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Photonics
Volume 11
Issue 8
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Photonics, Volume 11, Issue 8 (August 2024) – 98 articles

Cover Story (view full-size image): We utilize femtosecond laser-inscribed small-period long-period fiber grating (SP-LPFG) to induce a nonlinear polarization rotation (NPR) effect for mode-locking pulses in a normal dispersion erbium-doped fiber laser (EDFL). The SP-LPFG has a length of 2.5 mm and a period of 25 μm. At wavelengths of 1556 nm and 1561 nm, it exhibits polarization-dependent loss (PDL) values of 20 dB and 14.5 dB, respectively, sufficient to trigger the NPR mechanism. With pump power increased to 500 mW, the laser achieves normal dispersion mode-locked pulses centered at 1575 nm in the L-band, with a 3 dB bandwidth of 1.35 nm and a pulse width of 1.61 ps. This SP-LPFG holds promising applications, paving the way for efficient, compact, and stable normal-dispersion ultrafast fiber lasers. View this paper
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12 pages, 6964 KiB  
Article
Data-Driven Channel Modeling for End-to-End Visible Light DCO-OFDM Communication System Based on Experimental Data
by Bo Song, Yanwen Zhu, Yi Huang and Haiteng Zong
Photonics 2024, 11(8), 781; https://doi.org/10.3390/photonics11080781 - 22 Aug 2024
Viewed by 541
Abstract
End-to-end systems have been introduced to address the issue of independent signal processing module design in traditional communication systems, which prevents achieving global system optimization. However, research on indoor end-to-end Visible Light Communication (VLC) systems remains limited, especially regarding the channel modeling of [...] Read more.
End-to-end systems have been introduced to address the issue of independent signal processing module design in traditional communication systems, which prevents achieving global system optimization. However, research on indoor end-to-end Visible Light Communication (VLC) systems remains limited, especially regarding the channel modeling of high-speed, high-capacity Direct Current-biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) systems. This paper proposes three channel modeling methods for end-to-end DCO-OFDM VLC systems. The accuracy of the proposed methods is demonstrated through R-Square model fitting performance and data distribution analysis. The effectiveness of the proposed channel modeling methods is further validated by comparing the bit error rate (BER) performance of traditional receivers and existing deep learning (DL)-based receivers. The results show that the proposed methods can effectively mitigate both linear and nonlinear distortions. By employing these channel modeling methods, communication systems can reduce the spectral occupancy of pilot signals, thereby significantly lowering the complexity of traditional channel estimation methods. Thus, these methods are suitable for use in end-to-end VLC communication systems. Full article
(This article belongs to the Special Issue Advancements in Wireless Optical Communication: Integrating Visible Light and Beyond)
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15 pages, 4455 KiB  
Article
Design of Diffractive Neural Networks for Solving Different Classification Problems at Different Wavelengths
by Georgy A. Motz, Leonid L. Doskolovich, Daniil V. Soshnikov, Egor V. Byzov, Evgeni A. Bezus, Nikita V. Golovastikov and Dmitry A. Bykov
Photonics 2024, 11(8), 780; https://doi.org/10.3390/photonics11080780 - 22 Aug 2024
Viewed by 660
Abstract
We consider the problem of designing a diffractive neural network (DNN) consisting of a set of sequentially placed phase diffractive optical elements (DOEs) and intended for the optical solution of several given classification problems at different operating wavelengths, so that each classification problem [...] Read more.
We consider the problem of designing a diffractive neural network (DNN) consisting of a set of sequentially placed phase diffractive optical elements (DOEs) and intended for the optical solution of several given classification problems at different operating wavelengths, so that each classification problem is solved at the corresponding wavelength. The problem of calculating the DNN is formulated as the problem of minimizing a functional that depends on the functions of the diffractive microrelief height of the DOEs constituting the DNN and represents the error in solving the given classification problems at the operating wavelengths. We obtain explicit and compact expressions for the derivatives of this functional, and using them, we formulate a gradient method for the DNN calculation. Using this method, we design DNNs for solving the following three classification problems at three different wavelengths: the problem of classifying handwritten digits from the MNIST database, the problem of classifying fashion products from the Fashion MNIST database, and the problem of classifying ten handwritten letters from the EMNIST database. The presented simulation results of the designed DNNs demonstrate the high performance of the proposed method. Full article
(This article belongs to the Special Issue Recent Advances in Diffractive Optics)
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13 pages, 2257 KiB  
Article
Generation of UV Ellipsoidal Pulses by 3D Amplitude Shaping for Application in High-Brightness Photoinjectors
by Andreas Hoffmann, James Good, Matthias Gross, Mikhail Krasilnikov and Frank Stephan
Photonics 2024, 11(8), 779; https://doi.org/10.3390/photonics11080779 - 22 Aug 2024
Viewed by 683
Abstract
Photocathode laser pulse shaping is a crucial technology for enhancing the performance of X-ray free-electron lasers by optimizing the quality of electron beams generated from photocathodes within high-gradient radio frequency guns. By precisely shaping these laser pulses, it is possible to significantly reduce [...] Read more.
Photocathode laser pulse shaping is a crucial technology for enhancing the performance of X-ray free-electron lasers by optimizing the quality of electron beams generated from photocathodes within high-gradient radio frequency guns. By precisely shaping these laser pulses, it is possible to significantly reduce the transverse emittance of produced electron bunches. The optimal pulse shape is an ellipsoidal distribution, commonly referred to as the Kapchinskij–Vladimirskij profile. A pulse-shaping scheme utilizing a commercial Yb:KGW laser operating at 514 nm with a repetition rate of 1 MHz and duration of 260 fs has been developed for generating electron bunches with high peak and average power. This study presents the experimental realization of ellipsoidal pulses via three-dimensional amplitude shaping using spatial light modulators at 514 nm, followed by conversion to UV (257 nm) suitable for Cs 2Te photocathodes. The preservation of pulse shape and a high conversion efficiency during this process are investigated and our experiments pave the way for future emittance minimization for X-ray free-electron lasers. Full article
(This article belongs to the Special Issue Novel Ultraviolet Laser: Generation, Properties and Applications)
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13 pages, 2979 KiB  
Article
SMR–YOLO: Multi-Scale Detection of Concealed Suspicious Objects in Terahertz Images
by Yuan Zhang, Hao Chen, Zihao Ge, Yuying Jiang, Hongyi Ge, Yang Zhao and Haotian Xiong
Photonics 2024, 11(8), 778; https://doi.org/10.3390/photonics11080778 - 22 Aug 2024
Viewed by 671
Abstract
The detection of concealed suspicious objects in public places is a critical issue and a popular research topic. Terahertz (THz) imaging technology, as an emerging detection method, can penetrate materials without emitting ionizing radiation, providing a new approach to detecting concealed suspicious objects. [...] Read more.
The detection of concealed suspicious objects in public places is a critical issue and a popular research topic. Terahertz (THz) imaging technology, as an emerging detection method, can penetrate materials without emitting ionizing radiation, providing a new approach to detecting concealed suspicious objects. This study focuses on the detection of concealed suspicious objects wrapped in different materials such as polyethylene and kraft paper, including items like scissors, pistols, and blades, using THz imaging technology. To address issues such as the lack of texture details in THz images and the contour similarity of different objects, which can lead to missed detections and false alarms, we propose a THz concealed suspicious object detection model based on SMR–YOLO (SPD_Mobile + RFB + YOLO). This model, based on the MobileNext network, introduces the spatial-to-depth convolution (SPD-Conv) module to replace the backbone network, reducing computational and parameter load. The inclusion of the receptive field block (RFB) module, which uses a multi-branch structure of dilated convolutions, enhances the network’s depth features. Using the EIOU loss function to assess the accuracy of predicted box localization further optimizes convergence speed and localization accuracy. Experimental results show that the improved model achieved [email protected] and [email protected]:0.95 scores of 98.9% and 89.4%, respectively, representing improvements of 0.2% and 1.8% over the baseline model. Additionally, the detection speed reached 108.7 FPS, an improvement of 23.2 FPS over the baseline model. The model effectively identifies concealed suspicious objects within packages, offering a novel approach for detection in public places. Full article
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13 pages, 14855 KiB  
Article
Transmission Characteristics Analysis of a Twin-Waveguide Cavity
by Chanchan Luo, Ruiying Zhang, Ben Zhang, Bisheng Qin, Yanshuang Zhao, Bocang Qiu, Bohan Liu and Xiaoming Zhao
Photonics 2024, 11(8), 777; https://doi.org/10.3390/photonics11080777 - 21 Aug 2024
Viewed by 551
Abstract
The transmission spectrum of a twin-waveguide cavity is systematically analyzed based on coupled mode theory, using the transfer matrix method (TMM). The results show that the traveling-wave transmission spectra of the twin-waveguide cavity is entirely determined by the coherent coupling effect involving the [...] Read more.
The transmission spectrum of a twin-waveguide cavity is systematically analyzed based on coupled mode theory, using the transfer matrix method (TMM). The results show that the traveling-wave transmission spectra of the twin-waveguide cavity is entirely determined by the coherent coupling effect involving the parameters of the effective refractive indices of the upper and lower waveguides, the coupling length Lc, and the ratio of the cavity length L to the coupling length (L/Lc). Filters with single, double, or triple-notch filtering could be obtained by choosing an appropriate L/Lc value. When the facet reflection is taken into consideration, the traveling-wave transmission spectrum is modified by the Fabry––Perot (FP) resonance, making it a standing-wave transmission spectrum. As a result, resonance splitting has been observed in the transmission spectrum of twin-waveguide resonators with high facet reflectivity. Further analysis shows that such an abnormal resonance phenomenon can be attributed to the destructive interference between the two FP resonance modes of the upper and lower waveguide through coherent coupling. In addition, narrow bandwidth amplification has also been observed through asymmetric facet reflections. Undoubtedly, all these unique spectral characteristics should be beneficial to the twin-waveguide cavity, achieving many more functions and being widely used in photonic integration circuits (PICs). Full article
(This article belongs to the Special Issue Advances in Semiconductor Photonic Integrated Circuits)
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12 pages, 3924 KiB  
Article
A Denoise Network for Structured Illumination Microscopy with Low-Light Exposure
by Xin Liu, Jinze Li, Liangfeng Song, Kequn Zhuo, Kai Wen, Sha An, Ying Ma, Juanjuan Zheng and Peng Gao
Photonics 2024, 11(8), 776; https://doi.org/10.3390/photonics11080776 - 21 Aug 2024
Viewed by 797
Abstract
Super-resolution structured illumination microscopy (SR-SIM) is one of the important techniques that are most suitable for live-cell imaging. The reconstructed SR-SIM images are noisy once the raw images are recorded with low-light exposure. Here, we propose a new network (entitled the ND-SIM network) [...] Read more.
Super-resolution structured illumination microscopy (SR-SIM) is one of the important techniques that are most suitable for live-cell imaging. The reconstructed SR-SIM images are noisy once the raw images are recorded with low-light exposure. Here, we propose a new network (entitled the ND-SIM network) to denoise the SR images reconstructed using frequency-domain algorithms (FDAs). We demonstrate that ND-SIM can yield artifact-free SR images using raw images with an average photon count down to 20 per pixel while achieving comparable resolution to the ground truth (GT) obtained with high-light exposure. We can envisage that the ND-SIM will be widely applied for the long-term, super-resolution live-cell imaging of various bioprocesses in the future. Full article
(This article belongs to the Special Issue Advanced Quantitative Phase Microscopy: Techniques and Applications)
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12 pages, 2681 KiB  
Article
Analogue of High-Q Transparency Band and Sensitivity in All-Dielectric Metasurfaces Supporting Bound States in the Continuum
by Ling Shuai, Suxia Xie, Haoxuan Nan and Xin Guan
Photonics 2024, 11(8), 775; https://doi.org/10.3390/photonics11080775 - 20 Aug 2024
Viewed by 740
Abstract
Bound states in the continuum (BICs), which are characterized by their high-quality factor, have become a focal point in modern optical research. This study investigates BICs within a periodic array of dielectric resonators, specifically composed of a silicon rectangular bar coupled with four [...] Read more.
Bound states in the continuum (BICs), which are characterized by their high-quality factor, have become a focal point in modern optical research. This study investigates BICs within a periodic array of dielectric resonators, specifically composed of a silicon rectangular bar coupled with four silicon rectangular blocks. Through the analysis of mode coupling, we demonstrate that the interaction between the blocks significantly modulates the eigenmodes of the bar, causing a redshift in all modes and enabling the formation of electromagnetically induced transparency based on BICs (EIT-BIC). Unlike typical EIT mechanisms, this EIT-BIC arises from the coupling of “bright” and “dark” modes both from the rectangular bar, offering novel insights for nanophotonic and photonic device design. Further, our systematic exploration of BIC formation mechanisms and their sensing properties by breaking structural symmetries and changing environmental refractive indices has shed light on the underlying physics. This research not only consolidates a robust theoretical framework for understanding BIC behavior but also paves the way for high-quality factor resonator and sensor development, as well as the precise control of photonic states. The findings significantly deepen our understanding of these phenomena and hold substantial promise for future photonic applications. Full article
(This article belongs to the Special Issue Advanced Photonics Metamaterials and Metasurfaces: Science and Applications)
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7 pages, 2123 KiB  
Article
Prediction through Simulation-Based Corrective Models of Contra-Directional Couplers’ Experimental Results
by Inês Venâncio, Joana Tátá, Maria João Carvalhais, João Santos and António Teixeira
Photonics 2024, 11(8), 774; https://doi.org/10.3390/photonics11080774 - 20 Aug 2024
Viewed by 691
Abstract
Lithography variation presents one of the biggest challenges for photonic component optimization, especially for fabless designers. Lithography prediction models are a crucial tool for minimizing the necessary number of fabrication iterations for a device’s optimization. This paper presents one of these models specifically [...] Read more.
Lithography variation presents one of the biggest challenges for photonic component optimization, especially for fabless designers. Lithography prediction models are a crucial tool for minimizing the necessary number of fabrication iterations for a device’s optimization. This paper presents one of these models specifically adapted for the contra-directional coupler structure. Through the experimental characterization of devices with a specific range of design parameters, it was possible to observe how the lithography process impacts their performance. A correction model based on effective refractive index variation and its impact on the Bragg condition of the structure was developed to predict the performance variation of a device based on the expected design variation induced by fabrication. The contra-directional couplers fabricated at CORNERSTONE foundry show a tendency to be redshifted as the gap decreases, due to an increase in waveguide width as a result of a diffraction-limited lithography process. Based on these and other findings, it was possible to correlate the design parameters to the posterior fabricated structure and ultimately predict the expected experimental response. Full article
(This article belongs to the Special Issue Integrated Waveguide-Based Photonic Devices)
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14 pages, 24677 KiB  
Article
Rethinking of Underwater Image Restoration Based on Circularly Polarized Light
by Chao Wang, Zhiyang Wu, Zexiong Han, Junguang Wang, Haofeng Hu and Xiaobo Li
Photonics 2024, 11(8), 773; https://doi.org/10.3390/photonics11080773 - 20 Aug 2024
Cited by 1 | Viewed by 970
Abstract
Polarimetric imaging technology plays a crucial role in de-scattering, particularly in the field of underwater image restoration. Circularly polarized light (or the underlying circular polarization memory effect) has been proven to better preserve the polarization characteristics of detected light. Utilizing circularly polarized light [...] Read more.
Polarimetric imaging technology plays a crucial role in de-scattering, particularly in the field of underwater image restoration. Circularly polarized light (or the underlying circular polarization memory effect) has been proven to better preserve the polarization characteristics of detected light. Utilizing circularly polarized light as illumination can further enhance the effectiveness of polarization de-scattering techniques. After rethinking the advantages of circularly polarized light, this paper proposes a new method for underwater polarimetric imaging restoration that leverages the pre-processing of polarized sub-images and the correlation of polarization characteristics (i.e., the angle of polarization and degree of polarization). Additionally, to address the challenge of selecting scattering light parameters due to uneven light fields in target scenes, an intensity adjustment factor search algorithm is designed. This algorithm eliminates the need for the manual selection of scattering light parameters, effectively solving the problem of uneven illumination in restoration results. A series of experiments demonstrate that, compared to traditional algorithms, the proposed method offers superior detail restoration and higher robustness. Full article
(This article belongs to the Special Issue Optical Technology for Challenging Conditions：Methods and Applications)
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13 pages, 1037 KiB  
Article
Neural Network Equalisation for High-Speed Eye-Safe Optical Wireless Communication with 850 nm SM-VCSELs
by Isaac N. O. Osahon, Ioannis Kostakis, Denise Powell, Wyn Meredith, Mohamed Missous, Harald Haas, Jianming Tang and Sujan Rajbhandari
Photonics 2024, 11(8), 772; https://doi.org/10.3390/photonics11080772 - 20 Aug 2024
Viewed by 808
Abstract
In this paper, we experimentally illustrate the effectiveness of neural networks (NNs) as non-linear equalisers for multilevel pulse amplitude modulation (PAM-M) transmission over an optical wireless communication (OWC) link. In our study, we compare the bit-error-rate (BER) performances of two decision [...] Read more.
In this paper, we experimentally illustrate the effectiveness of neural networks (NNs) as non-linear equalisers for multilevel pulse amplitude modulation (PAM-M) transmission over an optical wireless communication (OWC) link. In our study, we compare the bit-error-rate (BER) performances of two decision feedback equalisers (DFEs)—a multilayer-perceptron-based DFE (MLPDFE), which is the NN equaliser, and a transversal DFE (TRDFE)—under two degrees of non-linear distortion using an eye-safe 850 nm single-mode vertical-cavity surface-emitting laser (SM-VCSEL). Our results consistently show that the MLPDFE delivers superior performance in comparison to the TRDFE, particularly in scenarios involving high non-linear distortion and PAM constellations with eight or more levels. At a forward error correction (FEC) threshold BER of 0.0038, we achieve bit rates of ~28 Gbps, ~29 Gbps, ~22.5 Gbps, and ~5 Gbps using PAM schemes with 2, 4, 8, and 16 levels, respectively, with the MLPDFE. Comparably, the TRDFE yields bit rates of ~28 Gbps and ~29 Gbps with PAM-2 and PAM-4, respectively. Higher PAM levels with the TRDFE result in BERs greater than 0.0038 for bit rates above 2 Gbps. These results highlight the effectiveness of the MLPDFE in optimising the performance of SM-VCSEL-based OWC systems across different modulation schemes and non-linear distortion levels. Full article
(This article belongs to the Special Issue Machine Learning Applied to Optical Communication Systems)
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15 pages, 8081 KiB  
Article
Polarization Analysis of Vertically Etched Lithium Niobate-on-Insulator (LNOI) Devices
by Chenyu Wang, Yuchen Liu, Jingyuan Qiu, Zhilin Ye, Dongjie Guo, Mengwen Chen, Zhijun Yin, Xiao-Hui Tian, Hua-Ying Liu, Shi-Ning Zhu and Zhenda Xie
Photonics 2024, 11(8), 771; https://doi.org/10.3390/photonics11080771 - 19 Aug 2024
Viewed by 839
Abstract
LNOI devices have emerged as prominent contributors to photonic integrated circuits (PICs), benefiting from their outstanding performance in electro-optics, acousto-optics, nonlinear optics, etc. Due to the physical properties and current etching technologies of LiNbO3, slanted sidewalls are typically formed in LNOI [...] Read more.
LNOI devices have emerged as prominent contributors to photonic integrated circuits (PICs), benefiting from their outstanding performance in electro-optics, acousto-optics, nonlinear optics, etc. Due to the physical properties and current etching technologies of LiNbO3, slanted sidewalls are typically formed in LNOI waveguides, causing polarization-related mode hybridization and crosstalk. Despite the low losses achieved with fabrication advancements in LNOI, such mode hybridization and crosstalk still significantly limit the device performance by introducing polarization-related losses. In this paper, we propose a vertically etched LNOI construction. By improving the geometrical symmetry in the waveguides, vertical sidewalls could adequately mitigate mode hybridization in common waveguide cross sections. Taking tapers and bends as representatives of PIC components, we then conducted theoretical modeling and simulations, which showed that vertical etching effectively exempts devices from polarization-related mode crosstalk. This not only improves the polarization purity and input mode transmittance but also enables lower polarization-related losses within more compact structures. As a demonstration of fabrication feasibility, we innovatively proposed a two-step fabrication technique, and successfully fabricated waveguides with vertical sidewalls. Such vertical etching technology facilitates the development of next-generation high-speed modulators, nonlinear optical devices, and other advanced photonic devices with lower losses and a smaller footprint, driving further innovations in both academic research and industrial applications. Full article
(This article belongs to the Special Issue Advances in Integrated Photonics: From Materials to Devices and Systems)
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8 pages, 1242 KiB  
Communication
Optical Frequency References at 1542 nm: Precision Spectroscopy of the R(106)50-0, R(100)49-0, R(84)47-0, R(59)45-0, P(82)47-0, and P(71)46-0 Lines of 127I2 at 514 nm
by Shogo Matsunaga, Yuta Isawa, Daisuke Akamatsu and Feng-Lei Hong
Photonics 2024, 11(8), 770; https://doi.org/10.3390/photonics11080770 - 19 Aug 2024
Viewed by 642
Abstract
Frequency-stabilized lasers are fundamental topics in research relating to optical frequency and wavelength standards. The absolute frequencies and hyperfine structures of the R(106)50-0, R(100)49-0, R(84)47-0, R(59)45-0, P(82)47-0, and P(71)46-0 lines of molecular iodine (127I2) at 514 nm were measured [...] Read more.
Frequency-stabilized lasers are fundamental topics in research relating to optical frequency and wavelength standards. The absolute frequencies and hyperfine structures of the R(106)50-0, R(100)49-0, R(84)47-0, R(59)45-0, P(82)47-0, and P(71)46-0 lines of molecular iodine (127I2) at 514 nm were measured using a frequency-stabilized laser based on modulation transfer spectroscopy. The hyperfine splitting of each line was fitted to a four-term Hamiltonian with an uncertainty of several kilohertz to obtain the hyperfine constants for the line. A total of 97 hyperfine transitions of the six lines were measured with an uncertainty of 5.6 kHz (fractionally 9.6 × 10−12). They can provide new optical frequency references for telecommunication and other applications. Full article
(This article belongs to the Special Issue Optical Communication, Sensing and Network)
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12 pages, 6023 KiB  
Article
Second Harmonic Generation in Apodized Chirped Periodically Poled Lithium Niobate Loaded Waveguides Based on Bound States in Continuum
by Junjie He, Mianjie Lin and Fei Ma
Photonics 2024, 11(8), 769; https://doi.org/10.3390/photonics11080769 - 18 Aug 2024
Viewed by 1031
Abstract
With the rapid development of optical communication and quantum information, the demand for efficient and broadband nonlinear frequency conversion has increased. At present, most single-frequency conversion processes in lithium niobate on insulator (LNOI) waveguides suffer from lateral leakage without proper design, leading to [...] Read more.
With the rapid development of optical communication and quantum information, the demand for efficient and broadband nonlinear frequency conversion has increased. At present, most single-frequency conversion processes in lithium niobate on insulator (LNOI) waveguides suffer from lateral leakage without proper design, leading to an additional increase in propagation loss. Achieving broadband frequency conversion also encounters this problem in that there are no relevant works that have solved this yet. In this paper, we theoretically propose an efficient and flat broadband second harmonic generation (SHG) in silicon nitride loaded apodized chirped periodically poled LNOI waveguides. By using a bound states in the continuum (BICs) mechanism to reduce the propagation loss and utilizing the characteristic that the BICs are insensitive to wavelength, an ultra-low-loss wave band of 80 nm is realized. Then, by employing an apodized chirped design, a flat broadband SHG is achieved. The normalized conversion efficiency (NCE) is approximately 222%W−1cm−2, and the bandwidth is about 100 nm. Moreover, the presented waveguides are simple and can be fabricated without direct etching of lithium niobate, exhibiting excellent fabrication tolerance. Our work may open a new avenue for exploring low-loss and flat broadband nonlinear frequency conversion on various on-chip integrated photonic platforms. Full article
(This article belongs to the Section Optoelectronics and Optical Materials)
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12 pages, 7222 KiB  
Article
Investigation of the Propagation Characteristics of Double-Ring Perfect Vortex Beams in Atmospheric Turbulence
by Xiang Xu, Chuankai Luo, Xianmei Qian and Wenyue Zhu
Photonics 2024, 11(8), 768; https://doi.org/10.3390/photonics11080768 - 16 Aug 2024
Viewed by 707
Abstract
Double-ring perfect vortex beams (DR–PVBs) have attracted increasing attention due to their unique characteristics of carrying independent information channels and exhibiting higher security and stability during propagation. In this study, we theoretically simulated and experimentally generated DR–PVBs with various topological charges. We investigated [...] Read more.
Double-ring perfect vortex beams (DR–PVBs) have attracted increasing attention due to their unique characteristics of carrying independent information channels and exhibiting higher security and stability during propagation. In this study, we theoretically simulated and experimentally generated DR–PVBs with various topological charges. We investigated the propagation characteristics of these beams under von Karman spectrum turbulence conditions through numerical simulations based on multiple-phase screen methods. The effects of different inner and outer ring topological charges and varying turbulence intensities on the intensity distribution, beam spreading, and beam wander of DR–PVBs over different propagation distances were examined and compared with double-ring Gaussian vortex beams (DR–GVBs). The simulation results indicate that within a propagation range of 0–500 m, the effective radius of DR–PVBs with different topological charges remains essentially unchanged and stable. For propagation distances exceeding 1000 m, DR–PVBs exhibit superior beam wander characteristics compared to DR–GVBs. Additionally, two occurrences of self-focusing effects were observed during propagation, each enhancing beam stability and reducing the beam spreading and beam wander of the DR–PVBs. This study provides valuable insights for applications of DR–PVBs in optical communication, optical manipulation, and optical measurement. Full article
(This article belongs to the Special Issue Optical Vortex: Fundamentals and Applications)
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15 pages, 7929 KiB  
Article
A Multi-Spectral Temperature Field Reconstruction Technology under a Sparse Projection
by Xuan Zhang and Yan Han
Photonics 2024, 11(8), 767; https://doi.org/10.3390/photonics11080767 - 16 Aug 2024
Viewed by 644
Abstract
In optical sparse projection reconstruction, the reconstruction of the tested field often requires the utilization of a priori knowledge to compensate for the lack of information due to the sparse projection angle. For situations where the radiation field of unknown materials is reconstructed [...] Read more.
In optical sparse projection reconstruction, the reconstruction of the tested field often requires the utilization of a priori knowledge to compensate for the lack of information due to the sparse projection angle. For situations where the radiation field of unknown materials is reconstructed or prior knowledge cannot be obtained, this paper proposes a multi-spectral temperature field reconstruction technology under a sparse projection. This technology utilizes the principles of multi-spectral temperature measurement technology, takes the correlation of radiation information between sub-regions of the temperature field as the optimization objective, and establishes statistical rules between the missing information by combining the equation constraint optimization algorithm and multi-spectral temperature measurement technology. Finally, the temperature field to be measured is reconstructed. The simulation and experimental tests show that, without any prior knowledge, the proposed method can reconstruct the temperature field under two projection angles, with an accuracy of 1.64~12.25%. Moreover, the projection angle is lower, and the robustness is stronger than that of the other methods. Full article
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15 pages, 4244 KiB  
Article
A Terahertz Point Source Meta-Sensor in Reflection Mode for Trace-Amount Bio-Sensing Applications
by Luwei Zheng, Masayoshi Tonouchi and Kazunori Serita
Photonics 2024, 11(8), 766; https://doi.org/10.3390/photonics11080766 - 16 Aug 2024
Viewed by 875
Abstract
Biosensors in the Terahertz (THz) region are attracting significant attention in the biomedical and chemical analysis fields owing to their potential for ultra-trace sensing of various solutions with high sensitivity. However, the development of compact, highly sensitive chips and methods for easy, rapid, [...] Read more.
Biosensors in the Terahertz (THz) region are attracting significant attention in the biomedical and chemical analysis fields owing to their potential for ultra-trace sensing of various solutions with high sensitivity. However, the development of compact, highly sensitive chips and methods for easy, rapid, and trace-amount measurements have been significantly hindered by the limited spatial resolution of THz waves and their strong absorption by water. In this study, we developed a nonlinear optical crystal (NLOC)-based compact THz sensor chip, and a near-field point THz source with a diameter of ~ϕ20 μm was locally generated via optical rectification. Here, only the single central meta-atom was excited. The reflective resonance responses highly depend on the array number and period of the meta-atom structures. The sensing performance was examined with several liquid biological samples, such as mineral water, DNA, and human blood. 1 μL of samples was directly dropped onto the meta-surface with an effective sensing area of 0.32 mm2 (564 μm × 564 μm). Obvious resonance frequency shifts were clearly observed. This research holds significance in advancing liquid bio-sample sensing methodologies by facilitating easy, rapid, and trace-amount measurements and promoting the development of compact and highly sensitive THz sensors tailored for liquid biological samples. Full article
(This article belongs to the Special Issue Nonlinear Optics and Hyperspectral Polarization Imaging)
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18 pages, 5517 KiB  
Article
FMCW Laser Ranging System Based on SiON Waveguides and IQ Demodulation Technology
by Leifu Zhou, Lijuan Zhang, Yanqing Qiu, Tingting Lang, Xiao Ma, Ting Chen and Lei Wang
Photonics 2024, 11(8), 765; https://doi.org/10.3390/photonics11080765 - 15 Aug 2024
Viewed by 833
Abstract
This study introduces the design of a nonlinear frequency-modulated continuous wave (FMCW) laser ranging system. In contrast to the commonly used triangular wave linear modulation, this study utilizes sinusoidal wave modulation. The frequency information of the original sinusoidal frequency-modulated signal is extracted using [...] Read more.
This study introduces the design of a nonlinear frequency-modulated continuous wave (FMCW) laser ranging system. In contrast to the commonly used triangular wave linear modulation, this study utilizes sinusoidal wave modulation. The frequency information of the original sinusoidal frequency-modulated signal is extracted using an on-chip interferometer based on SiON waveguides and IQ demodulation technology. After fitting the measured interference signal at equal frequency intervals, the corresponding distance information is derived using the fast Fourier transform (FFT). The principles underlying this method are thoroughly analyzed and derived, with its accuracy confirmed through experimental validation. Full article
(This article belongs to the Special Issue Silicon-Based Integrated Optics: From Design to Applications)
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15 pages, 2236 KiB  
Article
Highly Sensitive D-SPR Sensors with Optimized Metallic Thin Films for Bio-Analyte Detection
by John Ehiabhili, Radhakrishna Prabhu and Somasundar Kannan
Photonics 2024, 11(8), 764; https://doi.org/10.3390/photonics11080764 - 15 Aug 2024
Viewed by 749
Abstract
There is a growing need for precise and rapid detection methods in fields such as biomedical diagnostics, environmental monitoring, and chemical analysis. Surface plasmon resonance (SPR) sensors have been used for the detection and quantification of a wide range of analytes, including biomolecules, [...] Read more.
There is a growing need for precise and rapid detection methods in fields such as biomedical diagnostics, environmental monitoring, and chemical analysis. Surface plasmon resonance (SPR) sensors have been used for the detection and quantification of a wide range of analytes, including biomolecules, chemicals, and gases, in real-time. Despite the promising capabilities of SPR sensors, there remains a gap in creating a balance between having a large enough area to capture a significant number of analytes for detection and being small enough to ensure high sensitivity. This research aims to explore the design of a D-shaped SPR-based optical fiber sensor, focusing on the use of copper, gold, and silver thin films at optimized width and thickness of 10 µm and 45 nm, respectively, to improve the sensor’s performance. Employing a computational approach, this study examines the influence of the optimized width and refractive indices of metallic films on the sensor’s characteristics. The 10 µm width of the metallic thin film has been found to produce an optimal balance between the sensitivity and the dynamic range of the sensor. Leveraging on the ratio of the real and imaginary parts of the dielectric constant of the thin film metal provides insight into the optical properties and sensitivity at certain wavelengths. Within an analyte refractive index range of 1.37–1.42 and a wavelength range of 650–1200 nm, results indicate that silver outperforms gold and copper at the optimized width with a wavelength sensitivity, and detection accuracy of 12,300 nmRIU−1, and 3.075, respectively. By optimizing the width of the metal thin film at 10 µm, a highly sensitive D-SPR is designed, allowing for enhanced sensor detection capabilities for a wide range of bioanalytes. Full article
(This article belongs to the Special Issue Editorial Board Members’ Collection Series: Photonics Sensors)
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9 pages, 1275 KiB  
Article
Diamond-Based Fiber-Optic Fabry–Perot Interferometer with Ultrawide Refractive-Index Measurement Range
by Dewen Duan, Jianhao Yang, Yi Tang and Yi-Yuan Xie
Photonics 2024, 11(8), 763; https://doi.org/10.3390/photonics11080763 - 15 Aug 2024
Viewed by 1203
Abstract
The majority of Fabry–Perot interferometer (FPI) tip refractive index (RI) sensors utilize silica optical fiber as the cavity material, with an RI of approximately 1.45. This restricts their applicability in measuring the RI of liquids with an RI of approximately 1.45. Here, we [...] Read more.
The majority of Fabry–Perot interferometer (FPI) tip refractive index (RI) sensors utilize silica optical fiber as the cavity material, with an RI of approximately 1.45. This restricts their applicability in measuring the RI of liquids with an RI of approximately 1.45. Here, we propose a fiber-optic FPI-tip RI sensor by bonding a flat, thin diamond film onto the apex of a single-mode optical fiber. The FPI cavity is constructed from a diamond with an RI of approximately 2.4, theoretically enabling the sensor to achieve an ultrawide RI measurement range of 1 to 2.4. A theoretical comparison of its measurement performance was conducted with that of an FPI-tip RI sensor whose cavity is formed by silica fiber. Additionally, an experimental examination of the device’s RI measurement performance was conducted. The results show that the sensor has visibility to the RI unit of −0.4362/RIU in the RI range of 1.33 to 1.40. Combined with other narrow-RI-ranged high-sensitivity sensors, our proposed RI sensor has the potential for use in a wide range of applications. Full article
(This article belongs to the Special Issue Progress in Fiber Optic Sensors: Design and Applications)
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21 pages, 5489 KiB  
Article
Advancements in CMOS-Compatible Silicon Nitride Optical Modulators via Thin-Film Crystalline or Amorphous Silicon p–n Junctions
by Joaquín Hernández-Betanzos, Marçal Blasco-Solvas, Carlos Domínguez-Horna and Joaquín Faneca
Photonics 2024, 11(8), 762; https://doi.org/10.3390/photonics11080762 - 15 Aug 2024
Viewed by 3962
Abstract
This paper proposes two types of electro-refractive optical modulator structures as a fully CMOS-compatible alternative solution. These modulators leverage the properties of amorphous (top) and crystalline (bottom) silicon films surrounding silicon nitride waveguides operating in the C-band communications range at a wavelength of [...] Read more.
This paper proposes two types of electro-refractive optical modulator structures as a fully CMOS-compatible alternative solution. These modulators leverage the properties of amorphous (top) and crystalline (bottom) silicon films surrounding silicon nitride waveguides operating in the C-band communications range at a wavelength of 1550 nm. Various structures have been demonstrated and explored to compete with or surpass the current state-of-the-art performance of thermal tuners, the most widely used tuning mechanism in silicon nitride integrated photonics. Designs utilizing vertical and lateral p–n junctions with amorphous or crystalline films have been simulated and proposed. For the lateral p–n junctions, modulator lengths to achieve a π phase shift smaller than 287 μm have been demonstrated for the TE mode and that smaller than 1937 μm for the TM mode, reaching 168 μm in the case of a lateral p–n junction that is completely a p-doped region over or under the waveguide for TE, and 1107 μm for TM. Power consumption is higher for the TM modes than for the TE, being in the order of 100 mW for the former and lower than 23 mW for the latter. The modulators exhibit higher losses for amorphous material compared to crystalline, with losses smaller than 10.21 dB and 3.2 dB, respectively. The vertical p–n junctions present a larger footprint than the lateral ones, 5.03 mm for TE and 38.75 mm for TM, with losses lower than 3.16 dB and 3.95 dB, respectively, for the crystalline silicon. Also, their power consumption is on the order of 21 mW for TE and 164 mW for TM. Full article
(This article belongs to the Special Issue Group IV Photonics: Advances and Applications)
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20 pages, 3194 KiB  
Protocol
Transcranial Photobiomodulation for Executive Function in Bipolar Disorder (TPEB): Study Protocol
by David Richer Araujo Coelho, Aura Maria Hurtado Puerto, Willians Fernando Vieira, Carlos Alberto Lohmann, Muhammad Hamza Shahab, Maia Beth Gersten, Farzan Vahedifard, Kayla Marie McEachern, Julie A. Clancy and Paolo Cassano
Photonics 2024, 11(8), 761; https://doi.org/10.3390/photonics11080761 - 15 Aug 2024
Cited by 2 | Viewed by 1014
Abstract
Bipolar disorder (BD) is a debilitating psychiatric disorder characterized by mood disturbances and executive function deficits. Impairments in executive function, including impulsivity, significantly impact the daily lives of individuals with BD. Transcranial photobiomodulation (t-PBM) with near-infrared light offers a promising noninvasive neurostimulation approach [...] Read more.
Bipolar disorder (BD) is a debilitating psychiatric disorder characterized by mood disturbances and executive function deficits. Impairments in executive function, including impulsivity, significantly impact the daily lives of individuals with BD. Transcranial photobiomodulation (t-PBM) with near-infrared light offers a promising noninvasive neurostimulation approach to improve cognitive function. The Transcranial Photobiomodulation for Executive Function in Bipolar Disorder (TPEB) study aims to explore the potential of t-PBM in individuals with BD and executive function impairments. This study will include 20 adults with BD who will each receive one sham and one t-PBM session on the first day of stimulation (treatment day 1), followed by one daily t-PBM stimulation session for four days (treatment days 2 to 5). Cerebral blood flow changes will be evaluated using functional magnetic resonance imaging. Impulsivity, decision-making, and reward responsiveness will be assessed using the Barratt Impulsiveness Scale, the Iowa Gambling Task, and a gambling task that evaluates reward. The outcomes involve examining changes in cerebral blood flow, improvements in decision-making, and reductions in impulsivity and manic symptoms. The TPEB study aims to provide valuable insights into the potential of t-PBM as a therapeutic intervention to enhance executive function in BD. Full article
(This article belongs to the Section Biophotonics and Biomedical Optics)
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19 pages, 3849 KiB  
Article
Comparative Study of Quasi-Solid-State Dye-Sensitized Solar Cells Using Z907, N719, Photoactive Phenothiazine Dyes and PVDF-HFP Gel Polymer Electrolytes with Different Molecular Weights
by Rakesh A. Afre, Ka Yeon Ryu, Won Suk Shin and Diego Pugliese
Photonics 2024, 11(8), 760; https://doi.org/10.3390/photonics11080760 - 14 Aug 2024
Viewed by 1081
Abstract
The present study investigates the influence of photosensitizer selection and the polymer electrolyte composition on the performance of quasi-solid-state dye-sensitized solar cells (QsDSSCs). Two benchmark ruthenium dyes, N719 and Z907, alongside a novel photoactive phenothiazine dye were used. Each dye was incorporated into [...] Read more.
The present study investigates the influence of photosensitizer selection and the polymer electrolyte composition on the performance of quasi-solid-state dye-sensitized solar cells (QsDSSCs). Two benchmark ruthenium dyes, N719 and Z907, alongside a novel photoactive phenothiazine dye were used. Each dye was incorporated into a QsDSSC architecture employing poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) as the gel electrolyte matrix, with varying molecular weights, to investigate their impacts on the overall device performance and long-term stability. Our results demonstrated that the N719 dye exhibited the highest power conversion efficiency (PCE), attributed to its strong absorption in the visible spectrum and efficient electron injection into the TiO2 photoanode. Z907, on the other hand, showed moderate PCE due to its broader absorption profile but slower electron injection kinetics. The phenothiazine dye revealed promising PCE, with tunable absorption properties and efficient charge transfer. Furthermore, the impact of PVDF-HFP polymer gel electrolytes with varying molecular weights on cell stability was explored. The QsDSSC incorporating the PVH80 polymer with the phenothiazine dye exhibited reduced dye desorption, due to the effective dye molecules’ immobilization by the gel matrix, and consequently enhanced long-term stability over 600 h. This comparative study sheds light on the interplay between dye selection, the polymer gel’s properties, and QsDSSCs’ performance. These insights are crucial in designing robust and efficient QsDSSCs for practical applications. Full article
(This article belongs to the Special Issue Photophysical Processes in Non-fullerene Organic Solar Cells)
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10 pages, 1954 KiB  
Communication
Real-Time Massive Parallel Generation of Physical Random Bits Using Weak-Resonant-Cavity Fabry-Perot Laser Diodes
by Yongbo Wang, Xi Tang, Zhengmao Wu, Jiagui Wu and Guangqiong Xia
Photonics 2024, 11(8), 759; https://doi.org/10.3390/photonics11080759 - 14 Aug 2024
Viewed by 777
Abstract
We experimentally demonstrate a scheme for generating massively parallel and real-time physical random bits (PRBs) by using weak-resonant-cavity Fabry-Perot laser diodes (WRC-FPLDs) with optical feedback. By using external optical feedback to modify the nonlinear dynamic behavior of the longitudinal modes in WRC-FPLDs, the [...] Read more.
We experimentally demonstrate a scheme for generating massively parallel and real-time physical random bits (PRBs) by using weak-resonant-cavity Fabry-Perot laser diodes (WRC-FPLDs) with optical feedback. By using external optical feedback to modify the nonlinear dynamic behavior of the longitudinal modes in WRC-FPLDs, the chaotic behavior of each channel can be induced under suitable feedback strength. By filtering these longitudinal modes, a real-time PRBs at 10 Gbits/s can be generated by using field programmable gate array (FPGA) board for the real-time post-processing of a single-channel chaotic signal. Considering the presence of up to 70 longitudinal modes within a broad spectral range exceeding 40 nm, each of these modes can be used to extract chaotic time sequences for random number generation. Therefore, our PRB generation scheme has the potential to achieve a data throughput of over 700 Gbits/s. Full article
(This article belongs to the Special Issue Optical Technology for Challenging Conditions：Methods and Applications)
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16 pages, 4942 KiB  
Article
Three-Shot Dual-Frequency Fringe Scheme Based on Spatial Computer-Generated Moiré Fringe
by Hechen Zhang, Jin Zhou, Dan Jia, Jinlong Huang and Jing Yuan
Photonics 2024, 11(8), 758; https://doi.org/10.3390/photonics11080758 - 14 Aug 2024
Viewed by 758
Abstract
A highly robust dual-frequency hierarchical temporal phase unwrapping (DHTPU) based on the novel spatial computer-generated Moiré profilometry (SCGMP) is proposed. The method requires only three patterns: a high-frequency fringe to provide robust surface information, a multi-period low-frequency fringe to eliminate the 2π-phase ambiguities, [...] Read more.
A highly robust dual-frequency hierarchical temporal phase unwrapping (DHTPU) based on the novel spatial computer-generated Moiré profilometry (SCGMP) is proposed. The method requires only three patterns: a high-frequency fringe to provide robust surface information, a multi-period low-frequency fringe to eliminate the 2π-phase ambiguities, and a flat pattern to remove the average intensity of the two fringes. In decoding, different from traditional Moiré profilometries that rely on spectrum filters, SCGMP only employs spatial-domain calculations to extract the wrapped phase, thereby preserving more detailed information. Furthermore, we fully explore SCGMP’s capability to significantly alleviate phase ambiguity and provide an algorithm to determine the maximum measurable height range for a fixed system, enabling the direct extraction of the continuous basic phase from the multi-period low-frequency fringe. Consequently, the proposed basic phase exhibits an enhanced signal-to-noise ratio, compared to the traditional basic phase derived from the single-period fringes, effectively releasing the high-frequency restriction in the traditional DHTPU. The experimental results verify that the proposed DHTPU method has considerable accuracy and great potential for high-speed measurements, due to there being only three shots required. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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9 pages, 4645 KiB  
Communication
A 2.8 W Single-Frequency Laser Output at 1064 nm from a Gradient-Doped Composite Ceramic Non-Planar Ring Oscillator
by Mingwei Gao, Yibo Ding, Qing Wang, Lei Wang, Yuan Gao, Junping Wang, Haohao Ji, Jian Zhang and Chunqing Gao
Photonics 2024, 11(8), 757; https://doi.org/10.3390/photonics11080757 - 13 Aug 2024
Viewed by 923
Abstract
An efficient Nd: A YAG single-frequency laser was demonstrated using a gradient-doped ceramic non-planar ring oscillator (NPRO). A thermal model of the gradient-doped ceramic NPRO was built to analyze the temperature field and thermal focal length. By employing a gradient-doped gain structure, the [...] Read more.
An efficient Nd: A YAG single-frequency laser was demonstrated using a gradient-doped ceramic non-planar ring oscillator (NPRO). A thermal model of the gradient-doped ceramic NPRO was built to analyze the temperature field and thermal focal length. By employing a gradient-doped gain structure, the thermal distribution within the NPRO can be effectively smoothed to reduce thermal lensing effects. Up to 2.8 W of single-frequency output power at 1064 nm from the gradient-doped ceramic NPRO was obtained, with a slope efficiency of 38%. Full article
(This article belongs to the Special Issue Narrow Linewidth Laser Sources and Their Applications)
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12 pages, 12643 KiB  
Communication
Power Testing of Aspheric Lenses Based on Transmission Phase Deflectometric Method
by Qiong Wu, Xiaokun Wang, Shuangshuang Zhang, Wenhan Li, Yingjing Zhao, Chengchen Zhou, Donglin Xue and Xuejun Zhang
Photonics 2024, 11(8), 756; https://doi.org/10.3390/photonics11080756 - 13 Aug 2024
Viewed by 842
Abstract
Traditional methods for testing aspheric optical lenses struggle to achieve point-by-point testing across the full aperture of the lens. To facilitate the full-aperture, high-precision, and rapid testing of aspheric optical lenses, a power testing method of lenses based on the transmission phase deflectometric [...] Read more.
Traditional methods for testing aspheric optical lenses struggle to achieve point-by-point testing across the full aperture of the lens. To facilitate the full-aperture, high-precision, and rapid testing of aspheric optical lenses, a power testing method of lenses based on the transmission phase deflectometric technique was employed. This method determines the phase deviation of light caused by the lens, thereby fitting the transmission wavefront and quickly detecting the power distribution information of the lens. This paper constructs a power testing model based on the phase deflectometric technique, proposes a calibration method that combines DLT (direct linear transformation) and pinhole models to reduce system errors, and combines phase extraction and wavefront fitting for experimental verification. The experimental results are compared with those obtained from the commercial visual lens mapper (VM-2000). The central errors of spherical power and cylindrical power are 1% and 0.7%, respectively. This provides a reliable method for the full-aperture, high-precision, and rapid testing of aspheric optical lenses. Full article
(This article belongs to the Special Issue Optical Precision Manufacturing and Testing: Technologies and Trends)
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12 pages, 4615 KiB  
Article
The Method and Experiment of Micro-Crack Identification Using OFDR Strain Measurement Technology
by Bin Chen, Jun Yang, Dezhi Zhang, Wenxiang Liu, Jin Li and Min Zhang
Photonics 2024, 11(8), 755; https://doi.org/10.3390/photonics11080755 - 13 Aug 2024
Cited by 1 | Viewed by 919
Abstract
The precise evaluation of micro-crack sizes and locations is crucial for the safe operation of structures. Traditional detection techniques, however, suffer from low spatial resolution, making it difficult to accurately locate micrometer-scale cracks. A method and experimental study were proposed in this paper [...] Read more.
The precise evaluation of micro-crack sizes and locations is crucial for the safe operation of structures. Traditional detection techniques, however, suffer from low spatial resolution, making it difficult to accurately locate micrometer-scale cracks. A method and experimental study were proposed in this paper for identifying and locating micro-cracks using optical fiber strain sensing based on OFDR to address this issue. The feasibility of this method for micro-crack detection was verified by the combination of a polyimide-coated sensing optical fiber (PISOF) and tight sheath sensing optical fiber (TSSOF). A calculation method for micro-crack widths based on distributed optical fiber strain curves was established, and the test results of different optical fibers were compared. Through multiple verification experiments, it was found that the strain peak curves of both fiber types could accurately locate micro-cracks with a precision of 1 mm. Additionally, the crack widths could be obtained by processing the distributed strain curves using a computational model, enabling the accurate capture of micro-crack characteristics at the 10 μm level. A strong linear relationship was observed between the optical fiber stretching length and the crack width. Notably, the relative error in calculating the crack width from the strain curve of PI fiber was very small, while a linear relationship existed between the maximum strain value of the TSSOF and the crack width, allowing for the calculation of the crack width based on the maximum strain value. This further validated the feasibility of the method designed in this paper for the analysis of micro-crack characteristic parameters. Full article
(This article belongs to the Special Issue Fiber Optic Sensors: Science and Applications)
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18 pages, 704 KiB  
Article
Adaptive Resource Allocation for Emergency Communications with Unmanned Aerial Vehicle-Assisted Free Space Optical/Radio Frequency Relay System
by Yuanmo Lin, Yuxun Ai, Zhiyong Xu, Jingyuan Wang and Jianhua Li
Photonics 2024, 11(8), 754; https://doi.org/10.3390/photonics11080754 - 13 Aug 2024
Viewed by 1047
Abstract
This paper investigates the problem of coordinated resource allocation for multiple unmanned aerial vehicles (UAVs) to address the scarcity of communication resources in disaster-affected areas. UAVs carrying modules of free space optical (FSO) and radio frequency (RF) serve as relay nodes and edge [...] Read more.
This paper investigates the problem of coordinated resource allocation for multiple unmanned aerial vehicles (UAVs) to address the scarcity of communication resources in disaster-affected areas. UAVs carrying modules of free space optical (FSO) and radio frequency (RF) serve as relay nodes and edge offloading nodes, presenting an FSO/RF dual-hop framework. Considering the varying urgency levels of tasks, we assign task priorities and transform the proposed problem into distributed collaborative optimization problem. Based on the K-means algorithm and the multi-agent deep deterministic policy gradient (MADDPG) algorithm, we propose a UAV-coordinated K-means MADDPG (KMADDPG) to maximize the number of completed tasks while prioritizing high-priority tasks. Simulation results show that KMADDPG is 5% to 10% better than the benchmark DRL methods in convergence performance. Full article
(This article belongs to the Special Issue Coherent Transmission Systems in Optical Wireless Communication)
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15 pages, 5478 KiB  
Article
Design and Fabrication of High-Quality Two-Dimensional Silicon-Based Photonic Crystal Optical Cavity with Integrated Waveguides
by Sohail Muhammad, Dingwei Chen, Chengwei Xian, Jun Zhou, Zhongke Lei, Pengju Kuang, Zhe Li, Guangjun Wen and Yongjun Huang
Photonics 2024, 11(8), 753; https://doi.org/10.3390/photonics11080753 - 12 Aug 2024
Viewed by 812
Abstract
The emergences of silicon-based photonic crystal (PhC) waveguides and two-dimensional (2D) PhC line-defect optical cavities have revolutionized the field of integrated photonics. In this paper, we design and fabricate a high-quality (high-Q) 2D silicon-based PhC optical cavity with integrated waveguides. We employ the [...] Read more.
The emergences of silicon-based photonic crystal (PhC) waveguides and two-dimensional (2D) PhC line-defect optical cavities have revolutionized the field of integrated photonics. In this paper, we design and fabricate a high-quality (high-Q) 2D silicon-based PhC optical cavity with integrated waveguides. We employ the 2D finite-difference time-domain (FDTD) method to simulate the cavity, considering two different thicknesses: 0.5 μm and 0.25 μm. By optimizing the line-defect and air-slot widths for the integrated PhC waveguides, we are able to achieve remarkable Q-factors for the PhC optical cavity. With a silicon thickness of 0.5 μm, the high-Q achieves an impressively high value of 8.01 × 106, while at a silicon thickness of 0.25 μm, it achieves 1.91 × 107. This research highlights the importance of design optimization and fabrication techniques in achieving high-Q optical devices using PhC and silicon-based structures. Full article
(This article belongs to the Special Issue Emerging Trends in Photonic Crystals)
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8 pages, 5221 KiB  
Article
High-Energy Injection-Seeded Single-Frequency Er:YAG Laser at 1645 nm Pumped by a 1532 nm Fiber Laser
by Jiaze Wu, Youlun Ju, Jiawei Fan, Yiming Zhao, Kun Yang, Lijie Geng, Yuanxue Cai, Lei Song, Yaming Zhuang, Shuyun Wu and Xiaoming Duan
Photonics 2024, 11(8), 752; https://doi.org/10.3390/photonics11080752 - 12 Aug 2024
Viewed by 800
Abstract
A single-frequency, Q-switched Er:YAG laser, pumped by a 1532 nm fiber laser, has been demonstrated. At the pulse repetition frequency (PRF) of 200 Hz, the maximum single-frequency laser of 5.5 mJ is attained, and, correspondingly, the pulse width is 212 ns. Using the [...] Read more.
A single-frequency, Q-switched Er:YAG laser, pumped by a 1532 nm fiber laser, has been demonstrated. At the pulse repetition frequency (PRF) of 200 Hz, the maximum single-frequency laser of 5.5 mJ is attained, and, correspondingly, the pulse width is 212 ns. Using the heterodyne technique, the single-frequency laser spectrum’s full width at half maximum is determined to be 2.73 MHz. The experimental results show that the single-frequency laser has excellent beam quality factors (M2) of 1.18 and 1.21. Full article
(This article belongs to the Special Issue Single Frequency Fiber Lasers and Their Applications)
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