mayer, Jonathan Aguiar Soares and Dalton Soares Arantes

Photonics 2025, 12(4), 309; https://doi.org/10.3390/photonics12040309 - 27 Mar 2025

Abstract

Accurate and low-complexity phase estimation is crucial for optimal data recovery in coherent optical receivers, especially for applications in emerging scenarios such as low-margin optical networks, green networks, probabilistic-shaping modulation, the use of high-order modulation formats in hollow-core fibers, long-haul links operating at [...] Read more.

Accurate and low-complexity phase estimation is crucial for optimal data recovery in coherent optical receivers, especially for applications in emerging scenarios such as low-margin optical networks, green networks, probabilistic-shaping modulation, the use of high-order modulation formats in hollow-core fibers, long-haul links operating at low signal-to-noise ratios, and free-space optical links in low-earth orbit satellites, among others. These new developments demand highly efficient and reliable data transmission methods, even under stringent conditions of minimal operational excess margin. This paper introduces an efficient pilot-assisted phase estimation strategy for coherent optical receivers, integrating robust interpolation techniques for quasi-optimal operation. Our approach significantly enhances phase estimation accuracy, addressing the unique challenges posed by these new scenarios. Through comprehensive simulations, we illustrate our method’s superiority over conventional methods, showcasing marked improvements in computational complexity and bit error rate. The results highlight the critical role of sophisticated interpolation in bolstering pilot-assisted phase estimation, offering a promising technique for optimizing performance in next-generation coherent optical receivers. Full article

(This article belongs to the Special Issue Enabling Technologies for Optical Communications and Networking)

11 pages, 1972 KiB

Open AccessArticle

Research on Sugar Concentration Sensing Based on Real-Time Polarization and Interaction Effects

by Qiong Gong, Tongxiao Lyu, Song Ye and Xinqiang Wang

Photonics 2025, 12(4), 308; https://doi.org/10.3390/photonics12040308 - 27 Mar 2025

Abstract

This paper presents a non-contact method for detecting the sugar concentration in solutions based on real-time polarization characteristics and an interaction effects model. The feasibility of using polarization imaging technology for sugar concentration detection is analyzed. By analyzing the Stokes parameters, a linear [...] Read more.

This paper presents a non-contact method for detecting the sugar concentration in solutions based on real-time polarization characteristics and an interaction effects model. The feasibility of using polarization imaging technology for sugar concentration detection is analyzed. By analyzing the Stokes parameters, a linear regression model is developed to establish the interaction effects between sugar concentration and both the degree of linear polarization (DoLP) and the angle of linear polarization (AoLP). A three-beam polarization imaging system is used to simultaneously capture the polarization images of sucrose solutions with varying concentrations, and the SIFT algorithm is employed to eliminate image shifts. The results show a strong linear correlation between sugar concentration and both AoLP (R² = 0.998) and DoLP (R² = 0.968). The addition of the interaction effect model significantly improves the prediction accuracy, with an RMSE of 0.36934 and a relative error within ±5%. This method features a simple experimental setup, enables multi-angle simultaneous measurements, and offers advantages such as non-contact operation, ease of use, and high precision (average relative error < 5%). It provides a new approach for sugar concentration detection in food, industry, and other fields. Full article

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39 pages, 2349 KiB

Open AccessArticle

Quantum Entanglement of the Final Particles in the Resonant Trident Pair Production Process in a Strong Electromagnetic Wave

by Sergei P. Roshchupkin and Mikhail V. Shakhov

Photonics 2025, 12(4), 307; https://doi.org/10.3390/photonics12040307 - 27 Mar 2025

Abstract

The resonant trident pair production process in the collision of ultrarelativistic electrons with a strong electromagnetic wave was theoretically studied. Under resonant conditions, the intermediate virtual gamma-quantum became real. As a result, the original resonant trident pair production process effectively split into two [...] Read more.

The resonant trident pair production process in the collision of ultrarelativistic electrons with a strong electromagnetic wave was theoretically studied. Under resonant conditions, the intermediate virtual gamma-quantum became real. As a result, the original resonant trident pair production process effectively split into two first-order processes by the fine structure constant: the electromagnetic field-stimulated Compton effect and the electromagnetic field-stimulated Breit–Wheeler process. The kinematics of the resonant trident pair production process were studied in detail. It was shown that there are two different cases for the energies and outgoing angles of the final particles (an electron and an electron–positron pair) in which their quantum entanglement is realized. In the first case, energies and outgoing angles of the final ultrarelativistic particles are uniquely determined by the parameters of the electromagnetic field-stimulated Compton effect (the outgoing angle of the final electron and the quantum parameter of the Compton effect). In the second case, energies and outgoing angles of the final particles are uniquely determined by the electromagnetic field-stimulated Breit–Wheeler process (the electron–positron pair outgoing angle and the Breit–Wheeler quantum parameter). It was shown that in a sufficiently wide range of frequencies and intensities of a strong electromagnetic wave, and in the case of ultrarelativistic initial electrons, the differential probability of the resonant trident pair production process with simultaneous registration of the outgoing angles of the final particles can significantly (by several orders of magnitude) exceed the total probability of the electromagnetic field-stimulated Compton effect. Full article

(This article belongs to the Section Optical Interaction Science)

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29 pages, 11206 KiB

Open AccessReview

Digital Engineering in Diffractive Optics for Precision Laser Processing

by Serguei P. Murzin

Photonics 2025, 12(4), 306; https://doi.org/10.3390/photonics12040306 - 27 Mar 2025

Abstract

This article focuses on the application of digital engineering in diffractive optics for precision laser material processing. It examines methods for the development of diffractive optical elements (DOEs) and adaptive management approaches that enhance the accuracy and efficiency of laser processing. Key achievements [...] Read more.

This article focuses on the application of digital engineering in diffractive optics for precision laser material processing. It examines methods for the development of diffractive optical elements (DOEs) and adaptive management approaches that enhance the accuracy and efficiency of laser processing. Key achievements are highlighted in numerical modeling, machine learning applications, and geometry optimization of optical systems, along with the integration of dynamic DOEs with laser systems for adaptive beam control. The discussion includes the development of complex diffractive structures with improved characteristics and new optimization approaches. Special attention is given to the application of DOEs in micro- and nanostructuring, additive manufacturing technologies, and their integration into high-performance laser systems. Additionally, challenges related to the thermal stability of materials and the complexity of adaptive DOE control are explored, as well as the role of artificial intelligence in enhancing laser processing efficiency. Full article

(This article belongs to the Special Issue Diffractive Optics and Its Emerging Applications)

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12 pages, 3479 KiB

Open AccessCommunication

Compact Reflective Metasurface: Production of Broadband Vortex Beams in Millimeter Waves

by Asad Khan, Jinling Zhang, Muhammad Ishfaq, Ibrar Ahmad, Shahbaz Khan and Kamlesh Kumar Soothar

Photonics 2025, 12(4), 305; https://doi.org/10.3390/photonics12040305 - 26 Mar 2025

Abstract

A low-profile reflectarray has been designed in the Ka-band to efficiently generate wideband orbital angular momentum (OAM) vortex beams. The proposed design employs a reflective phase-shifting patch etched onto a dielectric substrate, featuring a three-square loop structure intersected by two transverse dipoles. This [...] Read more.

A low-profile reflectarray has been designed in the Ka-band to efficiently generate wideband orbital angular momentum (OAM) vortex beams. The proposed design employs a reflective phase-shifting patch etched onto a dielectric substrate, featuring a three-square loop structure intersected by two transverse dipoles. This unit cell achieves a 440° phase shift at 30 GHz with a minimal magnitude loss of (−0.25 dB), enabling high-efficiency reflectarray performance. The OAM vortex beam supports high-order phase distributions (

l = + 1, + 2, + 3, + 4

) modes, though fabrication and experimental validation focused on the

+ 1

mode. Measurements confirm that the reflectarray produces a high-purity OAM vortex beam for +1 mode, covering the operational frequency range from 27 to 39 GHz, and achieving a 40% bandwidth with a peak gain of 23.39 dBi at 33 GHz and an aperture efficiency of 17.38%. These results demonstrate the ability of the reflectarray to produce broadband directive OAM beams with robust performance, making it ideal for Ka-band communication systems. Full article

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

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10 pages, 1419 KiB

Open AccessArticle

Time-Domain Full-Field Confocal Optical Coherence Tomography with Digital Scanning

by Danielis Rutkauskas, Karolis Adomavičius and Egidijus Auksorius

Photonics 2025, 12(4), 304; https://doi.org/10.3390/photonics12040304 - 26 Mar 2025

Abstract

Full-field optical coherence tomography (FF-OCT) is a fast, en face interferometric technique that allows imaging inside a scattering tissue with high spatial resolution. However, camera-based detection, which lacks confocal gating, results in a suboptimal signal-to-noise ratio (SNR). To address this, we implemented a [...] Read more.

Full-field optical coherence tomography (FF-OCT) is a fast, en face interferometric technique that allows imaging inside a scattering tissue with high spatial resolution. However, camera-based detection, which lacks confocal gating, results in a suboptimal signal-to-noise ratio (SNR). To address this, we implemented a time-domain FF-OCT system that uses a digital micromirror device (DMD). The DMD allows us to scan multiple illumination spots across the sample and simultaneously realize confocal detection with multiple pinholes. Confocal imaging can also be demonstrated with line illumination and detection. Using a USAF target mounted behind a scattering layer, we demonstrate an order-of-magnitude improvement in SNR. Full article

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