Photonics

10 pages, 2997 KiB

Open AccessCommunication

Simultaneous Manipulation of the Temporal and Spatial Behaviors of Nanosecond Laser Based on Hybrid Q-Switching

by Haoxi Yang, Yuanji Li, Wenrong Wang, Jinxia Feng and Kuanshou Zhang

Photonics 2023, 10(2), 227; https://doi.org/10.3390/photonics10020227 - 20 Feb 2023

Viewed by 1753

A hybrid Q-switching method based on a special-shaped saturated absorber was proposed for simultaneous manipulation of the temporal and spatial behaviors of a solid-state pulse laser. The temporal–spatial rate equation model of the laser was given and used to optimize the design parameter [...] Read more.

A hybrid Q-switching method based on a special-shaped saturated absorber was proposed for simultaneous manipulation of the temporal and spatial behaviors of a solid-state pulse laser. The temporal–spatial rate equation model of the laser was given and used to optimize the design parameter of the saturated absorber. Best spatial intensity homogenization performance can be expected using an active-passive hybrid Q-switched laser, comprising a Pockels cell and a cylinder Cr:YAG crystal with one end cut as a spherical concave surface. The optimized laser pulse width could be narrowed to 2.39 ns and the laser radial intensity distribution became quasi-super-Gaussian distribution with a radial intensity distribution ratio of 0.91, while that for the Gaussian beam was 0.84. In principle, the laser coherence can be maintained, and the laser spatial intensity distribution can be kept in a long propagation distance. Full article

(This article belongs to the Special Issue Lasers and Dynamic of Systems)

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19 pages, 1619 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Revisiting Experimental Signatures of the Ponderomotive Force

by Bjorn Manuel Hegelich, Lance Labun and Ou Z. Labun

Photonics 2023, 10(2), 226; https://doi.org/10.3390/photonics10020226 - 20 Feb 2023

Cited by 4 | Viewed by 3785

Abstract

The classical theory of single-electron dynamics in focused laser pulses is the foundation of both the relativistic ponderomotive force (RPF), which underlies models of laser-collective-plasma dynamics, and the discovery of novel strong-field radiation dynamics. Despite this bedrock importance, consensus eludes the community as [...] Read more.

The classical theory of single-electron dynamics in focused laser pulses is the foundation of both the relativistic ponderomotive force (RPF), which underlies models of laser-collective-plasma dynamics, and the discovery of novel strong-field radiation dynamics. Despite this bedrock importance, consensus eludes the community as to whether acceleration of single electrons in vacuum has been observed in experimental conditions. We analyze an early experiment on the RPF with respect to several features that were neglected in modeling and that can restore consistency between theory predictions and experimental data. The right or wrong pulse profile function, laser parameters, or initial electron distribution can each make or break the agreement between predictions and data. The laser phase at which the electron’s interaction with the pulse begins has a large effect, explaining why much larger energies are achieved by electrons liberated in the focal region by photoionization from high-Z atoms and by electrons ejected from a plasma mirror. Finally, we compute the difference in a typical electron spectrum arising from fluctuating focal spot size in state-of-the-art ultra-relativistic laser facilities. Our results emphasize the importance of thoroughly characterizing laser parameters in order to achieve quantitatively accurate predictions and the precision required for discovery science. Full article

(This article belongs to the Special Issue Progress in Laser Accelerator and Future Prospects)

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13 pages, 13693 KiB

Open AccessArticle

Field Study of Photovoltaic Systems with Anti-Potential-Induced-Degradation Mechanism: UVF, EL, and Performance Ratio Investigations

by Mahmoud Dhimish and Ghadeer Badran

Photonics 2023, 10(2), 225; https://doi.org/10.3390/photonics10020225 - 19 Feb 2023

Cited by 6 | Viewed by 2134

Abstract

The potential-induced degradation (PID) of photovoltaic (PV) modules is one of the most extreme types of degradation in PV modules, where PID-affected modules can result in an almost 25% power reduction. Understanding how module defects impact PID is key to reducing the issue. [...] Read more.

The potential-induced degradation (PID) of photovoltaic (PV) modules is one of the most extreme types of degradation in PV modules, where PID-affected modules can result in an almost 25% power reduction. Understanding how module defects impact PID is key to reducing the issue. Therefore, this work investigates the impact of an anti-PID inverter on PV modules throughout three years of field operating conditions. We used electroluminescence (EL), ultraviolet fluorescence (UVF), and thermography imaging to explore the varieties of an anti-PID inverter connected to a PV string. It was discovered that a PV string with an anti-PID inverter could improve the output power of the modules by 5.8%. In addition, the performance ratio (PR) was equal to 91.2% and 87.8%, respectively, for PV strings with and without an anti-PID inverter. Full article

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12 pages, 11299 KiB

Open AccessArticle

Single-Pixel Hyperspectral Imaging via an Untrained Convolutional Neural Network

by Chen-Hui Wang, Hong-Ze Li, Shu-Hang Bie, Rui-Bing Lv and Xi-Hao Chen

Photonics 2023, 10(2), 224; https://doi.org/10.3390/photonics10020224 - 19 Feb 2023

Cited by 5 | Viewed by 2466

Abstract

Single-pixel hyperspectral imaging (HSI) has received a lot of attention in recent years due to its advantages of high sensitivity, wide spectral ranges, low cost, and small sizes. In this article, we perform a single-pixel HSI experiment based on an untrained convolutional neural [...] Read more.

Single-pixel hyperspectral imaging (HSI) has received a lot of attention in recent years due to its advantages of high sensitivity, wide spectral ranges, low cost, and small sizes. In this article, we perform a single-pixel HSI experiment based on an untrained convolutional neural network (CNN) at an ultralow sampling rate, where the high-quality retrieved images of the target objects can be achieved by every visible wavelength of a light source from 432 nm to 680 nm. Specifically, we integrate the imaging physical model of single-pixel HSI into a randomly initialized CNN, which allows the images to be reconstructed by relying solely on the interaction between the imaging physical process and the neural network without pre-training the neural network. Full article

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14 pages, 5216 KiB

Open AccessArticle

A Sparse Capture Light-Field Coding Algorithm Based on Target Pixel Matching for a Multi-Projector-Type Light-Field Display System

by Qingyu Meng, Haiyang Yu, Xiaoyu Jiang and Xinzhu Sang

Photonics 2023, 10(2), 223; https://doi.org/10.3390/photonics10020223 - 19 Feb 2023

Cited by 4 | Viewed by 1723

Abstract

The traditional light-field coding algorithm used in a multi-projector-type light-field display system requires sophisticated and complex three-dimensional modeling processes or parallax images obtained through dense capture. Here we propose an algorithm based on target pixel matching, which directly uses parallax images without a [...] Read more.

The traditional light-field coding algorithm used in a multi-projector-type light-field display system requires sophisticated and complex three-dimensional modeling processes or parallax images obtained through dense capture. Here we propose an algorithm based on target pixel matching, which directly uses parallax images without a complex modeling process, and can achieve a more accurate light-field reconstruction effect under sparse capture conditions. For the lack of capture information caused by sparse capture, this algorithm compares the pixel similarity of the captured images of the same object point on different cameras to accurately determine the real capture information of the object point at different depths, which is recorded as the target pixel, and then the target pixel is encoded according to the lighting path to obtain the correct projector image array (PIA). By comparing the quality of PIAs generated by the traditional light-field coding algorithm and the display effect after loading the PIAs into the actual display system, we proved the effectiveness of the algorithm. Full article

(This article belongs to the Special Issue Advances and Application of Imaging on Digital Holography)

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8 pages, 2058 KiB

Open AccessCommunication

Deep Sub–Wavelength Focusing Metalens at Terahertz Frequency

by Mengyu Yang, Xin Shen and Zhongquan Wen

Photonics 2023, 10(2), 222; https://doi.org/10.3390/photonics10020222 - 19 Feb 2023

Cited by 1 | Viewed by 1824

Abstract

With the benefits of non–invasive and low radiation, terahertz radiation has shown great potential in biomedical imaging applications. However, the low spatial resolution of the imaging system significantly affects its application in these fields. Although immersion techniques and super–oscillation theory have achieved considerable [...] Read more.

With the benefits of non–invasive and low radiation, terahertz radiation has shown great potential in biomedical imaging applications. However, the low spatial resolution of the imaging system significantly affects its application in these fields. Although immersion techniques and super–oscillation theory have achieved considerable success in improving the resolution of imaging systems, there are still problems with large focal spot sizes or large sidebands. Herein, a solid immersion lens based on super–oscillation is proposed to reduce the focal spot size when illuminated with circularly polarized light at a wavelength of 118.8 μm. The simulation results show that the lens can compress the full widths at half–maxima down to deep sub–wavelength scales, as small as 0.232 λ. At the same time, the maximum side–lobe ratio was 16.8%, which ensured that the device had a large field of view. The proposed method reveals new ideas in the field of super–resolution imaging. Full article

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17 pages, 2978 KiB

Open AccessReview

Recent Advances and Outlook in Single-Cavity Dual Comb Lasers

by Chenghong Zhang, Fangrui Qu, Peiwen Ou, Haoqi Sun, Shi He and Bo Fu

Photonics 2023, 10(2), 221; https://doi.org/10.3390/photonics10020221 - 18 Feb 2023

Cited by 15 | Viewed by 3830

Abstract

Dual-comb spectroscopy as an emerging tool for spectral analysis has been investigated in a wide range of applications, including absorption spectroscopy, light detection and ranging, and nonlinear spectral imaging. Two mutually coherent combs facilitate high-precision, high-resolution, and broadband spectroscopy. Recently, dual combs generated [...] Read more.

Dual-comb spectroscopy as an emerging tool for spectral analysis has been investigated in a wide range of applications, including absorption spectroscopy, light detection and ranging, and nonlinear spectral imaging. Two mutually coherent combs facilitate high-precision, high-resolution, and broadband spectroscopy. Recently, dual combs generated from a single cavity have become compelling options for dual-comb spectroscopy, enabling huge simplification to measuring systems. Here, we review the progress of single-cavity dual comb lasers in recent years and summarize the distinctive advantages of single-cavity dual combs. First, the principles of optical frequency comb and dual-comb spectroscopy are introduced in time and frequency domains. Then, the implementation techniques and typical applications of single-cavity dual comb lasers are discussed, including directional multiplexing, wavelength multiplexing, polarization multiplexing, and space multiplexing. Finally, an outlook on the development of single-cavity dual combs is presented. Full article

(This article belongs to the Special Issue Mode Locked Fiber Laser)

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15 pages, 1016 KiB

Open AccessArticle

Quantum Gate Generation in Two-Level Open Quantum Systems by Coherent and Incoherent Photons Found with Gradient Search

by Vadim N. Petruhanov and Alexander N. Pechen

Photonics 2023, 10(2), 220; https://doi.org/10.3390/photonics10020220 - 18 Feb 2023

Cited by 9 | Viewed by 1839

Abstract

In this work, we consider an environment formed by incoherent photons as a resource for controlling open quantum systems via an incoherent control. We exploit a coherent control in the Hamiltonian and an incoherent control in the dissipator which induces the time-dependent decoherence [...] Read more.

In this work, we consider an environment formed by incoherent photons as a resource for controlling open quantum systems via an incoherent control. We exploit a coherent control in the Hamiltonian and an incoherent control in the dissipator which induces the time-dependent decoherence rates

γ_{k} (t)

(via time-dependent spectral density of incoherent photons) for generation of single-qubit gates for a two-level open quantum system which evolves according to the Gorini–Kossakowski–Sudarshan–Lindblad (GKSL) master equation with time-dependent coefficients determined by these coherent and incoherent controls. The control problem is formulated as minimization of the objective functional, which is the sum of Hilbert-Schmidt norms between four fixed basis states evolved under the GKSL master equation with controls and the same four states evolved under the ideal gate transformation. The exact expression for the gradient of the objective functional with respect to piecewise constant controls is obtained. Subsequent optimization is performed using a gradient type algorithm with an adaptive step size that leads to oscillating behaviour of the gradient norm vs. iterations. Optimal trajectories in the Bloch ball for various initial states are computed. A relation of quantum gate generation with optimization on complex Stiefel manifolds is discussed. We develop methodology and apply it here for unitary gates as a testing example. The next step is to apply the method for generation of non-unitary processes and to multi-level quantum systems. Full article

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Graphical abstract

10 pages, 9545 KiB

Open AccessCommunication

Sub-40 GHz Broadband Polarization Chaos Generation Using Mutually Coupled Free-Running VCSELs

by Haofan Bian, Xiaomai Zhang, Pu Li, Zhiwei Jia, Li Ma, Bingjie Xu, Keith Alan Shore, Yuwen Qin and Yuncai Wang

Photonics 2023, 10(2), 219; https://doi.org/10.3390/photonics10020219 - 17 Feb 2023

Cited by 4 | Viewed by 1758

Abstract

We propose a simple method to generate broadband polarization chaos using two mutually coupled free-running vertical-cavity surface-emitting lasers (VCSELs). Specifically, we quantitatively investigate the effect of critical external parameters (bias current, frequency detuning and coupling coefficient) on the polarization chaos bandwidth in the [...] Read more.

We propose a simple method to generate broadband polarization chaos using two mutually coupled free-running vertical-cavity surface-emitting lasers (VCSELs). Specifically, we quantitatively investigate the effect of critical external parameters (bias current, frequency detuning and coupling coefficient) on the polarization chaos bandwidth in the scenarios of parallel injection and orthogonal injection, and reveal the physical mechanism of bandwidth enhancement in two scenarios. Final simulation results show that the bandwidth of chaotic signals obtained from parallel and orthogonal injection can reach 35.15 GHz and 32.96 GHz, respectively. Full article

(This article belongs to the Special Issue Advancements in Semiconductor Lasers)

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9 pages, 1678 KiB

Open AccessCommunication

Generalized Poincaré Beams in Tight Focus

by Victor V. Kotlyar, Alexey A. Kovalev and Alexey M. Telegin

Photonics 2023, 10(2), 218; https://doi.org/10.3390/photonics10020218 - 16 Feb 2023

Cited by 3 | Viewed by 1531

Abstract

We study the tight focus of generalized (hybrid) Poincaré beams. A conventional Poincaré beam is a coaxial superposition of two optical vortices, one with left circular polarization and a topological charge (TC) of m, while the other has a right circular polarization [...] Read more.

We study the tight focus of generalized (hybrid) Poincaré beams. A conventional Poincaré beam is a coaxial superposition of two optical vortices, one with left circular polarization and a topological charge (TC) of m, while the other has a right circular polarization and a TC of −m. The generalized Poincaré beams are also composed of two optical vortices, but their TCs are different, for instance, p and q. Here, we theoretically investigate the generalized Poincaré beams with the TCs p = m + 1 and q = −m in tight focus. In this case, both transverse components of the strength vector of the initial electric field have a topological charge of 1/2, and the beam itself is a cylindrical vector beam of fractional order m + 1/2. Analytical expressions are derived for the components of the strength vectors of the electric and magnetic field at the focus as well as for the intensity distribution, the longitudinal component of the spin angular momentum (SAM), and for the components of the Poynting vector (energy flow density). We show that the intensity at the focus has 2m − 1 local maxima residing evenly in a certain circle radius with the center on the optical axis. We also demonstrate that the radial spin and orbital Hall effects occur at the focus, i.e., the longitudinal SAM component has different signs in the circles of different radii, and the azimuthal component of the transverse Poynting vector also has different signs. Full article

(This article belongs to the Special Issue Lasers and Dynamic of Systems)

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28 pages, 26799 KiB

Open AccessArticle

Thermal Deformation Measurement of Aerospace Honeycomb Panel Based on Fusion of 3D-Digital Image Correlation and Finite Element Method

by Linghui Yang, Zezhi Fan, Ke Wang, Hui Sun, Shuotao Hu and Jigui Zhu

Photonics 2023, 10(2), 217; https://doi.org/10.3390/photonics10020217 - 16 Feb 2023

Cited by 1 | Viewed by 2408

Abstract

Aiming to solve the problem of the high-precision deformation measurement of large-scale satellite structures in manufacturing and testing environments, this paper proposes a measurement method based on the idea of fusing actual measurements with finite element analysis. The digital image correlation (DIC) method [...] Read more.

Aiming to solve the problem of the high-precision deformation measurement of large-scale satellite structures in manufacturing and testing environments, this paper proposes a measurement method based on the idea of fusing actual measurements with finite element analysis. The digital image correlation (DIC) method is used to obtain the high-precision deformation of the honeycomb panel, and the finite element method (FEM) model is introduced to remove the limitations of existing pure measurement methods. Data fusion based on a machine learning neural network is proposed to fuse high-precision deformation and physical information such as temperature to conduct multi-level training on FEM parameters. Through an interpolation of the analysis and calculation results after training, not only can the accuracy of the finite element be improved, but difference and extrapolation of the digital image correlation measurement results can be performed. In the experiments, the satellite on-orbit temperature data are substituted into the modified finite element model. The testing results shows that the prediction accuracy of the model under different temperature loads can be controlled within 10 μm under an 840 mm × 640 mm scale. A high predictive accuracy can be achieved for the revised model for the complete deformation of large structural sections. Full article

(This article belongs to the Special Issue Optical Measurement Systems)

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13 pages, 5316 KiB

Open AccessCommunication

High Extinction Ratio 4 × 2 Encoder Based on Electro-Optical Graphene Plasma Structure

by Aijun Zhu, Pengcheng Bu, Cong Hu, Junhao Niu and Rabi Mahapatra

Photonics 2023, 10(2), 216; https://doi.org/10.3390/photonics10020216 - 16 Feb 2023

Cited by 6 | Viewed by 1706

Abstract

In this paper, a plasmonic electro-optical encoder based on graphene at THz frequency is proposed. The surface plasmon polaritons (SPPs) in the graphene–insulator–metal structure are excited by an incident TM wave with a wavelength of 9.3 μm. Graphene plasma waveguides have extremely high [...] Read more.

In this paper, a plasmonic electro-optical encoder based on graphene at THz frequency is proposed. The surface plasmon polaritons (SPPs) in the graphene–insulator–metal structure are excited by an incident TM wave with a wavelength of 9.3 μm. Graphene plasma waveguides have extremely high confinement, relatively low losses, and high tunability. The switching mechanism is based on the application of an external voltage to locally change the chemical potential of the graphene for encoding. Setting the chemical potential to 1 eV allows SPPs to propagate while lowering the chemical potential to 0.1 eV prevents the SPPs from propagating. A 4 × 2 encoder with a minimum encoding extinction ratio (ER) of 37 dB, a maximum modulation depth (MD) of 99.99%, and a structure area of 0.8 μm² is proposed based on the design rules and simulations using the finite-difference time-domain (FDTD) method. In terms of the obtained results, the proposed structure can be used in optical integrated circuits. Full article

(This article belongs to the Special Issue Integrated Plasmonic Devices)

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12 pages, 2284 KiB

Open AccessArticle

Chromatic Dispersion Measurements of Single-Mode Fibers, Polarization-Maintaining Fibers, and Few-Mode Fibers Using a Frequency Domain Method

by Xin Chen, Jason E. Hurley, Jeffery S. Stone and Ming-Jun Li

Photonics 2023, 10(2), 215; https://doi.org/10.3390/photonics10020215 - 16 Feb 2023

Cited by 4 | Viewed by 3690

Abstract

Chromatic dispersion is an important fiber attribute affecting transmission performance over optical fibers. Various chromatic dispersion measurement methods have been developed primarily for single-mode fibers. In the literature, measurement techniques were also developed to characterize few-mode fibers and multi-mode fibers. These methods are [...] Read more.

Chromatic dispersion is an important fiber attribute affecting transmission performance over optical fibers. Various chromatic dispersion measurement methods have been developed primarily for single-mode fibers. In the literature, measurement techniques were also developed to characterize few-mode fibers and multi-mode fibers. These methods are often subject to some limitations. In this paper, a simple and robust measurement method for chromatic dispersion measurement of single-mode fibers, polarization--maintaining fibers, and few-mode fibers is presented using a frequency domain instrument and a vector network analyzer. The method is applied to all three types of fibers through one measurement methodology uniformly. Using a vector network analyzer, the measurement instrument obtains the complex transfer function of fiber transmission. The inverse Fourier transform of the measured complex transfer function is used to determine the group delays for each mode of the fiber. Although the sampling is highly under-sampled for the whole fiber link, through proper treatment of the data, we can de-alias the signals and obtain accurate values of the group delays of each mode. By measuring the group delays over different wavelengths, the data can yield the chromatic dispersion of each mode over the wavelength window. Full article

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

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9 pages, 2983 KiB

Open AccessCommunication

Generation of Vector Vortex Beams Based on the Optical Integration of Dynamic Phase and Geometric Phase

by Kuiming Zeng, Shanshan He, Xianping Wang and Hailu Luo

Photonics 2023, 10(2), 214; https://doi.org/10.3390/photonics10020214 - 15 Feb 2023

Cited by 4 | Viewed by 2266

Abstract

The phase and polarization of electromagnetic waves can be conveniently manipulated by the dynamic phase and geometric phase elements. Here, we propose a compact optical integration of dynamic phase and geometric phase to generate arbitrary vector vortex beams on a hybrid-order Poincaré sphere. [...] Read more.

The phase and polarization of electromagnetic waves can be conveniently manipulated by the dynamic phase and geometric phase elements. Here, we propose a compact optical integration of dynamic phase and geometric phase to generate arbitrary vector vortex beams on a hybrid-order Poincaré sphere. Two different technologies have been applied to integrate dynamic and geometric phase elements into a single glass plate to modulate the phase and polarization of light simultaneously. A spiral phase structure is made on one side of a glass substrate with optical lithography and a geometric phase metasurface structure is designed on the other side by femtosecond laser writing. The vector polarization is realized by the metasurface structure, while the vortex phase is generated by the spiral phase plate. Therefore, any desirable vector vortex beams on the hybrid-order Poincaré sphere can be generated. We believe that our scheme may have potential applications in future integrated optical devices for the generation of vector vortex beams due to its the high transmission efficiency and conversion efficiency. Full article

(This article belongs to the Special Issue Vortex Beams: Fundamentals and Applications)

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11 pages, 2870 KiB

Open AccessCommunication

Key Space Enhancement in Chaotic Secure Communication Utilizing Monolithically Integrated Multi-Section Semiconductor Lasers

by Feifan Zhang, Yuncai Wang, Yuehui Sun, Junpei Xu, Pu Li, Anbang Wang and Yuwen Qin

Photonics 2023, 10(2), 213; https://doi.org/10.3390/photonics10020213 - 15 Feb 2023

Cited by 2 | Viewed by 1748

Abstract

Chaotic secure communication schemes encounter a conflict of key space enhancement between the consistency and complexity of chaotic transceivers. In this paper, we propose a monolithically integrated multi-section semiconductor laser (MIMSL), used as a compact chaotic transceiver with an enhanced key space. The [...] Read more.

Chaotic secure communication schemes encounter a conflict of key space enhancement between the consistency and complexity of chaotic transceivers. In this paper, we propose a monolithically integrated multi-section semiconductor laser (MIMSL), used as a compact chaotic transceiver with an enhanced key space. The MIMSL consists of a distributed feedback (DFB) laser section, a semiconductor optical amplifier (SOA) section, two phase (P) sections and a passive optical waveguide. We simulate the dynamics of the MIMSL by applying the time-dependent coupled-wave equations for traveling-wave optical fields. Further, we numerically demonstrate a security enhancement of the unidirectional chaotic communication scheme using the MIMSL transceivers with independent high-speed modulation in the phase sections of the MIMSL. The security of our scheme depends not only on the difficulty of identifying the MIMSL structural parameters and the bias current of each section, but also on the phase shifts in two phase sections providing the additional dimension of security key space. Final simulation results show that a total of 2⁴⁸ key spaces can be achieved with a data rate of 2.5 Gb/s and an injection strength of 0.36. Full article

(This article belongs to the Special Issue Advancements in Semiconductor Lasers)

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10 pages, 3979 KiB

Open AccessCommunication

Subnanometer-Resolution Nanoparticle Sensing through the Strong Coupling between Surface Plasmon Polariton Whispering Gallery Resonances and Localized Surface Plasmon

by Han Yang and Yue-Gang Chen

Photonics 2023, 10(2), 212; https://doi.org/10.3390/photonics10020212 - 15 Feb 2023

Viewed by 1666

Abstract

High-resolution nanoparticle sensing is very important, and many schemes have been proposed to achieve this goal. Circular nanocavities in which surface plasmon polariton (SPP) whispering gallery mode (WGM) resonances were excited were designed to sense particles of ultra-small size and with high resolution. [...] Read more.

High-resolution nanoparticle sensing is very important, and many schemes have been proposed to achieve this goal. Circular nanocavities in which surface plasmon polariton (SPP) whispering gallery mode (WGM) resonances were excited were designed to sense particles of ultra-small size and with high resolution. Localized surface plasmon resonances (LSPRs) were excited when a metal particle was set in the circular cavity. The SPP WGM split into symmetric mode (SM) and antisymmetric mode (ASM) due to the LSPRs scattering into the SPPs. The strong coupling between SM resonance and LSPRs generated positive and opposite modes, which were sensitive to the variation in nanoparticle size and position. Even a small nanometer-sized metal particle introduced LSPRs and produced mode splitting. The WGM mode splitting induced by LSPRs reduced the sensing limit. The simulation results show that 1 nm changes in nanoparticle radius and position led to SM 11.8 nm and 10.2 nm wavelength shifts, respectively. This means that variations of 0.09 nm in size and 0.1 nm in position can be sensed with a 1 nm spectral resolution. The strong coupling between SPP WGM and LSPRs can be applied to sense at a subnanometer resolution. Full article

(This article belongs to the Special Issue Strong Light Fields Coupled with Plasmonic Nano-Structures)

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9 pages, 2730 KiB

Open AccessCommunication

Radiophotoluminescence Phenomenon of CaF₂ Ceramics Doped with Li

by Takumi Kato, Daisuke Nakauchi, Noriaki Kawaguchi and Takayuki Yanagida

Photonics 2023, 10(2), 211; https://doi.org/10.3390/photonics10020211 - 15 Feb 2023

Cited by 1 | Viewed by 1335

Abstract

The objective of this study is to examine the impact of Li-doping on the radiophotoluminescent (RPL) properties of CaF₂. Before ionizing irradiation, Li-doped CaF₂ exhibited no photoluminescence (PL) under excitation in the range of 250–700 nm. After ionizing irradiation, Li-doped [...] Read more.

The objective of this study is to examine the impact of Li-doping on the radiophotoluminescent (RPL) properties of CaF₂. Before ionizing irradiation, Li-doped CaF₂ exhibited no photoluminescence (PL) under excitation in the range of 250–700 nm. After ionizing irradiation, Li-doped CaF₂ displayed PL at 800 nm when excited at 390 and 610 nm. The decay time constant for this luminescence was determined to be 20 ns, which suggests that it is attributed to (F₂⁺)_A centers. All the Li-doped CaF₂ showed higher PL intensity than the non-doped CaF₂ did, with the highest intensity observed in the 0.5% Li-doped CaF₂. The 0.5% Li-doped CaF₂ was also found to have a minimum measurable dose of 14 μGy as an RPL dosimeter, and the RPL response monotonically increased to 10 Gy. As for radiation-induced luminescence other than RPL, the scintillation peak and the thermally stimulated luminescence (TSL) glow peak were mainly observed at 300 nm and 140 °C, respectively. Full article

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19 pages, 4958 KiB

Open AccessArticle

A Data Processing and Distribution System Based on Apache NiFi

by Karol Wnęk and Piotr Boryło

Photonics 2023, 10(2), 210; https://doi.org/10.3390/photonics10020210 - 15 Feb 2023

Cited by 2 | Viewed by 3373

Abstract

The monitoring of physical and logical networks is essential for the high availability of 5G/6G networks. This could become a challenge in 5G/6G deployments due to the heterogeneity of the optical layer. It uses equipment from multiple vendors, and, as a result, the [...] Read more.

The monitoring of physical and logical networks is essential for the high availability of 5G/6G networks. This could become a challenge in 5G/6G deployments due to the heterogeneity of the optical layer. It uses equipment from multiple vendors, and, as a result, the protocols and methods for gathering monitoring data usually differ. Simultaneously, to effectively support 5G/6G networks, the optical infrastructure should also be dense and ensure high throughput. Thus, vast numbers of photonic transceivers operating at up to 400 Gbps are needed to interconnect network components. In demanding optical solutions for 5G and beyond, enterprise-class equipment will be used—for example, high-capacity and high-density optical switches based on the SONiC distribution. These emerging devices produce vast amounts of data on the operational parameters of each optical transceiver, which should be effectively collected, processed, and analyzed. The aforementioned circumstances may lead to the necessity of using multiple independent monitoring systems dedicated to specific optical hardware. Apache NiFi can be used to address these potential issues. Its high configurability enables the aggregation of unstandardized log files collected from heterogenous devices. Furthermore, it is possible to configure Apache NiFi to absorb huge data streams about each of the thousands of transceivers comprising high-density optical switches. In this way, data can be preprocessed by using Apache NiFi and later uploaded to a dedicated system. In this paper, we focus on presenting the tool, its capabilities, and how it scales horizontally. The proven scalability is essential for making it usable in optical networks that support 5G/6G networks. Finally, we propose a unique optimization process that can greatly improve the performance and make Apache NiFi suitable for high-throughput and high-density photonic devices and optical networks. We also present some insider information on real-life implementations of Apache NiFi in commercial 5G networks that fully rely on optical networks. Full article

(This article belongs to the Special Issue Optical Technologies Supporting 5G/6G Mobile Networks)

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10 pages, 4229 KiB

Open AccessCommunication

Sub-kHz Narrow-Linewidth Single-Longitudinal-Mode Thulium-Doped Fiber Laser Utilizing Triple-Coupler Ring-Based Compound-Cavity Filter

by Biao Guan, Fengping Yan, Dandan Yang, Qi Qin, Ting Li, Chenhao Yu, Xiangdong Wang, Kazuo Kumamoto and Yuping Suo

Photonics 2023, 10(2), 209; https://doi.org/10.3390/photonics10020209 - 14 Feb 2023

Cited by 9 | Viewed by 1703

Abstract

This paper proposes and demonstrates a single-longitudinal-mode thulium-doped fiber laser using a passive triple-coupler ring-based compound-cavity filter (TCR-CC) and a uniform fiber Bragg grating. For the first time, the TCR-CC filter is used to select a single mode from dense longitudinal modes. Experimental [...] Read more.

This paper proposes and demonstrates a single-longitudinal-mode thulium-doped fiber laser using a passive triple-coupler ring-based compound-cavity filter (TCR-CC) and a uniform fiber Bragg grating. For the first time, the TCR-CC filter is used to select a single mode from dense longitudinal modes. Experimental results show that laser in the wavelength of 1941.28 nm can maintain exceptional stability with an optical signal-to-noise ratio of 74.1 dB. The measured maximum wavelength drift and power fluctuation are 0.01 nm and 0.45 dB, respectively. Meanwhile, the measured linewidth of the laser is 910 Hz, and the relative intensity noise is below −125.82 dB/Hz above 2 MHz frequencies. Full article

(This article belongs to the Special Issue Single Frequency Fiber Lasers and Their Applications)

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12 pages, 8720 KiB

Open AccessArticle

Silica-Titania Integrated Photonics Platform-Based 1 × 2 Demultiplexer Utilizing Two Serially Cascaded Racetrack Microrings for 1310 nm and 1550 nm Telecommunication Wavelengths

by Muhammad A. Butt, Muhammad Shahbaz, Łukasz Kozłowski, Andrzej Kaźmierczak and Ryszard Piramidowicz

Photonics 2023, 10(2), 208; https://doi.org/10.3390/photonics10020208 - 14 Feb 2023

Cited by 7 | Viewed by 1659

Abstract

Herein, a numerical analysis of a 1 × 2 demultiplexer based on a silica-titania integrated photonics platform is conducted via the finite element method. The structure is composed of two coupled racetrack microrings (RTMRs) and a subwavelength grating (SWG) structure for the demultiplexing [...] Read more.

Herein, a numerical analysis of a 1 × 2 demultiplexer based on a silica-titania integrated photonics platform is conducted via the finite element method. The structure is composed of two coupled racetrack microrings (RTMRs) and a subwavelength grating (SWG) structure for the demultiplexing of 1310 nm and 1550 nm telecommunication wavelengths. The material platform selected for this design is highly attractive due to its refined optical, physical, and chemical properties. Moreover, silica-titania sol-gel thin-films can be deposited on glass substrates with the dip-coating method. The proposed device has a small footprint of 84 × 125 μm² and offers crosstalk as low as ~−6.6 dB and ~−9.04 dB for 1550 nm and 1310 nm, respectively. We are convinced that this study promotes the use of the silica-titania platform for the development of low-cost on-chip optical communication devices for signal multiplexing and demultiplexing. Full article

(This article belongs to the Special Issue Editorial Board Members’ Collection Series: Photonics Sensors)

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14 pages, 3262 KiB

Open AccessArticle

Estimation Method Based on Extended Kalman Filter for Uncertain Phase Shifts in Phase-Measuring Profilometry

by Xin Lai, Yueyang Li, Xunren Li, Zhengdong Chen and Qican Zhang

Photonics 2023, 10(2), 207; https://doi.org/10.3390/photonics10020207 - 14 Feb 2023

Cited by 1 | Viewed by 1562

Abstract

Phase-measuring profilometry (PMP) is increasingly applied in high-accuracy three-dimensional shape measurement. However, various factors may result in the uncertainty of phase shift values in the PMP system, and phase errors induced by actual phase shift often bring about the reconstruction failure of a [...] Read more.

Phase-measuring profilometry (PMP) is increasingly applied in high-accuracy three-dimensional shape measurement. However, various factors may result in the uncertainty of phase shift values in the PMP system, and phase errors induced by actual phase shift often bring about the reconstruction failure of a measured object. A quadratic phase estimation method using the extended Kalman filter is proposed to remove the phase error introduced by uncertain phase shift. After eliminating the background and fringe modulation, the state estimation is employed to evaluate the quadratic phase coefficients in a selected mask window, and the phase shifts of adjacent fringe patterns can be estimated to compute the unwrapping phase. This paper presents a novel method for improving the accuracy of the PMP system influenced by phase shift errors, and the proposed region-wise method significantly enhances the reconstruction quality and efficiency. Experimental results show that the proposed algorithm effectively evaluates the actual phase shift and directly compensates the phase error, and has the advantages of high speed, high accuracy, and robustness. Full article

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10 pages, 3349 KiB

Open AccessCommunication

Movable Optical Frequency Ruler with Optical Activity

by Cheng-Mu Tsai, Jun-Hong Weng, Kuo-Wei Lin and Pin Han

Photonics 2023, 10(2), 206; https://doi.org/10.3390/photonics10020206 - 14 Feb 2023

Cited by 1 | Viewed by 1266

Abstract

Optical frequency rulers (OFR) are suggested for use as optical wavelength or frequency references for spectra manipulation or unknown wavelength measurement. In the past, complicated mechanisms that are not easy to utilize were used to make OFR, such as a double-slits with a [...] Read more.

Optical frequency rulers (OFR) are suggested for use as optical wavelength or frequency references for spectra manipulation or unknown wavelength measurement. In the past, complicated mechanisms that are not easy to utilize were used to make OFR, such as a double-slits with a high-speed fluid or an external circuit to control the liquid crystal birefringence. This work introduces a simple structure to produce an OFR, which should be easier to implement. It utilizes quartz block optical activity and two polarizers. Because of the strong wavelength dependence of the rotatory power, each wavelength component in the spectrum experiences a different amount of polarization angle rotation. Some components whose angles are perpendicular to that of the analyzer are filtered out and naturally form the OFR’s ticks. The numerical results show that those spectral ticks can be moved to higher or lower wavelengths by rotating the analyzer’s angle. This scheme provides another possibility for creating movable OFR with the merit of easy usage. Full article

(This article belongs to the Special Issue Women’s Special Issue Series: Photonics)

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26 pages, 16501 KiB

Open AccessReview

Working Mechanism and Progress of Electromagnetic Metamaterial Perfect Absorber

by Xiajun Liu, Feng Xia, Mei Wang, Jian Liang and Maojin Yun

Photonics 2023, 10(2), 205; https://doi.org/10.3390/photonics10020205 - 14 Feb 2023

Cited by 12 | Viewed by 4627

Abstract

Electromagnetic metamaterials are artificial subwavelength composites with periodic structures, which can interact strongly with the incident light to achieve effective control of the light field. Metamaterial absorbers can achieve nearly 100% perfect absorption of incident light at a specific frequency, so they are [...] Read more.

Electromagnetic metamaterials are artificial subwavelength composites with periodic structures, which can interact strongly with the incident light to achieve effective control of the light field. Metamaterial absorbers can achieve nearly 100% perfect absorption of incident light at a specific frequency, so they are widely used in sensors, optical switches, communication, and other fields. Based on the development history of metamaterials, this paper discusses the research background and significance of metamaterial perfect absorbers. Some perfect absorption mechanisms, such as impedance matching and coherent perfect absorption, are discussed. According to the functional division, the narrowband, dual frequency, multi-frequency, broadband, and tunable metamaterial perfect absorbers are briefly described. Full article

(This article belongs to the Special Issue Advanced Polarimetry and Polarimetric Imaging)

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15 pages, 5078 KiB

Open AccessArticle

High-Performance Polarization Imaging Reconstruction in Scattering System under Natural Light Conditions with an Improved U-Net

by Bing Lin, Xueqiang Fan, Dekui Li and Zhongyi Guo

Photonics 2023, 10(2), 204; https://doi.org/10.3390/photonics10020204 - 13 Feb 2023

Cited by 17 | Viewed by 2022

Abstract

Imaging through scattering media faces great challenges. Object information will be seriously degraded by scattering media, and the final imaging quality will be poor. In order to improve imaging quality, we propose using the transmitting characteristics of an object’s polarization information, to achieve [...] Read more.

Imaging through scattering media faces great challenges. Object information will be seriously degraded by scattering media, and the final imaging quality will be poor. In order to improve imaging quality, we propose using the transmitting characteristics of an object’s polarization information, to achieve imaging through scattering media under natural light using an improved U-net. In this paper, we choose ground glass as the scattering medium and capture the polarization images of targets through the scattering medium by a polarization camera. Experimental results show that the proposed model can reconstruct target information from highly damaged images, and for the same material object, the trained network model has a superior generalization without considering its structural shapes. Meanwhile, we have also investigated the effect of the distance between the target and the ground glass on the reconstructing performance, in which, and although the mismatch distance between the training set and the testing sample expands to 1 cm, the modified U-net can also efficaciously reconstruct the targets. Full article

(This article belongs to the Special Issue Advanced Polarimetry and Polarimetric Imaging)

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9 pages, 2760 KiB

Open AccessCommunication

Yb-Doped Mode-Locked Fiber Laser Based on an All-Fiber Interferometer Filter

by Fan Yang, Liqiang Zhang, Chenglin Bai, Shijie Ren, Zhen Tian, Yicun Yao and Minghong Wang

Photonics 2023, 10(2), 203; https://doi.org/10.3390/photonics10020203 - 13 Feb 2023

Cited by 2 | Viewed by 1771

Abstract

An interference filter is designed by fusing a segment of multi-core fiber (MCF) between two segments of multimode fibers (MMFs), which is then spliced between two segments of single mode fibers (SMFs). The light is split into the cladding and different cores of [...] Read more.

An interference filter is designed by fusing a segment of multi-core fiber (MCF) between two segments of multimode fibers (MMFs), which is then spliced between two segments of single mode fibers (SMFs). The light is split into the cladding and different cores of the MCF through the first segment of MMF, which is then coupled back into the core of SMF by the second segment of MMF. When the lengths of MCF are selected to be 4 mm and 10 mm, the 3 dB bandwidths of the filters around 1060 nm are 8.40 nm and 4.84 nm, respectively. Applying these filters in an Yb-doped fiber laser mode-locked by nonlinear polarization rotation, stable pulses have been obtained. Compared with the reported interference filters, the filter proposed in this paper has the advantages of simple fabrication process, compact structure and high environmental stability. Full article

(This article belongs to the Special Issue Mode Locked Fiber Laser)

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9 pages, 4301 KiB

Open AccessCommunication

Topology Optimization of Low-Loss Z-Bend 2D Photonic Crystal Waveguide

by Gang Liu, Fei Wang, Yongpan Gao, Baonan Jia, Xiaoning Guan, Pengfei Lu and Haizhi Song

Photonics 2023, 10(2), 202; https://doi.org/10.3390/photonics10020202 - 13 Feb 2023

Cited by 5 | Viewed by 1973

Abstract

In this article, we design a low-loss, high-bandwidth Z-bend photonic silicon crystal waveguide bending in a triangular lattice through topology optimization. Based on the topological optimization method, we change the relative position of air holes in the global scope to maximize the transmittance [...] Read more.

In this article, we design a low-loss, high-bandwidth Z-bend photonic silicon crystal waveguide bending in a triangular lattice through topology optimization. Based on the topological optimization method, we change the relative position of air holes in the global scope to maximize the transmittance and bandwidth of the waveguide. The simulation results indicate that the transmission characteristics can be effectively improved with our method. After the optimization, the loss of the waveguide can be reduced to −5 dB and the bandwidth can increase to 160 nm. Our research has great significance for further optimizing the propagation of light in photonic crystals. Full article

(This article belongs to the Section Quantum Photonics and Technologies)

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9 pages, 3038 KiB

Open AccessCommunication

Simulation Study of Phase-Driven Multichannel Nano-Optical Conveyor Belt Using Rectangular Gratings

by Chunyan Bai, Xiangcai Ma, Qian Cao, Saima Kanwal and Peizhen Qiu

Photonics 2023, 10(2), 201; https://doi.org/10.3390/photonics10020201 - 13 Feb 2023

Viewed by 1613

Abstract

A nano-optical conveyor belt is a unique type of near-field optical tweezer, capable not only of capturing nanoparticles, but also transporting them. In this study, we propose a multichannel nano-optical conveyer, based on a simple rectangular distributed grating array. The design was optimized [...] Read more.

A nano-optical conveyor belt is a unique type of near-field optical tweezer, capable not only of capturing nanoparticles, but also transporting them. In this study, we propose a multichannel nano-optical conveyer, based on a simple rectangular distributed grating array. The design was optimized by varying the number of slits in the gratings, and particle transport was achieved by adjusting the phase difference of the excitation beams. Simulation and calculation results indicate that multiple optical traps and parallel transport channels can be generated by exciting the gratings with four incident beams. The optical force and trapping potential were used to confirm that 20 nm metallic nanoparticles can be stably attracted to the traps and dynamically transported along channels by adjusting the phase of the excitation beams. Compared to existing nano-photon conveyors, this design boasts a straightforward structure and exceptional performance, offering a promising new approach to particle manipulation. Full article

(This article belongs to the Topic Applications of Photonics, Laser, Plasma and Radiation Physics)

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15 pages, 4753 KiB

Open AccessArticle

Optical Bottle Shaping Using Axicons with Amplitude or Phase Apodization

by Svetlana N. Khonina, Andrey V. Ustinov, Sergey I. Kharitonov, Sergey A. Fomchenkov and Alexey P. Porfirev

Photonics 2023, 10(2), 200; https://doi.org/10.3390/photonics10020200 - 13 Feb 2023

Cited by 7 | Viewed by 1983

Abstract

We investigate the formation of single and multiple optical bottle beams on the optical axis using a diffractive axicon with amplitude or phase apodization. The proposed approach allows one to control the location and the contrast of the boundaries of the generated dark [...] Read more.

We investigate the formation of single and multiple optical bottle beams on the optical axis using a diffractive axicon with amplitude or phase apodization. The proposed approach allows one to control the location and the contrast of the boundaries of the generated dark intensity regions on the optical axis. Experimental results obtained using a spatial light modulator are in good agreement with numerically obtained ones. We successfully used the designed and experimentally formed set of three optical bottle beams for trapping light-absorbing agglomerations of carbon nanoparticles in air under the action of photophoretic forces. This confirms the efficiency of the proposed approach for optical manipulation applications. Full article

(This article belongs to the Special Issue Diffractive Optics for Generation and Transformation of Structured Light)

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12 pages, 3887 KiB

Open AccessArticle

Comprehensive Analysis of Copper Plasma: A Laser-Induced Breakdown Spectroscopic Approach

by Asokan Ajith, Mohanachandran Nair Sindhu Swapna, Humberto Cabrera and Sankaranarayana Iyer Sankararaman

Photonics 2023, 10(2), 199; https://doi.org/10.3390/photonics10020199 - 12 Feb 2023

Cited by 3 | Viewed by 3154

Abstract

The emergence of diversified applications of laser-induced breakdown spectroscopy in the biomedical field, electronics, space physics, and material processing necessitates a comprehensive understanding of plasma parameters. The present work delineates the structure and evolution of copper plasma under different ambient pressures (0.01 mbar [...] Read more.

The emergence of diversified applications of laser-induced breakdown spectroscopy in the biomedical field, electronics, space physics, and material processing necessitates a comprehensive understanding of plasma parameters. The present work delineates the structure and evolution of copper plasma under different ambient pressures (0.01 mbar to 100 mbar) along with other plasma parameters. The study reveals the role of ambient pressure in the increase of plasma temperature (T_e), electron density (N_e), number of particles in the Debye sphere, plasma frequency, inverse bremsstrahlung absorption coefficient, electron thermal velocity, electron–ion collision frequency and in the decrease of Debye length (λ_D) and plasma skin depth (PSD). The experimental techniques and the theoretical explanations for the variation of plasma parameters and their applications are also detailed. As the ambient pressure increases, the motion of plasma species becomes restricted, resulting in the increase of T_e, calculated using the Boltzmann plot. From the values of λ_D, PSD, and N_e, it is understood that the copper plasma under investigation is thermally non-relativistic and satisfies McWhirter’s criterion, thus, revealing the local thermodynamic equilibrium condition of plasma. The effects of Debye shielding and stark broadening on the spectral lines are also investigated. Thus, the study helps bring newfangled dimensions to the application of plasma by exploring the possibility of tailoring plasma parameters. Full article

(This article belongs to the Special Issue Women’s Special Issue Series: Photonics)

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25 pages, 23292 KiB

Open AccessReview

Low-Illumination Image Enhancement Based on Deep Learning Techniques: A Brief Review

by Hao Tang, Hongyu Zhu, Linfeng Fei, Tingwei Wang, Yichao Cao and Chao Xie

Photonics 2023, 10(2), 198; https://doi.org/10.3390/photonics10020198 - 12 Feb 2023

Cited by 17 | Viewed by 8195

Abstract

As a critical preprocessing technique, low-illumination image enhancement has a wide range of practical applications. It aims to improve the visual perception of a given image captured without sufficient illumination. Conventional low-illumination image enhancement methods are typically implemented by improving image brightness, enhancing [...] Read more.

As a critical preprocessing technique, low-illumination image enhancement has a wide range of practical applications. It aims to improve the visual perception of a given image captured without sufficient illumination. Conventional low-illumination image enhancement methods are typically implemented by improving image brightness, enhancing image contrast, and suppressing image noise simultaneously. Nevertheless, recent advances in this area are dominated by deep-learning-based solutions, and consequently, various deep neural networks have been proposed and applied to this field. Therefore, this paper briefly reviews the latest low-illumination image enhancement, ranging from its related algorithms to its unsolved open issues. Specifically, current low-illumination image enhancement methods based on deep learning are first sorted out and divided into four categories: supervised learning methods, unsupervised learning methods, semi-supervised learning methods, and zero-shot learning methods. Then, existing low-light image datasets are summarized and analyzed. In addition, various quality assessment indices for low-light image enhancement are introduced in detail. We also compare 14 representative algorithms in terms of both objective evaluation and subjective evaluation. Finally, the future development trend of low-illumination image enhancement and its open issues are summarized and prospected. Full article

(This article belongs to the Special Issue Design and Applications of Optical Microscopy Imaging System)

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