Research

18 pages, 5384 KiB

Open AccessArticle

Numerical MCRT Simulation and Inter-Symbol Interference Analysis of Laser Multipath Transmission in Airborne Air-to-Air Channels

by Shiqi Wu, Yanan Li, Jun Wang, Haijun Zhou, Chengmao Tang, Yijie Guan, Fei Guo and Xiaofeng Li

Photonics 2024, 11(3), 280; https://doi.org/10.3390/photonics11030280 - 21 Mar 2024

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Abstract

To address the issues of signal waveform distortion and inter-symbol interference, both of which lead to performance degradation in airborne laser communication due to multipath transmission in airborne air-to-air channels, theoretical analysis and Monte Carlo ray tracing (MCRT) simulation methods were employed. Based [...] Read more.

To address the issues of signal waveform distortion and inter-symbol interference, both of which lead to performance degradation in airborne laser communication due to multipath transmission in airborne air-to-air channels, theoretical analysis and Monte Carlo ray tracing (MCRT) simulation methods were employed. Based on airborne application conditions, this research conducted numerical simulations of the Gaussian beam multipath transmission in an air-to-air channel, with a focus on analyzing the impact of meteorological conditions, communication distance, transmitter power, and initial pulse width on the pulse time spreading characteristics of received optical signals. In addition, an analysis of these parameters’ impact on the inter-symbol interference (ISI) and bit error rate (BER) was conducted. The research findings can serve as a reference for the design of anti-interference techniques in airborne laser communication links. Full article

(This article belongs to the Special Issue Free-Space Optical Communication: Physics and Applications)

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14 pages, 7842 KiB

Open AccessArticle

High-Resolution Image Processing of Probe-Based Confocal Laser Endomicroscopy Based on Multistage Neural Networks and Cross-Channel Attention Module

by Yufei Qiu, Haojie Zhang, Kun Yang, Tong Zhai, Yipeng Lu, Zhongwei Cao and Zhiguo Zhang

Photonics 2024, 11(2), 106; https://doi.org/10.3390/photonics11020106 - 25 Jan 2024

Viewed by 727

Abstract

Probe-based confocal laser endomicroscopy (pCLE) is a subcellular in vivo imaging technique that generates diagnostic images revealing malignant structural modifications in epithelial tissues. In the clinical diagnosis of probe confocal laser endomicroscopy (pCLE), the image background generally has the problems of dynamic blur [...] Read more.

Probe-based confocal laser endomicroscopy (pCLE) is a subcellular in vivo imaging technique that generates diagnostic images revealing malignant structural modifications in epithelial tissues. In the clinical diagnosis of probe confocal laser endomicroscopy (pCLE), the image background generally has the problems of dynamic blur or information loss, which is not conducive to achieving high-resolution and clear pCLE imaging. In recent years, deep learning technology has achieved remarkable results in image deblurring. For the task of recovering high-resolution pCLE images, the current methods still suffer from the following drawbacks: it is difficult to choose a strategy to make CNN converge at a deeper level and mainstream methods cannot handle the complex balance between spatial details and high-level feature information well when reconstructing clear images. In order to solve the problem, we propose a new cross-channel attention, multistage, high-resolution pCLE image deblurring structure. This methodology improves the supervised attention mechanism, enhances the ability of feature extraction and fusion capabilities, and improves the quality of image deblurring by adding cross-channel attention module (CAM) into the multistage neural networks’ architecture. The experimental results show that the average peak signal-to-noise ratio (PSNR) of the proposed model on the dataset is as high as 29.643 dB, and the structural similarity (SSIM) reaches 0.855. This method is superior to the prior algorithms in the visualization of recovered images, and the edge and texture details of the restored pCLE images are clearer. Full article

(This article belongs to the Special Issue Free-Space Optical Communication: Physics and Applications)

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17 pages, 928 KiB

Open AccessArticle

Cooperative Terrestrial–Underwater FSO System: Design and Performance Analysis

by Carmen Álvarez-Roa, María Álvarez-Roa, Thiago R. Raddo, Antonio Jurado-Navas and Miguel Castillo-Vázquez

Photonics 2024, 11(1), 58; https://doi.org/10.3390/photonics11010058 - 05 Jan 2024

Viewed by 973

Abstract

In this paper, we propose, design, and evaluate a new hybrid terrestrial–underwater optical communication link for providing high-speed connectivity between land and underwater systems. A device based on an amplify-and-forward strategy is considered and used for the hybrid optical link. A performance analysis [...] Read more.

In this paper, we propose, design, and evaluate a new hybrid terrestrial–underwater optical communication link for providing high-speed connectivity between land and underwater systems. A device based on an amplify-and-forward strategy is considered and used for the hybrid optical link. A performance analysis of the proposed hybrid system is then carried out, taking into account both the atmospheric and underwater channels and their respective degradation sources. Different networking scenarios and conditions are evaluated. To this end, the channel model of the terrestrial free-space optical (FSO) link is modeled using the Gamma–Gamma distribution, while the underwater optical link is modeled using the Weibull distribution. The former takes into account atmospheric and turbulence attenuation, geometric spread and pointing errors, while the latter takes into account underwater and turbulence attenuation and geometric spread. Accordingly, a new analytical closed-form expression for the bit error rate (BER), which depends on the cumulative distribution function of the holistic hybrid system, is derived. Analytical results show that pointing errors as well as atmospheric and oceanic turbulence seriously degrade the performance of the hybrid system. In addition, ocean turbulence leads to the occurrence of a BER floor in some scenarios. This is the first time that such a network is proposed and modeled under the assumption of critical channel impairments. Full article

(This article belongs to the Special Issue Free-Space Optical Communication: Physics and Applications)

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15 pages, 2682 KiB

Open AccessArticle

Transmission Characteristics and Spatial Coherence of Partially Coherent Light-Emitting Diode Array in the Ocean

by Zhifang Miao, Xiang’e Han, Qiyu Wang, Fang Lu and Qiwei Li

Photonics 2023, 10(9), 1049; https://doi.org/10.3390/photonics10091049 - 15 Sep 2023

Cited by 1 | Viewed by 560

Abstract

Underwater LED light sources are commonly implemented in array configurations with a wide-angle field of view, primarily catering to high-speed communication within a few meters. To increase transmission distance and mitigate oceanic turbulence effects, this paper focuses on the spatial coherence analysis of [...] Read more.

Underwater LED light sources are commonly implemented in array configurations with a wide-angle field of view, primarily catering to high-speed communication within a few meters. To increase transmission distance and mitigate oceanic turbulence effects, this paper focuses on the spatial coherence analysis of narrow-beam partially coherent light-emitting diode (PCLED) arrays, examining their average light intensity distribution, beam width, and spatial coherence during oceanic transmission. Based on the extended Huygens–Fresnel integral, the optical field models and spatial characteristics of the radial PCLED array are derived under oceanic conditions, considering parameters such as water attenuation coefficient, kinetic energy dissipation rate, temperature dissipation rate, temperature-to-salinity ratio, as well as the radial filling factor and the sub-beam spatial coherence length of the light source at different transmission distances. The simulations show that, as the spatial coherence length of the sub-beam decreases from hundreds to a few micrometers, the combining distance of the beam arrays also decreases. This reduction in coherence results in the average light intensity distribution degrading into a Gaussian-like distribution, with a significant five-fold decrease in peak intensity. Furthermore, the width of the array spreads, starting from distances of 7 m and 0 m, respectively. The radial PCLED beam array, with its sub-beam spatial coherence length inside micrometers, possesses inherent characteristics that suppress turbulence effects and has future extensive possibilities in the ocean. Full article

(This article belongs to the Special Issue Free-Space Optical Communication: Physics and Applications)

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8 pages, 2374 KiB

Open AccessCommunication

Quantitative Analysis of the Effect of Atmospheric Turbulence on a Bessel–Gaussian Beam

by Wei Wen

Photonics 2023, 10(8), 932; https://doi.org/10.3390/photonics10080932 - 14 Aug 2023

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Abstract

Suppressing the impact of atmospheric turbulence on laser beam propagation is a bottleneck problem in the application of free space optical communications, with the primary difficulty being the lack of a quantitative description of the effect of turbulence on a laser beam. In [...] Read more.

Suppressing the impact of atmospheric turbulence on laser beam propagation is a bottleneck problem in the application of free space optical communications, with the primary difficulty being the lack of a quantitative description of the effect of turbulence on a laser beam. In this paper, we propose a quantitative description of the effect of turbulence and express a quantitative analysis of the effect of atmospheric turbulence on Bessel–Gaussian beam based on the extended Huygens–Fresnel principle. The results of numerical analysis confirm the validity of the theoretical description. This study also shows that the influence of turbulence is stronger when the annular shape of the Bessel–Gaussian beam is more obvious. The method also provides theoretical guidance for reverse engineering the amplitude and phase distribution of the initial laser beam, thereby effectively reducing the impact of atmospheric turbulence on laser beam propagation. Full article

(This article belongs to the Special Issue Free-Space Optical Communication: Physics and Applications)

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13 pages, 3089 KiB

Open AccessArticle

Temporal Dynamics of an Asymmetrical Dielectric Nanodimer Wrapped with Graphene

by Xinchen Jiang, Yang Huang, Pujuan Ma, Alexander S. Shalin and Lei Gao

Photonics 2023, 10(8), 914; https://doi.org/10.3390/photonics10080914 - 09 Aug 2023

Cited by 1 | Viewed by 827

Abstract

We theoretically and numerically investigate the temporal dynamics of a nanodimer system consisting of a pair of graphene-wrapped dielectric nanospheres with tunable radii. Considering that symmetry breaks on resonant frequencies, we derive the temporal kinetic equations in an asymmetric form by utilizing the [...] Read more.

We theoretically and numerically investigate the temporal dynamics of a nanodimer system consisting of a pair of graphene-wrapped dielectric nanospheres with tunable radii. Considering that symmetry breaks on resonant frequencies, we derive the temporal kinetic equations in an asymmetric form by utilizing the dispersion relation method in dipole limit. The bifurcation diagrams achieved via the analysis on the linear instability and numerical solutions can quantitatively characterize the complex coexistences of stationary and dynamical behaviors in this dimer system, and the asymmetry apparently can increase the number of regimes with the periodic self-oscillation state or chaos. Furthermore, we find that the indefinite switching not only can be triggered among the stationary steady solutions, but it also universally exists among all the possible solutions in a coexistent regime. The switching can be tuned by applying a hard excitation signal with different durations and saturation values. Our results may provide new paths to realize a nonlinear nanophotonic device with tunable dynamical responses or even multi-functionalities. Full article

(This article belongs to the Special Issue Free-Space Optical Communication: Physics and Applications)

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10 pages, 2854 KiB

Open AccessCommunication

Second-Order Statistics of Partially Coherent Beams with Laguerre Non-Uniform Coherence Properties under Turbulence

by Yang Zhao, Zhiwen Yan, Yibo Wang, Liming Liu, Xinlei Zhu, Bohan Guo and Jiayi Yu

Photonics 2023, 10(7), 837; https://doi.org/10.3390/photonics10070837 - 20 Jul 2023

Cited by 1 | Viewed by 773

Abstract

We use the extended Huygens–Fresnel integral to analyze the propagation properties of a class of partially coherent beams with Laguerre non-uniform coherence properties (called Laguerre non-uniformly correlated beams) in free space and in a turbulent atmosphere. We focus on how different initial beam [...] Read more.

We use the extended Huygens–Fresnel integral to analyze the propagation properties of a class of partially coherent beams with Laguerre non-uniform coherence properties (called Laguerre non-uniformly correlated beams) in free space and in a turbulent atmosphere. We focus on how different initial beam orders and coherence lengths affect the propagation behavior of the beams, such as the evolution of intensity, degree of coherence, propagation factor, and beam wander. Our results show that non-uniform coherence properties play a role in resisting the degrading effects of turbulence. Furthermore, adjusting the initial beam parameter of the non-uniform coherence structure, i.e., increasing the beam order and decreasing the coherence, can further improve the turbulence resistance of the beams. Our results have potential applications in free-space optical communications. Full article

(This article belongs to the Special Issue Free-Space Optical Communication: Physics and Applications)

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9 pages, 5206 KiB

Open AccessCommunication

Inverse Energy Flux in Tight Focusing of Vector Vortex Beam

by Ruixiang Chen, Tiegen Song, Yuee Luo, Hehe Li and Xinzhong Li

Photonics 2023, 10(7), 743; https://doi.org/10.3390/photonics10070743 - 28 Jun 2023

Viewed by 694

Abstract

In this paper, we focus on the study of the negative energy flow in the tight focusing of a radially polarized vortex beam. We know that, because of the coupling of the polarization state and the vortex charge, the on-axis energy flow in [...] Read more.

In this paper, we focus on the study of the negative energy flow in the tight focusing of a radially polarized vortex beam. We know that, because of the coupling of the polarization state and the vortex charge, the on-axis energy flow in the focal region can be well modulated by changing the polarization order and the vortex charge of the incident vector beam. This shows that when the polarization order and the vortex charge satisfy the specific relation, the on-axis negative energy flow can be obtained in the focal region. Moreover, the initial phases of two polarization unit vectors also affect the evolution of the on-axis negative energy flow in the tight focusing of the radially polarized beam. The phase difference modulation of the two polarization unit vectors indicates two different modulations of the polarization state. Our work provides a more flexible modulation method for focal shaping and optical modulation. Full article

(This article belongs to the Special Issue Free-Space Optical Communication: Physics and Applications)

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9 pages, 4235 KiB

Open AccessCommunication

Self-Pilot Tone Based Adaptive Threshold RZ-OOK Decision for Free-Space Optical Communications

by Peng-Fei Lv and Yan-Qing Hong

Photonics 2023, 10(7), 714; https://doi.org/10.3390/photonics10070714 - 22 Jun 2023

Cited by 1 | Viewed by 709

Abstract

This paper studies a novel self-pilot tone based adaptive threshold return-to-zero on-off keying (RZ-OOK) decision for free-space optical (FSO) communications. RZ-OOK has the characteristics of impulse series in the spectrum. Therefore, these impulses can be utilized as the pilot tones of the transmitted [...] Read more.

This paper studies a novel self-pilot tone based adaptive threshold return-to-zero on-off keying (RZ-OOK) decision for free-space optical (FSO) communications. RZ-OOK has the characteristics of impulse series in the spectrum. Therefore, these impulses can be utilized as the pilot tones of the transmitted signal to convey the channel state information (CSI) of FSO links. Then, the CSI signal is extracted using a local oscillator (LO) with the frequencies of the impulse series and low pass filter. Finally, the adaptive threshold decision (ATD) is realized by assigning optimized weight factors into the extracted CSI signal. The proposed adaptive threshold RZ-OOK decision was studied in simulation under various pulse durations of RZ-OOK signal and frequencies of LO. Simulation results demonstrated that RZ-OOK with the proposed impulse tone-extracted CSI signal under optimized weight factor performs close to the conventional ATD under precise CSI knowledge. Full article

(This article belongs to the Special Issue Free-Space Optical Communication: Physics and Applications)

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8 pages, 866 KiB

Open AccessCommunication

Scintillation Increase Induced by Focusing (Invited)

by Jia Xu, Yaru Gao, Yangjian Cai and Taco D. Visser

Photonics 2023, 10(5), 604; https://doi.org/10.3390/photonics10050604 - 22 May 2023

Viewed by 806

Abstract

We show that the focusing of a random electromagnetic beam by a lens gives rise to a scintillation index at the geometrical focus that generally differs from that of the incident beam. In the examples we present, focusing produces a significant increase of [...] Read more.

We show that the focusing of a random electromagnetic beam by a lens gives rise to a scintillation index at the geometrical focus that generally differs from that of the incident beam. In the examples we present, focusing produces a significant increase of the index. This observation is of particular relevance for optical communication systems in which scintillation is a major cause of signal degradation. Full article

(This article belongs to the Special Issue Free-Space Optical Communication: Physics and Applications)

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