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Photonics
Volume 11
Issue 5
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Photonics, Volume 11, Issue 5 (May 2024) – 94 articles

Cover Story (view full-size image): Chirped pulse amplification (CPA) is the golden standard for obtaining powerful ultrashort laser pulses. However, wavelength-tunable CPA systems are rarely reported. The output parameters' flexibility is desirable in various fields such as biomedical imaging, sensing, nonlinear spectroscopy and optical parametric amplification. This work presents a 1720 nm–1800 nm tunable CPA system based on a Tm-doped fiber. The experimental results are empowered by numerical simulation, which suggests further steps for the improvement of system performance. This is the first demonstration of a wavelength-tunable CPA system beyond 1.1 µm, which may inspire the development of similar systems at other wavelengths. View this paper
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15 pages, 2530 KiB  
Article
Next-Generation Dual Transceiver FSO Communication System for High-Speed Trains in Neom Smart City
by Yehia Elsawy, Ayshah S. Alatawi, Mohamed Abaza, Azza Moawad and El-Hadi M. Aggoune
Photonics 2024, 11(5), 483; https://doi.org/10.3390/photonics11050483 - 20 May 2024
Viewed by 924
Abstract
Smart cities like Neom require efficient and reliable transportation systems to support their vision of sustainable and interconnected urban environments. High-speed trains (HSTs) play a crucial role in connecting different areas of the city and facilitating seamless mobility. However, to ensure uninterrupted communication [...] Read more.
Smart cities like Neom require efficient and reliable transportation systems to support their vision of sustainable and interconnected urban environments. High-speed trains (HSTs) play a crucial role in connecting different areas of the city and facilitating seamless mobility. However, to ensure uninterrupted communication along the rail lines, advanced communication systems are essential to expand the coverage range of each base station (BS) while reducing the handover frequency. This paper presents the dual transceiver free space optical (FSO) communication system as a solution to achieve these objectives in the operational environment of HSTs in Neom city. Our channel model incorporates log-normal (LN) and gamma–gamma (GG) distributions to represent channel impairments and atmospheric turbulence in the city. Furthermore, we integrated the siding loop model, providing valuable insights into the system in real-world scenarios. To assess the system’s performance, we formulated the received signal-to-noise ratio (SNR) of the network under assumed fading conditions. Additionally, we analyzed the system’s bit error rate (BER) analytically and through Monte Carlo simulation. A comparative analysis with reconfigurable intelligent surfaces (RIS) and relay-assisted FSO communications shows the superior coverage area and efficiency of the dual transceiver model. A significant reduction of up to 76% and 99% in the number of required BSs compared to RIS and relay, respectively, is observed. This reduction leads to fewer handovers and lower capital expenditure (CAPEX) costs. Full article
(This article belongs to the Special Issue Next-Generation Free-Space Optical Communication Technology)
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<p>Dual transceiver G2T communication link model.</p> Full article ">Figure 2
<p>Siding loop model.</p> Full article ">Figure 3
<p>Location of Neom City [<a href="#B35-photonics-11-00483" class="html-bibr">35</a>].</p> Full article ">Figure 4
<p>Received SNR vs. coverage distance through: (<b>a</b>) LN channel; (<b>b</b>) GG channel.</p> Full article ">Figure 5
<p>BER vs. received SNR of dual transceiver through: (<b>a</b>) LN channel; (<b>b</b>) GG channel for different propagation distances.</p> Full article ">Figure 6
<p>BER vs. received SNR of different models through: (<b>a</b>) LN channel; (<b>b</b>) GG channel.</p> Full article ">
11 pages, 2797 KiB  
Communication
Sensing Characteristic Analysis of All-Dielectric Metasurfaces Based on Fano Resonance in Near-Infrared Regime
by Yongpeng Zhao, Qingfubo Geng, Jian Liu and Zhaoxin Geng
Photonics 2024, 11(5), 482; https://doi.org/10.3390/photonics11050482 - 20 May 2024
Viewed by 733
Abstract
A novel, all-dielectric metasurface, featuring a missing wedge-shaped nanodisk, is proposed to investigate optical characteristics. By introducing symmetry-breaking to induce Fano resonance, the metasurface achieves an impressive Q-factor of 1202 in the near-infrared spectrum, with a remarkably narrow full width at half maximum [...] Read more.
A novel, all-dielectric metasurface, featuring a missing wedge-shaped nanodisk, is proposed to investigate optical characteristics. By introducing symmetry-breaking to induce Fano resonance, the metasurface achieves an impressive Q-factor of 1202 in the near-infrared spectrum, with a remarkably narrow full width at half maximum (FWHM) of less than 1 nm. The ability to adjust the wavelength resonance by manipulating the structure of the wedge-shaped nanodisk offers a simple and efficient approach for metasurface design. This breakthrough holds great potential for various applications in sensing and optical filtering, marking a significant advancement in the field of nanophotonics. Full article
(This article belongs to the Special Issue Advanced Photonic Sensing and Measurement II)
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<p>Schematic diagram of the all-dielectric metasurfaces and the working principle. (<b>a</b>) Array structure consists of the missing wedge-shaped nanodisk. (<b>b</b>) Top view and cross-section of a cell structure. (<b>c</b>) Top row shows the αe electric dipole mode and αm magnetic dipole mode of the incident x-polarized light excited nanodisk. (<b>d</b>) Bottom row shows symmetry-breaking and induced coupling of the αm magnetic dipole to the αe electric dipole.</p> Full article ">Figure 2
<p>Reflection spectra and electric field distribution (<span class="html-italic">Ey</span>) of a metasurfaces cell. (<b>a</b>) Reflection spectra of the whole nanodisk. (<b>b</b>) Reflection spectra of the missing wedge-shaped nanodisk. (<b>c</b>) Electric field distribution (<span class="html-italic">Ey</span>) at different wavelength positions (800 nm (M1), 950 nm (M2), 1000 nm (M3), 800 nm (M4), 938 nm (M5), and 1000 nm (M6)).</p> Full article ">Figure 3
<p>Reflection spectra at different slice angles and the radii of the nanodisk. (<b>a</b>) Reflection spectra of the missing wedge-shaped nanodisk with different <span class="html-italic">θ</span> (130–180°) at R = 160 nm. (<b>b</b>) Reflection spectra of the missing wedge-shaped nanodisk with different <span class="html-italic">R</span> (130–180 nm) at <span class="html-italic">θ</span> = 160°.</p> Full article ">Figure 4
<p>Electric field distribution of the missing wedge-shaped nanodisk (<span class="html-italic">θ</span> = 170° and <span class="html-italic">R</span> = 180 nm) at the resonance wavelength (<span class="html-italic">λ</span> = 938 nm). (<b>a</b>) Electric field distribution in the <span class="html-italic">x</span>–<span class="html-italic">y</span> plane. (<b>b</b>) Linear variation of the electric field distribution (normalized) in the <span class="html-italic">x</span>–<span class="html-italic">y</span> plane. (<b>c</b>) Electric field distribution in the <span class="html-italic">x–z</span> plane. (<b>d</b>) Linear variation of the electric field distribution (normalized) in the <span class="html-italic">x</span>–<span class="html-italic">z</span> plane.</p> Full article ">Figure 5
<p>Reflection spectra and <math display="inline"><semantics> <mrow> <mi>F</mi> <mi>W</mi> <mi>H</mi> <mi>M</mi> </mrow> </semantics></math> of the missing wedge-shaped silicon nanodisk at different parameters. (<b>a</b>) Reflection spectra changes with different refractive indices (<span class="html-italic">n</span> = 1.33–1.39) at <span class="html-italic">R</span> = 180 nm, <span class="html-italic">θ</span> = 170°. (<b>b</b>) Reflection spectra at different periods (<span class="html-italic">P</span> = 470–510 nm, at <span class="html-italic">n</span> = 1.33, <span class="html-italic">R</span> = 180 nm, <span class="html-italic">θ</span> = 170°). (<b>c</b>) Reflection spectra of the missing wedge-shaped nanodisk at different θ (130–180°) with <span class="html-italic">R</span> = 160 nm. (<b>d</b>) Reflectance spectra of the missing wedge-shaped nanodisk at different <span class="html-italic">R</span> (150–200 nm) with <span class="html-italic">θ</span> = 160°. (<b>e</b>) Evolution of the resonance wavelength and <math display="inline"><semantics> <mrow> <mi>F</mi> <mi>W</mi> <mi>H</mi> <mi>M</mi> </mrow> </semantics></math> at <span class="html-italic">θ</span> of 130–180°. (<b>f</b>) Evolution of the resonance wavelength and <span class="html-italic">FWHM</span> at <span class="html-italic">R</span> of 150–200 nm and <span class="html-italic">θ</span> of 180°.</p> Full article ">
15 pages, 3310 KiB  
Article
Training a Dataset Simulated Using RGB Images for an End-to-End Event-Based DoLP Recovery Network
by Changda Yan, Xia Wang, Xin Zhang, Conghe Wang, Qiyang Sun and Yifan Zuo
Photonics 2024, 11(5), 481; https://doi.org/10.3390/photonics11050481 - 20 May 2024
Viewed by 605
Abstract
Event cameras are bio-inspired neuromorphic sensors that have emerged in recent years, with advantages such as high temporal resolutions, high dynamic ranges, low latency, and low power consumption. Event cameras can be used to build event-based imaging polarimeters, overcoming the limited frame rates [...] Read more.
Event cameras are bio-inspired neuromorphic sensors that have emerged in recent years, with advantages such as high temporal resolutions, high dynamic ranges, low latency, and low power consumption. Event cameras can be used to build event-based imaging polarimeters, overcoming the limited frame rates and low dynamic ranges of existing systems. Since events cannot provide absolute brightness intensity in different angles of polarization (AoPs), degree of linear polarization (DoLP) recovery in non-division-of-time (non-DoT) event-based imaging polarimeters is an ill-posed problem. Thus, we need a data-driven deep learning approach. Deep learning requires large amounts of data for training, and constructing a dataset for event-based non-DoT imaging polarimeters requires significant resources, scenarios, and time. We propose a method for generating datasets using simulated polarization distributions from existing red–green–blue images. Combined with event simulator V2E, the proposed method can easily construct large datasets for network training. We also propose an end-to-end event-based DoLP recovery network to solve the problem of DoLP recovery using event-based non-DoT imaging polarimeters. Finally, we construct a division-of-time event-based imaging polarimeter simulating an event-based four-channel non-DoT imaging polarimeter. Using real-world polarization events and DoLP ground truths, we demonstrate the effectiveness of the proposed simulation method and network. Full article
(This article belongs to the Special Issue Innovations and Challenges in Polarization Imaging Detection Technology)
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<p>Single-pixel trigger for event cameras.</p> Full article ">Figure 2
<p>Comparison of output between a frame-based camera and an event camera. Data come from [<a href="#B42-photonics-11-00481" class="html-bibr">42</a>].</p> Full article ">Figure 3
<p>Visualization of DoLP and AoP.</p> Full article ">Figure 4
<p>Comparison of polarization visualization and simulation.</p> Full article ">Figure 5
<p>Process of simulating event-based DoLP dataset from RGB images.</p> Full article ">Figure 6
<p>End-to-end event-based DoLP recovery network model.</p> Full article ">Figure 7
<p>Experimental system.</p> Full article ">Figure 8
<p>A set of experimental data sequences.</p> Full article ">Figure 9
<p>Comparison of real-world experimental results.</p> Full article ">Figure 10
<p>Comparison of simulated experimental results.</p> Full article ">Figure 11
<p>Experiment of event number and temporal resolution.</p> Full article ">
9 pages, 4244 KiB  
Article
Carbon Dot-Decorated Polystyrene Microspheres for Whispering-Gallery Mode Biosensing
by Anton A. Starovoytov, Evgeniia O. Soloveva, Kamilla Kurassova, Kirill V. Bogdanov, Irina A. Arefina, Natalia N. Shevchenko, Tigran A. Vartanyan, Daler R. Dadadzhanov and Nikita A. Toropov
Photonics 2024, 11(5), 480; https://doi.org/10.3390/photonics11050480 - 20 May 2024
Viewed by 835
Abstract
Whispering gallery mode (WGM) resonators doped with fluorescent materials find impressive applications in biological sensing. They do not require special conditions for the excitation of WGM inside that provide the basis for in vivo sensing. Currently, the problem of materials for in vivo [...] Read more.
Whispering gallery mode (WGM) resonators doped with fluorescent materials find impressive applications in biological sensing. They do not require special conditions for the excitation of WGM inside that provide the basis for in vivo sensing. Currently, the problem of materials for in vivo WGM sensors are substantial since their fluorescence should have stable optical properties as well as they should be biocompatible. To address this we present WGM microresonators of 5–7 μm, where the dopant is made of carbon quantum dots (CDs). CDs are biocompatible since they are produced from carbon and demonstrate bright optical emission, which shows different bands depending on the excitation wavelength. The WGM sensors developed here were tested as label-free biosensors by detecting bovine serum albumin molecules. The results showed WGM frequency shifting, with the limit of detection down to 10−16 M level. Full article
(This article belongs to the Special Issue Advancements in Optical Metamaterials)
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<p>Spectra of water solutions: optical density (<b>a</b>) and normalized photoluminescence (<b>b</b>) of BSA (1), CDs (2), and their mixture excited at <math display="inline"><semantics> <mi>λ</mi> </semantics></math> = 488 nm (3).</p> Full article ">Figure 2
<p>(<b>a</b>) SEM image of a polystyrene microsphere. (<b>b</b>) Normalized photoluminescence spectra of CDs solution excited at 405 nm (curve 1) and 488 nm (curve 2), CD-doped microspheres excited at 405 nm (curve 3) and 488 nm (curve 4). Inset shows a microsphere image, corresponding to spectrum 3, which was acquired by a laser scanning confocal microscope.</p> Full article ">Figure 3
<p>(<b>a</b>) Emission spectra of polystyrene microspheres covered with carbon dots at different excitation intensities; (<b>b</b>) input-output characteristic defined as intensities of separate emission lines.</p> Full article ">Figure 4
<p>(<b>a</b>) Emission spectra of CDs doped microspheres before and after adding BSA molecules (<math display="inline"><semantics> <msup> <mn>10</mn> <mrow> <mo>−</mo> <mn>16</mn> </mrow> </msup> </semantics></math> M solution). WGM frequency shift extracted from emission spectra of CDs doped microspheres before and after adding BSA solution: (<b>b</b>)—<math display="inline"><semantics> <msup> <mn>10</mn> <mrow> <mo>−</mo> <mn>15</mn> </mrow> </msup> </semantics></math> M, (<b>c</b>)—<math display="inline"><semantics> <msup> <mn>10</mn> <mrow> <mo>−</mo> <mn>16</mn> </mrow> </msup> </semantics></math> M, (<b>d</b>)—<math display="inline"><semantics> <msup> <mn>10</mn> <mrow> <mo>−</mo> <mn>18</mn> </mrow> </msup> </semantics></math> M.</p> Full article ">Figure 5
<p>Dependence of WGM frequency shift vs. concentration of added BSA solution in logarithmic scale.</p> Full article ">
13 pages, 6278 KiB  
Article
Experimental Study on Evolution of Chemical Structure Defects and Secondary Contaminative Deposition during HF-Based Etching
by Xiao Shen, Feng Shi, Shuo Qiao, Xing Peng and Ying Xiong
Photonics 2024, 11(5), 479; https://doi.org/10.3390/photonics11050479 - 20 May 2024
Viewed by 684
Abstract
Post-processing based on HF etching has become a highly preferred technique in the fabrication of fused silica optical elements in various high-power laser systems. Previous studies have thoroughly examined and confirmed the elimination of fragments and contamination. However, limited attention has been paid [...] Read more.
Post-processing based on HF etching has become a highly preferred technique in the fabrication of fused silica optical elements in various high-power laser systems. Previous studies have thoroughly examined and confirmed the elimination of fragments and contamination. However, limited attention has been paid to nano-sized chemical structural defects and secondary precursors that arise during the etching process. Therefore, in this paper, a set of fused silica samples are prepared and undergo the etching process under different parameters. Subsequently, an atomic force microscope, scanning electron microscope and fluorescence spectrometer are applied to analyze sample surfaces, and then an LIDT test based on the R-on-1 method is applied. The findings revealed that appropriate etching configurations will lead to certain LIDT improvement (from initial 7.22 J/cm2 to 10.76 J/cm2), and HF-based etching effectively suppresses chemical structural defects, while additional processes are recommended for the elimination of micron- to nano-sized secondary deposition contamination. Full article
(This article belongs to the Special Issue New Perspectives in Optical Design)
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<p>Surface measurement of initial samples.</p> Full article ">Figure 2
<p>HF etching aided by megasonic acoustic field.</p> Full article ">Figure 3
<p>Etching schedule of each sample.</p> Full article ">Figure 4
<p>LIDT test platform.</p> Full article ">Figure 5
<p>Micro morphology of the samples under different etching depths.</p> Full article ">Figure 6
<p>Energy spectrum scanning results of test point one.</p> Full article ">Figure 7
<p>Energy-spectrum scanning results of test point two.</p> Full article ">Figure 8
<p>Chemical structure defects in fused silica substrate.</p> Full article ">Figure 9
<p>Fluorescence spectrum of the samples under different etching durations.</p> Full article ">Figure 10
<p>Relationship between LIDT, surface roughness and etching depth.</p> Full article ">Figure 11
<p>Relationship between roughness and etching depth.</p> Full article ">Figure 12
<p>Specific composition of superficial hydrolysis layer.</p> Full article ">
24 pages, 19086 KiB  
Article
Analysis of the Polarization Distribution and Spin Angular Momentum of the Interference Field Obtained by Co-Planar Beams with Linear and Circular Polarization
by Svetlana N. Khonina, Andrey V. Ustinov, Alexey P. Porfirev and Sergey V. Karpeev
Photonics 2024, 11(5), 478; https://doi.org/10.3390/photonics11050478 - 19 May 2024
Viewed by 585
Abstract
Interference of two and four light beams with linear or circular polarization is studied analytically and numerically based on the Richards–Wolf formalism. We consider such characteristics of the interference fields as the distribution of intensity, polarization, and spin angular momentum density. The generation [...] Read more.
Interference of two and four light beams with linear or circular polarization is studied analytically and numerically based on the Richards–Wolf formalism. We consider such characteristics of the interference fields as the distribution of intensity, polarization, and spin angular momentum density. The generation of light fields with 1D and 2D periodic structure of both intensity and polarization is demonstrated. We can control the periodic structure both by changing the polarization state of the interfering beams and by changing the numerical aperture of focusing. We consider examples with a basic configuration, as well as those with a certain symmetry in the polarization state of the interfering beams. In some cases, increasing the numerical aperture of the focusing system significantly affects the generated distributions of both intensity and polarization. Experimental results, obtained using a polarization video camera, are in good agreement with the simulation results. The considered light fields can be used in laser processing of thin films of photosensitive (as well as polarization-sensitive) materials in order to create arrays of various ordered nano- and microstructures. Full article
(This article belongs to the Special Issue Structured Light Beams: Science and Applications)
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Figure 1

Figure 1
<p>Explanations for the Richards–Wolf formulas: focusing a linearly polarized beam through a lens with focal length <span class="html-italic">f</span> and maximum azimuthal angle α.</p> Full article ">Figure 2
<p>Simulation results for two Gaussian beams with the same linear <span class="html-italic">x</span>-polarization: (<b>a</b>) input field; (<b>b</b>) intensity distributions of the <span class="html-italic">x</span>-, <span class="html-italic">y</span>-, and <span class="html-italic">z</span>-components of the electric field; and (<b>c</b>) distributions of the <span class="html-italic">x</span>-, <span class="html-italic">y</span>-, and <span class="html-italic">z</span>-components of the SAM density, as well as the pattern of the total intensity with the state of polarization (shown by arrows) in (<b>d</b>) the paraxial case and (<b>e</b>) in the case of sharp focusing.</p> Full article ">Figure 3
<p>(<b>a</b>–<b>e</b>) Simulation results for two Gaussian beams with the same linear <span class="html-italic">y</span>-polarization (the rest is as in <a href="#photonics-11-00478-f002" class="html-fig">Figure 2</a>).</p> Full article ">Figure 3 Cont.
<p>(<b>a</b>–<b>e</b>) Simulation results for two Gaussian beams with the same linear <span class="html-italic">y</span>-polarization (the rest is as in <a href="#photonics-11-00478-f002" class="html-fig">Figure 2</a>).</p> Full article ">Figure 4
<p>(<b>a</b>–<b>e</b>) Simulation results for two Gaussian beams with orthogonal linear polarization (the rest is as in <a href="#photonics-11-00478-f002" class="html-fig">Figure 2</a>).</p> Full article ">Figure 5
<p>(<b>a</b>–<b>e</b>) Simulation results for two Gaussian beams with opposite circular polarizations (the rest is as in <a href="#photonics-11-00478-f002" class="html-fig">Figure 2</a>).</p> Full article ">Figure 6
<p>(<b>a</b>–<b>e</b>) Simulation results for four Gaussian beams with the same linear x-polarizations (the rest as in <a href="#photonics-11-00478-f002" class="html-fig">Figure 2</a>).</p> Full article ">Figure 7
<p>(<b>a</b>–<b>e</b>) Simulation results for four Gaussian beams with identical circular polarizations (the rest is as in <a href="#photonics-11-00478-f002" class="html-fig">Figure 2</a>).</p> Full article ">Figure 8
<p>(<b>a</b>–<b>e</b>) Simulation results for four Gaussian beams with orthogonal linear polarizations of the radial type (the rest of the description is as in <a href="#photonics-11-00478-f002" class="html-fig">Figure 2</a>).</p> Full article ">Figure 8 Cont.
<p>(<b>a</b>–<b>e</b>) Simulation results for four Gaussian beams with orthogonal linear polarizations of the radial type (the rest of the description is as in <a href="#photonics-11-00478-f002" class="html-fig">Figure 2</a>).</p> Full article ">Figure 9
<p>(<b>a</b>–<b>e</b>) Simulation results for four Gaussian beams with orthogonal linear polarizations of azimuthal type (the rest of the description is as in <a href="#photonics-11-00478-f002" class="html-fig">Figure 2</a>).</p> Full article ">Figure 10
<p>(<b>a</b>–<b>e</b>) Simulation results for four Gaussian beams with linear polarizations of the spiral type (the rest of the description is as in <a href="#photonics-11-00478-f002" class="html-fig">Figure 2</a>).</p> Full article ">Figure 11
<p>(<b>a</b>–<b>e</b>) Simulation results for four Gaussian beams with two pairs of orthogonal circular polarizations (the rest of the description is as in <a href="#photonics-11-00478-f002" class="html-fig">Figure 2</a>).</p> Full article ">Figure 11 Cont.
<p>(<b>a</b>–<b>e</b>) Simulation results for four Gaussian beams with two pairs of orthogonal circular polarizations (the rest of the description is as in <a href="#photonics-11-00478-f002" class="html-fig">Figure 2</a>).</p> Full article ">Figure 12
<p>Experimental investigation of the generation of multi-beam interference patterns. (<b>a</b>) Optical setup: Laser is a cw solid-state laser MGL-U-532-1W; L1, L2, and L3 are spherical lenses (<span class="html-italic">f</span><sub>1</sub> = 100 mm, <span class="html-italic">f</span><sub>2</sub> = 125 mm, and <span class="html-italic">f</span><sub>3</sub> = 100 mm); M1 and M2 are mirrors, PT1 is a linear polarizer, and DOE is a diffractive optical element in the form of a binary diffractive grating for the splitting of incident laser beams; AM is an amplitude mask, PT2 is a polarizing element, MO1 and MO2 are micro-objectives (NA = 0.11 and 0.4), and PCAM is a ImagingSource DZK 33UX250 polarization video camera. (<b>b1</b>) Phase mask of the diffractive optical element (DOE) utilized to split an incident laser beam into four laser beams. (<b>b2</b>) Amplitude mask used for spatial filtering of the light field formed by the DOE. (<b>b3</b>) Local optical axis orientation of an S-waveplate utilized in the experiments for transformation of the polarization distribution of the incident light field. (<b>c</b>) Example of light field distributions (intensity and polarization vectors) of the experimentally generated light field corresponding to the modeling results presented in <a href="#photonics-11-00478-f009" class="html-fig">Figure 9</a>.</p> Full article ">Figure 13
<p>Intensity and polarization distributions of the experimentally generated two-beam interference patterns. White arrows and ellipses represent linear polarization vectors and polarization ellipses.</p> Full article ">Figure 14
<p>Intensity and polarization distributions of the experimentally generated four-beam interference patterns. White arrows and ellipses represent linear polarization vectors and polarization ellipses.</p> Full article ">
13 pages, 4516 KiB  
Article
Broadband High-Linear FMCW Light Source Based on Spectral Stitching
by Liang Sun, Xinguang Zhou, Haohao Zhao, Shichang Xu, Zihan Wu, Guohui Yuan and Zhuoran Wang
Photonics 2024, 11(5), 477; https://doi.org/10.3390/photonics11050477 - 19 May 2024
Viewed by 654
Abstract
The key to realizing a high-performance frequency-modulated continuous wave (FMCW) laser frequency-sweeping light source is how to extend the frequency-swept bandwidth and eliminate the effect of nonlinearity. To solve these issues, this paper designs a broadband high-linear FMCW frequency-sweeping light source system based [...] Read more.
The key to realizing a high-performance frequency-modulated continuous wave (FMCW) laser frequency-sweeping light source is how to extend the frequency-swept bandwidth and eliminate the effect of nonlinearity. To solve these issues, this paper designs a broadband high-linear FMCW frequency-sweeping light source system based on the combination of fixed temperature control and digital optoelectronic phase-locked loop (PLL), which controls the temperatures of the two lasers separately and attempts to achieve the coarse spectral stitching based on a time-division multiplexing scheme. Furthermore, we uses the PLL to correct the frequency error more specifically after the coarse stitching, which achieves the spectrum fine stitching and, meanwhile, realizes the nonlinearity correction. The experimental results show that our scheme can successfully achieve bandwidth expansion and nonlinearity correction, and the sweeping bandwidth is twice as much as that of the original single laser. The full-width half-maximum (FWHM) of the FMCW output is reduced from 150 kHz to 6.1 kHz, which exhibits excellent nonlinear correction performance. The relative error of the FMCW ranging system based on this frequency-swept light source is also reduced from 1.628% to 0.673%. Therefore, our frequency-swept light source with excellent performance has a promising application in the FMCW laser ranging system. Full article
(This article belongs to the Special Issue Advanced Lasers and Their Applications II)
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<p>Experimental system structure diagram.</p> Full article ">Figure 2
<p>Frequency diagrams of the reference and measurement return beams and beat signals during a single modulation period: (<b>a</b>) frequency diagrams of the reference and measurement return beams; (<b>b</b>) frequency diagram of the beat signal.</p> Full article ">Figure 3
<p>Sweep bandwidth expansion system.</p> Full article ">Figure 4
<p>The illustration of spectrum stitching.</p> Full article ">Figure 5
<p>Envelope signal plots before and after power equalization.</p> Full article ">Figure 6
<p>Residual error graph and sweep frequency graph: (<b>a</b>) before nonlinear correction; (<b>b</b>) after iterative algorithm correction; and (<b>c</b>) after phase-locked loop.</p> Full article ">Figure 7
<p>Spectrum diagrams of beat frequency signals.</p> Full article ">Figure 8
<p>Sweep frequency spectrum: (<b>a</b>) before expanding the sweep bandwidth; (<b>b</b>) after expanding the sweep bandwidth.</p> Full article ">Figure 9
<p>Residual error graph and sweep frequency graph: (<b>a</b>) before expanding the sweep bandwidth; (<b>b</b>) after expanding the sweep bandwidth.</p> Full article ">Figure 10
<p>Spectra of beat frequency signals.</p> Full article ">Figure 11
<p>Spectra of beat frequency signals prior to and following the displacement.</p> Full article ">Figure 12
<p>Spectrum diagrams of received signal: (<b>a</b>) before expanding the sweep bandwidth; (<b>b</b>) after expanding the sweep bandwidth.</p> Full article ">
12 pages, 3334 KiB  
Article
Chirped Integrated Bragg Grating Design
by José Ángel Praena and Alejandro Carballar
Photonics 2024, 11(5), 476; https://doi.org/10.3390/photonics11050476 - 19 May 2024
Viewed by 657
Abstract
We analyze the two classic methods for chirped Integrated Bragg Gratings (IBGs) in Silicon-on-Insulator technology using the transfer matrix method based on the effective refractive index (neff) technique, which translates the geometry of an IBG into a matrix of n [...] Read more.
We analyze the two classic methods for chirped Integrated Bragg Gratings (IBGs) in Silicon-on-Insulator technology using the transfer matrix method based on the effective refractive index (neff) technique, which translates the geometry of an IBG into a matrix of neff depending on the wavelength. We also implement a procedure that allows engineering of the chirped IBG parameters, given a required bandwidth (BW) and group delay (GD). Finally, a complementary method for designing chirped IBG is proposed, showing a significant improvement in the bandwidth of the device or a moderation in the variation of the geometrical parameters of the grating. Full article
(This article belongs to the Special Issue Silicon Photonics Devices and Integrated Circuits)
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<p>(<b>a</b>) Diagram of two Integrated Bragg Gratings (IBGs) with rectangular and sinusoidal corrugations and Δ<span class="html-italic">W</span> = 20 nm. The yellow arrow indicates the direction of propagation of the electromagnetic field. (<b>b</b>) Energy density distribution (normalized) of the quasi-transverse electric (TE) mode inside the strip waveguide (linear scale).</p> Full article ">Figure 2
<p>Effective refractive index obtained with numerical simulation as a function of wavelength (the discrete step wavelength is 2 nm) for different waveguide widths from 450 nm to 560 nm.</p> Full article ">Figure 3
<p>Effective refractive index obtained with numerical simulations as a function of waveguide width for different wavelengths from 1530 nm to 1570 nm (the discrete step width is 10 nm; for convenience, the sampling is marked only on the 1570 nm curve).</p> Full article ">Figure 4
<p>(<b>a</b>) Illustrative diagram of an IBG, showing incoming (orange), transmitted (yellow), and reflected (red) fields. (<b>b</b>) Schematic representation of the IBG sampling, and distribution of transmitted and reflected fields in a layer of size <span class="html-italic">dz</span> limited by two interfaces. <span class="html-italic">L</span> = <span class="html-italic">dz</span> × <span class="html-italic">n</span> (<span class="html-italic">n</span>, number of layers).</p> Full article ">Figure 5
<p>(<b>a</b>) Conceptual diagram of the apodized chirped IBG designed via Bragg grating period variation: initial Bragg period <span class="html-italic">Λ<sub>Bi</sub></span> = 312.5 nm and final <span class="html-italic">Λ<sub>Bf</sub></span> = 319.5 nm, <span class="html-italic">L</span> = 2258<span class="html-italic">Λ<sub>B</sub></span>, Δ<span class="html-italic">W</span> = 10 nm, <span class="html-italic">W</span><sub>0</sub> = 500 nm, with <span class="html-italic">tanh</span> as the apodization function (grating period and corrugation width sizes have been enlarged for the sake of illustration). (<b>b</b>) Simulated spectrum of reflectivity and GD.</p> Full article ">Figure 6
<p>(<b>a</b>) Conceptual diagram of the apodized chirped IBG designed via waveguide width variation; initial width <span class="html-italic">W</span><sub>0<span class="html-italic">i</span></sub> = 435 nm and final <span class="html-italic">W</span><sub>0<span class="html-italic">f</span></sub> = 546 nm, <span class="html-italic">L</span> = 2258<span class="html-italic">Λ<sub>B</sub></span>, Δ<span class="html-italic">W</span> = 10 nm, constant <span class="html-italic">Λ<sub>B</sub></span> = 316 nm and <span class="html-italic">tanh</span> as apodization function (grating period and corrugation width sizes have been enlarged for the sake of illustration). (<b>b</b>) Simulated spectrum of reflectivity and group delay (GD).</p> Full article ">Figure 7
<p>Simulation results for the new apodized chirped IBG design via Bragg grating period and waveguide width variation. IBG with initial <span class="html-italic">W</span><sub>0<span class="html-italic">i</span></sub> = 484.5 nm and <span class="html-italic">Λ<sub>Bi</sub></span> = 314 nm, and final <span class="html-italic">W</span><sub>0<span class="html-italic">f</span></sub> = 517.3 nm and <span class="html-italic">Λ<sub>Bf</sub></span> = 318 nm, <span class="html-italic">L</span> = 2258<span class="html-italic">Λ<sub>B</sub></span>, Δ<span class="html-italic">W</span> = 10 nm, constant <span class="html-italic">n<sub>eff</sub></span> = 2.4525, <span class="html-italic">Λ<sub>B</sub></span>= 316 nm, and <span class="html-italic">tanh</span> as the apodization function: (<b>a</b>) average internal power distribution for the reflected optical fields along the device length evaluated at each wavelength; and (<b>b</b>) spectrum of reflectivity and GD.</p> Full article ">
15 pages, 5146 KiB  
Article
A Fast Time Synchronization Method for Large Scale LEO Satellite Networks Based on A Bionic Algorithm
by Yue Xu, Tao Dong, Jie Yin, Ziyong Zhang, Zhihui Liu, Hao Jiang and Jing Wu
Photonics 2024, 11(5), 475; https://doi.org/10.3390/photonics11050475 - 19 May 2024
Viewed by 722
Abstract
A fast time synchronization method for large-scale LEO satellite networks based on a bionic algorithm is proposed. Because the inter-satellite links are continuously established and interrupted due to the relative motion of the satellites, the topology of the LEO satellite networks is time [...] Read more.
A fast time synchronization method for large-scale LEO satellite networks based on a bionic algorithm is proposed. Because the inter-satellite links are continuously established and interrupted due to the relative motion of the satellites, the topology of the LEO satellite networks is time varying. Firstly, according to the ephemeris information in navigation messages, a connection table which records the connections between satellites is generated. Then, based on the connection table, the current satellite network topology is calculated and generated. Furthermore, a bionic algorithm is used to select some satellites as time source nodes and calculate the hierarchy of the clock transmission tree. By taking the minimum level of the time transmission tree as the optimization objective, the time source nodes and the clock stratums of the whole satellite networks are obtained. Finally, the onboard computational center broadcasts the time layer table to all the satellites in the LEO satellite networks and the time synchronization links can be established or recovered fast. Full article
(This article belongs to the Special Issue Novel Advances in Optical Communications)
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<p>Schematic diagram of space and earth integration networks.</p> Full article ">Figure 2
<p>Time transfer trees: (<b>a</b>) the second group, (<b>b</b>) the fifth group in <a href="#photonics-11-00475-t001" class="html-table">Table 1</a>.</p> Full article ">Figure 3
<p>The curves of the number of iterations of the PSO algorithm and the traversal algorithm with respect to the minimum sum of the weighted time in the first time slice.</p> Full article ">Figure 4
<p>Schematic diagram of clock transmission path and topology of the first slice in LEO satellite networks.</p> Full article ">Figure 5
<p>Network topology for another time slice.</p> Full article ">Figure 6
<p>Time transfer trees of the fourth group in <a href="#photonics-11-00475-t004" class="html-table">Table 4</a>.</p> Full article ">Figure 7
<p>The curves of the number of iterations of the PSO algorithm and the traversal algorithm with respect to the minimum sum of the weighted time in another time slice.</p> Full article ">Figure A1
<p></p> Full article ">Figure A2
<p></p> Full article ">Figure A3
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12 pages, 4476 KiB  
Article
Polarization-Dependent Fiber Metasurface with Beam Collimating and Deflecting
by Yuemin Ma, Di Sang, Yi Lin, Qiang An, Zhanshan Sun and Yunqi Fu
Photonics 2024, 11(5), 474; https://doi.org/10.3390/photonics11050474 - 18 May 2024
Viewed by 608
Abstract
Metasurfaces can arbitrarily manipulate the amplitude, phase, and polarization of optical fields on subwavelength scales. Due to their arbitrary manipulation and compact size, the metasurface can be well integrated with optical fibers. Herein, we demonstrate a polarization-dependent metasurface using birefringent meta-atoms, which can [...] Read more.
Metasurfaces can arbitrarily manipulate the amplitude, phase, and polarization of optical fields on subwavelength scales. Due to their arbitrary manipulation and compact size, the metasurface can be well integrated with optical fibers. Herein, we demonstrate a polarization-dependent metasurface using birefringent meta-atoms, which can independently control X- and Y-polarization incident light. Each meta-atom allows for the division of phase into 16 steps ranging from 0 to 2π for both X- and Y-polarization, resulting in 256 nanopillars selected from the meta-atom library to satisfy the required phase. With the different effective refractive indices of the cuboid meta-atoms along the X- and Y-axis, we can achieve collimation of the X-polarization emitted beam from an optical fiber while deflecting orthogonally polarized light. As a result, the proposed metasurface collimates an X-polarized beam with a beam radius of 20 μm at z = 1 mm and 43.9 μm at z = 2 mm. Additionally, the metasurface can effectively deflect the Y-polarized beam to 36.01°, consistent with the results of the theoretical computation. The proposed metasurface exhibits a deflection efficiency of 55.6% for Y-polarized beams with a relative polarization efficiency of 82.2%, while the efficiency for the X-polarization is 71.4%. Our work is considered a promising application for optical communication, sensing, and quantum measurement. Full article
(This article belongs to the Special Issue Fiber Optic Sensors: Science and Applications)
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<p>Schematic of the birefringent meta-atom.</p> Full article ">Figure 2
<p>Simulation results of the birefringence meta-atom. (<b>a</b>) The transmission coefficients and (<b>b</b>) phase shifts of X-polarization. (<b>c</b>) The transmission coefficients and (<b>d</b>) phase shifts of Y-polarization. The green marks show the selected meta-atoms with phase shifts of 3<span class="html-italic">π</span>/4 and 13<span class="html-italic">π</span>/8 for X-polarization.</p> Full article ">Figure 3
<p>Schematic of the metasurface’s functions.</p> Full article ">Figure 4
<p>(<b>a</b>) The desired phase for X-polarization. (<b>b</b>) The desired phase for Y-polarization’s deflection. (<b>c</b>) The desired phase for Y-polarization.</p> Full article ">Figure 5
<p>The selected meta-atoms phase responses to the (<b>a</b>) X-polarization and (<b>b</b>) Y-polarization. The selected meta-atoms transmission to the (<b>c</b>) X-polarization and (<b>d</b>) Y-polarization.</p> Full article ">Figure 6
<p>Phase profiles of (<b>a</b>) X-polarized beam and Y-polarization beam along <span class="html-italic">x</span> = 0. (<b>b</b>) Schematic illustration of the metasurface. (<b>c</b>) The structure of the proposed metasurface.</p> Full article ">Figure 7
<p>(<b>a</b>) Intensity distributions of the deflected beam in the X-Z plane. (<b>b</b>) Far-field distribution. (<b>c</b>) Wavefront of the deflection beam.</p> Full article ">Figure 8
<p>(<b>a</b>) Uncollimated divergence of the outgoing light from an optical fiber. (<b>b</b>) Intensity distributions of the collimated beam in axial and cross-section. (<b>c</b>) Wavefront of the uncollimation beam. (<b>d</b>) Wavefront of the collimated beam.</p> Full article ">
12 pages, 3930 KiB  
Article
Nanosecond Laser Fabrication of Dammann Grating-like Structure on Glass for Bessel-Beam Array Generation
by Prasenjit Praharaj and Manoj Kumar Bhuyan
Photonics 2024, 11(5), 473; https://doi.org/10.3390/photonics11050473 - 18 May 2024
Viewed by 745
Abstract
The generation of optical beam arrays with prospective uses within the realms of microscopy, photonics, non-linear optics, and material processing often requires Dammann gratings. Here, we report the direct fabrication of one- and two-dimensional Dammann grating-like structures on soda lime glass using a [...] Read more.
The generation of optical beam arrays with prospective uses within the realms of microscopy, photonics, non-linear optics, and material processing often requires Dammann gratings. Here, we report the direct fabrication of one- and two-dimensional Dammann grating-like structures on soda lime glass using a nanosecond pulsed laser beam with a 1064 nm wavelength. Using the fabricated grating, an axicon lens, and an optical magnification system, we propose a scheme of generation of a diverging array of zero-order Bessel beams with a sub-micron-size central core, extending longitudinally over several hundred microns. Two different grating fabrication strategies are also proposed to control the number of Bessel beams in an array. It was demonstrated that Bessel beams of 12 degrees conical half-angle in an array of up to [5 × 5] dimensions can be generated using a suitable combination of Dammann grating, axicon lens and focusing optics. Full article
(This article belongs to the Special Issue Laser Processing and Modification of Materials)
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<p>Schematic diagram of nanosecond pulsed laser micromachining setup.</p> Full article ">Figure 2
<p>(<b>a</b>) Optical micrograph of trenches machined (on a single-pass basis) on the front surface of glass using nanosecond laser pulses of the following fluence levels: 191, 239, 335, 406, 478, 645 and 789 J/cm<sup>2</sup>. (<b>b</b>) As described above, 3D profilometric images of trenches. (<b>c</b>) Plot showing the variation of trench width with respect to laser fluence.</p> Full article ">Figure 3
<p>Schematics of Bessel beam generation and characterisation setups are shown in (<b>a</b>,<b>b</b>), respectively. (<b>c</b>) Cross-sectional profile image of generated Bessel beams of 12-degree conical half-angle. (<b>d</b>) Typical radial profile of generated Bessel beams showing concentric rings.</p> Full article ">Figure 4
<p>Cross-sectional images of zero-order Bessel beams in an array format. Bessel beams in array format were generated using axicon lens, telescope and different Dammann gratings (machined with a laser fluence of 335 J/cm<sup>2</sup>) of fixed period (P) of 200 µm, with various combinations of transparent zone (TZ) and opaque zone (OZ): (<b>a</b>) TZ = 180 µm, OZ = 20 µm; (<b>b</b>) TZ = 150 µm, OZ = 50 µm; and (<b>c</b>) TZ = 100 µm, OZ = 100 µm. Bessel beam arrays were also generated using Dammann gratings of defined parameters, i.e., TZ = OZ = 100 µm, which were machined with various laser fluence levels: (<b>d</b>) 335 J/cm<sup>2</sup>, (<b>e</b>) 478 J/cm<sup>2</sup>, and (<b>f</b>) 645 J/cm<sup>2</sup>. The insets show the optical micrographs and 3D profiles of Dammann gratings in each case.</p> Full article ">Figure 5
<p>Cross-sectional images of arrayed Bessel beams generated using an axicon lens, telescope, and Dammann gratings (machined with a laser fluence of 335 J/cm<sup>2</sup>) of the following parameters: (<b>a</b>) TZ = OZ = 60 µm, (<b>b</b>) TZ = OZ = 100 µm, (<b>c</b>) TZ = OZ = 200 µm, and (<b>d</b>) TZ = OZ = 400 µm. (<b>e</b>–<b>j</b>) Also shown here are the radial profiles of the Bessel beam array captured at three longitudinal distances, i.e., Z = 120 µm, 240 µm and 430 µm, respectively, corresponding to the cases (<b>b</b>,<b>d</b>). These longitudinal positions are marked as Z1, Z2, and Z3 on the images. The radial intensity profiles corresponding to images (<b>f</b>,<b>i</b>) are insets. (<b>k</b>) A table indicating the inter-Bessel beam separation (with respect to the central Bessel beam) at longitudinal distances of 120 µm, 240 µm, and 430 µm, as associated with two distinct gratings. (<b>l</b>) Plot showing the variation of diverging half-angle θ1 of Bessel beam arrays as a function of the grating period.</p> Full article ">Figure 6
<p>Optical micrographs of 2D Dammann gratings with periods of 200 μm fabricated using a laser fluence of 335 J/cm<sup>2</sup> with (<b>a</b>) line-scanning and (<b>b</b>) patch-scanning strategies. The corresponding 3D profiles of gratings are shown as insets. (<b>c</b>,<b>d</b>) The typical radial profiles of 2D Bessel beam arrays are generated using an axicon lens and gratings machined with both considered scanning strategies. The insets to (<b>c</b>,<b>d</b>) show the enlarged size of Bessel beams placed at the centre of the array.</p> Full article ">Figure 7
<p>(<b>a</b>) Optical micrograph of laser fabricated (using laser fluence of 335 J/cm<sup>2</sup>) 1D Dammann grating for the generation of a [1 × 5] Bessel beam array. (<b>b</b>–<b>d</b>) Also shown here are the radial profiles of the Bessel beam array captured at three longitudinal distances, i.e., Z = 190 µm, 295 µm, and 335 µm, respectively. The inset to (<b>d</b>) shows the central portion of the corresponding saturated image.</p> Full article ">
12 pages, 15402 KiB  
Article
Compact Low Loss Ribbed Asymmetric Multimode Interference Power Splitter
by Yanfeng Liang, Huanlin Lv, Baichao Liu, Haoyu Wang, Fangxu Liu, Shuo Liu, Yang Cong, Xuanchen Li and Qingxiao Guo
Photonics 2024, 11(5), 472; https://doi.org/10.3390/photonics11050472 - 17 May 2024
Viewed by 884
Abstract
Optical power splitters (OPSs) are utilized extensively in integrated photonic circuits, drawing significant interest in research on power splitters with adjustable splitting ratios. This paper introduces a compact, low-loss 1 × 2 asymmetric multimode interferometric (MMI) optical power splitter on a silicon-on-insulator (SOI) [...] Read more.
Optical power splitters (OPSs) are utilized extensively in integrated photonic circuits, drawing significant interest in research on power splitters with adjustable splitting ratios. This paper introduces a compact, low-loss 1 × 2 asymmetric multimode interferometric (MMI) optical power splitter on a silicon-on-insulator (SOI) platform. The device is simulated using the finite difference method (FDM) and eigenmode expansion solver (EME). It is possible to attain various output power splitting ratios by making the geometry of the MMI central section asymmetric relative to the propagation axis. Six distinct optical power splitters are designed with unconventional splitting ratios in this paper, which substantiates that the device can achieve any power splitter ratios (PSRs) in the range of 95:5 to 50:50. The dimensions of the multimode section were established at 2.9 × (9.5–10.9) μm. Simulation results show a range of unique advantages of the device, including a low extra loss of less than 0.4 dB, good fabrication tolerance, and power splitting ratio fluctuation below 3% across the 1500 nm to 1600 nm wavelength span. Full article
(This article belongs to the Special Issue Optical Fiber Communication Systems and Networks)
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<p>(<b>a</b>) Schematic structure of the designed device. the input and output ports (denoted as P<sub>1</sub> and P<sub>2</sub>), the dimensions of the various components: W<sub>mmi</sub> = 2.9 μm, L<sub>Taper</sub> = 2 μm, and W<sub>Taper</sub> = 850 nm. (<b>b</b>) Top view of the proposed design device. (I) Input single-mode region, (II) multimode interference coupling region W<sub>gap</sub> = 0.8 μm, (III) output energy region Δ<sub>1</sub> = Δ<sub>2</sub> = 0.35 μm. (<b>c</b>,<b>d</b>) The electric field strength distributions of the designed device with <span class="html-italic">θ</span> = 0 and <span class="html-italic">θ</span> ≠ 0. Normalized electric field intensity 1.0, 0.5, and 0.0.</p> Full article ">Figure 2
<p>(<b>a</b>) The curve of effective refractive index of TE<sub>0</sub> (black line), TM<sub>0</sub> (red line), TE<sub>1</sub> (blue line), and TM<sub>1</sub> (green line) as the input waveguide width increases. (<b>b</b>) Fundamental optical mode in a shallow etched configuration is utilized to realize the device. (<b>c</b>) Relationship between etching depth and normalized power.</p> Full article ">Figure 3
<p>(<b>a</b>) Relationship between the angle of the removal region and the power division ratio. (<b>b</b>–<b>g</b>) The relationship between the size of the six power splitter ratios device and the normalized transmission power.</p> Full article ">Figure 3 Cont.
<p>(<b>a</b>) Relationship between the angle of the removal region and the power division ratio. (<b>b</b>–<b>g</b>) The relationship between the size of the six power splitter ratios device and the normalized transmission power.</p> Full article ">Figure 4
<p>(<b>a</b>,<b>b</b>) The correlation between the length and width of the Taper structure and the normalized power. (<b>c</b>) Effect of presence or absence of the taper structure on the normalized power.</p> Full article ">Figure 5
<p>(<b>a</b>) Power splitting ratio (PSR) function of various devices within the wavelength range of 1500~1600 nm. (<b>b</b>) Device losses with different power ratios (pink, light blue, green, blue, red, and purple lines represent power distribution ratios of 50:50, 60:40, 70:30, 80:20, 90:10, 95:5, respectively) in the wavelength range of 1500 to 1600 nm.</p> Full article ">Figure 6
<p>Fabrication tolerances analysis for devices with different power splitter ratios. (<b>a</b>) The derivative of the PSR versus the etching depth per 5 nm (H) for devices with different PSRs. The inset illustrates a cross-section view of the ridge waveguide structure. The total thickness of the Si layer is fixed at 220 nm. (<b>b</b>) Derivative of the power splitter ratio with respect to the angle of the removed area per 0.1 degree for devices with different power ratios. The inset shows the top view of the proposed device. <span class="html-italic">θ</span> represents the angle of the removed region.</p> Full article ">
11 pages, 5670 KiB  
Article
Mechanical Assessment in Atherosclerosis Based on Photoacoustic Viscoelasticity Imaging
by Xingchao Zhang, Xiaohan Shi, Hui Wu, Caixun Bai, Junshan Xiu and Yue Zhao
Photonics 2024, 11(5), 471; https://doi.org/10.3390/photonics11050471 - 17 May 2024
Viewed by 735
Abstract
Early identification of vulnerable plaques is a major challenge in diagnosis and assessment of atherosclerosis. In atherosclerotic plaque development, the proportion change in components caused plaque mechanical property change and induced plaque rupture. In this paper, a photoacoustic viscoelasticity imaging (PAVEI) technique was [...] Read more.
Early identification of vulnerable plaques is a major challenge in diagnosis and assessment of atherosclerosis. In atherosclerotic plaque development, the proportion change in components caused plaque mechanical property change and induced plaque rupture. In this paper, a photoacoustic viscoelasticity imaging (PAVEI) technique was proposed to measure the viscosity–elasticity ratio of atherosclerotic plaque and evaluated for the potential in characterizing vulnerable plaques. Apolipoprotein E-knockout mice fed with a high-fat/high-cholesterol diet were chosen as the atherosclerotic model. Plaque component phantoms were examined to demonstrate the high efficiency of PAVEI in detecting the proportion change in components compared to single elasticity or viscosity detection. Finally, atherosclerotic plaques from mice aortas at different stages were imaged by PAVEI, which provided an insight into the compositional and functional characterization of vulnerability plaques and suggested its potential applications in the identification of high-risk plaques. Full article
(This article belongs to the Special Issue Advanced Techniques in Biomedical Optical Imaging)
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<p>Atherosclerotic tissue was excited with a sinusoidally modulated laser. This process generated a PA signal in the tissue at the same frequency as the excitation but with a delay in the relative phase. Through the recording and analysis of the PA phase delay, the viscoelasticity image of the atherosclerotic plaques could be reconstructed.</p> Full article ">Figure 2
<p>Average viscosity–elasticity ratios of phantoms containing various concentrations of lipid or collagen (<b>A</b>), and of phantoms mixed with different proportions of lipid and collagen (<b>B</b>). (<b>C</b>) Elasticity and viscosity measured by rheometer. (<b>D</b>) Comparison between the viscosity–elasticity ratios measured by PAVEI and rheometer.</p> Full article ">Figure 3
<p>Elasticity and viscosity of (<b>A</b>) phantoms mixed with different proportions of lipid and collagen and (<b>B</b>) the atherosclerotic artery and the control one measured by rheometer, and the viscosity–elasticity ratios measured by PAVEI.</p> Full article ">Figure 4
<p>(<b>A</b>) Cross-section PAVEI of the control artery compared to the atherosclerotic one. (<b>B</b>) Corresponding oil red O staining results. Bars = 100 µm. * <span class="html-italic">p</span> &lt; 0.01 for lipid plaque group vs control group. (<b>C</b>) Comparison of the viscosity–elasticity ratio measured by PAVEI. * <span class="html-italic">p</span> &lt; 0.01 for control artery vs lipid plaque.</p> Full article ">Figure 5
<p>(<b>A</b>) En-face PAVEI and (<b>B</b>) histology acquired from the luminal surface with different duration of HFC diet (4, 8, 12 weeks). The white dashed box showed the imaging area in Figure (<b>A</b>). Bars = 1 mm. (<b>C</b>) Comparison of the viscosity–elasticity ratio measured by PAVEI. * <span class="html-italic">p</span> &lt; 0.01 for 8-week group vs 4-week group; ** <span class="html-italic">p</span> &lt; 0.01 for 12-week group vs 8-week group.</p> Full article ">
10 pages, 2354 KiB  
Article
Enhanced Photon-Pair Generation Based on Thin-Film Lithium Niobate Doubly Resonant Photonic Crystal Cavity
by Jinmian Zhu, Fengli Liu, Fangheng Fu, Yuming Wei, Tiefeng Yang, Heyuan Guan and Huihui Lu
Photonics 2024, 11(5), 470; https://doi.org/10.3390/photonics11050470 - 17 May 2024
Viewed by 863
Abstract
In this work, a doubly resonant photonic crystal (PhC) cavity is proposed to enhance second harmonic generation (SHG) efficiency and photon pair generation rate (PGR). Through the exploration of geometry parameters, a band-edge mode within the light cone is identified as the first [...] Read more.
In this work, a doubly resonant photonic crystal (PhC) cavity is proposed to enhance second harmonic generation (SHG) efficiency and photon pair generation rate (PGR). Through the exploration of geometry parameters, a band-edge mode within the light cone is identified as the first harmonic (FH) mode, and a band-edge mode outside the light cone is designated as the second harmonic (SH). Subsequently, by increasing the layers of the core region, a heterostructure PhC cavity is designed. The results showcase a doubly resonant PhC cavity achieving a 133/W SHG efficiency and a photon pair generation rate of 3.7 × 108/s. The exceptional conversion efficiency is attributed to the high quality factors Q observed in the FH and SH modes with values of approximately 280,000 and 2100, respectively. The remarkably high Q factors compensate for nonlinear efficiency degradation caused by detuning, simultaneously making the manufacturing process easier and more feasible. This work is anticipated to provide valuable insights into efficient nonlinear conversion and photon pair generation rates. Full article
(This article belongs to the Special Issue Advances in Integrated Photonics)
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<p>(<b>a</b>) Schematic diagram of lithium niobate photonic crystal slab of thickness d = 350 nm. (<b>b</b>) The heterostructure cavity with lattice period of a = 770 nm and air holes with radii of r<sub>c</sub> = 250 nm, r<sub>t</sub> = 260 nm, r<sub>o</sub> = 270 nm.</p> Full article ">Figure 2
<p>(<b>a</b>) Photonic band structure for quasi-TE modes of lithium niobate photonic crystal slab with d = 0.45a, r = 0.33a, index of refraction n = 2.21. (<b>b</b>) Photonic band structure for quasi-TM modes of lithium niobate photonic crystal slab with d = 0.45a, r = 0.33a, index of refraction n = 2.25. The green markers in (<b>a</b>,<b>b</b>) highlight the selected doubly resonant modes, the red markers are band structure.</p> Full article ">Figure 3
<p>(<b>a</b>) The red line is the relationship between Q factor of the FH and core size. (<b>b</b>) The blue line is the relationship between Q factor of the SH and core size.</p> Full article ">Figure 4
<p>(<b>a</b>) Top view of the E field distribution of FH mode in the XOY plane at 1558.3 nm. (<b>b</b>) Top view of the E field distribution of SH mode in the XOY plane at 781.2 nm. (<b>c</b>) Top view of the H field distribution of FH mode in the XOY plane at 1558.3 nm. (<b>d</b>) Top view of the H field distribution of SH mode in the XOY plane at 781.2 nm. (<b>e</b>) Side view of the E field distribution of FH mode in the YOZ plane at 1558.3 nm. (<b>f</b>) Side view of the E field distribution of SH mode in the YOZ plane at 781.2 nm. (<b>g</b>) Side view of the H field distribution of FH mode in the YOZ plane at 1558.3 nm. (<b>h</b>) Side view of the H field distribution of SH mode in the YOZ plane at 781.2 nm. (<b>i</b>) Schematic diagram of the far-field mode in the XOY plane at 1558.3 nm. (<b>j</b>) The red line is the relationship between square of nonlinear overlapping factors and core region size.</p> Full article ">Figure 5
<p>(<b>a</b>) The red line is doubly resonant scattering spectrum at FH and the blue line is at SH (N<sub>c</sub> = 24). (<b>b</b>) The red line is the relationship between core size and SHG Efficiency and the blue line is the relationship between core size and PGR.</p> Full article ">
15 pages, 4537 KiB  
Article
High-Q Multiband Narrowband Absorbers Based on Two-Dimensional Graphene Metamaterials
by Aijun Zhu, Pengcheng Bu, Lei Cheng, Cong Hu and Rabi Mahapatra
Photonics 2024, 11(5), 469; https://doi.org/10.3390/photonics11050469 - 16 May 2024
Viewed by 824
Abstract
In this paper, an absorber with multi-band, tunable, high Q, and high sensitivity, based on terahertz periodic two-dimensional patterned graphene surface plasmon resonance (SPR), is proposed. The absorber consists of a bottom metal film separated by a periodically patterned graphene metamaterial structure and [...] Read more.
In this paper, an absorber with multi-band, tunable, high Q, and high sensitivity, based on terahertz periodic two-dimensional patterned graphene surface plasmon resonance (SPR), is proposed. The absorber consists of a bottom metal film separated by a periodically patterned graphene metamaterial structure and a SiO2 dielectric layer, where the patterned graphene layer is etched by “+” and “L” shapes and circles. It has simple structural features that can greatly simplify the fabrication process. We have analyzed the optical properties of a graphene surface plasmon perfect metamaterial absorber based on graphene in the terahertz region using the finite-difference method in time domain (FDTD). The results show that the absorber device exhibits three perfect absorption peaks in the terahertz bands of f1 = 1.55 THz, f2 = 4.19 THz, and f3 = 6.92 THz, with absorption rates as high as 98.70%, 99.63%, and 99.42%, respectively. By discussing the effects of parameters such as the geometrical dimensions of patterned graphene metamaterial structure “+” width W1, “L” width W2, circular width R, and the thickness of the dielectric layer on the absorption performance of absorber, as well as investigating the chemical potential and relaxation time of patterned-layer graphene material, it was found that the amplitude of the absorption peaks and the frequency of resonance of absorber devices can be dynamically adjusted. Finally, we simulated the spectra as the surrounding refractive index n varied to better evaluate the sensing performance of the structure, yielding structural sensitivities up to 382 GHz/RIU. Based on this study, we find that the results of our research will open new doors for the use of multi-band, tunable, polarization-independent metamaterial absorbers that are insensitive to large-angle oblique incidence. Full article
(This article belongs to the Special Issue Photonic Devices Based on Plasmonic or Dielectric Nanostructures)
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<p>(<b>a</b>) Schematic of the unit structure of a tunable perfect absorber based on periodically patterned graphene; (<b>b</b>) three-dimensional schematic of the absorber; (<b>c</b>) structural diagram of the top layer of graphene; (<b>d</b>) absorption spectra of the proposed perfect absorber (black solid line), the structure with only circular graphene arrays (red dashed line), the structure with only “L” shaped graphene arrays (blue dashed line), and the structure with only “+” shaped graphene arrays (green dashed line); (<b>e</b>) top view of the “+”-shaped graphene layer’s unit structure; (<b>f</b>) top view of four “L”-shaped graphene layer’s unit structures; (<b>g</b>) top view of a circular graphene layer’s unit structure; (<b>h</b>) top view of a patterned graphene layer’s unit structure.</p> Full article ">Figure 2
<p>(<b>a</b>–<b>c</b>) Electric field strength distribution at the top of the absorber in the x-y plane for different resonant frequencies; (<b>d</b>–<b>f</b>) electric field intensity distribution in the x-z plane at different resonant frequencies; (<b>g</b>–<b>i</b>) electric field intensity distribution in the y-z plane at different resonant frequencies. The resonant frequencies are f1 = 1.55 THz, f2 = 4.19 THz, and f3 = 6.92 THz.</p> Full article ">Figure 3
<p>The absorption spectra of fixed, patterned, circular, and “+” shape graphene parameters with the “L” shape’s width W2 changing from 0.20 μm to 0.30 μm.</p> Full article ">Figure 4
<p>Absorption spectra of the fixed patterned circular and “L” shape graphene parameters with the “+” shape’s length L1 changing from 1.3 μm to 2.1 μm.</p> Full article ">Figure 5
<p>Absorption spectra of fixed patterned rings and “L” shape graphene parameters with “+” shape’s width W1 changing from 0.1 μm to 2.1 μm.</p> Full article ">Figure 6
<p>Absorption spectra of the fixed patterned “+” and “L” shape graphene parameters with a change in the width of the rings from 0.4 μm to 0.8 μm.</p> Full article ">Figure 7
<p>Absorption spectra of the fixed patterned graphene for each parameter, with the thickness of the dielectric layer H2 changing from 2.02 μm to 7.02 μm.</p> Full article ">Figure 8
<p>(<b>a</b>) Absorption spectra obtained by changing the chemical potential of graphene from 0.6 to 1.4 eV; (<b>b</b>,<b>c</b>) the resonance frequency and peak absorption intensity spectra of the three modes with the change in chemical potential, respectively.</p> Full article ">Figure 9
<p>(<b>a</b>) Absorption spectra of the absorber for different relaxation times <span class="html-italic">τ</span>. (<b>b</b>) Absorption peak versus relaxation time <span class="html-italic">τ</span> for modes A, B, and C.</p> Full article ">Figure 10
<p>(<b>a</b>) Absorption spectra of mode A, mode B, and mode C at different refractive indices; (<b>b</b>,<b>c</b>) linear relationship between resonant frequency and refractive index; (<b>d</b>) linear relationship between absorption peaks and refractive index.</p> Full article ">Figure 11
<p>(<b>a</b>) Absorption spectra for different polarization angles; (<b>b</b>,<b>c</b>) the absorption spectra of incident light at angles of incidence ranging from 0°to 50° for TE polarization and TM polarization, respectively.</p> Full article ">
16 pages, 9701 KiB  
Article
Compact Quantum Random Number Generator Based on a Laser Diode and a Hybrid Chip with Integrated Silicon Photonics
by Xuyang Wang, Tao Zheng, Yanxiang Jia, Jin Huang, Xinyi Zhu, Yuqi Shi, Ning Wang, Zhenguo Lu, Jun Zou and Yongmin Li
Photonics 2024, 11(5), 468; https://doi.org/10.3390/photonics11050468 - 16 May 2024
Cited by 1 | Viewed by 735
Abstract
In this study, a compact and low-power-consumption quantum random number generator (QRNG) based on a laser diode and a hybrid chip with integrated silicon photonics is proposed and verified experimentally. The hybrid chip’s size is 8.8 × 2.6 × 1 mm3, [...] Read more.
In this study, a compact and low-power-consumption quantum random number generator (QRNG) based on a laser diode and a hybrid chip with integrated silicon photonics is proposed and verified experimentally. The hybrid chip’s size is 8.8 × 2.6 × 1 mm3, and the power of the entropy source is 80 mW. A common-mode rejection ratio greater than 40 dB was achieved using an optimized 1 × 2 multimode interferometer structure. A method for optimizing the quantum-to-classical noise ratio is presented. A quantum-to-classical noise ratio of approximately 9 dB was achieved when the photoelectron current is 1 μA using a balance homodyne detector with a high dark current GeSi photodiode. The proposed QRNG has the potential for use in scenarios of moderate MHz random number generation speed, with low power, small volume, and low cost prioritized. Full article
(This article belongs to the Topic Hybrid and Heterogeneous Integration on Photonic Circuits)
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<p>QRNG scheme and images: (<b>a</b>) scheme, (<b>b</b>) hybrid chip structure, (<b>c</b>) microphotograph of the hybrid chip, and (<b>d</b>) analog circuit. GC: grating coupler, CC: constant current, PD: photodiode, MMI: multimode interferometer, HPF: high-pass filter, PCB: printed circuit board, ADC: analog-to-digital converter, SPI: silicon photonics integrated.</p> Full article ">Figure 2
<p>BHD noise model. <math display="inline"><semantics> <msub> <mi>C</mi> <mi>TF</mi> </msub> </semantics></math>: total capacitance in the feedback loop; <math display="inline"><semantics> <msub> <mi>C</mi> <mi>TIN</mi> </msub> </semantics></math>: total capacitance in the input port.</p> Full article ">Figure 3
<p>Noise current densities versus frequency in BHD based on (<b>a</b>) GeSi and (<b>b</b>) InGaAs photodiodes.</p> Full article ">Figure 4
<p>Transimpedance gain mode and output noise voltage densities versus frequency: (<b>a</b>) transimpedance gain mode <math display="inline"><semantics> <mfenced separators="" open="|" close="|"> <mrow> <mi>G</mi> <mo>(</mo> <mi>f</mi> <mo>)</mo> </mrow> </mfenced> </semantics></math> and (<b>b</b>) output noise voltage densities.</p> Full article ">Figure 5
<p>Calculated and measured noise power of BHD based on two types of photodiodes. Calculated results with GeSi photodiode: (<b>a</b>) DC to 2 MHz and (<b>b</b>) DC to 10 MHz. Measurement results with GeSi photodiode: (<b>c</b>) DC to 2 MHz and (<b>d</b>) DC to 10 MHz. Measurement results with InGaAs photodiode: (<b>e</b>) DC to 2 MHz and (<b>f</b>) DC to 10 MHz.</p> Full article ">Figure 6
<p>Optimized 1 × 2 MMI structure: (<b>a</b>) size of 1 × 2 MMI. (<b>b</b>) Simulated power distribution at 1550 nm wavelength. (<b>c</b>) Transmission efficiency versus core length. (<b>d</b>) Transmission efficiency versus wavelength. The coordinates of black circles corresponding to optimized values are presented in (<b>c</b>,<b>d</b>).</p> Full article ">Figure 7
<p>CMRR measurement results of BHD based on (<b>a</b>) GeSi photodiode and (<b>b</b>) InGaAs photodiode.</p> Full article ">Figure 8
<p>The min entropies versus the ratio <math display="inline"><semantics> <mrow> <mi>R</mi> <mrow> <mfenced open="/" close=""> <mphantom> <mpadded width="0pt"> <mi>R</mi> <msub> <mi>σ</mi> <mi mathvariant="normal">Q</mi> </msub> </mpadded> </mphantom> </mfenced> </mrow> <msub> <mi>σ</mi> <mi mathvariant="normal">Q</mi> </msub> </mrow> </semantics></math>. (<b>a</b>) Average conditional min entropy; (<b>b</b>) Worst-case conditional min entropy.</p> Full article ">Figure 9
<p>(<b>a</b>) Time traces of obtained quantum and classical noise data, (<b>b</b>) histogram of obtained quantum and classical noise data, and (<b>c</b>) randomness test result.</p> Full article ">Figure A1
<p>Typical TIA circuit based on an operation amplifier with different noise: (<b>a</b>) without noise, (<b>b</b>) with noise voltage of TIA <math display="inline"><semantics> <msub> <mi>u</mi> <mi>NV</mi> </msub> </semantics></math>, and (<b>c</b>) with noise voltage of feedback resistance <math display="inline"><semantics> <msub> <mi>u</mi> <mi>RFT</mi> </msub> </semantics></math>.</p> Full article ">
12 pages, 5941 KiB  
Article
Boundary Feedback Fiber Random Microcavity Laser Based on Disordered Cladding Structures
by Hongyang Zhu, Bingquan Zhao, Zhi Liu, Zhen He, Lihong Dong, Hongyu Gao and Xiaoming Zhao
Photonics 2024, 11(5), 467; https://doi.org/10.3390/photonics11050467 - 16 May 2024
Viewed by 787
Abstract
The cavity form of complex microcavity lasers predominantly relies on disordered structures, whether found in nature or artificially prepared. These structures, characterized by disorder, facilitate random lasing through the feedback effect of the cavity boundary and the internal scattering medium via various mechanisms. [...] Read more.
The cavity form of complex microcavity lasers predominantly relies on disordered structures, whether found in nature or artificially prepared. These structures, characterized by disorder, facilitate random lasing through the feedback effect of the cavity boundary and the internal scattering medium via various mechanisms. In this paper, we report on a random fiber laser employing a disordered scattering cladding medium affixed to the inner cladding of a hollow-core fiber. The internal flowing liquid gain establishes a stable liquid-core waveguide environment, enabling long-term directional coupling output for random laser emission. Through theoretical analysis and experimental validation, we demonstrate that controlling the disorder at the cavity boundary allows liquid-core fiber random microcavities to exhibit random lasing output with different mechanisms. This provides a broad platform for in-depth research into the generation and control of complex microcavity lasers, as well as the detection of scattered matter within micro- and nanostructures. Full article
(This article belongs to the Special Issue Advancements in Fiber Lasers and Their Applications)
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<p>Configuration and basic features of the fiber random microcavity. (<b>a</b>) The axial microscopic image of HCF filled with suspension. (<b>b</b>) The axial microscopic images of optical fiber random microcavities with different degrees of disorder formed by airflow modification. (<b>c</b>,<b>d</b>) Two random microcavity cross-sections of fiber random microcavities with different degrees of disorder. (<b>e</b>) Sketch of the basic experimental setup. P, polarizer. PBS, polarization beam splitter. BS, beam splitter. PM, power meter. OBJ, microscope objective. M, mirror. AD, Adjustable diaphragm. CCD, charge-coupled device, S, spectrometer. MC, microfluidic controller. The inset is a microscopic image of the end surface of the sample. From outside to inside, there are walls of HCF, disordered inner cladding, and the gain region.</p> Full article ">Figure 2
<p>The spectral properties produced by a coherent random laser based on a localized regime. (<b>a</b>) The spectra of random laser with different pump energy densities. (<b>b</b>) The average photon counts at the peaks as the pump energy density changes. (<b>c</b>) The excitation spectrum of five consecutive pulses during long-term segmental measurement. (<b>d</b>) The changing trend of the average peak photon counts of five consecutive pulses collected at different periods.</p> Full article ">Figure 3
<p>The spectral properties of random laser based on diffusion and partially coherent random lasers. (<b>a</b>) The variation of incoherent random laser spectrum generated with pump power density at position 2. (<b>b</b>) The average photon counts at the peaks as the pump energy density changes. (<b>c</b>) The spectrum of partially coherent random laser excited with pump power density increasing. (<b>d</b>) The corresponding average photon counts at the peaks.</p> Full article ">Figure 4
<p>The PFT properties of random lasers based on strong localization and diffusive random lasers. (<b>a</b>) Power Fourier transform of the output spectrum produced by pumping position 1. The inset is the corresponding spectral information of a coherent random laser. (<b>b</b>) Power Fourier transform of the output spectrum produced by pumping position 2. The inset is the corresponding spectral information of a diffusive random laser. (<b>c</b>,<b>d</b>) The statistical characteristics of resonance peak intensity and equivalent optical cavity length in PFT, the red represents position 1 and the blue represents position 2.</p> Full article ">Figure 5
<p>The cold cavity theory analysis of boundary feedback fiber random microcavity. (<b>a</b>) The radial simulation structure of optical fiber random microcavity. The color scale represents the refractive index. (<b>b</b>,<b>c</b>) The change in optical power distribution in the radial section with the number of scattering particles <span class="html-italic">n</span>. The color scale represents the optical power. (<b>b</b>) <span class="html-italic">n</span> = 100. (<b>c</b>) <span class="html-italic">n</span> = 3000. (<b>d</b>) The axial simulation structure of optical fiber random microcavity. (<b>e</b>,<b>f</b>) The change in optical power distribution in the axial section with the number of scattering particles <span class="html-italic">n</span>, (<b>e</b>) <span class="html-italic">n</span> = 100. (<b>f</b>) <span class="html-italic">n</span> = 3000.</p> Full article ">Figure 6
<p>The theoretical spectral analysis of boundary feedback fiber random microcavity laser. (<b>a</b>–<b>c</b>) Spectral changes monitored from the fiber core to near the scattering boundary in the radial 2D model. (<b>d</b>) Radial section structure of optical fiber microcavity with regular boundary feedback. The color scale represents the refractive index. (<b>e</b>,<b>f</b>) Spectral changes monitored from the fiber core to near the scattering boundary in the radial 2D model.</p> Full article ">
11 pages, 2614 KiB  
Article
The Generation of Equal-Intensity and Multi-Focus Optical Vortices by a Composite Spiral Zone Plate
by Huaping Zang, Jingzhe Li, Chenglong Zheng, Yongzhi Tian, Lai Wei, Quanping Fan, Shaoyi Wang, Chuanke Wang, Juan Xie and Leifeng Cao
Photonics 2024, 11(5), 466; https://doi.org/10.3390/photonics11050466 - 15 May 2024
Viewed by 785
Abstract
We propose a new vortex lens for producing multiple focused coaxial vortices with approximately equal intensities along the optical axis, termed equal-intensity multi-focus composite spiral zone plates (EMCSZPs). In this typical methodology, two concentric conventional spiral zone plates (SZPs) of different focal lengths [...] Read more.
We propose a new vortex lens for producing multiple focused coaxial vortices with approximately equal intensities along the optical axis, termed equal-intensity multi-focus composite spiral zone plates (EMCSZPs). In this typical methodology, two concentric conventional spiral zone plates (SZPs) of different focal lengths were composited together and the alternate transparent and opaque zones were arranged with specific m-bonacci sequence. Based on the Fresnel–Kirchhoff diffraction theory, the focusing properties of the EMCSZPs were calculated in detail and the corresponding demonstration experiment was been carried out to verify our proposal. The investigations indicate that the EMCSZPs indeed exhibit superior performance, which accords well with our physical design. In addition, the topological charges (TCs) of the multi-focus vortices can be flexibly selected and controlled by optimizing the parameters of the zone plates. These findings which were demonstrated by the performed experiment may open new avenues towards improving the performance of biomedical imaging, quantum computation and optical manipulation. Full article
(This article belongs to the Special Issue Space Division Multiplexing Techniques)
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<p>(<b>a</b>) Geometrical construction of the Tetranacci sequence up to order <span class="html-italic">S</span> = 6, <span class="html-italic">m</span> = 4. The schematic view of SZPs with TCs of (<b>b</b>) <span class="html-italic">l</span> = 1. The schematic view of EMCSZPs with TCs of (<b>c</b>) <span class="html-italic">l</span> = 1. (<b>d</b>) <span class="html-italic">l</span> = 2.</p> Full article ">Figure 2
<p>(<b>a</b>) The inner zone plates for the first 29 rings of EMCSZPs. (<b>b</b>) Axial and transverse irradiance profiles of inner zone plates. (<b>c</b>) The outer zone plates and (<b>d</b>) axial and transverse irradiance profiles for the following 30 rings of EMCSZPs.</p> Full article ">Figure 3
<p>Far-field diffraction intensity distribution on the <span class="html-italic">y</span>-<span class="html-italic">z</span> (<span class="html-italic">x</span> = 0) plane and transverse irradiance and phase profiles on the <span class="html-italic">x</span>-<span class="html-italic">y</span> (<span class="html-italic">z</span> = 0) plane: (<b>a</b>) conventional SZPs, (<b>b</b>) EMCSZPs of <span class="html-italic">l</span> = 1, (<b>c</b>) EMCSZPs of <span class="html-italic">l</span> = 2. (<b>d</b>,<b>e</b>) The normalized vortex intensities along the dashed lines in (<b>b</b>,<b>c</b>).</p> Full article ">Figure 4
<p>EMCSZP schematic diagram for changing the focal lengths of inner and outer zone plates and EMCSZP axial irradiation diagram. We set the focal length of the inner zone plates as <span class="html-italic">f</span><sub>1</sub> and the focal length of the outer zone plates as <span class="html-italic">f</span><sub>2</sub>. (<b>a</b>) <span class="html-italic">f</span><sub>1</sub> = 260 mm, <span class="html-italic">f</span><sub>2</sub> = 310 mm, (<b>b</b>) <span class="html-italic">f</span><sub>1</sub> = 255 mm, <span class="html-italic">f</span><sub>2</sub> = 315 mm, (<b>c</b>) <span class="html-italic">f</span><sub>1</sub> = 250 mm, <span class="html-italic">f</span><sub>2</sub> = 320 mm.</p> Full article ">Figure 5
<p>(<b>a</b>) The calculated axial irradiance EMCSZPs in X-ray region. (<b>b</b>) Corresponding intensity profiles along the <span class="html-italic">z</span>-<span class="html-italic">x</span> plane (<span class="html-italic">y</span> = 0).</p> Full article ">Figure 6
<p>Experimental results of the EMCSZPs. (<b>a</b>) Schematic diagram of experimental setup. (<b>b</b>) The transverse irradiance and phase profiles on the <span class="html-italic">x</span>-<span class="html-italic">y</span> plane of <span class="html-italic">l</span> = 1. (<b>c</b>) The reconstructed axial irradiance results for <span class="html-italic">l</span> = 1 on the <span class="html-italic">y</span>-<span class="html-italic">z</span> plane (<span class="html-italic">x</span> = 0). (<b>d</b>) The transverse irradiance and phase profiles and (<b>e</b>) the reconstructed axial irradiance of <span class="html-italic">l</span> = 2.</p> Full article ">
14 pages, 2136 KiB  
Article
Simulation of a Pulsed Metastable Helium Lidar
by Jiaxin Lan, Yuli Han, Ruocan Zhao, Tingdi Chen, Xianghui Xue, Dongsong Sun, Hang Zhou, Zhenwei Liu and Yingyu Liu
Photonics 2024, 11(5), 465; https://doi.org/10.3390/photonics11050465 - 15 May 2024
Viewed by 870
Abstract
Measurements of atmosphere density in the upper thermosphere and exosphere are of great significance for studying space–atmosphere interactions. However, the region from 200 km to 1000 km has been a blind area for traditional ground-based active remote sensing techniques due to the limitation [...] Read more.
Measurements of atmosphere density in the upper thermosphere and exosphere are of great significance for studying space–atmosphere interactions. However, the region from 200 km to 1000 km has been a blind area for traditional ground-based active remote sensing techniques due to the limitation of facilities and the paucity of neutral atmosphere. To fulfill this gap, the University of Science and Technology of China is developing a powerful metastable helium resonance fluorescent lidar incorporating a 2 m aperture telescope, a high-energy 1083 nm pulsed laser, as well as a superconducting nanowire single-photon detector (SNSPD) with high quantum efficiency and low dark noise. The system is described in detail in this work. To evaluate the performance of the lidar system, numerical simulation is implemented. The results show that metastable helium density measurements can be achieved with a relative error of less than 20% above 370 km in winter and less than 200% in 270–460 km in summer, demonstrating the feasibility of metastable helium lidar. Full article
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<p>Backscattering cross-section of metastable helium (vertical wind speed is 0 and temperature is 1125 K).</p> Full article ">Figure 2
<p>Metastable helium number density profiles in winter and summer seasons for the Arecibo Observatory in Puerto Rico from model established by Waldrop et al. [<a href="#B26-photonics-11-00465" class="html-bibr">26</a>].</p> Full article ">Figure 3
<p>Schematic optical layout of metastable helium lidar (IF: interference filter; FPI: Fabry–Perot interferometer; SNSPD: superconducting nanowire single-photon detector; TDC: time-dependent single-photon counter; orange circle: PID loop).</p> Full article ">Figure 4
<p>Frequency stability of seed laser in 2 h. Blue line and red line in (<b>a</b>) are frequency of seed laser with and without stabilized, respectively, and (<b>b</b>) is locked frequency in detail.</p> Full article ">Figure 5
<p>Simulated transmittance of the combination of interference filter and FPI.</p> Full article ">Figure 6
<p>Transmission of FADOF.</p> Full article ">Figure 7
<p>The impact of pulse energy on saturation effect at different altitudes (pulse duration is 10 ns, laser spectral bandwidth is 300 MHz, beam divergence angle is 33 μrad).</p> Full article ">Figure 8
<p>Total photon counts estimated in winter and summer.</p> Full article ">Figure 9
<p>SNR estimated in winter and summer.</p> Full article ">Figure 10
<p>Total relative error.</p> Full article ">
11 pages, 4949 KiB  
Article
The Generation of Circularly Polarized Isolated Attosecond Pulses with Tunable Helicity from CO Molecules in Polarization Gating Laser Fields
by Shiju Chen, Hua Yuan, Feng Wang, Jiahang Song, Yue Zhao, Chunhui Yang, Tianxin Ou, Ru Zhang, Qiang Chang and Yuping Sun
Photonics 2024, 11(5), 464; https://doi.org/10.3390/photonics11050464 - 15 May 2024
Viewed by 801
Abstract
We theoretically demonstrate a scheme to generate circularly polarized (CP) isolated attosecond pulses (IAPs) with tunable helicity using a polarization gating laser field interacting with the CO molecule. The results show that a broadband CP supercontinuum is produced from the oriented CO molecule, [...] Read more.
We theoretically demonstrate a scheme to generate circularly polarized (CP) isolated attosecond pulses (IAPs) with tunable helicity using a polarization gating laser field interacting with the CO molecule. The results show that a broadband CP supercontinuum is produced from the oriented CO molecule, which supports the generation of an IAP with an ellipticity of 0.98 and a duration of 90 as. Furthermore, the helicity of the generated harmonics and IAP can be effectively controlled by modulating the laser field and the orientation angle of the CO molecule. Our method will advance research on chiral-specific dynamics and magnetic circular dichroism on the attosecond timescale. Full article
(This article belongs to the Topic Recent Advances in Nonlinear Optics and Nonlinear Optical Materials)
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<p>(<b>a</b>) The 3D plot of the PG laser field (purple line). The x, y components of the electric field (orange and blue lines). (<b>b</b>) The x, y components of the generated harmonic spectrum with the CO molecule oriented at <math display="inline"><semantics> <mrow> <mi>θ</mi> <mo>=</mo> <msup> <mrow> <mn>280</mn> </mrow> <mrow> <mo>∘</mo> </mrow> </msup> </mrow> </semantics></math>. The corresponding relative phases of the x, y components are also inserted as the blue circles. A black solid line indicates the relative phase of 0.5π. (<b>c</b>) The LCP and RCP components of the generated harmonic spectrum. (<b>d</b>,<b>e</b>) The corresponding time–frequency distributions of the x and y components of the harmonics. (<b>f</b>) The 3D plot of the electric field of the attosecond pulse generated by superposing the 360th- to 440th-order harmonics in (<b>b</b>).</p> Full article ">Figure 2
<p>(<b>a</b>–<b>c</b>) The LCP and RCP components of the harmonic spectra generated from the CO molecule oriented at <math display="inline"><semantics> <mrow> <mi>θ</mi> <mo>=</mo> <msup> <mrow> <mn>280</mn> </mrow> <mrow> <mo>∘</mo> </mrow> </msup> </mrow> </semantics></math> in the PG laser field for the time delays, <math display="inline"><semantics> <mrow> <msub> <mrow> <mi>T</mi> </mrow> <mrow> <mi>d</mi> </mrow> </msub> </mrow> </semantics></math>, of the laser fields of <math display="inline"><semantics> <mrow> <mn>1.6</mn> <msub> <mrow> <mi mathvariant="normal">T</mi> </mrow> <mrow> <mn>0</mn> </mrow> </msub> </mrow> </semantics></math>, <math display="inline"><semantics> <mrow> <mn>1.9</mn> <msub> <mrow> <mi mathvariant="normal">T</mi> </mrow> <mrow> <mn>0</mn> </mrow> </msub> </mrow> </semantics></math>, and <math display="inline"><semantics> <mrow> <mn>2.2</mn> <msub> <mrow> <mi mathvariant="normal">T</mi> </mrow> <mrow> <mn>0</mn> </mrow> </msub> </mrow> </semantics></math>, respectively. (<b>d</b>–<b>f</b>) The 3D plots of the electric fields of the attosecond pulses generated by superposing the 360th- to 440th-order harmonics in (<b>a</b>–<b>c</b>), respectively. In our simulations, except for the time delay, <math display="inline"><semantics> <mrow> <msub> <mrow> <mi>T</mi> </mrow> <mrow> <mi>d</mi> </mrow> </msub> </mrow> </semantics></math>, other parameters are the same as those in <a href="#photonics-11-00464-f001" class="html-fig">Figure 1</a>.</p> Full article ">Figure 3
<p>(<b>a</b>–<b>c</b>) The LCP and RCP components of the harmonic spectra generated from the CO molecule oriented at <math display="inline"><semantics> <mrow> <mi>θ</mi> <mo>=</mo> <mn>280</mn> <mo>°</mo> </mrow> </semantics></math> in the PG laser fields for CEPs, <math display="inline"><semantics> <mrow> <msub> <mrow> <mi>φ</mi> </mrow> <mrow> <mi>C</mi> <mi>E</mi> </mrow> </msub> </mrow> </semantics></math>, of <math display="inline"><semantics> <mrow> <mo>−</mo> <mn>0.3</mn> <mi mathvariant="sans-serif">π</mi> <mo>,</mo> </mrow> </semantics></math> <math display="inline"><semantics> <mrow> <mn>0.7</mn> <mi mathvariant="sans-serif">π</mi> </mrow> </semantics></math>, and <math display="inline"><semantics> <mrow> <mn>0.9</mn> <mi mathvariant="sans-serif">π</mi> </mrow> </semantics></math>, respectively. (<b>d</b>–<b>f</b>) The 3D plots of the electric fields of the attosecond pulses generated by superposing the harmonics from the 360th to 440th orders, 320th to 360th orders, and 640th to 700th orders in (<b>a</b>–<b>c</b>), respectively. In our simulations, except for the CEP, <math display="inline"><semantics> <mrow> <msub> <mrow> <mi>φ</mi> </mrow> <mrow> <mi>C</mi> <mi>E</mi> </mrow> </msub> </mrow> </semantics></math>, other parameters are the same as those in <a href="#photonics-11-00464-f001" class="html-fig">Figure 1</a>.</p> Full article ">Figure 4
<p>(<b>a</b>–<b>c</b>) The LCP and RCP components of the harmonic spectra generated in the PG laser field for orientation angles, <math display="inline"><semantics> <mrow> <mi>θ</mi> </mrow> </semantics></math>, of the CO molecules of <math display="inline"><semantics> <mrow> <mn>100</mn> <mo>°</mo> </mrow> </semantics></math>, <math display="inline"><semantics> <mrow> <mn>275</mn> <mo>°</mo> </mrow> </semantics></math>, and <math display="inline"><semantics> <mrow> <mn>285</mn> <mo>°</mo> </mrow> </semantics></math>, respectively. (<b>d</b>–<b>f</b>) The 3D plots of the electric fields of the attosecond pulses generated by superposing the harmonics from the 460th to 540th orders, 480th to 560th orders, and 310th to 390th orders in (<b>a</b>–<b>c</b>), respectively. In our simulations, except for the orientation angle, <math display="inline"><semantics> <mrow> <mi>θ</mi> </mrow> </semantics></math>, other parameters are the same as those in <a href="#photonics-11-00464-f001" class="html-fig">Figure 1</a>.</p> Full article ">Figure 5
<p>(<b>a</b>–<b>c</b>) The LCP and RCP components of the harmonic spectra generated in the PG laser field with the CO molecule oriented at <math display="inline"><semantics> <mrow> <mi>θ</mi> <mo>=</mo> <mn>280</mn> <mo>°</mo> </mrow> </semantics></math> for orientation degrees of 0.8, 0.6, and 0.4, respectively. (<b>d</b>–<b>f</b>) The 3D plots of the electric fields of the attosecond pulses generated by superposing the 660th- to 690th-order harmonics, the 490th- to 550th-order harmonics, and the 430th- to 490th-order harmonics in (<b>a</b>–<b>c</b>), respectively.</p> Full article ">Figure 6
<p>(<b>a</b>) The LCP and RCP components of the harmonic spectra generated from the CO molecule oriented at <math display="inline"><semantics> <mrow> <mi>θ</mi> <mo>=</mo> <mn>280</mn> <mo>°</mo> </mrow> </semantics></math> in the PG laser field for an intensity of <math display="inline"><semantics> <mrow> <mn>3</mn> <mo>×</mo> <msup> <mrow> <mn>10</mn> </mrow> <mrow> <mn>14</mn> </mrow> </msup> <mo> </mo> <mi mathvariant="normal">W</mi> <mo>/</mo> <msup> <mrow> <mi mathvariant="normal">c</mi> <mi mathvariant="normal">m</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msup> </mrow> </semantics></math> and a wavelength of 1300 nm. (<b>b</b>) The 3D plots of the electric fields of the attosecond pulses generated by superposing the harmonics from the 280th to 340th orders in (<b>a</b>). (<b>c</b>) Same as (<b>a</b>), but for an intensity of <math display="inline"><semantics> <mrow> <mn>2</mn> <mo>×</mo> <msup> <mrow> <mn>10</mn> </mrow> <mrow> <mn>14</mn> </mrow> </msup> <mo> </mo> <mi mathvariant="normal">W</mi> <mo>/</mo> <msup> <mrow> <mi mathvariant="normal">c</mi> <mi mathvariant="normal">m</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msup> </mrow> </semantics></math> and a wavelength of 1600 nm. (<b>d</b>) The 3D plots of the electric fields of the attosecond pulses generated by superposing the harmonics from the 340th to 400th orders in (<b>c</b>). In our simulations, except for the wavelength and intensity, other parameters are the same as those in <a href="#photonics-11-00464-f001" class="html-fig">Figure 1</a> of our manuscript.</p> Full article ">
15 pages, 6857 KiB  
Article
Generation of Propagation-Dependent OAM Self-Torque with Chirped Spiral Gratings
by Ruediger Grunwald, Mathias Jurke, Max Liebmann, Alexander Treffer and Martin Bock
Photonics 2024, 11(5), 463; https://doi.org/10.3390/photonics11050463 - 15 May 2024
Viewed by 821
Abstract
The application of non-uniform spiral gratings to control the structure, topological parameters and propagation of orbital angular momentum (OAM) beams was studied experimentally with coherent near-infrared light. Adapted digital spiral grating structures were programmed into the phase map of a high-resolution liquid-crystal-on-silicon spatial [...] Read more.
The application of non-uniform spiral gratings to control the structure, topological parameters and propagation of orbital angular momentum (OAM) beams was studied experimentally with coherent near-infrared light. Adapted digital spiral grating structures were programmed into the phase map of a high-resolution liquid-crystal-on-silicon spatial light modulator (LCoS-SLM). It is shown that characteristic spatio-spectral anomalies related to Gouy phase shift can be used as pointers to quantify rotational beam properties. Depending on the sign and gradient of spatially variable periods of chirped spiral gratings (CSGs), variations in rotation angle and angular velocity were measured as a function of the propagation distance. Propagation-dependent self-torque is introduced in analogy to known local self-torque phenomena of OAM beams as obtained by the superposition of temporally chirped or phase-modulated wavepackets. Applications in metrology, nonlinear optics or particle trapping are conceivable. Full article
(This article belongs to the Special Issue Structured Light Beams: Science and Applications)
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Figure 1

Figure 1
<p>Selected types of uniform and non-uniform spiral gratings used in singular optics experiments in spectral domain (schematically); (<b>a</b>) case (A,1), (<b>b</b>) (A,2), (<b>c</b>) (B,2) and (B,4) with positive spatial chirp for each azimuthal angle, (<b>d</b>) (C,1), (<b>e</b>) (A,3) torus with negative spatial chirp, (<b>f</b>) (B,3), (<b>g</b>) (D,1), (<b>h</b>) (D,3), (<b>i</b>) (D,4). Patterns are transferred into calibrated SLM phase maps.</p> Full article ">Figure 2
<p>Experimental setup for the rotational control of OAM beams in spectral domain (schematically). (L = laser, BF = rotatable narrow band-pass filter, RP = rotatable broadband polarizer, BE = beam expander, M = high-reflectance mirror, SLM = spatial light modulator, PC1 = computer for programming SLM, SPG = spiral phase grating, <span class="html-italic">p</span>(<span class="html-italic">r</span>) = radially dependent grating period, OAM = OAM beam, bSE and rSE = blue-shifted and red-shifted spectral eyes, CAM = movable camera with 50× and 100× objective lenses, PC2 = computer for data processing, <math display="inline"><semantics> <mrow> <mi>φ</mi> </mrow> </semantics></math>(<span class="html-italic">z</span>) = local azimuthal rotation angle, <math display="inline"><semantics> <mrow> <mi>Ω</mi> <mo>(</mo> <mi>z</mi> <mo>)</mo> </mrow> </semantics></math> = local angular velocity of spectral Gouy rotation).</p> Full article ">Figure 3
<p>Theoretical perfectly linear curve progression of radially variable SPG periods with (<b>a</b>) negative (type B,3) and (<b>b</b>) positive (type B,2) spatial frequency chirp d<span class="html-italic">ν</span>/d<span class="html-italic">r</span> (parameters for near-realistic conditions, number of spiral rotations: 37 and 55, along minor axes: increasing period between 29 µm and 55 µm, decreasing period between 58 µm and 34 µm, respectively). Depending on incident angle <span class="html-italic">α</span>, gratings are stretched in <span class="html-italic">x</span>-direction (perpendicular to the tilt axis) by a factor of cos<span class="html-italic">α</span>. Periods are compared for two tilt angles (<span class="html-italic">α</span> = 0°: circular SPG, <span class="html-italic">α</span> = 45°: elliptical SPG). Colors symbolize spatial frequency chirp (decreasing from blue to red, increasing from red to blue).</p> Full article ">Figure 4
<p>Programmed SPG periods as used in experiments for beam shaping with SLM: (<b>a</b>) SPG-1 with negative spatial frequency chirp and 37 periods along the minor axes, increasing between 29 µm and 55 µm (type B,3), see <a href="#photonics-11-00463-f001" class="html-fig">Figure 1</a>b; (<b>b</b>) SPG-2 with positive spatial frequency chirp and 55 periods, decreasing from 58 µm to 34 µm (type B,2), see <a href="#photonics-11-00463-f001" class="html-fig">Figure 1</a>c.</p> Full article ">Figure 5
<p>SPG periods as used in experiments determined by image analysis: SPG with (<b>a</b>) negative spatial frequency chirp or (<b>b</b>) positive spatial frequency chirp (number of spiral rotations: 37 and 55, along minor axes: increasing period between 29 µm and 55 µm, decreasing period between 58 µm and 34 µm, respectively). Colors symbolize spatial frequency chirp (decreasing from blue to red, increasing from red to blue).</p> Full article ">Figure 6
<p>Radially dependent “focal distances” <span class="html-italic">z<sub>F</sub></span>(<span class="html-italic">r</span>) (i.e., points of intersection of diffracted rays with the propagation axis) for perfect symmetric SPGs with negative (<b>a</b>) and positive chirp (<b>b</b>) for a wavelength of 787 nm (for geometrical parameters, see <a href="#photonics-11-00463-f003" class="html-fig">Figure 3</a>a,b). Colors symbolize spatial frequency chirp (decreasing from blue to red, increasing from red to blue).</p> Full article ">Figure 7
<p>Composite grating of type (D,4) consisting of an elliptical SPG (23 windings) centered in a surrounding toroidal SPG (18 windings) of different spatial frequency (periods in the directions of minor and major axes: 17 µm and 45 µm for the inner SPG, 21 µm and 54 µm for the torus, respectively).</p> Full article ">Figure 8
<p>Selection of “spectral eyes” by a narrowband filter: spatial OAM beam intensity distributions for three filter curves with maximum transmission at (<b>a</b>) 782.08 nm, (<b>b</b>) 795.15 nm and (<b>c</b>) 798.22 nm. Simultaneous detection of blue-shifted and red-shifted parts at maximum contrast and signal was found between 784 nm and 787 nm, indicated by distinct double maxima in (<b>b</b>,<b>e</b>) (50× objective lens, negatively chirped SPG, <span class="html-italic">z</span> = 10 mm, distance between “spectral eyes”: 32 µm).</p> Full article ">Figure 9
<p>Aberrations for spiral gratings at oblique incidence with structural and angular mismatch: (<b>a</b>) diamond-shaped distortion and higher diffraction orders up to m = ±3 of a uniform, non-adapted SPG at an incident angle of α = 45°; (<b>b</b>) distortion for a non-uniform SPG designed for α = 45° at an incident angle of α = 51°.</p> Full article ">Figure 10
<p>Angular rotation of “spectral eyes” generated with positively chirped SPG-2 varying with increasing propagation distance from (<b>a</b>–<b>d</b>) 20.0 mm to 20.6 mm for a selected sequence of intensity maps (AOI = 50 × 50 µm<sup>2</sup>, red and blue circles: red- and blue-shifted parts of the spectral anomaly, the line indicates the actual angle of rotation).</p> Full article ">Figure 11
<p>Accumulated Gouy phase shift (black) and Gouy rotation angle (red) (schematically) for SPGs with (<b>a</b>) a constant period, (<b>b</b>) a negatively chirped period, (<b>c</b>) a positively chirped period and (<b>d</b>) a composite structure consisting of two zones with different constant periods separated by a spatial gap (yellow). A higher spatial frequency is assumed for the central part.</p> Full article ">Figure 12
<p>Experimentally determined propagation-dependent rotation angles <span class="html-italic">φ</span>(<span class="html-italic">z</span>): (<b>a</b>) SPG-1 (negative spatial frequency chirp), decreasing steepness; (<b>b</b>) SPG-2 (positive spatial frequency chirp), increasing steepness; (<b>c</b>) SPG-3 (composite structure), complex dynamics. Characteristic ramps are caused by accumulated Gouy phase shift (see <a href="#photonics-11-00463-f011" class="html-fig">Figure 11</a>).</p> Full article ">Figure 13
<p>Angular gradients <span class="html-italic">φ(z)</span> calculated from angular rotation ramps in <a href="#photonics-11-00463-f012" class="html-fig">Figure 12</a>a–c: (<b>a</b>) SPG-1 (negative spatial frequency chirp); (<b>b</b>) SPG-2 (positive spatial frequency chirp); (<b>c</b>) SPG-3 (composite structure). Colors symbolize the spatial frequency chirp (decreasing from blue to red, increasing from red to blue). The dotted line in (<b>c</b>) marks the theoretical focal distance corresponding to the rim of the inner spiral calculated on the basis of Equation (2).</p> Full article ">
11 pages, 4572 KiB  
Article
Characterizing Extreme Events in a Fabry–Perot Laser with Optical Feedback
by Shanshan Ge, Yu Huang, Kun Li, Pei Zhou, Penghua Mu, Xin Zhu and Nianqiang Li
Photonics 2024, 11(5), 462; https://doi.org/10.3390/photonics11050462 - 15 May 2024
Viewed by 667
Abstract
The study of extreme events (EEs) in photonics has expanded significantly due to straightforward implementation conditions. EEs have not been discussed systematically, to the best of our knowledge, in the chaotic dynamics of a Fabry–Perot laser with optical feedback, so we address this [...] Read more.
The study of extreme events (EEs) in photonics has expanded significantly due to straightforward implementation conditions. EEs have not been discussed systematically, to the best of our knowledge, in the chaotic dynamics of a Fabry–Perot laser with optical feedback, so we address this in the current contribution. Herein, we not only find EEs in all modes but also divide the EEs in total output into two categories for further discussion. The two types of EEs have similar statistical features to conventional rogue waves. The occurrence probability of EEs undergoes a saturation effect as the feedback strength increases. Additionally, we analyze the influence of feedback strength, feedback delay, and pump current on the probability of EEs defined by two criteria of EEs and find similar trends. We hope that this work contributes to a deep understanding and serves as inspiration for further research into various multimode semiconductor laser systems. Full article
(This article belongs to the Special Issue Advanced Lasers and Their Applications II)
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Figure 1

Figure 1
<p>(<b>a</b>) Bifurcation diagrams plotting extrema of the total intensity and the corresponding results of the 0-1 test for chaos (the magenta line), where <math display="inline"><semantics> <mrow> <msub> <mi>τ</mi> <mi>f</mi> </msub> <mo>=</mo> <mn>2</mn> <mrow> <mo> </mo> <mi>ns</mi> </mrow> </mrow> </semantics></math>. (<b>b</b>) Intensity time series, where (<b>b1</b>) <math display="inline"><semantics> <mrow> <msub> <mi>k</mi> <mi>f</mi> </msub> <mo>=</mo> <mn>3</mn> <msup> <mrow> <mrow> <mo> </mo> <mi>ns</mi> </mrow> </mrow> <mrow> <mo>−</mo> <mn>1</mn> </mrow> </msup> </mrow> </semantics></math>, (<b>b2</b>) <math display="inline"><semantics> <mrow> <msub> <mi>k</mi> <mi>f</mi> </msub> <mo>=</mo> <mn>4</mn> <msup> <mrow> <mrow> <mo> </mo> <mi>ns</mi> </mrow> </mrow> <mrow> <mo>−</mo> <mn>1</mn> </mrow> </msup> </mrow> </semantics></math>, and (<b>b3</b>) <math display="inline"><semantics> <mrow> <msub> <mi>k</mi> <mi>f</mi> </msub> <mo>=</mo> <mn>10</mn> <msup> <mrow> <mrow> <mo> </mo> <mi>ns</mi> </mrow> </mrow> <mrow> <mo>−</mo> <mn>1</mn> </mrow> </msup> </mrow> </semantics></math>. The red line is the threshold of <span class="html-italic">AI</span> = 2.</p> Full article ">Figure 2
<p>(<b>a1</b>,<b>a2</b>) Intensity time series, where <math display="inline"><semantics> <mrow> <msub> <mi>k</mi> <mi>f</mi> </msub> <mo>=</mo> <mn>30</mn> <msup> <mrow> <mrow> <mo> </mo> <mi>ns</mi> </mrow> </mrow> <mrow> <mo>−</mo> <mn>1</mn> </mrow> </msup> <mo>,</mo> <msub> <mi>τ</mi> <mi>f</mi> </msub> <mo>=</mo> <mn>2</mn> <mrow> <mo> </mo> <mi>ns</mi> </mrow> </mrow> </semantics></math>. The dashed lines represent the threshold of <span class="html-italic">AI =</span> 2. (<b>b1</b>,<b>b2</b>,<b>c1</b>,<b>c2</b>) Zoom of time series presented in panels (<b>a1</b>), and the colored squares mark EEs in corresponding output.</p> Full article ">Figure 3
<p>(<b>a</b>) The PDF of the time series in <a href="#photonics-11-00462-f002" class="html-fig">Figure 2</a>(<b>a1</b>), where the dashed line represents the threshold of <span class="html-italic">AI.</span> (<b>b</b>) Waiting times between consecutive EEs in <math display="inline"><semantics> <mrow> <msub> <mi>I</mi> <mrow> <mi>t</mi> <mi>o</mi> <mi>t</mi> <mi>a</mi> <mi>l</mi> </mrow> </msub> </mrow> </semantics></math>, for either (<b>c</b>) type 1 or (<b>d</b>) type 2. The dashed lines are fitting curves.</p> Full article ">Figure 4
<p>(<b>a</b>) DMR of the five modes as a function of <math display="inline"><semantics> <mrow> <msub> <mi>k</mi> <mi>f</mi> </msub> </mrow> </semantics></math>, (<b>b</b>) relative number of EEs in <math display="inline"><semantics> <mrow> <msub> <mi>I</mi> <mrow> <mi>t</mi> <mi>o</mi> <mi>t</mi> <mi>a</mi> <mi>l</mi> </mrow> </msub> </mrow> </semantics></math> and <math display="inline"><semantics> <mrow> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>1</mn> <mo>,</mo> <mi>M</mi> <mn>2</mn> <mo>,</mo> <mi>M</mi> <mn>3</mn> <mo>,</mo> <mi>M</mi> <mn>4</mn> <mo>,</mo> <mi>M</mi> <mn>5</mn> </mrow> </msub> </mrow> </semantics></math>, (<b>c</b>) relative number of two types of EEs in <math display="inline"><semantics> <mrow> <msub> <mi>I</mi> <mrow> <mi>t</mi> <mi>o</mi> <mi>t</mi> <mi>a</mi> <mi>l</mi> </mrow> </msub> </mrow> </semantics></math>, and (<b>d</b>) average intensity of the two types of EEs. The feedback delay is set at <math display="inline"><semantics> <mrow> <msub> <mi>τ</mi> <mi>f</mi> </msub> <mo>=</mo> <mn>2</mn> <mrow> <mo> </mo> <mi>ns</mi> </mrow> </mrow> </semantics></math>.</p> Full article ">Figure 5
<p>Maps of the relative number of EEs in the <math display="inline"><semantics> <mrow> <mo stretchy="false">(</mo> <msub> <mi>k</mi> <mi>f</mi> </msub> <mo>,</mo> <msub> <mi>τ</mi> <mi>f</mi> </msub> <mo stretchy="false">)</mo> </mrow> </semantics></math> plane. (<b>a</b>) for <math display="inline"><semantics> <mrow> <msub> <mi>I</mi> <mrow> <mi>t</mi> <mi>o</mi> <mi>t</mi> <mi>a</mi> <mi>l</mi> </mrow> </msub> </mrow> </semantics></math>, and (<b>b</b>–<b>f</b>) for <math display="inline"><semantics> <mrow> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>1</mn> </mrow> </msub> <mo>,</mo> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>2</mn> </mrow> </msub> <mo>,</mo> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>3</mn> </mrow> </msub> <mo>,</mo> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>4</mn> </mrow> </msub> <mo>,</mo> <mrow> <mo> </mo> <mi>and</mi> <mo> </mo> </mrow> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>5</mn> </mrow> </msub> </mrow> </semantics></math>, respectively. Here, the EE is defined by <span class="html-italic">AI</span> = 2, and <math display="inline"><semantics> <mrow> <mi>I</mi> <mo>/</mo> <msub> <mi>I</mi> <mrow> <mi>t</mi> <mi>h</mi> </mrow> </msub> <mo>=</mo> <mn>2</mn> </mrow> </semantics></math>.</p> Full article ">Figure 6
<p>Maps of the relative number of EEs in the <math display="inline"><semantics> <mrow> <mo stretchy="false">(</mo> <msub> <mi>k</mi> <mi>f</mi> </msub> <mo>,</mo> <mi>I</mi> <mo>/</mo> <msub> <mi>I</mi> <mrow> <mi>t</mi> <mi>h</mi> </mrow> </msub> <mo stretchy="false">)</mo> </mrow> </semantics></math> plane. (<b>a</b>) for <math display="inline"><semantics> <mrow> <msub> <mi>I</mi> <mrow> <mi>t</mi> <mi>o</mi> <mi>t</mi> <mi>a</mi> <mi>l</mi> </mrow> </msub> </mrow> </semantics></math>, and (<b>b</b>–<b>f</b>) for <math display="inline"><semantics> <mrow> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>1</mn> </mrow> </msub> <mo>,</mo> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>2</mn> </mrow> </msub> <mo>,</mo> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>3</mn> </mrow> </msub> <mo>,</mo> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>4</mn> </mrow> </msub> <mo>,</mo> <mrow> <mo> </mo> <mi>and</mi> <mo> </mo> </mrow> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>5</mn> </mrow> </msub> </mrow> </semantics></math>, respectively. Here the EE is defined by <span class="html-italic">AI</span> = 2, and the feedback delay is set at 2 ns.</p> Full article ">Figure 7
<p>Intensity time series of total output (<b>a1</b>) and mode 1-mode 5 (<b>b1</b>–<b>f1</b>), and (<b>a2</b>–<b>f2</b>) are the corresponding PDFs. The red dashed lines represent the threshold of <span class="html-italic">AI</span> = 2, and the black ones stand for the threshold of <math display="inline"><semantics> <mrow> <mrow> <mo>〈</mo> <mi>H</mi> <mo>〉</mo> </mrow> <mo>+</mo> <mn>5</mn> <mi>σ</mi> </mrow> </semantics></math>. The other parameters are the same as those in <a href="#photonics-11-00462-f002" class="html-fig">Figure 2</a>.</p> Full article ">Figure 8
<p>Maps of the relative number of EEs in the <math display="inline"><semantics> <mrow> <mo stretchy="false">(</mo> <msub> <mi>k</mi> <mi>f</mi> </msub> <mo>,</mo> <msub> <mi>τ</mi> <mi>f</mi> </msub> <mo stretchy="false">)</mo> </mrow> </semantics></math> plane. (<b>a</b>) for <math display="inline"><semantics> <mrow> <msub> <mi>I</mi> <mrow> <mi>t</mi> <mi>o</mi> <mi>t</mi> <mi>a</mi> <mi>l</mi> </mrow> </msub> </mrow> </semantics></math>, and (<b>b</b>–<b>f</b>) for <math display="inline"><semantics> <mrow> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>1</mn> </mrow> </msub> <mo>,</mo> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>2</mn> </mrow> </msub> <mo>,</mo> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>3</mn> </mrow> </msub> <mo>,</mo> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>4</mn> </mrow> </msub> <mo>,</mo> <mrow> <mo> </mo> <mi>and</mi> <mo> </mo> </mrow> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>5</mn> </mrow> </msub> </mrow> </semantics></math>, respectively. Here the EE is defined by <math display="inline"><semantics> <mrow> <mrow> <mo>〈</mo> <mi>H</mi> <mo>〉</mo> </mrow> <mo>+</mo> <mn>5</mn> <mi>σ</mi> </mrow> </semantics></math>, and <math display="inline"><semantics> <mrow> <mi>I</mi> <mo>/</mo> <msub> <mi>I</mi> <mrow> <mi>t</mi> <mi>h</mi> </mrow> </msub> <mo>=</mo> <mn>2</mn> </mrow> </semantics></math>.</p> Full article ">Figure 9
<p>Maps of the relative number of EEs in the <math display="inline"><semantics> <mrow> <mo stretchy="false">(</mo> <msub> <mi>k</mi> <mi>f</mi> </msub> <mo>,</mo> <mi>I</mi> <mo>/</mo> <msub> <mi>I</mi> <mrow> <mi>t</mi> <mi>h</mi> </mrow> </msub> <mo stretchy="false">)</mo> </mrow> </semantics></math> plane. (<b>a</b>) for <math display="inline"><semantics> <mrow> <msub> <mi>I</mi> <mrow> <mi>t</mi> <mi>o</mi> <mi>t</mi> <mi>a</mi> <mi>l</mi> </mrow> </msub> </mrow> </semantics></math>, and (<b>b</b>–<b>f</b>) for <math display="inline"><semantics> <mrow> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>1</mn> </mrow> </msub> <mo>,</mo> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>2</mn> </mrow> </msub> <mo>,</mo> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>3</mn> </mrow> </msub> <mo>,</mo> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>4</mn> </mrow> </msub> <mo>,</mo> <mrow> <mo> </mo> <mi>and</mi> <mo> </mo> </mrow> <msub> <mi>I</mi> <mrow> <mi>M</mi> <mn>5</mn> </mrow> </msub> </mrow> </semantics></math>, respectively. Here the EE is defined by <math display="inline"><semantics> <mrow> <mrow> <mo>〈</mo> <mi>H</mi> <mo>〉</mo> </mrow> <mo>+</mo> <mn>5</mn> <mi>σ</mi> </mrow> </semantics></math>, and <math display="inline"><semantics> <mrow> <msub> <mi>τ</mi> <mi>f</mi> </msub> <mo>=</mo> <mn>2</mn> <mrow> <mo> </mo> <mi>ns</mi> </mrow> </mrow> </semantics></math>.</p> Full article ">
11 pages, 2457 KiB  
Article
Integrated Analysis of Line-Of-Sight Stability of Off-Axis Three-Mirror Optical System
by Yatao Lu, Bin Sun, Gui Mei, Qinglei Zhao, Zhongshan Wang, Yang Gao and Shuxin Wang
Photonics 2024, 11(5), 461; https://doi.org/10.3390/photonics11050461 - 15 May 2024
Viewed by 634
Abstract
As a space camera works in orbit, the stress rebound caused by gravity inevitably results in the deformation of its optomechanical structure, and the relative position change between different optical components will affect the Line-Of-Sight pointing of the camera. In this paper, the [...] Read more.
As a space camera works in orbit, the stress rebound caused by gravity inevitably results in the deformation of its optomechanical structure, and the relative position change between different optical components will affect the Line-Of-Sight pointing of the camera. In this paper, the optical sensitivity calculation of a space camera’s Line-Of-Sight pointing is realized based on the optomechanical constraint equations, and the Line-Of-Sight equations are constructed using the second type of response (DRESP2) method to realize an optomechanical integrated analysis of the camera’s Line-Of-Sight stability at the structural finite element solver level. The verification results show that the Line-Of-Sight stability error is 6.38%, meaning that this method can identify the sensitive optical elements of the optical system efficiently and quickly. Thus, the method in this paper has important significance as a reference for the analysis of the Line-Of-Sight stability of complex optical systems. Full article
(This article belongs to the Special Issue Recent Industrial Approaches of Optical Metasurfaces: Applications and Trends)
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<p>A simple example of an off-axis triplex optical system.</p> Full article ">Figure 2
<p>The optical model diagram of an off-axis three-mirror optical system.</p> Full article ">Figure 3
<p>The finite element model diagram of an off-axis three-mirror optical system.</p> Full article ">Figure 4
<p>Flow chart of the integrated optomechanical analysis of Line-Of-Sight pointing.</p> Full article ">Figure 5
<p>Radial LOS error computed as the vector sum.</p> Full article ">Figure 6
<p>Line-Of-Sight error components caused by the translation of each optical element under gravity loading conditions in three directions.</p> Full article ">Figure 7
<p>Line-Of-Sight error components caused by the rotation of each optical element under gravity loading conditions in three directions.</p> Full article ">
16 pages, 3641 KiB  
Review
Features of Adaptive Phase Correction of Optical Wave Distortions under Conditions of Intensity Fluctuations
by Vladimir Lukin
Photonics 2024, 11(5), 460; https://doi.org/10.3390/photonics11050460 - 14 May 2024
Viewed by 818
Abstract
An analysis of the features of measurements and correction of phase distortions in optical waves propagating in the atmosphere at various levels of turbulence was performed. It is shown that with increasing intensity fluctuations, the limiting capabilities of phase correction decrease, and the [...] Read more.
An analysis of the features of measurements and correction of phase distortions in optical waves propagating in the atmosphere at various levels of turbulence was performed. It is shown that with increasing intensity fluctuations, the limiting capabilities of phase correction decrease, and the phase of an optical wave that has passed through a turbulence layer consists of two components: potential and vortex. It was found that even in the region of weak fluctuations there is an overlap of spectral filtering functions for intensity and phase fluctuations. Areas of turbulence inhomogeneities have been identified that will have mutual influence and negatively affect the operation of the phase meter. It is noted that correlation functions, both phase and intensity, are less susceptible to this compared to structural functions. The results of experimental studies on the reconstruction of the wavefront of laser radiation distorted by atmospheric turbulence using a Shack–Hartmann wavefront sensor during vignetting and central screening of the entrance pupil in the optical system are presented. Studies have been carried out on the propagation of laser radiation along a horizontal atmospheric path for various levels of turbulence. The results are analyzed in terms of Zernike polynomials. Full article
(This article belongs to the Special Issue Advances in Structured Light Generation and Manipulation)
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<p>Dependence of the Strehl ratio <span class="html-italic">SR</span> on the scintillation index during phase correction of fluctuations in the potential phase of the “receiving” signal (according to scheme 2).</p> Full article ">Figure 2
<p>The same as in <a href="#photonics-11-00460-f001" class="html-fig">Figure 1</a> for a phase conjugation system operating “for transmission” according to scheme 4.</p> Full article ">Figure 3
<p>Comparison of experimental data from the Lincoln Laboratory (USA) and calculations [<a href="#B20-photonics-11-00460" class="html-bibr">20</a>,<a href="#B22-photonics-11-00460" class="html-bibr">22</a>].</p> Full article ">Figure 4
<p>Spectral filtering functions for correlation functions of phase and intensity fluctuations, calculated using Formula (9).</p> Full article ">Figure 5
<p>Spectral filter in <a href="#photonics-11-00460-f005" class="html-fig">Figure 5</a>. Spectral filtering functions for the structure functions of phase and intensity, calculated using Formula (10).</p> Full article ">Figure 5 Cont.
<p>Spectral filter in <a href="#photonics-11-00460-f005" class="html-fig">Figure 5</a>. Spectral filtering functions for the structure functions of phase and intensity, calculated using Formula (10).</p> Full article ">Figure 6
<p>Appearance of a pattern of focal spots in the Shack–Hartmann sensor with weak (<b>left</b>) and strong (<b>right</b>) intensity fluctuations in the optical wave.</p> Full article ">Figure 7
<p>Illumination distribution of focal spots across the aperture of the wavefront sensor at frame with number 369.</p> Full article ">Figure 8
<p>Illumination distribution of focal spots across the aperture of the wavefront sensor at frame with number 306.</p> Full article ">Figure 9
<p>Time dependence of the normalized number of subapertures that form images with maximum illumination below the threshold (this threshold of illumination value is 1.5 times greater than background).</p> Full article ">Figure 10
<p>Experimental scheme.</p> Full article ">Figure 11
<p>Simultaneous measurements with two sensors WFS1 and WFS2 at full aperture.</p> Full article ">Figure 12
<p>Simultaneous measurements of the mode components of the phase front: WFS1 with a central shielding coefficient of 13% and WFS2—without shielding.</p> Full article ">
9 pages, 6506 KiB  
Article
Influence of the Experimental Setup on Electromagnetic Pulses in the VHF Band at Relativistic High-Power Laser Facilities
by Michael Ehret, Luca Volpe, Jon Imanol Apiñaniz, Maria Dolores Rodríguez-Frías and Giancarlo Gatti
Photonics 2024, 11(5), 459; https://doi.org/10.3390/photonics11050459 - 14 May 2024
Cited by 1 | Viewed by 840
Abstract
We present experimental results for the controlled mitigation of the electromagnetic pulses (EMPs) produced in the interactions of a 1 PW high-power 30 fs Ti:Sa laser VEGA-3 with solid-density targets transparent to laser-forward-accelerated relativistic electrons. This study aims at the band of very [...] Read more.
We present experimental results for the controlled mitigation of the electromagnetic pulses (EMPs) produced in the interactions of a 1 PW high-power 30 fs Ti:Sa laser VEGA-3 with solid-density targets transparent to laser-forward-accelerated relativistic electrons. This study aims at the band of very high frequencies (VHFs), i.e., those in the hundreds of MHz, which comprise the fundamental cavity modes of the rectangular VEGA-3 vacuum chamber. We demonstrate mode suppression by a tailoring of the laser-produced space charge distribution. Full article
(This article belongs to the Special Issue Metrology at High-Power Laser Facilities: Primary and Secondary Laser-Driven Sources)
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<p>Experimental setup sketch (<b>left</b>) and computer-aided drawing (<b>right</b>) of the pulsed laser interaction with a solid-density target located off-center with respect to the rectangular vacuum vessel, which served as the interaction chamber. The EMPs were detected with a B-field antenna located behind the interaction region (in a laser forward direction) and above the path of the horizontal plane. A LiF crystal was located on a motorized stage to be moved in and out of the path of the laser-accelerated charged particles from the target rear side.</p> Full article ">Figure 2
<p>A first setup (<b>A</b>) that comprises only the aluminum target and allows for a free propagation of accelerated species until they reach the chamber wall. The setup change (<b>B</b>) introduces an obstacle on the path of relativistic electrons, which are intended to change the seed space charge distribution and thus the build up of cavity modes.</p> Full article ">Figure 3
<p>The residual pressure of the detected molecules in a high vacuum chamber for commercially available EMP antennas from Aaronia.</p> Full article ">Figure 4
<p>The time-integrated spectrum of the magnetic field derived from the oscilloscope recordings of B-field antenna measurements in the VEGA-3 interaction chamber for shots on solid targets, with an indication of identified rectangular cavity modes and the resonance of the vertical stalks that hold optics (<math display="inline"><semantics> <mrow> <mi>λ</mi> <mo>/</mo> <mn>4</mn> </mrow> </semantics></math>). Note that the signal denotes the measurement by the oscilloscope, and the corrected signal denotes the input signal from the antenna, which was calculated from the measurement.</p> Full article ">Figure 5
<p>The time-integrated spectrum of the magnetic field derived from the oscilloscope recordings of B-field antenna measurements in the VEGA-3 interaction chamber for shots on solid targets with an inserted LiF beam dump shortly behind the solid target, with an indication of the identified rectangular cavity modes and the resonance of the vertical stalks that hold optics (<math display="inline"><semantics> <mrow> <mi>λ</mi> <mo>/</mo> <mn>4</mn> </mrow> </semantics></math>). Note that the signal denotes the measurement by the oscilloscope, and the corrected signal denotes the input signal from the antenna, which was calculated from the measurement.</p> Full article ">
15 pages, 13683 KiB  
Article
A 3D Reconstruction Method Based on Homogeneous De Bruijn-Encoded Structured Light
by Weimin Li and Songlin Li
Photonics 2024, 11(5), 458; https://doi.org/10.3390/photonics11050458 - 14 May 2024
Viewed by 722
Abstract
Structured light three-dimensional reconstruction is one of the important methods for non-contact acquisition of sparse texture object surfaces. Variations in ambient illumination and disparities in object surface reflectance can significantly impact the fidelity of three-dimensional reconstruction, introducing considerable inaccuracies. We introduce a robust [...] Read more.
Structured light three-dimensional reconstruction is one of the important methods for non-contact acquisition of sparse texture object surfaces. Variations in ambient illumination and disparities in object surface reflectance can significantly impact the fidelity of three-dimensional reconstruction, introducing considerable inaccuracies. We introduce a robust method for color speckle structured light encoding, which is based on a variant of the De Bruijn sequence, termed the Homogeneous De Bruijn Sequence. This innovative approach enhances the reliability and accuracy of structured light techniques for three-dimensional reconstruction by utilizing the distinctive characteristics of Homogeneous De Bruijn Sequences. Through a pruning process applied to the De Bruijn sequence, a structured light pattern with seven distinct color patches is generated. This approach ensures a more equitable distribution of speckle information. Full article
(This article belongs to the Section Optical Interaction Science)
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<p>Partial construction sequence of the De Bruijn-directed graph. In the green box, the overlapping elements of the two vertices are consistent, which can serve as a path in the De Bruijn directed graph. In the red box, the overlapping elements of the two vertices are inconsistent, and thus cannot serve as a path in the De Bruijn directed graph.</p> Full article ">Figure 2
<p>Flow diagram of the three-dimensional reconstruction algorithm based on a Homogeneous De Bruijn sequence.</p> Full article ">Figure 3
<p>Directed Graph of the P(3,3) Sequence Generated Based on S1 = {R, G, M}. Solid arrows indicate paths that satisfy homogeneous constraints. Red dashed arrows show paths that violate these constraints and should be pruned.</p> Full article ">Figure 4
<p>The decoding results of the captured images. (<b>a</b>) Original image. (<b>b</b>) Clustering result obtained using the k-means method [<a href="#B24-photonics-11-00458" class="html-bibr">24</a>]. (<b>c</b>) Clustering result of our proposed method.</p> Full article ">Figure 5
<p>The images captured by the stereo camera. (<b>a</b>) Reconstruction target object. (<b>b</b>) Image captured by the left camera after projecting the speckle pattern. (<b>c</b>) Image captured by the right camera after projecting the speckle pattern.</p> Full article ">Figure 6
<p>Experimental results of different speckles. (<b>a</b>) Speckle generated by random three-channel grayscale values. (<b>b</b>) Speckles generated by De Bruijn but containing repetitive subsequences. (<b>c</b>) Speckles generated by integrating black pixel blocks into the center of each binarized grayscale channel. (<b>d</b>) Speckle generated by Homogeneous De Bruijn Sequence. (<b>e</b>–<b>h</b>) represent the spherical crown point clouds reconstructed from the speckle projections of (<b>a</b>–<b>d</b>), respectively.</p> Full article ">Figure 7
<p>The error distribution of the experimental method. (<b>a</b>) positional distribution of errors. (<b>b</b>) quantitative distribution of errors.</p> Full article ">Figure 8
<p>The reconstruction of a mask. (<b>a</b>) object left view. (<b>b</b>) object right view. (<b>c</b>) original mask model image. (<b>d</b>) reconstructed point cloud.</p> Full article ">Figure 9
<p>The mesh reconstruction of a mask. (<b>a</b>) mesh view 1. (<b>b</b>) mesh view 2. The part within the dashed box will be enlarged in <a href="#photonics-11-00458-f010" class="html-fig">Figure 10</a>.</p> Full article ">Figure 10
<p>Comparison of mask mesh details. (<b>a</b>) details of mask mesh. (<b>b</b>) details of original mask model image.</p> Full article ">Figure 11
<p>The reconstruction of fan blades. (<b>a</b>) object left view. (<b>b</b>) object right view. (<b>c</b>) original fan blades image. (<b>d</b>) reconstructed point cloud.</p> Full article ">Figure 12
<p>The reconstruction of a human hand. (<b>a</b>) object left view. (<b>b</b>) object right view. (<b>c</b>) reconstructed point cloud. (<b>d</b>) reconstructed mesh.</p> Full article ">Figure 12 Cont.
<p>The reconstruction of a human hand. (<b>a</b>) object left view. (<b>b</b>) object right view. (<b>c</b>) reconstructed point cloud. (<b>d</b>) reconstructed mesh.</p> Full article ">Figure 13
<p>The reconstruction of a human face. (<b>a</b>) object left view. (<b>b</b>) object right view. (<b>c</b>) reconstructed point cloud. (<b>d</b>) reconstructed mesh.</p> Full article ">
10 pages, 5013 KiB  
Article
Mode Heterogeneous Multimode Power Splitter Based on Cascaded Mode-Dependent Splitters and Converters
by Xin Xu, Hongliang Chen, Xin Fu and Lin Yang
Photonics 2024, 11(5), 457; https://doi.org/10.3390/photonics11050457 - 14 May 2024
Viewed by 654
Abstract
To the best of our knowledge, a novel concept of mode heterogeneity for the design of multimode devices is presented in this paper and applied to the design of scalable multimode power splitters. Based on a cascade of mode-dependent splitters and converters, we [...] Read more.
To the best of our knowledge, a novel concept of mode heterogeneity for the design of multimode devices is presented in this paper and applied to the design of scalable multimode power splitters. Based on a cascade of mode-dependent splitters and converters, we achieve beam splitting and mode conversion for four modes from TE0 to TE3 in the bandwidth from 1525 nm to 1560 nm. The measurements of the device at 1550 nm show excellent performance, with the insertion loss ranging from 0.16 dB to 0.63 dB, crosstalk all below −16.71 dB, and power uniformity between 0.026 dB and 0.168 dB. Full article
(This article belongs to the Special Issue Optical Communication, Sensing and Network)
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<p>(<b>a</b>) Three-dimensional schematic of the mode heterogeneous multimode power splitter and cross-sectional view of each stage. (<b>b</b>) Width dispersion curves for <math display="inline"><semantics> <mrow> <mi>T</mi> <msub> <mi>E</mi> <mn>0</mn> </msub> </mrow> </semantics></math>,<math display="inline"><semantics> <mrow> <mi>T</mi> <msub> <mi>E</mi> <mn>1</mn> </msub> </mrow> </semantics></math>,<math display="inline"><semantics> <mrow> <mi>T</mi> <msub> <mi>E</mi> <mn>2</mn> </msub> </mrow> </semantics></math>, and <math display="inline"><semantics> <mrow> <mi>T</mi> <msub> <mi>E</mi> <mn>3</mn> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 2
<p>The Ey distribution of the supermodes within the MDSC at stage 2, when target mode <math display="inline"><semantics> <mrow> <mi>T</mi> <msub> <mi>E</mi> <mn>2</mn> </msub> </mrow> </semantics></math> is input from waveguide 4 (<b>a</b>) and non-target modes are input from waveguide 4 (<b>b</b>) and waveguide 1 (<b>c</b>).</p> Full article ">Figure 3
<p>The schematic structure for MDSC optimization in the first three (<b>a</b>) and the fourth (<b>e</b>) stages. (<b>b</b>–<b>d</b>,<b>f</b>) The transmission efficiency with the matching length for different optimized structures at 1550 nm.</p> Full article ">Figure 4
<p>The simulated insertion loss and crosstalk with the wavelength and light propagation at 1550 nm in the MH-MPS when the inputs are the <math display="inline"><semantics> <mrow> <mi>T</mi> <msub> <mi>E</mi> <mn>0</mn> </msub> </mrow> </semantics></math> (<b>a</b>), <math display="inline"><semantics> <mrow> <mi>T</mi> <msub> <mi>E</mi> <mn>1</mn> </msub> </mrow> </semantics></math> (<b>b</b>), <math display="inline"><semantics> <mrow> <mi>T</mi> <msub> <mi>E</mi> <mn>2</mn> </msub> </mrow> </semantics></math> (<b>c</b>), and <math display="inline"><semantics> <mrow> <mi>T</mi> <msub> <mi>E</mi> <mn>3</mn> </msub> </mrow> </semantics></math> (<b>d</b>) modes.</p> Full article ">Figure 5
<p>Simulated insertion loss and crosstalk on fabrication error of −10 nm (<b>a</b>) and 10 nm (<b>b</b>) and at temperatures of 300 K (<b>c</b>) and 350 K (<b>d</b>).</p> Full article ">Figure 6
<p>Microscope image of the fabricated device and multimode bending used for normalization.</p> Full article ">Figure 7
<p>The normalized transmission (NT) of the upper and lower ports, crosstalk (CT), insertion loss (IL), and power uniformity (PU) with the wavelength when the input modes are <math display="inline"><semantics> <mrow> <mi>T</mi> <msub> <mi>E</mi> <mn>0</mn> </msub> </mrow> </semantics></math> (<b>a</b>,<b>b</b>), <math display="inline"><semantics> <mrow> <mi>T</mi> <msub> <mi>E</mi> <mn>1</mn> </msub> </mrow> </semantics></math> (<b>c</b>,<b>d</b>), <math display="inline"><semantics> <mrow> <mi>T</mi> <msub> <mi>E</mi> <mn>2</mn> </msub> </mrow> </semantics></math> (<b>e</b>,<b>f</b>), and <math display="inline"><semantics> <mrow> <mi>T</mi> <msub> <mi>E</mi> <mn>3</mn> </msub> </mrow> </semantics></math> (<b>g</b>,<b>h</b>).</p> Full article ">
13 pages, 8880 KiB  
Article
Exploring the Origin of Lissajous Geometric Modes from the Ray Tracing Model
by Xin-Liang Zheng, Yu-Han Fang, Wei-Che Chung, Cheng-Li Hsieh and Yung-Fu Chen
Photonics 2024, 11(5), 456; https://doi.org/10.3390/photonics11050456 - 13 May 2024
Viewed by 714
Abstract
In this paper, we use the geometric optics and discuss the path of laser beam in a simple laser (concave-plano) cavity with the birefringence crystal. In specific lengths of the laser cavity, we can observe various types of Lissajous-like structural laser modes that [...] Read more.
In this paper, we use the geometric optics and discuss the path of laser beam in a simple laser (concave-plano) cavity with the birefringence crystal. In specific lengths of the laser cavity, we can observe various types of Lissajous-like structural laser modes that can be simulated using our ray tracing model. At the end of this paper, we provide an adjusted ABCD matrix. With the adjusted ABCD matrix and iterative calculation, we can obtain the 3D trajectories which are similar to the experimental results. These structural laser modes can be realized by a Nd:YVO4 solid-state laser with off-axis pumping. From the comparison between the experimental data and the numerical data, we clarify the relationship between the 3D Lissajous-like structural laser modes and ray trajectory in the laser cavity. Full article
(This article belongs to the Special Issue Emerging Topics in Structured Light)
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Figure 1

Figure 1
<p>Experimental setup for exciting the high-order mode of the laser pattern.</p> Full article ">Figure 2
<p>Plotting the reflection ray path by a concave mirror of an <math display="inline"><semantics> <mrow> <mi>R</mi> <mo>=</mo> <mn>20</mn> </mrow> </semantics></math> mm radius curvature, where <math display="inline"><semantics> <mrow> <msub> <mi mathvariant="sans-serif">θ</mi> <mi>R</mi> </msub> <mo>=</mo> <msup> <mrow> <mi>sin</mi> </mrow> <mrow> <mo>−</mo> <mn>1</mn> </mrow> </msup> <mfenced> <mrow> <mfrac> <mrow> <mi mathvariant="normal">Δ</mi> <mi>x</mi> </mrow> <mi>R</mi> </mfrac> </mrow> </mfenced> </mrow> </semantics></math>, <math display="inline"><semantics> <mrow> <mi mathvariant="normal">Δ</mi> <mi>x</mi> </mrow> </semantics></math> indicates the distance between the incident point and the center point of the mirror, <math display="inline"><semantics> <mi mathvariant="sans-serif">ϕ</mi> </semantics></math> and <math display="inline"><semantics> <msup> <mi mathvariant="sans-serif">ϕ</mi> <mo>′</mo> </msup> </semantics></math> indicate the ray angle relative to <math display="inline"><semantics> <mrow> <msub> <mover accent="true"> <mi>a</mi> <mo stretchy="false">^</mo> </mover> <mi>z</mi> </msub> </mrow> </semantics></math> and <math display="inline"><semantics> <mrow> <mo>−</mo> <msub> <mover accent="true"> <mi>a</mi> <mo stretchy="false">^</mo> </mover> <mi>z</mi> </msub> </mrow> </semantics></math>, respectively.</p> Full article ">Figure 3
<p>(<b>a</b>) The three round-trip ray trajectories in specific cavity length <math display="inline"><semantics> <mrow> <msub> <mi>L</mi> <mrow> <mi>c</mi> <mi>a</mi> <mi>v</mi> </mrow> </msub> </mrow> </semantics></math> (the distance between concave mirror and crystal); (<b>b</b>) The thirty round-trip ray trajectories in specific cavity length <math display="inline"><semantics> <mrow> <msub> <mi>L</mi> <mrow> <mi>c</mi> <mi>a</mi> <mi>v</mi> </mrow> </msub> </mrow> </semantics></math>; (<b>c</b>) The three hundred round-trip ray trajectories in specific cavity length <math display="inline"><semantics> <mrow> <msub> <mi>L</mi> <mrow> <mi>c</mi> <mi>a</mi> <mi>v</mi> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 4
<p>The use of the ray tracing model with refractive index <math display="inline"><semantics> <mrow> <msub> <mi>n</mi> <mrow> <mi>e</mi> <mi>f</mi> <mi>f</mi> <mi>x</mi> </mrow> </msub> <mo>=</mo> <msub> <mi>n</mi> <mi>e</mi> </msub> </mrow> </semantics></math> in vertical polarization to quantize the periodic property. The above figure represents the simple periodic trajectories simulated by the ray tracing model and the bottom image illustrates several observed geometric modes in the <math display="inline"><semantics> <mrow> <msub> <mover accent="true"> <mi>a</mi> <mo stretchy="false">^</mo> </mover> <mi>x</mi> </msub> </mrow> </semantics></math> axis in the experiment.</p> Full article ">Figure 5
<p>By comparing experiments data and simulation results, it can be noticed that the refractive index shows <math display="inline"><semantics> <mrow> <msub> <mi>n</mi> <mrow> <mi>e</mi> <mi>f</mi> <mi>f</mi> <mi>y</mi> </mrow> </msub> <mo>=</mo> <msub> <mi>n</mi> <mi>e</mi> </msub> <mo>+</mo> <mn>0.54</mn> </mrow> </semantics></math> in vertical polarization. The following image illustrates several observed geometric modes in the <math display="inline"><semantics> <mrow> <msub> <mover accent="true"> <mi>a</mi> <mo stretchy="false">^</mo> </mover> <mi>y</mi> </msub> </mrow> </semantics></math> axis in the experiment.</p> Full article ">Figure 6
<p>(<b>a</b>) To discuss the ray trajectory in the crystal, we need to consider the refraction in both the x-z plane and the y-z plane, respectively, in the 3D situation; (<b>b</b>) the displacement in the x-z plane.</p> Full article ">Figure 7
<p>(<b>a</b>) To discuss the ray trajectory reflected by the concave mirror, we need to decompose the propagation direction into radius direction <math display="inline"><semantics> <mrow> <msub> <mover accent="true"> <mi>a</mi> <mo stretchy="false">^</mo> </mover> <mi mathvariant="sans-serif">ρ</mi> </msub> </mrow> </semantics></math> and azimuthal direction <math display="inline"><semantics> <mrow> <msub> <mover accent="true"> <mi>a</mi> <mo stretchy="false">^</mo> </mover> <mi mathvariant="sans-serif">ϕ</mi> </msub> </mrow> </semantics></math>; (<b>b</b>) the decomposition in the x-y plane where <math display="inline"><semantics> <mrow> <msub> <mover accent="true"> <mi>v</mi> <mo stretchy="false">⇀</mo> </mover> <mrow> <mi>x</mi> <mi>y</mi> <mo>−</mo> <mi>p</mi> <mi>l</mi> <mi>a</mi> <mi>n</mi> <mi>e</mi> </mrow> </msub> </mrow> </semantics></math> indicates the velocity vector on the x-y plane.</p> Full article ">Figure 8
<p>(<b>a</b>) The schematic of the end-side pumping with off-axis displacement; (<b>b</b>) The Lissajous-like trajectory inside the laser cavity is simulated using the 3D ray tracing model, and the trajectory changes in the rays from the laser cavity after leaving it during their free space propagation also agree with the experimental observation.</p> Full article ">Figure 9
<p>(<b>a</b>) Experimental and theoretical results for <math display="inline"><semantics> <mrow> <msub> <mi>L</mi> <mrow> <mi>c</mi> <mi>a</mi> <mi>v</mi> </mrow> </msub> </mrow> </semantics></math> versus <math display="inline"><semantics> <mi>ζ</mi> </semantics></math> for the emergence of the Lissajous-like structural laser modes around <math display="inline"><semantics> <mrow> <msub> <mi>L</mi> <mrow> <mi>P</mi> <mo>/</mo> <mi>Q</mi> <mo>=</mo> <mn>1</mn> <mo>/</mo> <mn>3</mn> </mrow> </msub> </mrow> </semantics></math> and <math display="inline"><semantics> <mrow> <msub> <mi>L</mi> <mrow> <mi>P</mi> <mo>/</mo> <mi>Q</mi> <mo>=</mo> <mn>2</mn> <mo>/</mo> <mn>5</mn> </mrow> </msub> </mrow> </semantics></math>. (<b>b</b>) Lissajous-like structural laser modes observed in the experiment. (<b>c</b>) Lissajous parametric surfaces calculated using <math display="inline"><semantics> <mrow> <msub> <mi>x</mi> <mi>s</mi> </msub> <mo>=</mo> <mi>A</mi> <mi>cos</mi> <mfenced close="]" open="["> <mrow> <mfenced> <mrow> <mn>2</mn> <mi mathvariant="sans-serif">π</mi> <mi>s</mi> <mi>P</mi> <mo>/</mo> <mi>Q</mi> </mrow> </mfenced> <mfenced> <mrow> <mn>1</mn> <mo>−</mo> <mrow> <mi>q</mi> <mo>/</mo> <mi>M</mi> </mrow> </mrow> </mfenced> </mrow> </mfenced> </mrow> </semantics></math> and <math display="inline"><semantics> <mrow> <msub> <mi>y</mi> <mi>s</mi> </msub> <mo>=</mo> <mi>B</mi> <mi>cos</mi> <mfenced close="]" open="["> <mrow> <mfenced> <mrow> <mn>2</mn> <mi mathvariant="sans-serif">π</mi> <mi>s</mi> <mi>P</mi> <mo>/</mo> <mi>Q</mi> </mrow> </mfenced> <mfenced> <mrow> <mn>1</mn> <mo>+</mo> <mrow> <mi>p</mi> <mo>/</mo> <mi>M</mi> </mrow> </mrow> </mfenced> </mrow> </mfenced> </mrow> </semantics></math> with <math display="inline"><semantics> <mrow> <mi>P</mi> <mo>/</mo> <mi>Q</mi> <mo>=</mo> <mn>1</mn> <mo>/</mo> <mn>3</mn> </mrow> </semantics></math> and <math display="inline"><semantics> <mrow> <mn>2</mn> <mo>/</mo> <mn>5</mn> </mrow> </semantics></math>, <math display="inline"><semantics> <mrow> <mi>s</mi> <mo>=</mo> <mn>0</mn> <mo>,</mo> <mn>1</mn> <mo>,</mo> <mn>2</mn> <mo>⋯</mo> <mi>M</mi> <mi>Q</mi> <mo>−</mo> <mn>1</mn> </mrow> </semantics></math>, <math display="inline"><semantics> <mrow> <mi>A</mi> <mo>=</mo> <mi>B</mi> <mo>=</mo> <mn>10</mn> </mrow> </semantics></math>, <math display="inline"><semantics> <mrow> <mi>M</mi> <mo>=</mo> <mn>400</mn> </mrow> </semantics></math> and indices <math display="inline"><semantics> <mrow> <mfenced> <mrow> <mi>p</mi> <mo>,</mo> <mi>q</mi> </mrow> </mfenced> </mrow> </semantics></math>.</p> Full article ">Figure 10
<p>The points in the figure represent the cavity lengths corresponding to the Lissajous-like structural laser modes observed experimentally under different off-axis pumping conditions, and the lines represent the cavity lengths at which the Lissajous trajectories appear in the 3D ray tracing model under different off-axis pumping conditions.</p> Full article ">Figure 11
<p>(<b>a</b>) The distortion Lissajous-like structural laser modes observed in the experiment; (<b>b</b>) the far field of the ray trajectories at the end side of crystal calculated using the 3D ray tracing model.</p> Full article ">Figure 12
<p>The adjusted ABCD matrix for a round trip in the 3D laser cavity.</p> Full article ">Figure 13
<p>Given cavity length <math display="inline"><semantics> <mrow> <msub> <mi>L</mi> <mrow> <mi>c</mi> <mi>a</mi> <mi>v</mi> </mrow> </msub> </mrow> </semantics></math> and 2D off-axis pumping <math display="inline"><semantics> <mrow> <mi mathvariant="normal">Δ</mi> <mi>x</mi> </mrow> </semantics></math> and <math display="inline"><semantics> <mrow> <mi mathvariant="normal">Δ</mi> <mi>y</mi> </mrow> </semantics></math>, the far field of the ray trajectories at the end side of crystal calculated using the adjusted ABCD matrix are shown in the above figure. The lower row is the far field of the distortion 2D HG modes in the experiments.</p> Full article ">Figure 14
<p>Given cavity length <math display="inline"><semantics> <mrow> <msub> <mi>L</mi> <mrow> <mi>c</mi> <mi>a</mi> <mi>v</mi> </mrow> </msub> </mrow> </semantics></math> and 2D off-axis pump <math display="inline"><semantics> <mrow> <mi mathvariant="normal">Δ</mi> <mi>x</mi> </mrow> </semantics></math> and <math display="inline"><semantics> <mrow> <mi mathvariant="normal">Δ</mi> <mi>y</mi> </mrow> </semantics></math>, the far field of the ray trajectories at the end side of the crystal calculated using the adjusted ABCD matrix are shown in the above figure. The lower row is the far field of the distortion Lissajous-like structural laser modes observed in experiments with cavity lengths near <math display="inline"><semantics> <mrow> <msub> <mi>L</mi> <mrow> <mi>c</mi> <mi>a</mi> <mi>v</mi> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">
14 pages, 5228 KiB  
Article
Analytical Model of Point Spread Function under Defocused Degradation in Diffraction-Limited Systems: Confluent Hypergeometric Function
by Feijun Song, Qiao Chen, Xiongxin Tang and Fanjiang Xu
Photonics 2024, 11(5), 455; https://doi.org/10.3390/photonics11050455 - 13 May 2024
Viewed by 729
Abstract
In recent years, optical systems near the diffraction limit have been widely used in high-end applications. Evidently, an analytical solution of the point spread function (PSF) will help to enhance both understanding and dealing with the imaging process. This paper analyzes the Fresnel [...] Read more.
In recent years, optical systems near the diffraction limit have been widely used in high-end applications. Evidently, an analytical solution of the point spread function (PSF) will help to enhance both understanding and dealing with the imaging process. This paper analyzes the Fresnel diffraction of diffraction-limited optical systems in defocused conditions. For this work, an analytical solution of the defocused PSF was obtained using the series expansion of the confluent hypergeometric functions. The analytical expression of the defocused optical transfer function is also presented herein for comparison with the PSF. Additionally, some characteristic parameters for the PSF are provided, such as the equivalent bandwidth and the Strehl ratio. Comparing the PSF obtained using the fast Fourier transform algorithm of an optical system with known, detailed parameters to the analytical solution derived in this paper using only the typical parameters, the root mean square errors of the two methods were found to be less than 3% in the weak and medium defocus range. The attractive advantages of the universal model, which is independent of design details, objective types, and applications, are discussed. Full article
(This article belongs to the Special Issue Emerging Topics in High-Power Laser and Light–Matter Interactions)
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Figure 1

Figure 1
<p>Schematic diagram of the defocus system.</p> Full article ">Figure 2
<p>The PSF distribution under different degrees of defocusing.</p> Full article ">Figure 3
<p>The sampling values of the PSF curve vs. the term’s truncation number N of the series equation, Equation (19). All terms from <span class="html-italic">n</span> = 0 to <span class="html-italic">n</span> = N are kept, but the terms with <span class="html-italic">n</span> &gt; N are omitted.</p> Full article ">Figure 4
<p>OTF curves for different defocus states.</p> Full article ">Figure 5
<p>The characteristic parameter (<b>a</b>) Equivalent bandwidth and (<b>b</b>) equivalent linewidth.</p> Full article ">Figure 6
<p>Schematic of the two-dimensional structure of the equivalent bandwidth.</p> Full article ">Figure 7
<p>Diagram of the SR as a function of the defocus amount.</p> Full article ">Figure 8
<p>OTF versus frequency.</p> Full article ">Figure 9
<p>The curve of the relationship between the SR and resolution at OTF = 0.2 for various defocus positions. (<b>a</b>) SR vs. resolution; (<b>b</b>) ln(SR) vs. resolution.</p> Full article ">Figure 10
<p>The 2D system layouts: (<b>a</b>) system 1; (<b>b</b>) system 2; (<b>c</b>) system 3.</p> Full article ">Figure 11
<p>Comparison of the analytical solution and ray tracing results for system 1 at different defocus levels. (<b>a</b>) MTF; (<b>b</b>) PSF.</p> Full article ">Figure 12
<p>Comparison of the analytical solution and ray tracing results for system 2 at different defocus levels. (<b>a</b>) MTF; (<b>b</b>) PSF.</p> Full article ">Figure 13
<p>Comparison of the analytical solution and ray tracing results for system 3 at different defocus levels. (<b>a</b>) MTF; (<b>b</b>) PSF.</p> Full article ">
15 pages, 10765 KiB  
Article
Dual-Polarization Conversion and Coding Metasurface for Wideband Radar Cross-Section Reduction
by Saima Hafeez, Jianguo Yu, Fahim Aziz Umrani, Yibo Huang, Wang Yun and Muhammad Ishfaq
Photonics 2024, 11(5), 454; https://doi.org/10.3390/photonics11050454 - 11 May 2024
Viewed by 916
Abstract
Modern stealth application systems require integrated meta-devices to operate effectively and have gained significant attention recently. This research paper proposes a 1-bit coding metasurface (CM) design. The fundamental component of the proposed CM is integrated to convert linearly polarized incoming electromagnetic waves into [...] Read more.
Modern stealth application systems require integrated meta-devices to operate effectively and have gained significant attention recently. This research paper proposes a 1-bit coding metasurface (CM) design. The fundamental component of the proposed CM is integrated to convert linearly polarized incoming electromagnetic waves into their orthogonal counterpart within frequency bands of 12.37–13.03 GHz and 18.96–32.37 GHz, achieving a polarization conversion ratio exceeding 99%. Furthermore, it enables linear-to-circular polarization conversion from 11.80 to 12.29, 13.17 to 18.44, and 33.33 to 40.35 GHz. A second element is produced by rotating a fundamental component by 90°, introducing a phase difference of π (pi) between them. Both elements are arranged in an array using a random aperiodic coding sequence to create a 1-bit CM for reducing the radar cross-section (RCS). The planar structure achieved over 10 dB RCS reduction for polarized waves in the frequency bands of 13.1–13.8 GHz and 20.4–30.9 GHz. A prototype was fabricated and tested, with the experimental results showing a good agreement with the simulated outcomes. The proposed design holds potential applications in radar systems, reflector antennas, stealth technologies, and satellite communication. Full article
(This article belongs to the Special Issue Advanced Photonics Metamaterials and Metasurfaces: Science and Applications)
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Figure 1

Figure 1
<p>The schematic diagram of the proposed unit cell: (<b>a</b>) top view, (<b>b</b>) side view of proposed meta-atom, and (<b>c</b>) isometric view and simulation setup.</p> Full article ">Figure 2
<p>The simulated result of proposed unit cell: (<b>a</b>) reflection coefficient in magnitude (<math display="inline"><semantics> <mrow> <mfenced close="|" open="|"> <mrow> <msub> <mi>r</mi> <mrow> <mi>x</mi> <mi>y</mi> </mrow> </msub> </mrow> </mfenced> <mo>,</mo> <mfenced close="|" open="|"> <mrow> <msub> <mi>r</mi> <mrow> <mi>y</mi> <mi>y</mi> </mrow> </msub> </mrow> </mfenced> </mrow> </semantics></math>) and (<b>b</b>) polarization conversion efficiency (%).</p> Full article ">Figure 3
<p>(<b>a</b>) Calculated results of phase difference (deg) and amplitude ratio (<math display="inline"><semantics> <mrow> <msub> <mi>r</mi> <mrow> <mi>x</mi> <mi>y</mi> </mrow> </msub> <mo>/</mo> <msub> <mi>r</mi> <mrow> <mi>y</mi> <mi>y</mi> </mrow> </msub> </mrow> </semantics></math>); (<b>b</b>) axial ratio (AR <math display="inline"><semantics> <mo>≤</mo> </semantics></math> 3 dB).</p> Full article ">Figure 4
<p>Simulated results at oblique incidences: (<b>a</b>) PCR and (<b>b</b>) axial ratio (AR ≤ 3 dB).</p> Full article ">Figure 5
<p>(<b>a</b>) Proposed meta-atom with <span class="html-italic">UV</span>-coordinates, and (<b>b</b>) magnitude (<math display="inline"><semantics> <mrow> <msub> <mi>r</mi> <mi>u</mi> </msub> </mrow> </semantics></math> and <math display="inline"><semantics> <mrow> <msub> <mi>r</mi> <mi>v</mi> </msub> </mrow> </semantics></math>) and reflection phase difference <math display="inline"><semantics> <mrow> <mfenced> <mrow> <mo>Δ</mo> <mi>φ</mi> <mo>=</mo> <mfenced close="|" open="|"> <mrow> <msub> <mi>φ</mi> <mi>u</mi> </msub> <mo>−</mo> <msub> <mi>φ</mi> <mi>v</mi> </msub> </mrow> </mfenced> </mrow> </mfenced> </mrow> </semantics></math> in <span class="html-italic">u</span>- and <span class="html-italic">v</span>-polarized waves.</p> Full article ">Figure 6
<p>Surface current distribution at different resonance frequencies: (<b>a</b>) 12.47 GHz, (<b>b</b>) 20.08 GHz, and (<b>c</b>) 32.56 GHz.</p> Full article ">Figure 7
<p>Layout of meta-atom: (<b>a</b>) ‘1’ element, (<b>b</b>) ‘0’ element, and (<b>c</b>) reflection phases of ‘1’ and ‘0’ with their phase difference.</p> Full article ">Figure 8
<p>Configuration of 1-bit coding metasurface (CM) with <math display="inline"><semantics> <mrow> <mn>5</mn> <mo>×</mo> <mn>5</mn> </mrow> </semantics></math> meta-atoms of ‘1’ and ‘0’.</p> Full article ">Figure 9
<p>(<b>a</b>) Monostatic RCS reduction of PEC and MS under a normal <span class="html-italic">x</span>-polarized wave, and (<b>b</b>) RCS reduction in different polarization states.</p> Full article ">Figure 10
<p>Three-dimensional scattering performance of (<b>a</b>–<b>c</b>) coding metasurface and (<b>d</b>–<b>f</b>) PEC under normal incident x-polarized wave at 12.64, 23.32 GHz, and 26.06 GHz.</p> Full article ">Figure 11
<p>RCS reduction of 1-bit coding metasurface at different incident angles (θ°).</p> Full article ">Figure 12
<p>(<b>a</b>) Prototype of 1-bit CM, and (<b>b</b>) experimental setup.</p> Full article ">Figure 13
<p>Measured and simulated results of (<b>a</b>) (AR ≤ 3 dB) and (<b>b</b>) monostatic RCS reduction.</p> Full article ">
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