Editor’s Choice Articles

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to readers, or important in the respective research area. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

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35 pages, 12619 KiB  
Review
Polarization Lidar: Principles and Applications
by Xudong Liu, Liping Zhang, Xiaoyu Zhai, Liye Li, Qingji Zhou, Xue Chen and Xiaobo Li
Photonics 2023, 10(10), 1118; https://doi.org/10.3390/photonics10101118 - 04 Oct 2023
Cited by 3 | Viewed by 2600
Abstract
Traditional lidar techniques mainly rely on the backscattering/echo light intensity and spectrum as information sources. In contrast, polarization lidar (P-lidar) expands the dimensions of detection by utilizing the physical property of polarization. By incorporating parameters such as polarization degree, polarization angle, and ellipticity, [...] Read more.
Traditional lidar techniques mainly rely on the backscattering/echo light intensity and spectrum as information sources. In contrast, polarization lidar (P-lidar) expands the dimensions of detection by utilizing the physical property of polarization. By incorporating parameters such as polarization degree, polarization angle, and ellipticity, P-lidar enhances the richness of physical information obtained from target objects, providing advantages for subsequent information analysis. Over the past five decades, the application fields of P-lidar have rapidly expanded, starting from its early use in atmospheric remote sensing to later applications in oceanic remote sensing. This review first provides a brief introduction to the basic principles of both polarization and P-lidar, along with typical systems. It then explores the applications of P-lidar in various remote sensing fields, including atmospheric, oceanic, and terrestrial domains. Additionally, we propose potential research directions based on current cutting-edge applications, with the aims of providing critical insights to researchers in the fields of polarization and lidar and inspiring further exciting ideas. Full article
(This article belongs to the Special Issue Advanced Polarimetry and Polarimetric Imaging)
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11 pages, 9874 KiB  
Article
Optimizing Self-Seeded Perfluorooctane SBS Compressor Configurations to Achieve ~90 ps High-Energy Pulses
by Aleksej M. Rodin, Augustė Černeckytė, Paulius Mackonis and Augustinas Petrulėnas
Photonics 2023, 10(9), 1060; https://doi.org/10.3390/photonics10091060 - 20 Sep 2023
Cited by 1 | Viewed by 771
Abstract
Three different stimulated Brillouin scattering (SBS) configurations in perfluorooctane were experimentally compared to achieve the ultimate compression of ~1.1 ns pulses from a commercially available Nd:YAG mini-laser. These schemes contained either a focusing lens and a plane feedback mirror, a spherical mirror, or [...] Read more.
Three different stimulated Brillouin scattering (SBS) configurations in perfluorooctane were experimentally compared to achieve the ultimate compression of ~1.1 ns pulses from a commercially available Nd:YAG mini-laser. These schemes contained either a focusing lens and a plane feedback mirror, a spherical mirror, or variable pulse splitting to provide self-seeding of the SBS. In the optimal configuration with a focusing lens and return mirror, 93 ps pulses with an energy of 9.5 mJ were achieved at the output of the double-pass phase-conjugated Nd:YAG amplifier. The resulting diffraction-free, high-quality beams with M2~1.2 and excellent pointing stability are of practical interest for scientific, medical, and industrial applications. Full article
(This article belongs to the Special Issue Photonics: Theory, Technologies and Applications)
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11 pages, 2657 KiB  
Article
High-Linear Frequency-Swept Lasers with Data-Driven Control
by Haohao Zhao, Dachao Xu, Zihan Wu, Liang Sun, Guohui Yuan and Zhuoran Wang
Photonics 2023, 10(9), 1056; https://doi.org/10.3390/photonics10091056 - 18 Sep 2023
Viewed by 889
Abstract
The frequency-swept laser (FSL) is applied widely in various sensing systems in the scientific and industrial fields, especially in the light detection and ranging (Lidar) area. However, the inherent nonlinearity limits its performance in application systems, especially in the broadband frequency-swept condition. In [...] Read more.
The frequency-swept laser (FSL) is applied widely in various sensing systems in the scientific and industrial fields, especially in the light detection and ranging (Lidar) area. However, the inherent nonlinearity limits its performance in application systems, especially in the broadband frequency-swept condition. In this work, from the perspective of data-driven control, we adopt the reinforcement learning-based broadband frequency-swept linearization method (RL-FSL) to optimize the control policy and generate the modulation signals. The nonlinearity measurement system and the system simulator are established. Since the powerful learning ability of the reinforcement learning algorithm, the linearization policy is optimized off-line and the generated modulation signals reduce the nonlinearity almost 20 times, compared to the case without control. In the long-term operation, the regular updated modulation signals perform better than the traditional iteration results, demonstrating the efficiency of the proposed data-driven control method in application systems. Therefore, the RL-FSL method has the potential to be the candidate of optical system control. Full article
(This article belongs to the Special Issue Artificial Intelligence and Machine Learning in Photonics)
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12 pages, 4493 KiB  
Article
Structurally Stable Astigmatic Vortex Beams with Super-High Orbital Angular Momentum (ABCD Matrix Approach)
by Alexander Volyar, Mikhail Bretsko, Server Khalilov and Yana Akimova
Photonics 2023, 10(9), 1048; https://doi.org/10.3390/photonics10091048 - 15 Sep 2023
Cited by 1 | Viewed by 958
Abstract
We have demonstrated efficiency of employing the ABCD matrix approach to transform higher-order structured Laguerre–Gaussian (sLG) beams into structurally stable astigmatic sLG (asLG) beams, highlighting their dynamics at propagating. Radical transformations of the beam structure by a cylindrical lens form not only orbital [...] Read more.
We have demonstrated efficiency of employing the ABCD matrix approach to transform higher-order structured Laguerre–Gaussian (sLG) beams into structurally stable astigmatic sLG (asLG) beams, highlighting their dynamics at propagating. Radical transformations of the beam structure by a cylindrical lens form not only orbital angular momentum (OAM) fast oscillations and bursts, but also make the asLG beams structurally unstable in propagation through cylindrical and spherical lenses when focusing paraxially. But, if the spherical lens performs a Fourier transform of the asLG beam after a cylindrical lens, the symmetric beam emerges at the lens focal plane with a sharp OAM dip; then, the OAM restores its former astigmatism, becoming structurally stable at the far diffraction domain. By investigating the beam structure at the focal area, we have showed that the OAM sharp dip is associated with nothing less than the process of dividing the OAM into the vortex and astigmatic constitutes predicted by Anan’ev and Bekshaev. Full article
(This article belongs to the Special Issue Advances and Application of Structured Light)
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15 pages, 4860 KiB  
Article
Gyrotropic Crystals as a Basis for Creation of Helical Polychromatic Singular Beams
by Yuriy Egorov and Alexander Rubass
Photonics 2023, 10(9), 1044; https://doi.org/10.3390/photonics10091044 - 14 Sep 2023
Viewed by 859
Abstract
In this work, studies are carried out in the field of optical singular beams that have passed through gyrotropic crystals. We have experimentally shown that singular beams with a helical intensity distribution are formed when passing through a system of two gyrotropic crystals [...] Read more.
In this work, studies are carried out in the field of optical singular beams that have passed through gyrotropic crystals. We have experimentally shown that singular beams with a helical intensity distribution are formed when passing through a system of two gyrotropic crystals with opposite values of the gyration coefficient. It is shown that the system is capable of generating optical vortices with a double topological charge in one of the components of circular polarization when light propagates through two gyrotropic crystals. Full article
(This article belongs to the Special Issue Optical Communication, Sensing and Network)
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8 pages, 1480 KiB  
Communication
High-Responsivity Planar Photodetector Based on Methylammonium Lead Bromide Perovskite Thin Film
by Pavlo Mai, Julien Houel, Nathan Dreveton, Benoit Mahler and Alban Gassenq
Photonics 2023, 10(9), 1043; https://doi.org/10.3390/photonics10091043 - 14 Sep 2023
Viewed by 4235
Abstract
We have fabricated planar interdigitated photodetectors exhibiting high responsivity. These detectors are based on thin layers of methylammonium lead bromide (MAPbBr3) at 90 nm thickness. MAPbBr3 thin films were first characterized on glass (borosilicate) substrates using absorption and photoluminescence measurements [...] Read more.
We have fabricated planar interdigitated photodetectors exhibiting high responsivity. These detectors are based on thin layers of methylammonium lead bromide (MAPbBr3) at 90 nm thickness. MAPbBr3 thin films were first characterized on glass (borosilicate) substrates using absorption and photoluminescence measurements showing a high absorption edge at 521 nm and strong emission at 530 nm, as expected. MAPbBr3 thin films were then deposited on top of interdigitated electrodes, hence producing planar photodetectors with responsivity up to 0.4 A/W. Such higher performances were attributed to the interdigitated design, low crack density (0.05 µm−2), and lower resistivity (20 MΩ.cm) compared to MAPbBr3 single crystal. Therefore, this work highlights MAPbBr3 thin films as very promising for photodetection applications. Full article
(This article belongs to the Special Issue Perovskite Materials and Optoelectronic Applications)
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17 pages, 7013 KiB  
Article
Design and Implementation of a Non-Common-View Axis Alignment System for Airborne Laser Communication
by Chenghu Ke, Yuting Shu, Xizheng Ke, Meimiao Han and Ruidong Chen
Photonics 2023, 10(9), 1037; https://doi.org/10.3390/photonics10091037 - 11 Sep 2023
Cited by 1 | Viewed by 791
Abstract
This paper proposes a non-common-view axis alignment method for the alignment requirements of airborne laser communication systems. The system consists of a ground transmitting end and an airborne relay terminal. The ground transmitting end uses a camera and a pan-tilt for image tracking, [...] Read more.
This paper proposes a non-common-view axis alignment method for the alignment requirements of airborne laser communication systems. The system consists of a ground transmitting end and an airborne relay terminal. The ground transmitting end uses a camera and a pan-tilt for image tracking, while the airborne relay end uses a two-dimensional mirror to control the beam to achieve non-common-view axis alignment between the transmitting and receiving sides. The working principle and process of both the transmitter and receiver of the non-common-view axis alignment system for airborne laser communication were compared with traditional wireless optical alignment methods. The design process of the two-dimensional mirror used in this paper is introduced, the scanning trajectory of the two-dimensional mirror is simulated and analyzed according to the beam scanning principle, and the field experiment link is set up to carry out the airborne laser communication experiment. The experimental results show that when the link distance is 10 m, the tracking errors of the system in the azimuth and pitch directions are 19.02 µrad and 22.35 µrad respectively, and the amplitude of the electrical signal output by the signal detector is 84.0 mV; When the link distance is 20 m, the tracking errors of the system in the azimuth and pitch directions are 39.66 µrad and 33.94 µrad respectively, and the amplitude of the electrical signal output by the signal detector is 23.0 mV. Using this method, the alignment can be completed without data return, and the establishment of the reverse link can also be realized while the transmission link is quickly established, and there is no need for an air stability platform. The feasibility of the application of the non-common-view axis alignment method to the airborne laser communication system is verified. Full article
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12 pages, 750 KiB  
Article
Optical Bistability and Thermal Mode Hopping in External Cavity Feedback Semiconductor Lasers
by Da Meng, Haiyang Yu, Yejin Zhang and Jianguo Liu
Photonics 2023, 10(9), 1027; https://doi.org/10.3390/photonics10091027 - 08 Sep 2023
Cited by 1 | Viewed by 832
Abstract
The external cavity feedback semiconductor laser shows obvious optical bistability in practice. That is, at the same operating temperature, different temperature tuning directions will lead to different laser mode outputs. This will lead to optical mode hopping and unstable output wavelength of the [...] Read more.
The external cavity feedback semiconductor laser shows obvious optical bistability in practice. That is, at the same operating temperature, different temperature tuning directions will lead to different laser mode outputs. This will lead to optical mode hopping and unstable output wavelength of the laser. In this work, we measured this phenomenon and explained it through a theoretical model with thermal synchronization considered. Our measured results show obvious bistability and temperature tuning direction dependence. In order to explain this phenomenon, we considered the physical characteristics of each optoelectronic component in the external cavity, and found that the temperature changes of each component were not synchronized over time. After establishing a model that takes into account the thermal asynchrony phenomenon, we have effectively explained this phenomenon. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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9 pages, 3147 KiB  
Communication
Inverse Design of Ultra-Compact and Low-Loss Optical Phase Shifters
by Junpeng Liao, Ye Tian, Zhe Kang and Xiaowei Zhang
Photonics 2023, 10(9), 1030; https://doi.org/10.3390/photonics10091030 - 08 Sep 2023
Viewed by 918
Abstract
In previous works, inverse design methods have primarily focused on manipulating the optical power to achieve specific design targets. In this paper, we use the inverse design method to enable the precise engineering of the optical phase. As a proof of concept, we [...] Read more.
In previous works, inverse design methods have primarily focused on manipulating the optical power to achieve specific design targets. In this paper, we use the inverse design method to enable the precise engineering of the optical phase. As a proof of concept, we present a series of phase shifters (PSs) with varying phase shifts, which are inversely designed and theoretically validated on the silicon-on-insulator (SOI) platform. The designed PSs exhibit remarkable accuracy, with deviations of below 1°. These proposed PSs feature an ultra-compact footprint of 3 µm in length and offer superior fabrication tolerances compared to conventional structures. Over the 1535–1565 nm bandwidth, the discrepancy between the actual and target phase shifts remains below ±1° for all phase shifters, while the insertion loss is consistently below 0.035 dB. Moreover, the feasibility of the designed five PSs is verified using 2 × 2 multimode interference couplers (MMI). Full article
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21 pages, 5976 KiB  
Review
Functionalized Chiral Twisted Optical Fibers: A Review
by Yifan Zhang, Boyao Li, Tianrong Huang, Guiyao Zhou and Yaoyao Liang
Photonics 2023, 10(9), 1025; https://doi.org/10.3390/photonics10091025 - 07 Sep 2023
Viewed by 997
Abstract
With an increase in the volume of information exchange and perception, the demands for intelligent, miniaturized, and integrated optical devices for information acquisition are also increasing. As the core component of optical networks for transmitting information, further optimization of their structural characteristics to [...] Read more.
With an increase in the volume of information exchange and perception, the demands for intelligent, miniaturized, and integrated optical devices for information acquisition are also increasing. As the core component of optical networks for transmitting information, further optimization of their structural characteristics to generate richer optical characteristics and apply them to information exchange and optical field control has become a key research hotspot. The introduction of chiral twist characteristics has led to new phenomena and applications in optical field transmission and the transformation of traditional optical fibers or microstructured optical fibers (MOF). Therefore, this review mainly begins with the principle of chiral optical fibers, introduces their preparation and latest application scenarios, and finally discusses their potential future development prospects. Full article
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12 pages, 7172 KiB  
Article
Imaging through a Scattering Medium under Different Intensities of Ambient Light Interference
by Yantong Zhang, Huiling Huang, Feibin Wu, Jun Han, Yi Yang and Ruyi Li
Photonics 2023, 10(9), 1023; https://doi.org/10.3390/photonics10091023 - 07 Sep 2023
Viewed by 935
Abstract
Many solutions for imaging through a scattering medium are sensitive to noise, which can lead to degradation or even to a failure of the image quality. This is especially the case in practical application scenarios, which are always filled with changing ambient light [...] Read more.
Many solutions for imaging through a scattering medium are sensitive to noise, which can lead to degradation or even to a failure of the image quality. This is especially the case in practical application scenarios, which are always filled with changing ambient light interference; as such, the traditional methods are difficult to practically apply. Therefore, in this paper, a spatial-frequency dual-domain learning neural network is designed for reconstructing the target of a speckle pattern under different intensities of ambient light interference. The network is mainly based on two modules. One module is designed from two perspectives, frequency domain denoising and the spatial-frequency spectrum of the speckle pattern. Another module is a dual-feature fusion attention module, which is used to improve the accuracy of the network. The experimental results demonstrate that the network is capable of reconstructing complex targets with high quality under varying intensities of interfering light. Furthermore, it is not constrained by the optical memory effect, exhibiting remarkable robustness and generalizability. The research based on this paper provides a feasible path for the practical application of scattering imaging methods. Full article
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16 pages, 2686 KiB  
Article
Kramers–Kronig Transmission with a Crosstalk-Dependent Step Multiple-Input Multiple-Output Volterra Equalizer in a Seven-Core Fiber
by Feng Tian, Tianze Wu, Chao Yu, Chuxuan Wang, Mohai Yue, Ran Gao, Qi Zhang, Zhipei Li, Qinghua Tian, Fu Wang and Xiangjun Xin
Photonics 2023, 10(9), 1017; https://doi.org/10.3390/photonics10091017 - 05 Sep 2023
Cited by 1 | Viewed by 733
Abstract
In this paper, we experimentally demonstrate a net bit rate of 261.7 Gbit/s in a seven-core transmission system with a Kramers–Kronig (KK) receiver. The 10 GBaud 16-level quadrature amplitude modulation (QAM) signal is transmitted over a 2.5 km seven-core fiber, and the relationship [...] Read more.
In this paper, we experimentally demonstrate a net bit rate of 261.7 Gbit/s in a seven-core transmission system with a Kramers–Kronig (KK) receiver. The 10 GBaud 16-level quadrature amplitude modulation (QAM) signal is transmitted over a 2.5 km seven-core fiber, and the relationship between carrier-to-signal power ratio, signal power, frequency spacing, and optical power is analyzed. Moreover, a multiple-input multiple-output (MIMO) Volterra equalization algorithm with crosstalk-dependent steps is proposed to compensate for inter-core crosstalk and impairments induced by other devices. Compared to the single-input single-output (SISO) Volterra equalizer, the CSPR can be reduced by 1.3 dB, and the received power gain can reach up to 0.7 dB. Full article
(This article belongs to the Special Issue Optical Fiber Transmission Systems)
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22 pages, 1161 KiB  
Article
Comparison between the Propagation Properties of Bessel–Gauss and Generalized Laguerre–Gauss Beams
by Colin J. R. Sheppard and Miguel A. Porras
Photonics 2023, 10(9), 1011; https://doi.org/10.3390/photonics10091011 - 04 Sep 2023
Cited by 1 | Viewed by 1242
Abstract
The connections between Laguerre–Gauss and Bessel–Gauss beams, and between Hermite–Gauss and cosine-Gauss beams are investigated. We review different asymptotic expressions for generalized Laguerre and Hermite polynomials of large radial/transverse order. The amplitude variations of generalized Laguerre–Gauss beams, including standard and elegant Laguerre–Gauss beams [...] Read more.
The connections between Laguerre–Gauss and Bessel–Gauss beams, and between Hermite–Gauss and cosine-Gauss beams are investigated. We review different asymptotic expressions for generalized Laguerre and Hermite polynomials of large radial/transverse order. The amplitude variations of generalized Laguerre–Gauss beams, including standard and elegant Laguerre–Gauss beams as special cases, are compared with Bessel–Gauss beams. Bessel–Gauss beams can be well-approximated by elegant Laguerre–Gauss beams. For non-integral values of the Laguerre function radial order, a generalized Laguerre–Gauss beam with integer order matches the width of the central lobe well, even for low radial orders. Previous approximations are found to be inaccurate for large azimuthal mode number (topolgical charge), and an improved approximation for this case is also introduced. Full article
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17 pages, 4198 KiB  
Review
Machine Learning for Self-Coherent Detection Short-Reach Optical Communications
by Qi Wu, Zhaopeng Xu, Yixiao Zhu, Yikun Zhang, Honglin Ji, Yu Yang, Gang Qiao, Lulu Liu, Shangcheng Wang, Junpeng Liang, Jinlong Wei, Jiali Li, Zhixue He, Qunbi Zhuge and Weisheng Hu
Photonics 2023, 10(9), 1001; https://doi.org/10.3390/photonics10091001 - 31 Aug 2023
Cited by 2 | Viewed by 1527
Abstract
Driven by emerging technologies such as the Internet of Things, 4K/8K video applications, virtual reality, and the metaverse, global internet protocol traffic has experienced an explosive growth in recent years. The surge in traffic imposes higher requirements for the data rate, spectral efficiency, [...] Read more.
Driven by emerging technologies such as the Internet of Things, 4K/8K video applications, virtual reality, and the metaverse, global internet protocol traffic has experienced an explosive growth in recent years. The surge in traffic imposes higher requirements for the data rate, spectral efficiency, cost, and power consumption of optical transceivers in short-reach optical networks, including data-center interconnects, passive optical networks, and 5G front-haul networks. Recently, a number of self-coherent detection (SCD) systems have been proposed and gained considerable attention due to their spectral efficiency and low cost. Compared with coherent detection, the narrow-linewidth and high-stable local oscillator can be saved at the receiver, significantly reducing the hardware complexity and cost of optical modules. At the same time, machine learning (ML) algorithms have demonstrated a remarkable performance in various types of optical communication applications, including channel equalization, constellation optimization, and optical performance monitoring. ML can also find its place in SCD systems in these scenarios. In this paper, we provide a comprehensive review of the recent progress in SCD systems designed for high-speed optical short- to medium-reach transmission links. We discuss the diverse applications and the future perspectives of ML for these SCD systems. Full article
(This article belongs to the Special Issue Machine Learning Applied to Optical Communication Systems)
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13 pages, 850 KiB  
Article
Higher-Order Optomechanical Nonlinearity Based on the Mechanical Effect of Light
by Qin Wu and Hao-Jin Sun
Photonics 2023, 10(9), 991; https://doi.org/10.3390/photonics10090991 - 30 Aug 2023
Viewed by 615
Abstract
Nonlinear cavity optomechanics based on the mechanical effect of light has recently received considerable attention due to its potential applications in high-precision metrology. In this work, we theoretically studied the third-order optomechanical nonlinearity by using a perturbative approach, and an analytical solution is [...] Read more.
Nonlinear cavity optomechanics based on the mechanical effect of light has recently received considerable attention due to its potential applications in high-precision metrology. In this work, we theoretically studied the third-order optomechanical nonlinearity by using a perturbative approach, and an analytical solution is given, which can be extended to cases of higher-order optomechanical nonlinearity. Furthermore, the generation of a third-order sideband is analyzed in detail, and the results show that the amplitude of the third-order sideband shows a high dependence on the control field detuning, suggesting that the high-order nonlinear intensity can be enhanced by properly adjusting the detuning of the laser field rather than by a strong laser drive. In addition to providing insight into optomechanical nonlinearity, the analytical description of third-order optomechanical nonlinearity based on the mechanical effects of light may find applications in ultra-high precision measurement under low power conditions. Full article
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11 pages, 8213 KiB  
Article
Microcomb-Driven Optical Convolution for Car Plate Recognition
by Zhenming He, Junwei Cheng, Xinyu Liu, Bo Wu, Heng Zhou, Jianji Dong and Xinliang Zhang
Photonics 2023, 10(9), 972; https://doi.org/10.3390/photonics10090972 - 25 Aug 2023
Cited by 1 | Viewed by 938
Abstract
The great success of artificial intelligence (AI) calls for higher-performance computing accelerators, and optical neural networks (ONNs) with the advantages of high speed and low power consumption have become competitive candidates. However, most of the reported ONN architectures have demonstrated simple MNIST handwritten [...] Read more.
The great success of artificial intelligence (AI) calls for higher-performance computing accelerators, and optical neural networks (ONNs) with the advantages of high speed and low power consumption have become competitive candidates. However, most of the reported ONN architectures have demonstrated simple MNIST handwritten digit classification tasks due to relatively low precision. A microring resonator (MRR) weight bank can achieve a high-precision weight matrix and can increase computing density with the assistance of wavelength division multiplexing (WDM) technology offered by dissipative Kerr soliton (DKS) microcomb sources. Here, we implement a car plate recognition task based on an optical convolutional neural network (CNN). An integrated DKS microcomb was used to drive an MRR weight-bank-based photonic processor, and the computing precision of one optical convolution operation could reach 7 bits. The first convolutional layer was realized in the optical domain, and the remaining layers were performed in the electrical domain. Totally, the optoelectronic computing system (OCS) could achieve a comparable performance with a 64-bit digital computer for character classification. The error distribution obtained from the experiment was used to emulate the optical convolution operation of other layers. The probabilities of the softmax layer were slightly degraded, and the robustness of the CNN was reduced, but the recognition results were still acceptable. This work explores an MRR weight-bank-based OCS driven by a soliton microcomb to realize a real-life neural network task for the first time and provides a promising computational acceleration scheme for complex AI tasks. Full article
(This article belongs to the Special Issue Optical Computing and Optical Neural Networks)
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9 pages, 5327 KiB  
Communication
Single-Shot Phase-Contrast Imaging with a Single Grating
by Xin Liu, Lang Liu, Jianheng Huang, Yaohu Lei and Ji Li
Photonics 2023, 10(9), 968; https://doi.org/10.3390/photonics10090968 - 24 Aug 2023
Cited by 1 | Viewed by 843
Abstract
In the field of X-ray phase-contrast imaging, a time-saving approach and preservation of details are crucial factors for obtaining phase-contrast images. In this manuscript, a single grating imaging system is proposed to perform the X-ray phase-contrast imaging. Instead of the time-consuming phase-stepping method, [...] Read more.
In the field of X-ray phase-contrast imaging, a time-saving approach and preservation of details are crucial factors for obtaining phase-contrast images. In this manuscript, a single grating imaging system is proposed to perform the X-ray phase-contrast imaging. Instead of the time-consuming phase-stepping method, this system uses a single-shot algorithm to retrieve the distribution of samples’ attenuation and phase gradient. Unlike the single-shot Fourier transform algorithm, which truncates the high-frequency component of the image and reduces the spatial resolution, our method can retrieve the attenuation and phase information images with the same spatial resolution as the images acquired directly by the X-ray detector used. Furthermore, by using a large-size X-ray detector (29 cm × 23 cm), the imaging system can be configured as either a microscopic instrument or a normal large field-of-view imaging system. Finally, a series of experiments were performed to validate the feasibility of the proposed method. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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13 pages, 5176 KiB  
Essay
Integrated Encapsulation and Implementation of a Linear-Mode APD Detector for Single-Pixel Imaging Lidar
by Akang Lv, Kee Yuan, Jian Huang, Dongfeng Shi, Shiguo Zhang, Yafeng Chen and Zixin He
Photonics 2023, 10(9), 970; https://doi.org/10.3390/photonics10090970 - 24 Aug 2023
Viewed by 922
Abstract
Single-pixel imaging lidar is a novel technology that leverages single-pixel detectors without spatial resolution and spatial light modulators to capture images by reconstruction. This technique has potential imaging capability in non-visible wavelengths compared with surface array detectors. An avalanche photodiode (APD) is a [...] Read more.
Single-pixel imaging lidar is a novel technology that leverages single-pixel detectors without spatial resolution and spatial light modulators to capture images by reconstruction. This technique has potential imaging capability in non-visible wavelengths compared with surface array detectors. An avalanche photodiode (APD) is a device in which the internal photoelectric effect and the avalanche multiplication effect are exploited to detect and amplify optical signals. An encapsulated APD detector, with an APD device as the core, is the preferred photodetector for lidar due to its high quantum efficiency in the near-infrared waveband. However, research into APD detectors in China is still in the exploratory period, when most of the work focuses on theoretical analysis and experimental verification. This is a far cry from foreign research levels in key technologies, and the required near-infrared APD detectors with high sensitivity and low noise have to be imported at a high price. In this present study, an encapsulated APD detector was designed in a linear mode by integrating a bare APD tube, a bias power circuit, a temperature control circuit and a signal processing circuit, and the corresponding theoretical analysis, circuit design, circuit simulation and experimental tests were carried out. Then, the APD detector was applied in the single-pixel imaging lidar system. The study showed that the bias power circuit could provide the APD with an operating voltage of DC 1.6 V to 300 V and a ripple voltage of less than 4.2 mV. Not only that, the temperature control circuit quickly changed the operating state of the Thermo Electric Cooler (TEC) to stabilize the ambient temperature of the APD and maintain it at 25 ± 0.3 °C within 5 h. The signal processing circuit was designed with a multi-stage amplification cascade structure, effectively raising the gain of signal amplification. By comparison, the trial also suggested that the encapsulated APD detector and the commercial Licel detector had a good agreement on the scattered signal, such as a repetition rate and pulse width response under the same lidar environment. Therefore, target objects in real atmospheric environments could be imaged by applying the encapsulated APD detector to the near-infrared single-pixel imaging lidar system. Full article
(This article belongs to the Topic Advances in Optical Sensors)
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15 pages, 3271 KiB  
Article
Fourier Single-Pixel Imaging Based on Online Modulation Pattern Binarization
by Xinding Jiang, Ziyi Tong, Zhongyang Yu, Pengfei Jiang, Lu Xu, Long Wu, Mingsheng Chen, Yong Zhang, Jianlong Zhang and Xu Yang
Photonics 2023, 10(9), 963; https://doi.org/10.3390/photonics10090963 - 23 Aug 2023
Cited by 2 | Viewed by 834
Abstract
Down-sampling Fourier single-pixel imaging is typically achieved by truncating the Fourier spectrum, where exclusively the low-frequency Fourier coefficients are extracted while discarding the high-frequency components. However, the truncation of the Fourier spectrum can lead to an undesired ringing effect in the reconstructed result. [...] Read more.
Down-sampling Fourier single-pixel imaging is typically achieved by truncating the Fourier spectrum, where exclusively the low-frequency Fourier coefficients are extracted while discarding the high-frequency components. However, the truncation of the Fourier spectrum can lead to an undesired ringing effect in the reconstructed result. Moreover, the original Fourier single-pixel imaging necessitated grayscale Fourier basis patterns for illumination. This requirement limits imaging speed because digital micromirror devices (DMDs) generate grayscale patterns at a lower refresh rate. In order to solve the above problem, a fast and high-quality Fourier single-pixel imaging reconstruction method is proposed in the paper. In the method, the threshold binarization of the Fourier base pattern is performed online to improve the DMD refresh rate, and the reconstruction quality of Fourier single-pixel imaging at a low-sampling rate is improved by generating an adversarial network. This method enables fast reconstruction of target images with higher quality despite low-sampling rates. Compared with conventional Fourier single-pixel imaging, numerical simulation and experimentation demonstrate the effectiveness of the proposed method. Notably, this method is particularly significant for fast Fourier single-pixel imaging applications. Full article
(This article belongs to the Special Issue Nonlinear Optics and Hyperspectral Polarization Imaging)
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11 pages, 3096 KiB  
Communication
Frequency Comb Fiber Generator Based on Photonic Bandgap Amplifier
by Aleksei Abramov, Dmitry Korobko and Igor Zolotovskii
Photonics 2023, 10(9), 965; https://doi.org/10.3390/photonics10090965 - 23 Aug 2023
Viewed by 772
Abstract
We report on a model of a fiber frequency comb generator that develops an approach to harmonically mode-locked fiber laser design based on dissipative four-wave mixing. In our version of this approach, we assume an amplifying one-dimensional photonic crystal as a key cavity [...] Read more.
We report on a model of a fiber frequency comb generator that develops an approach to harmonically mode-locked fiber laser design based on dissipative four-wave mixing. In our version of this approach, we assume an amplifying one-dimensional photonic crystal as a key cavity element combining the properties of an intra-cavity filter and a power amplifier. Using standard equations describing the signal transformation in the ring cavity and the output fiber cascade, we have demonstrated the possibility of the application of the proposed model as a generator of broadband frequency comb with controllable line spacing. Full article
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11 pages, 2285 KiB  
Article
An Efficient Silicon Grating Coupler for a 2 μm Waveband Based on a Polysilicon Overlay
by Yifan Wu, Yuzhou Wu, Jiefeng Xu, Xi Wang, You Wu, Yanglin Chen, Jiahang Li and Ke Xu
Photonics 2023, 10(8), 952; https://doi.org/10.3390/photonics10080952 - 20 Aug 2023
Viewed by 1108
Abstract
The short-wavelength mid-infrared spectral range of the 2 μm waveband has the advantages of low transmission loss and broad gain bandwidth, making it a promising candidate for the next optical fiber communication window. It is thus highly desired to develop high-performance silicon photonic [...] Read more.
The short-wavelength mid-infrared spectral range of the 2 μm waveband has the advantages of low transmission loss and broad gain bandwidth, making it a promising candidate for the next optical fiber communication window. It is thus highly desired to develop high-performance silicon photonic components in this waveband. Here, an efficient dual-layer grating coupler was designed on a 220 nm thick silicon-on-insulator based on raised polysilicon to address the low directionality issue. For the fiber tilted at an angle of 10°, the grating coupler’s simulated coupling efficiency reaches 80.3% (−0.95 dB) at a wavelength of 2002 nm. The 1 dB bandwidth is 66 nm. The structure is completely compatible with the standard silicon photonic fabrication process, making it suitable for large volume fabrication. Full article
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14 pages, 5257 KiB  
Article
Simple Method of Light Field Calculation for Shaping of 3D Light Curves
by Svetlana N. Khonina, Alexey P. Porfirev, Sergey G. Volotovskiy, Andrey V. Ustinov and Sergey V. Karpeev
Photonics 2023, 10(8), 941; https://doi.org/10.3390/photonics10080941 - 17 Aug 2023
Viewed by 977
Abstract
We propose a method for generating three-dimensional light fields with given intensity and phase distributions using purely phase transmission functions. The method is based on a generalization of the well-known approach to the design of diffractive optical elements that focus an incident laser [...] Read more.
We propose a method for generating three-dimensional light fields with given intensity and phase distributions using purely phase transmission functions. The method is based on a generalization of the well-known approach to the design of diffractive optical elements that focus an incident laser beam into an array of light spots in space. To calculate purely phase transmission functions, we use amplitude encoding, which made it possible to implement the designed elements using a single spatial light modulator. The generation of light beams in the form of rings, spirals, Lissajous figures, and multi-petal “rose” distributions uniformly elongated along the optical axis in the required segment is demonstrated. It is also possible to control the three-dimensional structure of the intensity and phase of the shaped light fields along the propagation axis. The experimentally generated intensity distributions are in good agreement with the numerically obtained results and show high potential for the application of the proposed method in laser manipulation with nano- and microparticles, as well as in laser material processing. Full article
(This article belongs to the Special Issue Light Focusing and Optical Vortices)
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18 pages, 8696 KiB  
Article
Analysis and Prediction of Image Quality Degradation Caused by Diffraction of Infrared Optical System Turning Marks
by Haokun Ye, Jianping Zhang, Shangnan Zhao, Mingxin Liu and Xin Zhang
Photonics 2023, 10(8), 937; https://doi.org/10.3390/photonics10080937 - 17 Aug 2023
Viewed by 943
Abstract
This paper addresses the issue of reduced image quality due to annular turning marks formed by single-point diamond turning (SPDT) during the processing of metal-based mirrors and infrared lenses. An ideal single-point diamond turning marks diffraction action model to quantitatively analyze the impact [...] Read more.
This paper addresses the issue of reduced image quality due to annular turning marks formed by single-point diamond turning (SPDT) during the processing of metal-based mirrors and infrared lenses. An ideal single-point diamond turning marks diffraction action model to quantitatively analyze the impact of turning marks diffraction on imaging quality degradation is proposed. Based on this model, a fast estimation algorithm for the optical modulation transfer function of the system under turning marks diffraction (TMTF) is proposed. The results show that the TMTF algorithm achieves high computational accuracy, with a relative error of only 3% in diffraction efficiency, while being hundreds of times faster than rigorous coupled wave analysis (RCWA). This method is significant for reducing manufacturing costs and improving production efficiency, as it avoids the problem of being unable to compute large-size optical systems due to computational resource and time constraints. Full article
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13 pages, 3680 KiB  
Communication
Vertical Multi-Junction Laser Power Converters with 61% Efficiency at 30 W Output Power and with Tolerance to Beam Non-Uniformity, Partial Illumination, and Beam Displacement
by Simon Fafard and Denis Masson
Photonics 2023, 10(8), 940; https://doi.org/10.3390/photonics10080940 - 17 Aug 2023
Cited by 10 | Viewed by 1490
Abstract
Stable and reliable optical power converting devices are obtained using vertical multi-junction laser power converters. They are based on the GaAs and the InP material systems and are used for power-over-fiber or power-beaming applications. This study demonstrates that, in addition to providing the [...] Read more.
Stable and reliable optical power converting devices are obtained using vertical multi-junction laser power converters. They are based on the GaAs and the InP material systems and are used for power-over-fiber or power-beaming applications. This study demonstrates that, in addition to providing the overall best conversion efficiencies with output voltages ideal for various applications, these semiconductor photovoltaic devices are very tolerant to beam non-uniformity, partial illumination, or beam displacement variations. Examples are given with two tight beams, each covering as little as ~7% of the cell area. An optical input power of 10 W was converted with still an efficiency of Eff ~59.4%. For an input power of 20 W, the illuminated area was set to ~22% without significantly affecting the conversion efficiency of Eff ~60%. Remarkably, for a beam diameter at ~65% of the chip length (i.e., covering ~35% of the chip area), a converted power of 29.5 W was obtained using a 12-junction GaAs device with a conversion efficiency of 61%. For a 10 junction InP-based device, an efficiency of Eff = 51.1% was obtained at an output voltage reaching as high as Voc = 5.954 V for an average optical intensity of 69 W/cm2 and an illumination area of ~57%. Full article
(This article belongs to the Topic Photovoltaic Materials and Devices)
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0 pages, 4065 KiB  
Article
A 1083 nm Narrow-Linewidth DFB Semiconductor Laser for Quantum Magnetometry
by Mengying Wu, Haiyang Yu, Wenyu Wang, Shaojie Li, Yulian Cao and Jianguo Liu
Photonics 2023, 10(8), 934; https://doi.org/10.3390/photonics10080934 - 15 Aug 2023
Cited by 1 | Viewed by 964
Abstract
A 1083 nm laser, corresponding to a characteristic spectral line of 3He 23S1-23P, is the core light source for spin-exchange optical pumping-free technology, and thus has important developmental significance. In this paper, precise wavelength 1083.34 nm [...] Read more.
A 1083 nm laser, corresponding to a characteristic spectral line of 3He 23S1-23P, is the core light source for spin-exchange optical pumping-free technology, and thus has important developmental significance. In this paper, precise wavelength 1083.34 nm semiconductor lasers with 285 mW output power, −144.73 dBc/Hz RIN noise and 30.9952 kHz linewidth have been successfully achieved via reasonable chips design, high-quality epitaxial growth process and ultra-low reflectivity coating fabrication. All the results show the highest output power and ultra-narrow linewidth of the single-frequency 1083 nm DFB semiconductor laser achieved in this paper, which can fully satisfy the requirement of quantum magnetometers. Full article
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11 pages, 4176 KiB  
Article
Dual-Band Laser Stealth Based on Quasi Photonic Crystals
by Man Yuan, Jianjing Zhao, Xinye Liao and Xin He
Photonics 2023, 10(8), 931; https://doi.org/10.3390/photonics10080931 - 14 Aug 2023
Cited by 1 | Viewed by 810
Abstract
A quasi photonic crystal (QPC) dual-band absorber for laser stealth is designed and numerically studied. It consists of a defective two-dimensional photonic crystal on a thick Ni film. The defective photonic crystal is a continuous Ge layer with air holes, but some of [...] Read more.
A quasi photonic crystal (QPC) dual-band absorber for laser stealth is designed and numerically studied. It consists of a defective two-dimensional photonic crystal on a thick Ni film. The defective photonic crystal is a continuous Ge layer with air holes, but some of the holes are periodically removed. Under a normal incidence that is perpendicular to the plane of the defects, the absorptivity can achieve 92.8% at the 1.064 μm wavelength and 93.2% at the 1.55 μm wavelength. Within large incident angles (<45 degrees), the dual-band absorptivity is still >80%. Additionally, the emissivity is as low as 5.8%~20.0% and 2.8%~5.8% in the 3–5 μm and 8–14 μm atmospheric windows. It is found that the introduced defects can couple the incidence into the structure and lead to spectral peaks (electromagnetic resonances) even without the bottom Ni film. With the help of the high-loss Ni film, the resonances are transformed into magnetic or/and electric modes of different orders. As a result, the QPC generates four absorption peaks. They are superimposed in pairs, resulting in enhanced absorption of the two laser wavelengths. Full article
(This article belongs to the Special Issue Advances in Photonic Materials and Technologies)
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14 pages, 9207 KiB  
Article
Experimental Comparison of Carrier Phase Recovery Algorithms for Uniform and Probabilistically Shaped QAM in a 324.1 Gb/S Fiber-mm-Wave Integration System at W-Band
by Junhao Zhang, Jiao Zhang, Qingsong Wang, Jian Chen, Wei Luo, Shitong Xiang, Yuancheng Cai, Bingchang Hua, Mingzheng Lei, Yucong Zou, Liang Tian, Xingyu Chen and Min Zhu
Photonics 2023, 10(8), 927; https://doi.org/10.3390/photonics10080927 - 12 Aug 2023
Viewed by 838
Abstract
We have experimentally implemented a photonics-aided large-capacity fiber-mm-wave wireless communication system employing a simple dual-polarized single-input single-output (SISO) wireless based on polarization multiplexing at the W-band. To compare the performance of different algorithms, 18G-baud, and 35G-baud 16-level quadrature-amplitude-modulation (16QAM), probabilistically shaped 16QAM (PS-16QAM), [...] Read more.
We have experimentally implemented a photonics-aided large-capacity fiber-mm-wave wireless communication system employing a simple dual-polarized single-input single-output (SISO) wireless based on polarization multiplexing at the W-band. To compare the performance of different algorithms, 18G-baud, and 35G-baud 16-level quadrature-amplitude-modulation (16QAM), probabilistically shaped 16QAM (PS-16QAM), 64QAM and PS-64QAM signal using different carrier phase recovery (CPR) algorithms are transmitted in the system. Moreover, we compare the Viterbi–Viterbi (VV), improved new algorithm based on VV (NVV), blind phase search (BPS), and two-stage BPS algorithms’ computational complexity to better compare different algorithms. Using the experiment result, we can demonstrate that the BPS algorithm is about half a magnitude better than the NVV algorithm for PS-QAM signals, while the NVV algorithm has the lowest computational complexity. Additionally, we also achieve error-free wireless transmission at a net data rate of 324.1 Gb/s with the bit error ratio (BER) below the forward-error correction (FEC) threshold of 1 × 10−2 assuming soft-decision forward-error correction (SD-FEC) when using the BPS algorithm. Full article
(This article belongs to the Special Issue Coherent Transmission Systems in Optical Wireless Communication)
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13 pages, 4973 KiB  
Article
LiNEV: Visible Light Networking for Connected Vehicles
by Osama Saied, Omprakash Kaiwartya, Mohammad Aljaidi, Sushil Kumar, Mufti Mahmud, Rupak Kharel, Farah Al-Sallami and Charalampos C. Tsimenidis
Photonics 2023, 10(8), 925; https://doi.org/10.3390/photonics10080925 - 11 Aug 2023
Viewed by 1482
Abstract
DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) has been introduced to visible light networking framework for connected vehicles (LiNEV) systems as a modulation and multiplexing scheme. This is to overcome the light-emitting diode (LED) bandwidth limitation, as well as to reduce the inter-symbol [...] Read more.
DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) has been introduced to visible light networking framework for connected vehicles (LiNEV) systems as a modulation and multiplexing scheme. This is to overcome the light-emitting diode (LED) bandwidth limitation, as well as to reduce the inter-symbol interference caused by the multipath road fading. Due to the implementation of the inverse fast Fourier transform, DC-OFDM suffers from its large peak-to-average power ratio (PAPR), which degrades the performance in LiNEV systems, as the LEDs used in the vehicles’ headlights have a limited optical power-current linear range. To tackle this issue, discrete Fourier transform spread-optical pulse amplitude modulation (DFTS-OPAM) has been proposed as an alternative modulation scheme for LiNEV systems instead of DCO-OFDM. In this paper, we investigate the system performance of both schemes considering the light-emitting diode linear dynamic range and LED 3 dB modulation bandwidth limitations. The simulation results indicate that DCO-OFDM has a 9 dB higher PAPR value compared with DFTS-OPAM. Additionally, it is demonstrated that DCO-OFDM requires an LED with a linear range that is twice the one required by DFTS-OPAM for the same high quadrature amplitude modulation (QAM) order. Furthermore, the findings illustrate that when the signal bandwidth of both schemes significantly exceeds the LED modulation bandwidth, DCO-OFDM outperforms DFTS-OPAM, as it requires a lower signal-to-noise ratio at a high QAM order. Full article
(This article belongs to the Special Issue Advances in Visible Light Communication)
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9 pages, 2089 KiB  
Communication
Inverse Design of Broadband Absorption in the Visible with Plasmonic Multilayered Planar Structures
by Davi F. Rêgo, Igor L. Gomes de Souza, Vitaly F. Rodriguez-Esquerre and Gilliard N. Malheiros-Silveira
Photonics 2023, 10(8), 922; https://doi.org/10.3390/photonics10080922 - 11 Aug 2023
Cited by 1 | Viewed by 751
Abstract
Plasmonic structures based on stacked layers of metal and dielectric materials excel as broadband absorbers because of the nonlinear relationship between the compound materials’ dispersion characteristics and the multilayered structure’s actual performance. In this work, radiation absorption along the plasmonic absorber is studied. [...] Read more.
Plasmonic structures based on stacked layers of metal and dielectric materials excel as broadband absorbers because of the nonlinear relationship between the compound materials’ dispersion characteristics and the multilayered structure’s actual performance. In this work, radiation absorption along the plasmonic absorber is studied. Broadband absorptance spectra play an important role in applications such as photovoltaics, detectors, modulators, and emitters. We propose and analyze plasmonic stacked structures that exhibit high broadband absorption. For this purpose, an inverse design approach has been implemented using a conventional genetic algorithm as a global optimizer in conjunction with a pattern search as a local optimizer. The proposed strategy found structures with absorption covering the visible spectrum, maintaining its performance for high incident angles. Full article
(This article belongs to the Section Optoelectronics and Optical Materials)
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23 pages, 16196 KiB  
Article
Nonlocal Hydrodynamic Model with Viscosive Damping and Generalized Drude–Lorentz Term
by Milan Burda and Ivan Richter
Photonics 2023, 10(8), 913; https://doi.org/10.3390/photonics10080913 - 09 Aug 2023
Viewed by 704
Abstract
The response of plasmonic metal particles to an electromagnetic wave produces significant features at the nanoscale level. Different properties of the internal composition of a metal, such as its ionic background and the free electron gas, begin to manifest more prominently. As the [...] Read more.
The response of plasmonic metal particles to an electromagnetic wave produces significant features at the nanoscale level. Different properties of the internal composition of a metal, such as its ionic background and the free electron gas, begin to manifest more prominently. As the dimensions of the nanostructures decrease, the classical local theory gradually becomes inadequate. Therefore, Maxwell’s equations need to be supplemented with a relationship determining the dynamics of current density which is the essence of nonlocal plasmonic models. In this field of physics, the standard (linearized) hydrodynamic model (HDM) has been widely adopted with great success, serving as the basis for a variety of simulation methods. However, ongoing efforts are also being made to expand and refine it. Recently, the GNOR (general nonlocal optical response) modification of the HDM has been used, with the intention of incorporating the influence of electron gas diffusion. Clearly, from the classical description of fluid dynamics, a close relationship between viscosive damping and diffusion arises. This offers a relevant motivation for introducing the GNOR modification in an alternative manner. The standard HDM and its existing GNOR modification also do not include the influence of interband electron transitions in the conduction band and other phenomena that are part of many refining modifications of the Drude–Lorentz and other models of metal permittivity. In this article, we present a modified version of GNOR-HDM that incorporates the viscosive damping of the electron gas and a generalized Drude–Lorentz term. In the selected simulations, we also introduce Landau damping, which corrects the magnitude of the standard damping constant of the electron gas based on the size of the nanoparticle. We have chosen a spherical particle as a suitable object for testing and comparing HD models and their modifications because it allows the calculation of precise analytical solutions for the interactions and, simultaneously, it is a relatively easily fabricated nanostructure in practice. Our contribution also includes our own analytical method for solving the HDM interaction of a plane wave with a spherical particle. This method forms the core of calculations of the characteristic quantities, such as the extinction cross-sections and the corresponding components of electric fields and current densities. Full article
(This article belongs to the Special Issue Editorial Board Members’ Collection Series: Photonics Sensors)
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19 pages, 5900 KiB  
Article
Optical Trapping of Chiral Particles by Dual Laser Beams
by Jing Bai, Cheng-Xian Ge and Zhen-Sen Wu
Photonics 2023, 10(8), 905; https://doi.org/10.3390/photonics10080905 - 04 Aug 2023
Cited by 1 | Viewed by 958
Abstract
In this paper, an analytical method for studying the radiation force (RF) of chiral spheres generated by dual laser beams is presented under the framework of generalized Lorenz–Mie theory (GLMT). According to the coordinate transformation relations, the arbitrarily incident laser beam is represented [...] Read more.
In this paper, an analytical method for studying the radiation force (RF) of chiral spheres generated by dual laser beams is presented under the framework of generalized Lorenz–Mie theory (GLMT). According to the coordinate transformation relations, the arbitrarily incident laser beam is represented by vector spherical harmonic functions (VSHFs) in the sphere system. The entire induced field expression coefficients of dual laser beams can be obtained by superposition of each illuminated field. Based on the momentum conservation theory, the concrete expression of lateral and axial RF on chiral sphere is derived. The current theories are shown to be valid by comparison with the existing reference. To investigate the stable capture state of chiral sphere, the influences of the corresponding parameters of chiral particles and dual laser beams on the trapping and manipulation are investigated in detail. The analytical study on the RF of dual laser beams on chiral particles is an efficient method for improving optical tweezers technology and can become an encouraging approach to realize the high accuracy operation of chiral particles. Full article
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10 pages, 6637 KiB  
Communication
The Effect of Electron Escape Rate on the Nonlinear Dynamics of Quantum Dot Lasers under Optical Feedback
by Qingqing Wang, Zhengmao Wu, Yanfei Zheng and Guangqiong Xia
Photonics 2023, 10(8), 878; https://doi.org/10.3390/photonics10080878 - 28 Jul 2023
Viewed by 736
Abstract
When theoretically investigating the nonlinear dynamics of quantum dot lasers (QDLs), the parameter value of the electron escape rate (Ce) is sometimes approximated to zero to simplify the calculation. However, the value of Ce is dependent on the energy [...] Read more.
When theoretically investigating the nonlinear dynamics of quantum dot lasers (QDLs), the parameter value of the electron escape rate (Ce) is sometimes approximated to zero to simplify the calculation. However, the value of Ce is dependent on the energy interval between the ground state (GS) and the excited state (ES) in the conduction band and is affected by the operation temperature. As a result, such simplified approximation treatments may lead to inaccurate results. In this study, after considering the effect of Ce, we investigate the nonlinear dynamics of QDLs with and without optical feedback based on the asymmetric electron-hole carrier rate equation model. The simulation results show that without optical feedback, the lasing conditions for ES and GS in free-running QDLs are dependent on the value of Ce. A larger Ce is more helpful for the ES emission, and the GS emission will stop lasing if Ce is large enough. Through analyzing the dynamical characteristics of GS and ES in QDLs with optical feedback under different Ce values, it can be found that the dynamical characteristics are strongly correlative with Ce. Full article
(This article belongs to the Special Issue Lasers and Dynamic of Systems)
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14 pages, 797 KiB  
Review
Atmospheric Pre-Compensation of Ground-to-Space Communications with Adaptive Optics: Past, Present and Future—A Field Review
by Noelia Martinez
Photonics 2023, 10(7), 858; https://doi.org/10.3390/photonics10070858 - 24 Jul 2023
Viewed by 1437
Abstract
Conventionally used in astronomy, adaptive optics (AO) systems measure and correct for turbulence and, therefore, have the capability to mitigate the impact of the atmosphere on the ground-to-space communication links. Historically, there have been two main streams, respectively, advocating to use or not [...] Read more.
Conventionally used in astronomy, adaptive optics (AO) systems measure and correct for turbulence and, therefore, have the capability to mitigate the impact of the atmosphere on the ground-to-space communication links. Historically, there have been two main streams, respectively, advocating to use or not use adaptive optics on optical communications. This paper aims to present a comprehensive review of the field of AO-based uplink pre-compensation. It will cover the technical challenges associated with uplink correction, provide an overview of the state-of-the-art research and demonstrations from the early days to the present, and discuss the future prospects of AO-based uplink pre-compensation and potential trade-offs between performance, cost, and operational considerations. Full article
(This article belongs to the Special Issue New Perspectives in Free-Space Optical Communications and Networks)
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15 pages, 2130 KiB  
Article
High-Order Harmonics Generation in Selenium-Containing Plasmas
by Rashid A. Ganeev
Photonics 2023, 10(7), 854; https://doi.org/10.3390/photonics10070854 - 24 Jul 2023
Cited by 2 | Viewed by 742
Abstract
The studies of the high-order harmonics generated in Se-containing plasmas are reported. The ablation of selenium in a vacuum allowed for the formation of a plasma demonstrating high-order harmonics generation and resonance enhancement of the harmonic at the shortest wavelength reported so far [...] Read more.
The studies of the high-order harmonics generated in Se-containing plasmas are reported. The ablation of selenium in a vacuum allowed for the formation of a plasma demonstrating high-order harmonics generation and resonance enhancement of the harmonic at the shortest wavelength reported so far (λ ≈ 22.9 nm, Eph ≈ 54.14 eV). This harmonic corresponds to the 35th order of the 800-nm-class lasers. The influence of the presence of selenium in the molecular state (ZnSe and HgSe) on the suppression of the resonance effect during harmonics generation in plasma is studied. The enhanced 35th harmonic was analyzed by different methods of plasma formation using nanosecond, picosecond, and femtosecond pulses. The enhancement factor of the resonance-enhanced harmonic was measured to be 32× compared with the neighboring lower-order harmonics in the case of the picosecond-pulses-induced Se plasma. The role of the strong ionic transition of Se in the region of 22.7 nm in the observation of the resonance-induced enhancement of a single harmonic is discussed. Full article
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15 pages, 6480 KiB  
Article
Research on Indoor Visible Light Location Based on Fusion Clustering Algorithm
by Chenghu Ke, Yuting Shu and Xizheng Ke
Photonics 2023, 10(7), 853; https://doi.org/10.3390/photonics10070853 - 23 Jul 2023
Viewed by 833
Abstract
Aiming at the problem of large positioning errors in the boundary area, a new location fingerprint location method based on a fusion clustering algorithm is proposed. This clustering-based method embodies the idea of rough location first and then fine location. Firstly, the edge [...] Read more.
Aiming at the problem of large positioning errors in the boundary area, a new location fingerprint location method based on a fusion clustering algorithm is proposed. This clustering-based method embodies the idea of rough location first and then fine location. Firstly, the edge regions of the received signal strength (RSS) samples which are greatly affected by reflection are divided using the k-medoids algorithm, and then the center part is clustered via density-based spatial clustering of applications with noise (DBSCAN). In the actual location estimation stage, the points to be measured can only be located in one of the classified areas, and combined with the optimal k-nearest neighbor algorithm (WOKNN) to match the location. The results show that the average positioning error of the algorithm is 13 cm in an indoor environment of 5 m × 5 m × 3 m. Compared with the traditional method without clustering, the positioning accuracy of the edge area is increased by 21%, and the overall improvement is 33.8%, which proves that the proposed algorithm effectively improves the efficiency of real-time positioning and indoor positioning accuracy. Full article
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10 pages, 3291 KiB  
Communication
Efficient 1054 nm Raman Random Fiber Laser
by Pan Wang, Shengtao Lin, Jiaojiao Zhang, Xingyu Bao, Longqun Ni, Yifei Qi and Zinan Wang
Photonics 2023, 10(7), 851; https://doi.org/10.3390/photonics10070851 - 22 Jul 2023
Cited by 2 | Viewed by 979
Abstract
Low–coherence laser is regarded as the key to mitigating laser–plasma instability (LPI) in laser–driven inertial confinement fusion (ICF), where LPI can decrease the laser energy coupled to the target. With the merits of low coherence, high spectral stability, and flexible output characteristics, the [...] Read more.
Low–coherence laser is regarded as the key to mitigating laser–plasma instability (LPI) in laser–driven inertial confinement fusion (ICF), where LPI can decrease the laser energy coupled to the target. With the merits of low coherence, high spectral stability, and flexible output characteristics, the Raman random fiber laser (RRFL) is considered to be a candidate light source in ICF. In this paper, the 1054 nm RRFL with high slope efficiency is achieved for the first time. In the RRFL pump source design section, we have optimized the ytterbium–doped fiber (YDF) length by simulation and amplified the power by Master Oscillator Power Amplifier (MOPA) to realize a 1011 nm YDF laser with 47.3 dB optical signal–to–noise ratio (OSNR). In terms of RRFL cavity design, a fiber loop mirror and Rayleigh scattering in the HI 1060 Flex fiber provide wideband point feedback and random distributed feedback, respectively. Based on this system, we achieve an RRFL output with 0.4 nm half–maximum full width, 182% slope efficiency, and 41.3 dB OSNR. This work will provide guidance for the application of RRFL in high–energy–density physics research. Full article
(This article belongs to the Special Issue High Power Fiber Laser and Amplifiers)
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10 pages, 1770 KiB  
Article
All-Fiber Narrow-Bandwidth Mode-Locked Laser Based on Polarization-Dependent Helical Long-Period Grating
by Ying Wan, Chen Jiang, Zuxing Zhang, Yaya Mao, Jianxin Ren, Jianxiang Wen and Yunqi Liu
Photonics 2023, 10(7), 842; https://doi.org/10.3390/photonics10070842 - 21 Jul 2023
Cited by 1 | Viewed by 1075
Abstract
As a crucial component of nonlinear polarization rotation (NPR) mode locking, optical fiber gratings offer advantages such as polarization modulation capability, stability, fiber compatibility, and preparation maturity, making them a vital technological foundation for achieving NPR mode locking. Here, a polarization-maintaining fiber helical [...] Read more.
As a crucial component of nonlinear polarization rotation (NPR) mode locking, optical fiber gratings offer advantages such as polarization modulation capability, stability, fiber compatibility, and preparation maturity, making them a vital technological foundation for achieving NPR mode locking. Here, a polarization-maintaining fiber helical long-period grating (PMF-HLPG) was designed and fabricated as a polarizer using the CO2-laser direct-write technique to realize the NPR effect. A homemade fiber Bragg grating (FBG) was also introduced into the laser system to enable a narrow-bandwidth lasing output and wavelength tunability. Based on the PMF-HLPG and FBG mentioned above, an all-fiber mode-locked laser with a spectra bandwidth of 0.15 nm was constructed to generate stable short pulses with a fundamental repetition rate of 12.7122 MHz and a pulse duration of 30.08 ps. In particular, its signal-to-noise ratio is up to 84.5 dB, showing the high stability of the laser. Further, the operating wavelength of the laser can be tuned from 1559.65 nm to 1560.29 nm via heating the FBG while maintaining its mode-locked state with stability. The results indicate that the PMF-HLPG could be used as a polarizer to meet the NPR mechanism for ultrashort pulse laser applications in optical communication, optical sensing, and biomedical imaging. Full article
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8 pages, 4421 KiB  
Communication
Photonic-Assisted Microwave Frequency Measurement Using High Q-Factor Microdisk with High Accuracy
by Mengyao Zhao, Wenyu Wang, Lei Shi, Chicheng Che and Jianji Dong
Photonics 2023, 10(7), 847; https://doi.org/10.3390/photonics10070847 - 21 Jul 2023
Cited by 2 | Viewed by 1045
Abstract
Frequency measurement plays a crucial role in radar, communication, and various applications. The photonic-assisted frequency measurement method offers several advantages, including resistance to electromagnetic interference, broad bandwidth, and low power consumption. Notably, frequency-to-time mapping enables the measurement of various microwave signal types, such [...] Read more.
Frequency measurement plays a crucial role in radar, communication, and various applications. The photonic-assisted frequency measurement method offers several advantages, including resistance to electromagnetic interference, broad bandwidth, and low power consumption. Notably, frequency-to-time mapping enables the measurement of various microwave signal types, such as single-frequency, multiple-frequency, frequency hopping, and chirped signals. However, the accuracy of this method is currently limited due to the absence of resonant devices with high-quality factors, which are essential for achieving higher-precision measurements. In this work, a frequency measurement method based on an ultrahigh-quality-factor microdisk is proposed. By establishing a correlation between the time difference and the frequency to be measured, a reduction in measurement error to below 10 MHz within a frequency measurement range of 3 GHz is realized. Our work introduces a new approach to frequency measurement using optical devices, opening new possibilities in this field. Full article
(This article belongs to the Special Issue Microwave Photonic Techniques)
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10 pages, 4213 KiB  
Communication
Study on Cortisol Sensing Principle Based on Fluorophore and Aptamer Competitive Assay on Polymer Optical Fiber
by Vivek Semwal, Jonas Højgaard, Emil Møller, Ole Bang and Jakob Janting
Photonics 2023, 10(7), 840; https://doi.org/10.3390/photonics10070840 - 20 Jul 2023
Viewed by 1049
Abstract
In this study, we present a polymer optical fiber fluorophore/aptamer competitive assay-based cortisol sensing principle. We developed a low-cost, two-fiber perpendicular design for fluorophore-based sensing with less input light interference and high output signal intensity. The design is suitable for narrow stokes shift [...] Read more.
In this study, we present a polymer optical fiber fluorophore/aptamer competitive assay-based cortisol sensing principle. We developed a low-cost, two-fiber perpendicular design for fluorophore-based sensing with less input light interference and high output signal intensity. The design is suitable for narrow stokes shift fluorophores. We have demonstrated the cortisol sensing principle based on the competition between tagged and normal cortisol. To date, the sensing design has exhibited a slow response, and we identified possible modifications for improvement. Our estimation shows that with miniaturization and a modified sensor assay compartment design, a less than one-hour response time can be achieved. The reported sensing principle and low-cost new design will be helpful for the future development of fluorophore-based fiber optic aptasensors that can potentially be used in a wet environment for online sensing. Full article
(This article belongs to the Special Issue Editorial Board Members’ Collection Series: Photonics Sensors)
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9 pages, 2483 KiB  
Communication
Non-Contact Real-Time Measurement of Refractive Index of Flowing Liquid Based on Optical Coherence Tomography
by Hongbo Fu, Weijian Gao, Zixin Lin, Zhemin Zeng, Wen Shi and Jian Zhang
Photonics 2023, 10(7), 841; https://doi.org/10.3390/photonics10070841 - 20 Jul 2023
Viewed by 907
Abstract
The refractive index (RI) is one of the liquid’s most important physical parameters of liquid. The change of RI usually indicates the change of liquid properties, and this relation leads to broad application. This paper proposes a method based on optical coherence tomography [...] Read more.
The refractive index (RI) is one of the liquid’s most important physical parameters of liquid. The change of RI usually indicates the change of liquid properties, and this relation leads to broad application. This paper proposes a method based on optical coherence tomography (OCT) to measure the RI of liquid in real time. A series of targeted image-processing techniques play a vital role in this method. Benefiting from the advantages of OCT, this method does not require the operators to come into direct contact with liquids. Moreover, the principles and techniques adopted in this method imply that the measuring range of RI of unknown liquids is unlimited. The effectiveness of this method is validated by the experiment measuring the RI of sucrose liquids with gradient concentration. Another experiment thoroughly verifies this method’s feasibility in predicting sucrose concentration based on real-time measured RI. This simple and easy proposed method indicates an immense potential application prospect in the industry and scientific research. Full article
(This article belongs to the Special Issue Advanced Techniques in Biomedical Optical Imaging)
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10 pages, 4068 KiB  
Communication
Effect of Contact Angle on Friction Properties of Superhydrophobic Nickel Surface
by Junyuan Huang, Zhiwei Zhu, Ling Zhang, Dongdong Guo, Zhen Niu and Wei Zhang
Photonics 2023, 10(7), 829; https://doi.org/10.3390/photonics10070829 - 17 Jul 2023
Cited by 2 | Viewed by 1019
Abstract
Due to their excellent performance, superhydrophobic materials have received a lot of attention and research in friction reduction and wear resistance. However, the effect of different contact angles of superhydrophobicity on friction and wear properties has not been thoroughly studied. In this paper, [...] Read more.
Due to their excellent performance, superhydrophobic materials have received a lot of attention and research in friction reduction and wear resistance. However, the effect of different contact angles of superhydrophobicity on friction and wear properties has not been thoroughly studied. In this paper, a nanosecond pulsed laser was used to realize the preparation of a superhydrophobic nickel surface, which indeed reduced the coefficient of friction but also increased the wear volume when compared to the unprocessed surface. As the contact angle of the superhydrophobic nickel surface increased, the coefficient of friction gradually increased, and the wear volume decreased gradually in superhydrophobic nickel surfaces. When the laser energy density was 1 J/cm2, the contact angle was 150.3° and the minimum friction coefficient was 0.4. However, when the laser energy density was 50 J/cm2, the maximum contact angle was 156.4° and the minimum wear volume was 4.23 × 107 μm3. The friction direction also influenced the tribological properties of the superhydrophobic-textured surface. This method makes it possible to process superhydrophobic surfaces with more suitable friction and wear properties. Full article
(This article belongs to the Special Issue Fiber Laser and Their Applications)
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9 pages, 1961 KiB  
Article
A Large Area Wide Bandwidth THz Phase Shifter Plate for High Intensity Field Applications
by Can Koral, Zahra Mazaheri and Antonello Andreone
Photonics 2023, 10(7), 825; https://doi.org/10.3390/photonics10070825 - 15 Jul 2023
Cited by 1 | Viewed by 800
Abstract
We present the design, fabrication, and experimental test of a THz all-dielectric phase shifter plate. The design consists of two wave plate zones coupled in a perpendicular orientation with respect to each other. A large surface area device is realized by an additive [...] Read more.
We present the design, fabrication, and experimental test of a THz all-dielectric phase shifter plate. The design consists of two wave plate zones coupled in a perpendicular orientation with respect to each other. A large surface area device is realized by an additive manufacturing technique using Acrylonitrile Butadiene Styrene (ABS). Its characteristics are analytically evaluated and experimentally measured in the THz band using time domain spectroscopy and imaging routines. The proposed design enables the creation of quasi-ideal phase retardation in between the two planes with good uniformity on a large surface area. We also achieve the flexibility to select the plane of symmetry around the chosen central axes of choice with a sensitive control over the electromagnetic field polarization direction without inducing any temporal shifts in between the wave front components of the traversed beam. Due to its inherent simplicity and robustness, the phase shifter can be easily scaled at higher frequencies and potentially used in several advanced applications, including free-electron laser (FEL) systems where an accurate polarization control of high intensity beams is required. Full article
(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)
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10 pages, 5044 KiB  
Communication
Engineering Substrate-Mediated Localized Surface Plasmons in Gold Nanodiscs
by Anisha Chirumamilla, Maria H. Salazar, Deyong Wang, Peter K. Kristensen, Duncan S. Sutherland, Manohar Chirumamilla, Vladimir N. Popok and Kjeld Pedersen
Photonics 2023, 10(7), 821; https://doi.org/10.3390/photonics10070821 - 14 Jul 2023
Viewed by 865
Abstract
A variety of nanostructures capable of generating strong local electromagnetic fields (hot spots) in interaction with radiation have been under intensive investigation towards plasmonic applications in surface-enhanced Raman scattering (SERS), biosensing, broadband absorbers, thermophotovoltaics, photocatalysis, etc. In many cases, these nanostructures are formed [...] Read more.
A variety of nanostructures capable of generating strong local electromagnetic fields (hot spots) in interaction with radiation have been under intensive investigation towards plasmonic applications in surface-enhanced Raman scattering (SERS), biosensing, broadband absorbers, thermophotovoltaics, photocatalysis, etc. In many cases, these nanostructures are formed on a surface or embedded into a near-surface layer of the dielectric substrate, making some part of the field dissipate into the bulk and not contribute to the desired plasmonic functionality. To reduce such losses, the interface between the metallic nanostructures and the dielectric environment should be engineered. In the current work, Au nanodiscs are fabricated on Si posts of very small diameter (pin-shaped structures), enabling them to decouple the strong optical near fields localized at the nanodiscs from the bulk Si substrate. The Si post diameter is optimized by adjusting the gas flow rates in reactive-ion etching, resulting in a minimum post diameter of 20 nm at the nanodisc interface. The effect of this diameter on the localized surface plasmon resonance of the nanodisc is investigated with linear optical spectroscopic measurements, where a significant spectral blue shift of the resonance band is noticed compared to similar discs formed on bulk Si surface. The experimental results are compared with modelling where a 3-fold increase in the electric field enhancement is demonstrated. The fabricated pin-shaped nanostructures are tested in SERS measurements showing a significant increase in the enhancement factor in the order of 106. Thus, this work suggests a way of engineering 3D morphology to tune the substrate influence on the plasmonic properties of nanostructures and to develop efficient nanofabrication technologies. Full article
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19 pages, 5968 KiB  
Review
Principle and Recent Development in Photonic Time-Stretch Imaging
by Guoqing Wang, Yuan Zhou, Rui Min, E Du and Chao Wang
Photonics 2023, 10(7), 817; https://doi.org/10.3390/photonics10070817 - 13 Jul 2023
Cited by 2 | Viewed by 1355
Abstract
Inspiring development in optical imaging enables great applications in the science and engineering industry, especially in the medical imaging area. Photonic time-stretch imaging is one emerging innovation that attracted a wide range of attention due to its principle of one-to-one-to-one mapping among space-wavelength-time [...] Read more.
Inspiring development in optical imaging enables great applications in the science and engineering industry, especially in the medical imaging area. Photonic time-stretch imaging is one emerging innovation that attracted a wide range of attention due to its principle of one-to-one-to-one mapping among space-wavelength-time using dispersive medium both in spatial and time domains. The ultrafast imaging speed of the photonics time-stretch imaging technique achieves an ultrahigh frame rate of tens of millions of frames per second, which exceeds the traditional imaging methods in several orders of magnitudes. Additionally, regarding ultrafast optical signal processing, it can combine several other optical technologies, such as compressive sensing, nonlinear processing, and deep learning. In this paper, we review the principle and recent development of photonic time-stretch imaging and discuss the future trends. Full article
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11 pages, 2282 KiB  
Article
Two-Stage Link Loss Optimization of Divergent Gaussian Beams for Narrow Field-of-View Receivers in Line-of-Sight Indoor Downlink Optical Wireless Communication (Invited)
by Xinda Yan, Yuzhe Wang, Chao Li, Fan Li, Zizheng Cao and Eduward Tangdiongga
Photonics 2023, 10(7), 815; https://doi.org/10.3390/photonics10070815 - 13 Jul 2023
Cited by 2 | Viewed by 1089
Abstract
The predominant focus of research in high-speed optical wireless communication (OWC) lies in line-of-sight (LOS) links with narrow infrared beams. However, the implementation of precise tracking and steering necessitates delicate active devices, thereby presenting a formidable challenge in establishing a cost-effective wireless transmission. [...] Read more.
The predominant focus of research in high-speed optical wireless communication (OWC) lies in line-of-sight (LOS) links with narrow infrared beams. However, the implementation of precise tracking and steering necessitates delicate active devices, thereby presenting a formidable challenge in establishing a cost-effective wireless transmission. Other than using none-line-of-sight (NLOS) links with excessive link losses and multi-path distortions, the simplification of the tracking and steering process can be alternatively achieved through the utilization of divergent optical beams in LOS. This paper addresses the issue by relaxing the stringent link budget associated with divergent Gaussian-shaped optical beams and narrow field-of-view (FOV) receivers in LOS OWC through the independent optimization of geometrical path loss and fiber coupling loss. More importantly, the geometrical path loss is effectively mitigated by modifying the transverse intensity distribution of the optical beam using manipulations of multi-mode fibers (MMFs) in an all-fiber configuration. In addition, the sufficiently excited higher order modes (HOMs) of MMFs enable a homogenized distribution of received optical powers (ROPs) within the coverage area, which facilitates the mobility of end-users. Comparative analysis against back-to-back links without free-space transmission demonstrates the proposed scheme’s ability to achieve low power penalties. With the minimized link losses, experimental results demonstrate a 10 Gbps error-free (BER < 10−13) LOS OWC downlink transmission at 2.5 m over an angular range of 10° × 10° without using any optical pre-amplifications at a typical PIN receiver. The proposed scheme provides a simple and low-cost solution for high-speed and short-range indoor wireless applications. Full article
(This article belongs to the Special Issue Advances in Visible Light Communication)
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17 pages, 45939 KiB  
Article
Route to Chaos in a Unidirectional Ring of Three Diffusively Coupled Erbium-Doped Fiber Lasers
by José Octavio Esqueda de la Torre, Juan Hugo García-López, Rider Jaimes-Reátegui, Guillermo Huerta-Cuellar, Vicente Aboites and Alexander N. Pisarchik
Photonics 2023, 10(7), 813; https://doi.org/10.3390/photonics10070813 - 12 Jul 2023
Cited by 3 | Viewed by 767
Abstract
We numerically investigate the dynamics of a ring consisting of three unidirectionally coupled Erbium-Doped Fiber Lasers (EDLFs) without external pump modulation. The study focuses on the system behavior as the coupling strength is varied, employing a six-dimensional mathematical model that includes three variables [...] Read more.
We numerically investigate the dynamics of a ring consisting of three unidirectionally coupled Erbium-Doped Fiber Lasers (EDLFs) without external pump modulation. The study focuses on the system behavior as the coupling strength is varied, employing a six-dimensional mathematical model that includes three variables for laser intensities and three variables for population inversions of all lasers. Our primary objective is to understand the system evolution towards chaos from a stable equilibrium in the ring, considering the impact of increasing coupling strength. To analyze the system’s behavior, we employ various techniques such as time series analysis, power spectra, Poincaré sections, bifurcation diagrams, and Lyapunov exponents. During the transition to chaos, the system undergoes a Hopf bifurcation and a series of torus bifurcations. An essential aspect of this study is the exploration of a rotating wave propagating along the ring, where the wave nature (periodic, quasiperiodic, or chaotic) depends on the coupling strength. Additionally, we observe the coexistence of periodic and chaotic orbits within a specific range of the coupling strength. However, for very strong coupling, this bistability disappears, resulting in a monostable system with a single limit cycle. This regime exhibits potential for applications that demand short laser pulses with a substantial increase in peak power, reaching nearly 20 times higher levels compared to the continuous mode when the lasers are uncoupled. This discovery holds particular importance for optical communication systems, especially considering the attenuation optical signals experience when transmitted over long distances. Full article
(This article belongs to the Special Issue Lasers and Dynamic of Systems)
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20 pages, 4453 KiB  
Article
Hierarchical Feature Enhancement Algorithm for Multispectral Infrared Images of Dark and Weak Targets
by Shuai Yang, Zhihui Zou, Yingchao Li, Haodong Shi and Qiang Fu
Photonics 2023, 10(7), 805; https://doi.org/10.3390/photonics10070805 - 11 Jul 2023
Cited by 1 | Viewed by 642
Abstract
A multispectral infrared zoom optical system design and a single-frame hierarchical guided filtering image enhancement algorithm are proposed to address the technical problems of low contrast, blurred edges, and weak signal strength of single-spectrum infrared imaging of faint targets, which are easily drowned [...] Read more.
A multispectral infrared zoom optical system design and a single-frame hierarchical guided filtering image enhancement algorithm are proposed to address the technical problems of low contrast, blurred edges, and weak signal strength of single-spectrum infrared imaging of faint targets, which are easily drowned out by noise. The multispectral infrared zoom optical system, based on the theory of complex achromatic and mechanical positive group compensation, can simultaneously acquire multispectral image information for faint targets. The single-frame hierarchical guided filtering image enhancement algorithm, which extracts the background features and detailed features of faint targets in a hierarchical manner and then weights fusion, effectively enhances the target and suppresses the interference of complex background and noise. Solving multi-frame processing increases data storage and real-time challenges. The experimental verification of the optical system design and image enhancement algorithm proposed in this paper separately verified that the experimental enhancement was significant, with the combined use improving Mean Square Error (MSE) by 14.32, Signal-Noise Ratio (SNR) by 11.64, Peak Signal-to-Noise Ratio (PSNR) by 12.78, and Structural Similarity (SSIM) by 14.0% compared to guided filtering. This research lays the theoretical foundation for the research of infrared detection and tracking technology for clusters of faint targets. Full article
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12 pages, 379 KiB  
Article
Energy Efficiency Optimization for SLIPT-Enabled NOMA System
by Danyang Chen, Qingxuan Wang, Jianping Wang, Zhao Li, Shuai Wu, Rui Hao, Kai Fan, Huimin Lu and Jianli Jin
Photonics 2023, 10(7), 791; https://doi.org/10.3390/photonics10070791 - 09 Jul 2023
Cited by 2 | Viewed by 746
Abstract
For the upcoming sixth generation (6G) networks, the application of simultaneous lightwave information and power transfer (SLIPT) in a non-orthogonal multiple access (NOMA) system is a potential solution to improve energy efficiency (EE). In this paper, we propose a novel SLIPT-enabled NOMA multi-user [...] Read more.
For the upcoming sixth generation (6G) networks, the application of simultaneous lightwave information and power transfer (SLIPT) in a non-orthogonal multiple access (NOMA) system is a potential solution to improve energy efficiency (EE). In this paper, we propose a novel SLIPT-enabled NOMA multi-user system with power splitting (PS) protocol and investigate the effect of system parameters on EE. In addition, to enhance the energy harvesting and information receiving performance of the proposed system, we build up an optimization framework that aims to maximize the EE of the system by jointly optimizing the power allocation of the users and the PS coefficient. We introduce a two-step particle swarm optimization (PSO) algorithm to solve this problem while satisfying the constraints of maximum transmit power, the minimum achievable data rate, and the minimum harvested energy. The numerical results demonstrate the SLIPT-enabled NOMA system using PSO algorithm has significantly improved up to 3.83 ×106 bit/s/J in terms of EE over the traditional orthogonal multiple access (OMA) systems. Full article
(This article belongs to the Special Issue State-of-the-Art Optical Communication in China)
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15 pages, 4542 KiB  
Article
Angle-Dependent Transport Theory-Based Ray Transfer Function for Non-Contact Diffuse Optical Tomographic Imaging
by Stephen Hyunkeol Kim, Jingfei Jia and Andreas H. Hielscher
Photonics 2023, 10(7), 767; https://doi.org/10.3390/photonics10070767 - 03 Jul 2023
Viewed by 693
Abstract
This work presents a generalized angle-dependent ray transfer function that can accurately map the angular and spatial distribution of light intensities on the tissue surface onto a camera image plane in a non-contact camera-based imaging system. The method developed here goes beyond existing [...] Read more.
This work presents a generalized angle-dependent ray transfer function that can accurately map the angular and spatial distribution of light intensities on the tissue surface onto a camera image plane in a non-contact camera-based imaging system. The method developed here goes beyond existing ray transfer models that apply to angle-averaged tomographic data alone. The angle-dependent ray transfer operator was constructed using backward ray tracing based on radiation surface theory. The proposed method was validated using numerical phantoms and experimental data from an actual non-contact imaging system. Full article