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Photonics, Volume 8, Issue 6 (June 2021) – 60 articles

Cover Story (view full-size image): The ultrafast direct laser writing (DLW) technique can be used for generating complex 3D optical functions in bulk glasses. The 3D-embedded nature of the laser-fabricated optical element brings several advantages in terms of prototype stability, lifetime and complexity. Tuning the optical characteristics of the generated optical function via index engineering demands a thorough knowledge and an optimal control of the material response to the ultrafast laser pulse. Here, we present an overview of some recent results concerning large-mode-area light transport and extraction in sulfur-based chalcogenide mid-infrared glasses, placing emphasis on the glass response to ultrafast lasers. We then demonstrate the utilization of the achieved optimized local index changes for building efficient and compact embedded linear and nonlinear optical functions. View this paper
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12 pages, 3245 KiB  
Article
Design of a Surface Plasmon Resonance Temperature Sensor with Multi-Wavebands Based on Conjoined-Tubular Anti-Resonance Fiber
by Qiming Wang, Xuenan Zhang, Xin Yan, Fang Wang and Tonglei Cheng
Photonics 2021, 8(6), 231; https://doi.org/10.3390/photonics8060231 - 21 Jun 2021
Cited by 21 | Viewed by 2137
Abstract
In this work, a surface plasmon resonance (SPR) temperature sensor based on a con-joined-tubular anti-resonance optical fiber (CTF) was theoretically designed and analyzed using the finite element method. The CTF cladding was composed of eight pairs of conjoined tubes, and one or two [...] Read more.
In this work, a surface plasmon resonance (SPR) temperature sensor based on a con-joined-tubular anti-resonance optical fiber (CTF) was theoretically designed and analyzed using the finite element method. The CTF cladding was composed of eight pairs of conjoined tubes, and one or two holes of the tubes were selectively coated with gold to generate the SPR effect. Alcohol was injected into the core of the CTF to work as the sensing medium using vapor deposition. The proposed sensing structure exhibited excellent birefringence and produced more than six resonant peaks in different wavebands of the X and Y polarization. The positions of those resonant peaks were sensitive to temperature change, and the simulated sensitivity was about 3.2–3.6 nm/°C. The multiple working wavebands of the proposed sensing structure could be used for self-verification. Moreover, the influence of structural parameters on sensing performance was analyzed in detail. Possessing features of high sensitivity, good birefringence, multiple measuring wavebands, and self-verification, the proposed CTF-based SPR sensor has great potential in practical applications such as biological research and chemical sensing. Full article
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11 pages, 519 KiB  
Article
Performance Analysis of Dual-Hop Mixed Power Line Communication/Free-Space Optical Cooperative Systems
by Manh Le-Tran and Sunghwan Kim
Photonics 2021, 8(6), 230; https://doi.org/10.3390/photonics8060230 - 21 Jun 2021
Cited by 6 | Viewed by 2081
Abstract
In this paper, we study a mixed cooperative communication system consisting of power line communication (PLC) and free-space optical communication (FSO) links, where the PLC link suffers from log-normal fading and is affected by both impulsive and background noises. Meanwhile, the FSO link [...] Read more.
In this paper, we study a mixed cooperative communication system consisting of power line communication (PLC) and free-space optical communication (FSO) links, where the PLC link suffers from log-normal fading and is affected by both impulsive and background noises. Meanwhile, the FSO link undergoes Gamma-Gamma fading with both atmospheric turbulence and pointing errors. More specifically, we present closed-form expressions for the probability density function and the cumulative distribution function of the end-to-end signal-to-noise ratio of the proposed model. Consequently, the outage probability and the bit error rate (BER) performance are derived in terms of univariate Fox-H and bivariate Fox-H functions. Finally, the analytical results are verified using Monte Carlo simulations, providing useful insights into the capabilities of the proposed system. Full article
(This article belongs to the Special Issue Optical Wireless Communications Systems)
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11 pages, 6967 KiB  
Article
Efficient and Noise Robust Photon-Counting Imaging with First Signal Photon Unit Method
by Kangjian Hua, Bo Liu, Zhen Chen, Liang Fang and Huachuang Wang
Photonics 2021, 8(6), 229; https://doi.org/10.3390/photonics8060229 - 19 Jun 2021
Cited by 12 | Viewed by 2477
Abstract
Efficient photon-counting imaging in low signal photon level is challenging, especially when noise is intensive. In this paper, we report a first signal photon unit (FSPU) method to rapidly reconstruct depth image from sparse signal photon counts with strong noise robustness. The method [...] Read more.
Efficient photon-counting imaging in low signal photon level is challenging, especially when noise is intensive. In this paper, we report a first signal photon unit (FSPU) method to rapidly reconstruct depth image from sparse signal photon counts with strong noise robustness. The method consists of acquisition strategy and reconstruction strategy. Different statistic properties of signal and noise are exploited to quickly distinguish signal unit during acquisition. Three steps, including maximum likelihood estimation (MLE), anomaly censorship and total variation (TV) regularization, are implemented to recover high quality images. Simulations demonstrate that the method performs much better than traditional photon-counting methods such as peak and cross-correlation methods, and it also has better performance than the state-of-the-art unmixing method. In addition, it could reconstruct much clearer images than the first photon imaging (FPI) method when noise is severe. An experiment with our photon-counting LIDAR system was conducted, which indicates that our method has advantages in sparse photon-counting imaging application, especially when signal to noise ratio (SNR) is low. Without the knowledge of noise distribution, our method reconstructed the clearest depth image which has the least mean square error (MSE) as 0.011, even when SNR is as low as −10.85 dB. Full article
(This article belongs to the Special Issue Smart Pixels and Imaging)
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19 pages, 587 KiB  
Article
A Versatile Quantum Simulator for Coupled Oscillators Using a 1D Chain of Atoms Trapped near an Optical Nanofiber
by Daniela Holzmann, Matthias Sonnleitner and Helmut Ritsch
Photonics 2021, 8(6), 228; https://doi.org/10.3390/photonics8060228 - 19 Jun 2021
Cited by 3 | Viewed by 2300
Abstract
The transversely confined propagating light modes of a nanophotonic optical waveguide or nanofiber can effectively mediate infinite-range forces. We show that for a linear chain of particles trapped within the waveguide’s evanescent field, transverse illumination with a suitable set of laser frequencies should [...] Read more.
The transversely confined propagating light modes of a nanophotonic optical waveguide or nanofiber can effectively mediate infinite-range forces. We show that for a linear chain of particles trapped within the waveguide’s evanescent field, transverse illumination with a suitable set of laser frequencies should allow the implementation of a coupled-oscillator quantum simulator with time-dependent and widely controllable all-to-all interactions. Using the example of the energy spectrum of oscillators with simulated Coulomb interactions, we show that different effective coupling geometries can be emulated with high precision by proper choice of laser illumination conditions. Similarly, basic quantum gates can be selectively implemented between arbitrarily chosen pairs of oscillators in the energy as well as in the coherent-state basis. Key properties of the system dynamics and states can be monitored continuously by analysis of the out-coupled fiber fields. Full article
(This article belongs to the Special Issue Quantum Technologies in Electrodynamic Resonators and Waveguides)
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14 pages, 3602 KiB  
Article
Sharp Focusing of a Hybrid Vector Beam with a Polarization Singularity
by Victor V. Kotlyar, Sergey S. Stafeev and Anton G. Nalimov
Photonics 2021, 8(6), 227; https://doi.org/10.3390/photonics8060227 - 18 Jun 2021
Cited by 11 | Viewed by 2122
Abstract
The key result of this work is the use of the global characteristics of the polarization singularities of the entire beam as a whole, rather than the analysis of local polarization, Stokes and Poincare–Hopf indices. We extend Berry’s concept of the topological charge [...] Read more.
The key result of this work is the use of the global characteristics of the polarization singularities of the entire beam as a whole, rather than the analysis of local polarization, Stokes and Poincare–Hopf indices. We extend Berry’s concept of the topological charge of scalar beams to hybrid vector beams. We discuss tightly focusing a new type of nth-order hybrid vector light field comprising n C-lines (circular polarization lines). Using a complex Stokes field, it is shown that the field polarization singularity index equals n/2 and does not preserve in the focal plane. The intensity and Stokes vector components in the focal plane are expressed analytically. It is theoretically and numerically demonstrated that at an even n, the intensity pattern at the focus is symmetrical, and instead of C-lines, there occur C-points around which axes of polarization ellipses are rotated. At n = 4, C-points characterized by singularity indices 1/2 and ‘lemon’-type topology are found at the focus. For an odd source field order n, the intensity pattern at the focus has no symmetry, and the field becomes purely vectorial (with no elliptical polarization) and has n V-points, around which linear polarization vectors are rotating. Full article
(This article belongs to the Special Issue Polarized Light and Optical Systems)
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12 pages, 1112 KiB  
Article
Numerical Simulations on Polarization Quantum Noise Squeezing for Ultrashort Solitons in Optical Fiber with Enlarged Mode Field Area
by Arseny A. Sorokin, Elena A. Anashkina, Joel F. Corney, Vjaceslavs Bobrovs, Gerd Leuchs and Alexey V. Andrianov
Photonics 2021, 8(6), 226; https://doi.org/10.3390/photonics8060226 - 18 Jun 2021
Cited by 12 | Viewed by 2073
Abstract
Broadband quantum noise suppression of light is required for many applications, including detection of gravitational waves, quantum sensing, and quantum communication. Here, using numerical simulations, we investigate the possibility of polarization squeezing of ultrashort soliton pulses in an optical fiber with an enlarged [...] Read more.
Broadband quantum noise suppression of light is required for many applications, including detection of gravitational waves, quantum sensing, and quantum communication. Here, using numerical simulations, we investigate the possibility of polarization squeezing of ultrashort soliton pulses in an optical fiber with an enlarged mode field area, such as large-mode area or multicore fibers (to scale up the pulse energy). Our model includes the second-order dispersion, Kerr and Raman effects, quantum noise, and optical losses. In simulations, we switch on and switch off Raman effects and losses to find their contribution to squeezing of optical pulses with different durations (0.1–1 ps). For longer solitons, the peak power is lower and a longer fiber is required to attain the same squeezing as for shorter solitons, when Raman effects and losses are neglected. In the full model, we demonstrate optimal pulse duration (~0.4 ps) since losses limit squeezing of longer pulses and Raman effects limit squeezing of shorter pulses. Full article
(This article belongs to the Special Issue Novel Materials and Technologies for Fiber Lasers)
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12 pages, 3330 KiB  
Article
Influence of the Surface Roughness of a Silicon Disk Resonator on Its Q-Factor
by Anastasia V. Yakuhina, Alexey S. Kadochkin, Dmitry V. Gorelov, Vyacheslav V. Svetukhin, Sergey S. Generalov and Vladimir V. Amelichev
Photonics 2021, 8(6), 225; https://doi.org/10.3390/photonics8060225 - 17 Jun 2021
Cited by 3 | Viewed by 2311
Abstract
This article presents a silicon disk resonator of the whispering-gallery-mode (WGM) type. The calculated Q-factor of the silicon WGM resonator was 107. Two methods of studying the surface roughness of a silicon WGM resonator with a nonlinear profile by means of [...] Read more.
This article presents a silicon disk resonator of the whispering-gallery-mode (WGM) type. The calculated Q-factor of the silicon WGM resonator was 107. Two methods of studying the surface roughness of a silicon WGM resonator with a nonlinear profile by means of Helios 650 scanning electron microscope and Bruker atomic force microscope (AFM) are presented. The results obtained by the two methods agreed well with each other. A comparison of the surface roughness values of WGM resonators manufactured using different technological approaches is presented. Based on the obtained data, a preliminary estimated Q-factor calculation of the resonators was performed, which was refined by numerical calculation using the finite-difference time-domain (FDTD) method. The effect of the surface roughness of the resonator on its Q-factor was found. Reducing the surface roughness of the resonator from 30 nm to 1–2 nm led to an increase in its Q-factor from 104 to 107. Full article
(This article belongs to the Special Issue Photonic Integration: Technologies and Applications)
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11 pages, 7167 KiB  
Article
Edge and Contrast Enhancement Using Spatially Incoherent Correlation Holography Techniques
by Vijayakumar Anand, Joseph Rosen, Soon Hock Ng, Tomas Katkus, Denver P Linklater, Elena P Ivanova and Saulius Juodkazis
Photonics 2021, 8(6), 224; https://doi.org/10.3390/photonics8060224 - 16 Jun 2021
Cited by 13 | Viewed by 2826
Abstract
Image enhancement techniques (such as edge and contrast enhancement) are essential for many imaging applications. In incoherent holography techniques such as Fresnel incoherent correlation holography (FINCH), the light from an object is split into two, each of which is modulated differently from one [...] Read more.
Image enhancement techniques (such as edge and contrast enhancement) are essential for many imaging applications. In incoherent holography techniques such as Fresnel incoherent correlation holography (FINCH), the light from an object is split into two, each of which is modulated differently from one another by two different quadratic phase functions and coherently interfered to generate the hologram. The hologram can be reconstructed via a numerical backpropagation. The edge enhancement procedure in FINCH requires the modulation of one of the beams by a spiral phase element and, upon reconstruction, edge-enhanced images are obtained. An optical technique for edge enhancement in coded aperture imaging (CAI) techniques that does not involve two-beam interference has not been established yet. In this study, we propose and demonstrate an iterative algorithm that can yield from the experimentally recorded point spread function (PSF), a synthetic PSF that can generate edge-enhanced reconstructions when processed with the object hologram. The edge-enhanced reconstructions are subtracted from the original reconstructions to obtain contrast enhancement. The technique has been demonstrated on FINCH and CAI methods with different spectral conditions. Full article
(This article belongs to the Special Issue Holography)
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8 pages, 3147 KiB  
Article
Sensitivity-Tunable Oscillator-Accelerometer Based on Optical Fiber Bragg Grating
by Xiangpeng Xiao, Jinpeng Tao, Qingguo Song, Yuezhen Sun, Jiang Yang and Zhijun Yan
Photonics 2021, 8(6), 223; https://doi.org/10.3390/photonics8060223 - 15 Jun 2021
Cited by 4 | Viewed by 2207
Abstract
We demonstrate a fiber Bragg grating (FBG)-based oscillator-accelerometer in which the acceleration sensitivity can be tuned by controlling the location of the mass oscillator. We theoretically and experimentally investigated the performance of the proposed accelerometer. Theoretical analysis showed that both the mass and [...] Read more.
We demonstrate a fiber Bragg grating (FBG)-based oscillator-accelerometer in which the acceleration sensitivity can be tuned by controlling the location of the mass oscillator. We theoretically and experimentally investigated the performance of the proposed accelerometer. Theoretical analysis showed that both the mass and location of the oscillator affect the sensitivity and resonant frequency of the accelerometer. To simplify the analysis, a nondimensional parameter, P, was introduced to tune the sensitivity of the FBG-based oscillator-accelerometer, which is related to the location of the mass oscillator. Numerical analysis showed that the accelerometer sensitivity is linearly proportional to the P parameter. In the experiment, six FBG-based oscillator-accelerometers with different P parameters (0.125, 0.25, 0.375, 0.5, 0.625, 0.75) were fabricated and tested. The experimental results agree very well with the numerical analysis, in which the sensitivity of the proposed accelerometer linearly increased with the increase in parameter P (7.6 pm/g, 15.8 pm/g, 19.3 pm/g, 25.4 pm/g, 30.6 pm/g, 35.7 pm/g). The resonance frequency is quadratically proportional to parameter P, and the resonance frequency reaches the minimum of 440 Hz when P is equal to 0.5. The proposed oscillator-accelerometer showed very good orthogonal vibration isolation. Full article
(This article belongs to the Special Issue Advancements in Fiber Bragg Grating Research)
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9 pages, 2478 KiB  
Article
Super-Resolution Imaging by Dielectric Superlenses: TiO2 Metamaterial Superlens versus BaTiO3 Superlens
by Rakesh Dhama, Bing Yan, Cristiano Palego and Zengbo Wang
Photonics 2021, 8(6), 222; https://doi.org/10.3390/photonics8060222 - 15 Jun 2021
Cited by 22 | Viewed by 3702
Abstract
All-dielectric superlens made from micro and nano particles has emerged as a simple yet effective solution to label-free, super-resolution imaging. High-index BaTiO3 Glass (BTG) microspheres are among the most widely used dielectric superlenses today but could potentially be replaced by a new [...] Read more.
All-dielectric superlens made from micro and nano particles has emerged as a simple yet effective solution to label-free, super-resolution imaging. High-index BaTiO3 Glass (BTG) microspheres are among the most widely used dielectric superlenses today but could potentially be replaced by a new class of TiO2 metamaterial (meta-TiO2) superlens made of TiO2 nanoparticles. In this work, we designed and fabricated TiO2 metamaterial superlens in full-sphere shape for the first time, which resembles BTG microsphere in terms of the physical shape, size, and effective refractive index. Super-resolution imaging performances were compared using the same sample, lighting, and imaging settings. The results show that TiO2 meta-superlens performs consistently better over BTG superlens in terms of imaging contrast, clarity, field of view, and resolution, which was further supported by theoretical simulation. This opens new possibilities in developing more powerful, robust, and reliable super-resolution lens and imaging systems. Full article
(This article belongs to the Special Issue Photonic Jet: Science and Application)
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11 pages, 1199 KiB  
Article
Light Confinement with Structured Beams in Gold Nanoparticle Suspensions
by Argelia Balbuena Ortega, Felix E. Torres-González, Valentin López Gayou, Raul Delgado Macuil, Gaetano Assanto and Karen Volke-Sepulveda
Photonics 2021, 8(6), 221; https://doi.org/10.3390/photonics8060221 - 15 Jun 2021
Cited by 3 | Viewed by 2596
Abstract
We carry out an experimental campaign to investigate the nonlinear self-defocusing propagation of singular light beams with various complex structures of phase and intensity in a colloidal suspension of gold nanoparticles with a plasmonic resonance near the laser wavelength (532nm). Studying optical vortices [...] Read more.
We carry out an experimental campaign to investigate the nonlinear self-defocusing propagation of singular light beams with various complex structures of phase and intensity in a colloidal suspension of gold nanoparticles with a plasmonic resonance near the laser wavelength (532nm). Studying optical vortices embedded in Gaussian beams, Bessel vortices and Bessel-cosine (necklace) beams, we gather evidence that while intense vortices turn into two-dimensional dark solitons, all structured wavepackets are able to guide a weak Gaussian probe of different wavelength (632.8 nm) along the dark core. The probe confinement also depends on the topological charge of the singular pump. Full article
(This article belongs to the Special Issue Singular Optics)
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11 pages, 2560 KiB  
Article
Stretchable and Transparent Metal Nanowire Microelectrodes for Simultaneous Electrophysiology and Optogenetics Applications
by Jinbi Tian, Zexu Lin, Zhiyuan Chen, Sofian N. Obaid, Igor R. Efimov and Luyao Lu
Photonics 2021, 8(6), 220; https://doi.org/10.3390/photonics8060220 - 15 Jun 2021
Cited by 10 | Viewed by 3630
Abstract
Recently developed optically transparent microelectrode technology provides a promising approach for simultaneous high-resolution electrical and optical biointerfacing with tissues in vivo and in vitro. A critically unmet need is designing high-performance stretchable platforms for conformal biointerfacing with mechanically active organs. Here, we report [...] Read more.
Recently developed optically transparent microelectrode technology provides a promising approach for simultaneous high-resolution electrical and optical biointerfacing with tissues in vivo and in vitro. A critically unmet need is designing high-performance stretchable platforms for conformal biointerfacing with mechanically active organs. Here, we report silver nanowire (Ag NW) stretchable transparent microelectrodes and interconnects that exhibit excellent electrical and electrochemical performance, high optical transparency, superior mechanical robustness and durability by a simple selective-patterning process. The fabrication method allows the direct integration of Ag NW networks on elastomeric substrates. The resulting Ag NW interface exhibits a low sheet resistance (Rsh) of 1.52–4.35 Ω sq−1, an advantageous normalized electrochemical impedance of 3.78–6.04 Ω cm2, a high optical transparency of 61.3–80.5% at 550 nm and a stretchability of 40%. The microelectrode arrays (MEAs) fabricated with this approach exhibit uniform electrochemical performance across all channels. Studies on mice demonstrate that both pristine and stretched Ag NW microelectrodes can achieve high-fidelity electrophysiological monitoring of cardiac activity with/without co-localized optogenetic pacing. Together, these results pave the way for developing stretchable and transparent metal nanowire networks for high-resolution opto-electric biointerfacing with mechanically active organs, such as the heart. Full article
(This article belongs to the Special Issue Bio-Integrated Photonic Materials and Devices)
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10 pages, 2917 KiB  
Communication
Holography and the Luxury Industry
by V. Michael Bove, Jr. and Nicole A. Reader
Photonics 2021, 8(6), 219; https://doi.org/10.3390/photonics8060219 - 13 Jun 2021
Cited by 2 | Viewed by 4229
Abstract
The luxury goods industry and holography have a lengthy history together. In this article, we review the applications of holography to the industry and the relevant technical requirements, in particular when the hologram itself is the luxury item, when the hologram is used [...] Read more.
The luxury goods industry and holography have a lengthy history together. In this article, we review the applications of holography to the industry and the relevant technical requirements, in particular when the hologram itself is the luxury item, when the hologram is used to promote luxury items, and when the hologram is used for authentication of luxury items. We then explore some possible scenarios for the evolution of this relationship. Full article
(This article belongs to the Special Issue Holography)
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14 pages, 7823 KiB  
Article
Generation of Multiple Vector Optical Bottle Beams
by Svetlana N. Khonina, Alexey P. Porfirev, Sergey G. Volotovskiy, Andrey V. Ustinov, Sergey A. Fomchenkov, Vladimir S. Pavelyev, Siegmund Schröter and Michael Duparré
Photonics 2021, 8(6), 218; https://doi.org/10.3390/photonics8060218 - 12 Jun 2021
Cited by 18 | Viewed by 3668
Abstract
We propose binary diffractive optical elements, combining several axicons of different types (axis-symmetrical and spiral), for the generation of a 3D intensity distribution in the form of multiple vector optical ‘bottle’ beams, which can be tailored by a change in the polarization state [...] Read more.
We propose binary diffractive optical elements, combining several axicons of different types (axis-symmetrical and spiral), for the generation of a 3D intensity distribution in the form of multiple vector optical ‘bottle’ beams, which can be tailored by a change in the polarization state of the illumination radiation. The spatial dynamics of the obtained intensity distribution with different polarization states (circular and cylindrical of various orders) were investigated in paraxial mode numerically and experimentally. The designed binary axicons were manufactured using the e-beam lithography technique. The proposed combinations of optical elements can be used for the generation of vector optical traps in the field of laser trapping and manipulation, as well as for performing the spatial transformation of the polarization state of laser radiation, which is crucial in the field of laser-matter interaction for the generation of special morphologies of laser-induced periodic surface structures. Full article
(This article belongs to the Special Issue Polarized Light and Optical Systems)
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18 pages, 4144 KiB  
Article
Crosstalk Reduction in Voxels for a See-Through Holographic Waveguide by Using Integral Imaging with Compensated Elemental Images
by Jiwoon Yeom, Yeseul Son and Kwangsoon Choi
Photonics 2021, 8(6), 217; https://doi.org/10.3390/photonics8060217 - 11 Jun 2021
Cited by 12 | Viewed by 2882
Abstract
The representation of three-dimensional volumetric pixels, voxels, is an important issue for the near-to-eye displays (NEDs) to solve the vergence-accommodation conflict problem. Although the holographic waveguides using holographic optical element (HOE) couplers are promising technologies for NEDs with the ultra-thin structure and high [...] Read more.
The representation of three-dimensional volumetric pixels, voxels, is an important issue for the near-to-eye displays (NEDs) to solve the vergence-accommodation conflict problem. Although the holographic waveguides using holographic optical element (HOE) couplers are promising technologies for NEDs with the ultra-thin structure and high transparency, most of them have presented only a single and fixed depth plane. In this paper, we analyze the imaging characteristics of holographic waveguides, particularly to represent the arbitrary voxels and investigate the voxel duplication problem arising from the non-collimated light from the voxels. In order to prevent the image crosstalk arising from the voxel duplication, we propose an adjustment method for the emission angle profile of voxels by using the integral imaging technique. In the proposed method, the sub-regions of elemental images, which correspond to the duplicated voxels, are masked in order to optimize the emission angle of integrated voxels. In the experimental verification, a see-through integral imaging system, based on the organic light-emitting diode display and a holographic waveguide with the thickness of 5 mm, was constructed. The fabricated HOE in the waveguide showed high diffraction efficiency of 72.8 %, 76.6%, and 72.5 % for 460 nm, 532 nm, and 640 nm lasers, respectively. By applying the masked elemental images, the proposed method resulted in a reduced crosstalk in the observed voxels by 2.35 times. The full-color experimental results of see-through holographic waveguide with integral imaging are provided, whereby the observed 3D images are presented clearly without the ghost images due to the voxel duplication problem. Full article
(This article belongs to the Special Issue Materials, Methods and Models for Holographic Optical Elements)
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20 pages, 3721 KiB  
Article
High-Density Dynamics of Laser Wakefield Acceleration from Gas Plasmas to Nanotubes
by Bradley Scott Nicks, Ernesto Barraza-Valdez, Sahel Hakimi, Kyle Chesnut, Genevieve DeGrandchamp, Kenneth Gage, David Housley, Gregory Huxtable, Gerard Lawler, Daniel Lin, Pratik Manwani, Eric Nelson, Gabriel Player, Michael Seggebruch, James Sweeney, Joshua Tanner, Kurt Thompson and Toshiki Tajima
Photonics 2021, 8(6), 216; https://doi.org/10.3390/photonics8060216 - 11 Jun 2021
Cited by 5 | Viewed by 2929
Abstract
The electron dynamics of laser wakefield acceleration (LWFA) is examined in the high-density regime using particle-in-cell simulations. These simulations model the electron source as a target of carbon nanotubes. Carbon nanotubes readily allow access to near-critical densities and may have other advantageous properties [...] Read more.
The electron dynamics of laser wakefield acceleration (LWFA) is examined in the high-density regime using particle-in-cell simulations. These simulations model the electron source as a target of carbon nanotubes. Carbon nanotubes readily allow access to near-critical densities and may have other advantageous properties for potential medical applications of electron acceleration. In the near-critical density regime, electrons are accelerated by the ponderomotive force followed by the electron sheath formation, resulting in a flow of bulk electrons. This behavior represents a qualitatively distinct regime from that of low-density LWFA. A quantitative entropy index for differentiating these regimes is proposed. The dependence of accelerated electron energy on laser amplitude is also examined. For the majority of this study, the laser propagates along the axis of the target of carbon nanotubes in a 1D geometry. After the fundamental high-density physics is established, an alternative, 2D scheme of laser acceleration of electrons using carbon nanotubes is considered. Full article
(This article belongs to the Special Issue Functional Nanophotonic Materials and Structures)
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20 pages, 5501 KiB  
Article
Structural and Optical Properties of InAsSbBi Grown by Molecular Beam Epitaxy on Offcut GaSb Substrates
by Rajeev R. Kosireddy, Stephen T. Schaefer, Marko S. Milosavljevic and Shane R. Johnson
Photonics 2021, 8(6), 215; https://doi.org/10.3390/photonics8060215 - 11 Jun 2021
Cited by 1 | Viewed by 2147
Abstract
Three InAsSbBi samples are grown by molecular beam epitaxy at 400 °C on GaSb substrates with three different offcuts: (100) on-axis, (100) offcut 1° toward [011], and (100) offcut 4° toward [011]. The samples are investigated using X-ray diffraction, Nomarski optical microscopy, atomic [...] Read more.
Three InAsSbBi samples are grown by molecular beam epitaxy at 400 °C on GaSb substrates with three different offcuts: (100) on-axis, (100) offcut 1° toward [011], and (100) offcut 4° toward [011]. The samples are investigated using X-ray diffraction, Nomarski optical microscopy, atomic force microscopy, transmission electron microscopy, and photoluminescence spectroscopy. The InAsSbBi layers are 210 nm thick, coherently strained, and show no observable defects. The substrate offcut is not observed to influence the structural and interface quality of the samples. Each sample exhibits small lateral variations in the Bi mole fraction, with the largest variation observed in the on-axis growth. Bismuth rich surface droplet features are observed on all samples. The surface droplets are isotropic on the on-axis sample and elongated along the [011¯] step edges on the 1° and 4° offcut samples. No significant change in optical quality with offcut angle is observed. Full article
(This article belongs to the Special Issue Near- and Mid-Infrared Photonics Technologies)
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11 pages, 5557 KiB  
Communication
SOI-Based Multi-Channel AWG with Fiber Bragg Grating Sensing Interrogation System
by Siming Weng, Pei Yuan, Wei Zhuang, Dongliang Zhang, Fei Luo and Lianqing Zhu
Photonics 2021, 8(6), 214; https://doi.org/10.3390/photonics8060214 - 10 Jun 2021
Cited by 17 | Viewed by 3574
Abstract
For the development of minimized and high-rate photonic-integrated fiber Bragg grating interrogation (FBGI) systems, arrayed waveguide grating (AWG) has been widely used as one of the critical components. In this paper, we present an 8-channel SOI-based AWG for a photonic integrated FBG interrogation [...] Read more.
For the development of minimized and high-rate photonic-integrated fiber Bragg grating interrogation (FBGI) systems, arrayed waveguide grating (AWG) has been widely used as one of the critical components. In this paper, we present an 8-channel SOI-based AWG for a photonic integrated FBG interrogation microsystem. The channel spacing of the AWG is designed to be 3 nm to meet a high-dynamic-range demodulation requirement. The core size of the fabricated AWG is about 335 × 335 μm2. The simulation results and experimental results are in high agreement, showing that AWG has a fine transmission spectrum with crosstalk below −16 dB, nonuniformity below 0.4 dB, insertion loss below −6.35 dB, 3 dB bandwidth about 1.3 nm and 10 dB bandwidth of 2.3 nm. The proposed AWG can be applied perfectly to the SOI-based AWG demodulation microsystem, exhibiting a large dynamic range of 1.2 nm, the resolution for measurements is 1.27 pm and a high accuracy of 20.6 pm. Full article
(This article belongs to the Special Issue Fiber-Optic Sensors)
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8 pages, 1780 KiB  
Article
Terahertz Time-Domain Polarimetry for Principal Optical Axes of Anisotropic Crystals
by Alexander Mamrashev, Fedor Minakov, Nazar Nikolaev and Valery Antsygin
Photonics 2021, 8(6), 213; https://doi.org/10.3390/photonics8060213 - 10 Jun 2021
Cited by 12 | Viewed by 2342
Abstract
We propose a method for measuring the terahertz properties for two principal optical axes of anisotropic crystals without optical activity using terahertz time-domain spectroscopy (THz-TDS). The method put forward in this paper utilizes the inherent polarization sensitivity of the THz-TDS electro-optic detection system. [...] Read more.
We propose a method for measuring the terahertz properties for two principal optical axes of anisotropic crystals without optical activity using terahertz time-domain spectroscopy (THz-TDS). The method put forward in this paper utilizes the inherent polarization sensitivity of the THz-TDS electro-optic detection system. We demonstrate the practical application of the method by measuring the temperature dependence of the refractive index and the absorption coefficient of a lithium triborate crystal for three optical axes. Full article
(This article belongs to the Special Issue Terahertz Spectroscopy and Imaging)
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17 pages, 5011 KiB  
Article
Using Machine Learning Algorithms for Accurate Received Optical Power Prediction of an FSO Link over a Maritime Environment
by Antonios Lionis, Konstantinos Peppas, Hector E. Nistazakis, Andreas Tsigopoulos, Keith Cohn and Athanassios Zagouras
Photonics 2021, 8(6), 212; https://doi.org/10.3390/photonics8060212 - 10 Jun 2021
Cited by 23 | Viewed by 3815
Abstract
The performance prediction of an optical communications link over maritime environments has been extensively researched over the last two decades. The various atmospheric phenomena and turbulence effects have been thoroughly explored, and long-term measurements have allowed for the construction of simple empirical models. [...] Read more.
The performance prediction of an optical communications link over maritime environments has been extensively researched over the last two decades. The various atmospheric phenomena and turbulence effects have been thoroughly explored, and long-term measurements have allowed for the construction of simple empirical models. The aim of this work is to demonstrate the prediction accuracy of various machine learning (ML) algorithms for a free-space optical communication (FSO) link performance, with respect to real time, non-linear atmospheric conditions. A large data set of received signal strength indicators (RSSI) for a laser communications link has been collected and analyzed against seven local atmospheric parameters (i.e., wind speed, pressure, temperature, humidity, dew point, solar flux and air-sea temperature difference). The k-nearest-neighbors (KNN), tree-based methods-decision trees, random forest and gradient boosting- and artificial neural networks (ANN) have been employed and compared among each other using the root mean square error (RMSE) and the coefficient of determination (R2) of each model as the primary performance indices. The regression analysis revealed an excellent fit for all ML models, indicative of their ability to offer a significant improvement in FSO performance modeling as compared to traditional regression models. The best-performing R2 model found to be the ANN approach (0.94867), while random forests achieved the most optimal RMSE result (7.37). Full article
(This article belongs to the Special Issue Optical Wireless Communications Systems)
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29 pages, 8403 KiB  
Review
Multiscale Laser Written Photonic Structures in Bulk Chalcogenide Glasses for Infrared Light Transport and Extraction
by Ciro D’Amico, Guillermo Martin, Johann Troles, Guanghua Cheng and Razvan Stoian
Photonics 2021, 8(6), 211; https://doi.org/10.3390/photonics8060211 - 10 Jun 2021
Cited by 14 | Viewed by 3888
Abstract
Direct ultrafast laser processing is nowadays considered the most flexible technique allowing to generate complex 3D optical functions in bulk glasses. The fact that the built-in optical element is embedded in the material brings several advantages in terms of prototype stability and lifetime, [...] Read more.
Direct ultrafast laser processing is nowadays considered the most flexible technique allowing to generate complex 3D optical functions in bulk glasses. The fact that the built-in optical element is embedded in the material brings several advantages in terms of prototype stability and lifetime, but equally in terms of complexity and number of possible applications, due to the 3D design. The generated optical functions, and in particular the single mode character of the light guiding element alongside the accessibility toward different spectral windows, depend on the refractive index contrast that can be achieved within the material transparency window and on the characteristic dimensions of the optical modification. In particular, the accessibility to the infrared and mid-infrared spectral domains, and to the relevant applications in sensing and imaging, requires increasing the cross-section of the guiding element in order to obtain the desired normalized frequency. Moreover, efficient signal extraction from the transported light requires nanometer size void-like index structures. All this demands a thorough knowledge and an optimal control of the material response within the interaction with the ultrafast laser pulse. We present here an overview of some recent results concerning large-mode-area light transport and extraction in sulfur-based chalcogenide mid-infrared glasses, putting emphasis on the study of the glass response to ultrafast lasers. We then demonstrate the utilization of the achieved optimized local index modifications for building efficient and compact embedded spectrometers (linear optical functions) and saturable absorbers (nonlinear optical functions) for integrated photonic applications in the infrared and mid-infrared spectral ranges. Full article
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12 pages, 2600 KiB  
Article
Analysis and Investigation of Dual-Polarized Color LED Based Visible Light Communication System
by Yun-Cheng Yang, Chien-Hung Yeh, Shien-Kuei Liaw, Chi-Wai Chow, Wei-Hung Hsu and Bo-Yin Wang
Photonics 2021, 8(6), 210; https://doi.org/10.3390/photonics8060210 - 10 Jun 2021
Cited by 5 | Viewed by 2485
Abstract
To increase the data capacity of a light-emitting diode (LED) based visible light communication (VLC) transmission, a polarization-division-multiplexing (PMD) green (G)- and blue (B)- light-based transmitter (Tx) module is demonstrated here. It was demonstrated that we can achieve 1200 and 1120 Mbps VLC [...] Read more.
To increase the data capacity of a light-emitting diode (LED) based visible light communication (VLC) transmission, a polarization-division-multiplexing (PMD) green (G)- and blue (B)- light-based transmitter (Tx) module is demonstrated here. It was demonstrated that we can achieve 1200 and 1120 Mbps VLC capacities based on dual-polarized G- and a B-LED based light wave after 3 and 4 m free-space link lengths, respectively, at exceedingly low illuminance. Based on the presented VLC system, paired G-LEDs or B-LEDs with dual-polarization can also be applied on the VLC-Tx side for doubling and delivering VLC data. According to the obtained results, the largest polarization offset angle of 50° between two polarizers (POLs) can be allowed experimentally to provide optimal VLC traffic. Moreover, the relationships of polarization offset, the illuminance of LED and maximum achieved VLC capacity are also performed and analyzed. Full article
(This article belongs to the Special Issue Optical Network and Access Technologies)
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13 pages, 2186 KiB  
Article
Near Unity PLQY and High Stability of Barium Thiocyanate Based All-Inorganic Perovskites and Their Applications in White Light-Emitting Diodes
by Gopi Chandra Adhikari, Saroj Thapa, Yang Yue, Hongyang Zhu and Peifen Zhu
Photonics 2021, 8(6), 209; https://doi.org/10.3390/photonics8060209 - 9 Jun 2021
Cited by 10 | Viewed by 4348
Abstract
All-inorganic lead halide perovskite (CsPbX3) nanocrystals (NCs) have emerged as a highly promising new generation of light emitters due to their extraordinary photophysical properties. However, the performance of these semiconducting NCs is undermined due to the inherent toxicity of lead and [...] Read more.
All-inorganic lead halide perovskite (CsPbX3) nanocrystals (NCs) have emerged as a highly promising new generation of light emitters due to their extraordinary photophysical properties. However, the performance of these semiconducting NCs is undermined due to the inherent toxicity of lead and long-term environmental stability. Here, we report the addition of B-site cation and X-site anion (pseudo-halide) concurrently using Ba(SCN)2 (≤50%) in CsPbX3 NCs to reduce the lead and improve the photophysical properties and stability. The as-grown particles demonstrated an analogous structure with an almost identical lattice constant and a fluctuation of particle size without altering the morphology of particles. Photoluminescence quantum yield is enhanced up to near unity (~98%) by taking advantage of concomitant doping at the B- and X-site of the structure. Benefitted from the defect reductions and stronger bonding interaction between Pb2+ and SCN ions, Ba(SCN)2-based NCs exhibit improved stability towards air and moisture compared to the host NCs. The doped NCs retain higher PLQY (as high as seven times) compared to the host NCs) when stored in an ambient atmosphere for more than 176 days. A novel 3D-printed multiplex color conversion layer was used to fabricate a white light-emitting diode (LED). The obtained white light shows a correlated color temperature of 6764 K, a color rendering index of 87, and luminous efficacy of radiation of 333 lm/W. In summary, this work proposes a facile route to treat sensitive lead halide perovskite NCs and to fabricate LEDs by using a low-cost large-scale 3-D printing method, which would serve as a foundation for fabricating high-quality optoelectronic devices for near future lighting technologies. Full article
(This article belongs to the Special Issue Wide Bandgap Semiconductor Photonic Devices)
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10 pages, 2161 KiB  
Article
Key Distribution Scheme for Optical Fiber Channel Based on SNR Feature Measurement
by Xiangqing Wang, Jie Zhang, Bo Wang, Kongni Zhu, Haokun Song, Ruixia Li and Fenghui Zhang
Photonics 2021, 8(6), 208; https://doi.org/10.3390/photonics8060208 - 9 Jun 2021
Cited by 3 | Viewed by 2392
Abstract
With the increase in the popularity of cloud computing and big data applications, the amount of sensitive data transmitted through optical networks has increased dramatically. Furthermore, optical transmission systems face various security risks at the physical level. We propose a novel key distribution [...] Read more.
With the increase in the popularity of cloud computing and big data applications, the amount of sensitive data transmitted through optical networks has increased dramatically. Furthermore, optical transmission systems face various security risks at the physical level. We propose a novel key distribution scheme based on signal-to-noise ratio (SNR) measurements to extract the fingerprint of the fiber channel and improve the physical level of security. The SNR varies with time because the fiber channel is affected by many physical characteristics, such as dispersion, polarization, scattering, and amplifier noise. The extracted SNR of the optical fiber channel can be used as the basis of key generation. Alice and Bob can obtain channel characteristics by measuring the SNR of the optical fiber channel and generate the consistent key by quantization coding. The security and consistency of the key are guaranteed by the randomness and reciprocity of the channel. The simulation results show that the key generation rate (KGR) can reach 25 kbps, the key consistency rate (KCR) can reach 98% after key post-processing, and the error probability of Eve’s key is ~50%. In the proposed scheme, the equipment used is simple and compatible with existing optic fiber links. Full article
(This article belongs to the Special Issue Photonics for Emerging Applications in Communication and Sensing)
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11 pages, 3231 KiB  
Article
Elasticity Changes in the Crystalline Lens during Oxidative Damage and the Antioxidant Effect of Alpha-Lipoic Acid Measured by Optical Coherence Elastography
by Hongqiu Zhang, Manmohan Singh, Achuth Nair, Kirill V. Larin and Salavat R. Aglyamov
Photonics 2021, 8(6), 207; https://doi.org/10.3390/photonics8060207 - 8 Jun 2021
Cited by 9 | Viewed by 3414
Abstract
Age-related cataracts are one of the most prevalent causes of visual impairment around the world. Understanding the mechanisms of cataract development and progression is essential to enable early clinical diagnosis and treatment to preserve visual acuity. Reductive chemicals are potential medicines effective on [...] Read more.
Age-related cataracts are one of the most prevalent causes of visual impairment around the world. Understanding the mechanisms of cataract development and progression is essential to enable early clinical diagnosis and treatment to preserve visual acuity. Reductive chemicals are potential medicines effective on cataract treatment. In this work, we investigated the cataract-induced oxidative damage in the crystalline lens and a kind of reductant, α-lipoic acid (ALA), ability to reduce the damage. We created oxidative environment to investigate the relationship between the progression of oxidative cataract and lenticular biomechanical properties measured by dynamic optical coherence elastography in porcine crystalline lenses ex vivo. The efficacy of ALA to minimize the stiffening of the lens was also quantified. The results showed a significant increase in Young’s modulus of the lens due to the formation of the oxidative cataract. We found a statistically significant difference between Young’s modulus of the lenses stored in phosphate-buffered saline and ALA solution after incubation in H2O2 solution for 3 h (43.0 ± 9.0 kPa versus 20.7 ± 3.5 kPa, respectively). These results show that the lens stiffness increases during oxidative cataract formation, and ALA has the potential to reverse stiffening of the lens caused by oxidative damage. Full article
(This article belongs to the Special Issue Tissue Optics)
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9 pages, 1376 KiB  
Communication
Dispersion Analysis of Twist-Symmetric Dielectric Waveguides
by Pilar Castillo-Tapia, Kwinten Van Gassen, Qiao Chen, Francisco Mesa, Zvonimir Sipus and Oscar Quevedo-Teruel
Photonics 2021, 8(6), 206; https://doi.org/10.3390/photonics8060206 - 8 Jun 2021
Cited by 7 | Viewed by 2688
Abstract
We propose a circular twist-symmetric dielectric waveguide that is polarization-selective. In the practical implementation of optical fibers, a selective circular polarization is more convenient than its linearly polarized counterpart where previous knowledge of the emitted polarization from the transmitter is unknown. The analysis [...] Read more.
We propose a circular twist-symmetric dielectric waveguide that is polarization-selective. In the practical implementation of optical fibers, a selective circular polarization is more convenient than its linearly polarized counterpart where previous knowledge of the emitted polarization from the transmitter is unknown. The analysis of the waveguide was conducted with three methods: an eigenmode approach, simulation of a truncated structure, and the so-called multimodal transfer-matrix method (MMTMM). The presented simulations demonstrate that the operational band can be manipulated by tuning the parameters of the structure. Furthermore, the MMTMM allows for a direct and accurate calculation of the attenuation constant of the rejected circular polarization. Full article
(This article belongs to the Special Issue Advances in Complex Media Electromagnetics)
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28 pages, 3517 KiB  
Review
On-Chip Integrated Photonic Devices Based on Phase Change Materials
by Muhammad Shemyal Nisar, Xing Yang, Liangjun Lu, Jianping Chen and Linjie Zhou
Photonics 2021, 8(6), 205; https://doi.org/10.3390/photonics8060205 - 7 Jun 2021
Cited by 26 | Viewed by 6669
Abstract
Phase change materials present a unique type of materials that drastically change their electrical and optical properties on the introduction of an external electrical or optical stimulus. Although these materials have been around for some decades, they have only recently been implemented for [...] Read more.
Phase change materials present a unique type of materials that drastically change their electrical and optical properties on the introduction of an external electrical or optical stimulus. Although these materials have been around for some decades, they have only recently been implemented for on-chip photonic applications. Since their reinvigoration a few years ago, on-chip devices based on phase change materials have been making a lot of progress, impacting many diverse applications at a very fast pace. At present, they are found in many interesting applications including switches and modulation; however, phase change materials are deemed most essential for next-generation low-power memory devices and neuromorphic computational platforms. This review seeks to highlight the progress thus far made in on-chip devices derived from phase change materials including memory devices, neuromorphic computing, switches, and modulators. Full article
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12 pages, 4395 KiB  
Article
Holographic Display System to Suppress Speckle Noise Based on Beam Shaping
by Di Wang, Yi-Wei Zheng, Nan-Nan Li and Qiong-Hua Wang
Photonics 2021, 8(6), 204; https://doi.org/10.3390/photonics8060204 - 6 Jun 2021
Cited by 4 | Viewed by 2687
Abstract
In this paper, a holographic system to suppress the speckle noise is proposed. Two spatial light modulators (SLMs) are used in the system, one of which is used for beam shaping, and the other is used for reproducing the image. By calculating the [...] Read more.
In this paper, a holographic system to suppress the speckle noise is proposed. Two spatial light modulators (SLMs) are used in the system, one of which is used for beam shaping, and the other is used for reproducing the image. By calculating the effective viewing angle of the reconstructed image, the effective hologram and the effective region of the SLM are calculated accordingly. Then, the size of the diffractive optical element (DOE) is calculated accordingly. The dynamic DOEs and effective hologram are loaded on the effective regions of the two SLMs, respectively, while the wasted areas of the two SLMs are performed with zero-padded operations. When the laser passes through the first SLM, the light can be modulated by the effective DOEs. When the modulated beam illuminates the second SLM which is loaded with the effective hologram, the image is reconstructed with better quality and lower speckle noise. Moreover, the calculation time of the hologram is reduced. Experiments indicate the validity of the proposed system. Full article
(This article belongs to the Special Issue Holography)
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12 pages, 6527 KiB  
Communication
A Comparison Study of Data Link with Medium-Wavelength Infrared Pulsed and CW Quantum Cascade Lasers
by Janusz Mikołajczyk
Photonics 2021, 8(6), 203; https://doi.org/10.3390/photonics8060203 - 5 Jun 2021
Cited by 3 | Viewed by 2716
Abstract
In this paper, a comparison study of a quantum cascade laser used for signal transmission by free-space optics is presented. The main goal is to define the capabilities of medium-wavelength infrared lasers operated in pulsed or continuous wave (cw) mode through testing and [...] Read more.
In this paper, a comparison study of a quantum cascade laser used for signal transmission by free-space optics is presented. The main goal is to define the capabilities of medium-wavelength infrared lasers operated in pulsed or continuous wave (cw) mode through testing and analyzing a laboratory setup of a data link operated at wavelengths of 4.5 µm (pulsed, peak power 3 W) and 4.8 µm (cw, average power ~20 mW). In this spectral range, the link budget is also defined by radiation attenuation in the atmosphere (absorption, scattering, and turbulence interaction). The performed measurements define unique operational aspects of the quantum cascade lasers considering on–off keying modulation. The registered light pulse changes for different parameters of driving current signals determine some limitations in both rate and data range. Finally, we present eye diagrams of the signals obtained using two data links. Full article
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15 pages, 2133 KiB  
Article
Electromagnetic Wave Scattering from a Moving Medium with Stationary Interface across the Interluminal Regime
by Zoé-Lise Deck-Léger, Xuezhi Zheng and Christophe Caloz
Photonics 2021, 8(6), 202; https://doi.org/10.3390/photonics8060202 - 5 Jun 2021
Cited by 12 | Viewed by 2704
Abstract
This paper extends current knowledge on electromagnetic wave scattering from bounded moving media in several regards. First, it complements the usual dispersion relation of moving media, ω(θk) (θk: phase velocity direction, associated with the wave vector, [...] Read more.
This paper extends current knowledge on electromagnetic wave scattering from bounded moving media in several regards. First, it complements the usual dispersion relation of moving media, ω(θk) (θk: phase velocity direction, associated with the wave vector, k), with the equally important impedance relation, η(θS) (θS: group velocity direction, associated with the Poynting vector, S). Second, it explains the interluminal-regime phenomenon of double-downstream wave transmission across a stationary interface between a regular medium and the moving medium, assuming motion perpendicular to the interface, and shows that the related waves are symmetric in terms of the energy refraction angle, while being asymmetric in terms of the phase refraction angle, with one of the waves subject to negative refraction, and shows that the wave impedances of the two transmitted waves are equal. Third, it generalizes the problem to the case where the medium moves obliquely with respect to the interface. Finally, it highlights the connection between this problem and a spacetime modulated medium. Full article
(This article belongs to the Special Issue Advances in Complex Media Electromagnetics)
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