Photonics

3 pages, 145 KiB

Open AccessEditorial

Introduction to the Special Issue on Structured Light Coherence

by Olga Korotkova and Franco Gori

Photonics 2021, 8(10), 457; https://doi.org/10.3390/photonics8100457 - 19 Oct 2021

Cited by 2 | Viewed by 1477

Statistical optics, and optical coherence in particular, developed into a stand-alone branch of physical optics in the second half of the 20th century and has found a number of ground-breaking applications in astronomical measurements, medical diagnostics, environmental remote sensing, and wireless communications [...] [...] Read more.

Statistical optics, and optical coherence in particular, developed into a stand-alone branch of physical optics in the second half of the 20th century and has found a number of ground-breaking applications in astronomical measurements, medical diagnostics, environmental remote sensing, and wireless communications [...] Full article

(This article belongs to the Special Issue Structured Light Coherence)

16 pages, 2660 KiB

Open AccessArticle

LBFA: A Load-Balanced and Fragmentation-Aware Resource Allocation Algorithm in Space-Division Multiplexing Elastic Optical Networks

by Shengyu Zhang, Kwan-Lawrence Yeung and Along Jin

Photonics 2021, 8(10), 456; https://doi.org/10.3390/photonics8100456 - 19 Oct 2021

Cited by 12 | Viewed by 2367

Abstract

We consider a space-division multiplexing elastic optical network (SDM-EON) that supports super-channels (SChs). A Sch comprises a set of contiguous frequency slots on multiple cores in a multi-core fiber. The problem of finding a lightpath using SChs involves routing, modulation, spectrum and core [...] Read more.

We consider a space-division multiplexing elastic optical network (SDM-EON) that supports super-channels (SChs). A Sch comprises a set of contiguous frequency slots on multiple cores in a multi-core fiber. The problem of finding a lightpath using SChs involves routing, modulation, spectrum and core assignment (RMSCA). To minimize the request blocking probability (RBP), two critical issues must be addressed. First, routing and modulation assignment (RMA) should not cause hotspots, or overutilized links. Second, spectrum and core assignment (SCA) should aim at minimizing fragmentation, or small frequency slot blocks that can hardly be utilized by future requests. In this paper, a pre-computation method is first proposed for better load balancing in RMA. Then an efficient fragmentation-aware SCA is proposed based on a new fragmentation metric that measures both the spectral and spatial fragmentation. With the enhanced RMA and SCA, a joint load-balanced and fragmentation-aware algorithm called LBFA is designed to solve the RMSCA problem. As compared with the existing algorithms, simulation results show that our LBFA provides significant reduction in RBP. Full article

(This article belongs to the Special Issue Optical Network and Access Technologies)

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9 pages, 2828 KiB

Open AccessCommunication

Ultra-Compact and Ultra-Broadband Polarization-Insensitive Mach–Zehnder Interferometer in Silicon-on-Insulator Platform for Quantum Internet Application

by Jingjing Zhang, Zhaojian Zhang, Chao Ma, Xuefeng Chen, Liping Liu, Wei Zhao, Xiaoxian Song, Haiting Zhang, Yu Yu, Huan Chen and Junbo Yang

Photonics 2021, 8(10), 455; https://doi.org/10.3390/photonics8100455 - 19 Oct 2021

Cited by 2 | Viewed by 2650

Abstract

Polarization dependence in integrated silicon photonics has a detrimental effect on the manipulation of quantum state with different polarizations in the quantum technology. Those limits have profound implications for further technological developments, especially in quantum photonic internet. Here, we propose a polarization-independent Mach–Zehnder [...] Read more.

Polarization dependence in integrated silicon photonics has a detrimental effect on the manipulation of quantum state with different polarizations in the quantum technology. Those limits have profound implications for further technological developments, especially in quantum photonic internet. Here, we propose a polarization-independent Mach–Zehnder interferometer (MZI) structure based on a 340 nm-thick silicon-on-insulator (SOI) platform. The MZI facilitates low loss, broad operating bandwidth, and large tolerance of the fabrication imperfection. We achieve an excess loss of <10% and an extinction radio of >18 in the 100 nm bandwidth (1500∼1600 nm) for both transverse electric (TE) and transverse magnetic (TM) modes. We numerically demonstrate an interference visibility of 99% and a polarization-independent loss (PDL) of 0.03 for both polarizations at 1550 nm. Furthermore, by using the principle of phase compensation and self-image, we shorten the length of the waveguide taper by almost an order of magnitude with the transmission of >95% for both TE and TM polarizations. Up to now, the proposed structure could significantly improve the integration and promote the development of monolithic integrated quantum internet. Full article

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

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10 pages, 8719 KiB

Open AccessCommunication

Multi-View Optical Image Fusion and Reconstruction for Defogging without a Prior In-Plane

by Yuru Huang, Yikun Liu, Haishan Liu, Yuyang Shui, Guanwen Zhao, Jinhua Chu, Guohai Situ, Zhibing Li, Jianying Zhou and Haowen Liang

Photonics 2021, 8(10), 454; https://doi.org/10.3390/photonics8100454 - 18 Oct 2021

Cited by 4 | Viewed by 2333

Abstract

Image fusion and reconstruction from muldti-images taken by distributed or mobile cameras need accurate calibration to avoid image mismatching. This calibration process becomes difficult in fog when no clear nearby reference is available. In this work, the fusion of multi-view images taken in [...] Read more.

Image fusion and reconstruction from muldti-images taken by distributed or mobile cameras need accurate calibration to avoid image mismatching. This calibration process becomes difficult in fog when no clear nearby reference is available. In this work, the fusion of multi-view images taken in fog by two cameras fixed on a moving platform is realized. The positions and aiming directions of the cameras are determined by taking a close visible object as a reference. One camera with a large field of view (FOV) is applied to acquire images of a short-distance object which is still visible in fog. This reference is then adopted to the calibration of the camera system to determine the positions and pointing directions at each viewpoint. The extrinsic parameter matrices are obtained with these data, which are applied for the image fusion of distant images captured by another camera beyond visibility. The experimental verification was carried out in a fog chamber and the technique is shown to be valid for imaging reconstruction in fog without a prior in-plane. The synthetic image, accumulated and averaged by ten-view images, is shown to perform potential applicability for fog removal. The enhanced structure similarity is discussed and compared in detail with conventional single-view defogging techniques. Full article

(This article belongs to the Special Issue Smart Pixels and Imaging)

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16 pages, 4546 KiB

Open AccessArticle

Study of the Performance of Deep Learning-Based Channel Equalization for Indoor Visible Light Communication Systems

by Pu Miao, Weibang Yin, Hui Peng and Yu Yao

Photonics 2021, 8(10), 453; https://doi.org/10.3390/photonics8100453 - 18 Oct 2021

Cited by 16 | Viewed by 3199

Abstract

The inherent impairments of visible light communication (VLC) in terms of nonlinearity of light-emitting diode (LED) and the optical multipath restrict bit error rate (BER) performance. In this paper, a model-driven deep learning (DL) equalization scheme is proposed to deal with the severe [...] Read more.

The inherent impairments of visible light communication (VLC) in terms of nonlinearity of light-emitting diode (LED) and the optical multipath restrict bit error rate (BER) performance. In this paper, a model-driven deep learning (DL) equalization scheme is proposed to deal with the severe channel impairments. By imitating the block-by-block signal processing block in orthogonal frequency division multiplexing (OFDM) communication, the proposed scheme employs two subnets to replace the signal demodulation module in traditional system for learning the channel nonlinearity and the symbol de-mapping relationship from the training data. In addition, the conventional solution and algorithm are also incorporated into the system architecture to accelerate the convergence speed. After an efficient training, the distorted symbols can be implicitly equalized into the binary bits directly. The results demonstrate that the proposed scheme can address the overall channel impairments efficiently and can recover the original symbols with better BER performance. Moreover, it can still work robustly when the system is complicated by serious distortions and interference, which demonstrates the superiority and validity of the proposed scheme in channel equalization. Full article

(This article belongs to the Special Issue Optical Wireless Communication (OWC) Systems)

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9 pages, 1355 KiB

Open AccessArticle

Diode Laser Lithotription Technique Based on Optothermal Converter

by Olga S. Streltsova, Evgeny V. Grebenkin, Nikita M. Bityurin, Vladimir I. Bredikhin, Vadim V. Elagin, Vasily V. Vlasov and Vladislav A. Kamensky

Photonics 2021, 8(10), 452; https://doi.org/10.3390/photonics8100452 - 18 Oct 2021

Cited by 3 | Viewed by 2547

Abstract

Purpose: evaluation of the efficiency of the “hot spot” method for the fragmentation of urinary stones. Materials and methods: A retrospective analysis of clinical records of 1666 patients with urolithiasis who underwent percutaneous nephrolithoextraction/tripsy and contact ureterolithotripsy/extraction in the period from 2014 to [...] Read more.

Purpose: evaluation of the efficiency of the “hot spot” method for the fragmentation of urinary stones. Materials and methods: A retrospective analysis of clinical records of 1666 patients with urolithiasis who underwent percutaneous nephrolithoextraction/tripsy and contact ureterolithotripsy/extraction in the period from 2014 to 2017 at the urology clinic was performed to assess the incidence of postoperative infectious and inflammatory complications. The research objects were postoperative urinary stones (n-78). The X-ray density and linear dimensions of the stones were determined. Stone fragmentation was performed with a continuous-wave diode laser operating at wavelengths of 0.81 μm, 0.97 μm, and 1.47 μm. An absorbing coating of micro-size graphite powder was applied on the working tip of the optical fiber. In vitro fragmentation was carried out in liquid. Results: A group of patients (224/1666) (13.4 ± 0.86%) was identified, who developed infectious and inflammatory complications after: percutaneous nephrolithotripsy, 123/361 (34.1 ± 2.5%) cases; percutaneous nephrolithoextraction, 59/240 (24.6 ± 2.78%); contact ureterolithotripsy, 23/294 (7.8 ± 1.57%); and ureterolithoextraction, 19/771 (2.5 ± 0.56%). In liquid, the “hot spot” technique made it possible to fragment stones with an X-ray density of up to 1000 HU at a laser wavelength of 0.81 µm, up to 1400 HU at 0.97 µm, and up to 1400 HU at 1.47 µm. Full article

(This article belongs to the Special Issue Topical Problems of Biophotonics)

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Graphical abstract

15 pages, 3386 KiB

Open AccessArticle

TDTS: Three-Dimensional Traffic Scheduling in Optical Fronthaul Networks with Conv-LSTM

by Bowen Bao, Zhen Xu, Chao Li, Zhengjie Sun, Sheng Liu and Yunbo Li

Photonics 2021, 8(10), 451; https://doi.org/10.3390/photonics8100451 - 18 Oct 2021

Cited by 3 | Viewed by 1973

Abstract

Given the more intensive deployments of emerging Internet of Things applications with beyond-fifth-generation communication, the access network becomes bandwidth-hungry to support more kinds of services, requiring higher resource utilization of the optical fronthaul network. To enhance resource utilization, this study novelly proposed a [...] Read more.

Given the more intensive deployments of emerging Internet of Things applications with beyond-fifth-generation communication, the access network becomes bandwidth-hungry to support more kinds of services, requiring higher resource utilization of the optical fronthaul network. To enhance resource utilization, this study novelly proposed a three-dimensional traffic scheduling (TDTS) scheme in the optical fronthaul network. Specifically, large and mixed traffic data with multiple different requirements were firstly divided according to three-dimensions parameters of traffic requests, i.e., arriving time, transmission tolerance delay, and bandwidth requirements, forming eight types of traffic model. Then, historical traffic data with division results were put into convolutional-long short-term memory (Conv-LSTM) strategy for traffic prediction, obtaining a clear traffic pattern. Next, the traffic processing order was supported by a priority evaluation factor that was measured by traffic status of the link and network characteristics comprehensively. Finally, following the priority, the proposed TDTS scheme assigned the resource to traffic requests according to their results of traffic division, prediction, and processing order with the shortest path routing and first-fit spectrum allocation policies. Simulation results demonstrated that the proposed TDTS scheme, on the premise of accurate traffic prediction, could outperform conventional resource-allocation schemes in terms of blocking probability and resource utilization. Full article

(This article belongs to the Special Issue Machine Learning in the Era of Computing and Network Integration)

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24 pages, 12492 KiB

Open AccessArticle

A Preliminary Assessment of an FBG-Based Hard Landing Monitoring System

by Angela Brindisi, Cristian Vendittozzi, Lidia Travascio, Luigi Di Palma, Michele Ignarra, Vincenzo Fiorillo and Antonio Concilio

Photonics 2021, 8(10), 450; https://doi.org/10.3390/photonics8100450 - 17 Oct 2021

Cited by 8 | Viewed by 2985

Abstract

In aeronautics, hard landing is a critical condition as the aircraft approaches the runway with a vertical velocity that exceeds 2 m/s. Beyond that level, the energy that should be then absorbed by the whole structure could cause severe damage to the landing [...] Read more.

In aeronautics, hard landing is a critical condition as the aircraft approaches the runway with a vertical velocity that exceeds 2 m/s. Beyond that level, the energy that should be then absorbed by the whole structure could cause severe damage to the landing gear and the whole structural system. This document reports on the set-up, execution and results of a preparatory test campaign performed on a small landing gear (LG) demonstrator instrumented with a fibre-optic sensor system. In detail, a leaf spring landing gear was released from a drop tower to detect information about the strain state and the related acceleration history of some specific components during the impact. The objective of the present research is the development of a method for assessing whether hard landing is experienced, and to what extent. Deformation measurements through an integrated Fibre-Bragg Grating (FBG) network allowed retrieving impact velocity by a devoted, original algorithm. The proposed preliminary methodology is the base for assessing a more complex procedure to correlate structural response to the energy entering the structure during the touchdown event. Full article

(This article belongs to the Special Issue Optical Sensing)

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10 pages, 630 KiB

Open AccessCommunication

Modal Analysis of Pseudo-Schell Model Sources

by Massimo Santarsiero, Rosario Martínez-Herrero, Gemma Piquero, Juan Carlos González de Sande and Franco Gori

Photonics 2021, 8(10), 449; https://doi.org/10.3390/photonics8100449 - 17 Oct 2021

Cited by 7 | Viewed by 1676

Abstract

All pseudo-Schell model sources have been shown to possess the same continuous set of circularly symmetric modes, all of them presenting a conical wavefront. For keeping energy at a finite level, the mode amplitude along the radial coordinate is modulated by a decreasing [...] Read more.

All pseudo-Schell model sources have been shown to possess the same continuous set of circularly symmetric modes, all of them presenting a conical wavefront. For keeping energy at a finite level, the mode amplitude along the radial coordinate is modulated by a decreasing exponential function. A peculiar property of such modes is that they exist in the Laplace transform’s realm. After a brief discussion of the near-zone, we pass to the far-zone, where the field can be evaluated in closed form. The corresponding features of the intensity distribution are discussed. Full article

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24 pages, 12558 KiB

Open AccessArticle

Transmission of 3D Holographic Information via Conventional Communication Channels and the Possibility of Multiplexing in the Implementation of 3D Hyperspectral Images

by Sergey A. Shoydin and Artem L. Pazoev

Photonics 2021, 8(10), 448; https://doi.org/10.3390/photonics8100448 - 15 Oct 2021

Cited by 5 | Viewed by 2895

Abstract

This paper shows the possibility of transmitting 3D holographic information in real time with a TV frame rate over conventional radio channels by transmitting two two-dimensional signals in two image modes: depth map and surface texture of the object (mask + texture). The [...] Read more.

This paper shows the possibility of transmitting 3D holographic information in real time with a TV frame rate over conventional radio channels by transmitting two two-dimensional signals in two image modes: depth map and surface texture of the object (mask + texture). The authors point out that it is similar to compression through eliminating the carrier and it is inherently similar to SSB (single-sideband modulation) but has higher resolution ability in reconstructing 3D images. It is also shown that such technology for transmitting 3D holographic information is in good agreement with the tasks of both aggregating and multiplexing 3D images when they are transferred from one part of the electromagnetic spectrum of radiation to another and the creation of hyperspectral 3D images. Full article

(This article belongs to the Special Issue Holography)

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10 pages, 2280 KiB

Open AccessReview

Cascaded Generation in Multimode Diode-Pumped Graded-Index Fiber Raman Lasers

by Alexey G. Kuznetsov, Ilya N. Nemov, Alexey A. Wolf, Ekaterina A. Evmenova, Sergey I. Kablukov and Sergey A. Babin

Photonics 2021, 8(10), 447; https://doi.org/10.3390/photonics8100447 - 15 Oct 2021

Cited by 5 | Viewed by 1953

Abstract

We review our recent experimental results on the cascaded Raman conversion of highly multimode laser diode (LD) pump radiation into the first- and higher-order Stokes radiation in multimode graded-index fibers. A linear cavity composed of fiber Bragg gratings (FBGs) inscribed in the fiber [...] Read more.

We review our recent experimental results on the cascaded Raman conversion of highly multimode laser diode (LD) pump radiation into the first- and higher-order Stokes radiation in multimode graded-index fibers. A linear cavity composed of fiber Bragg gratings (FBGs) inscribed in the fiber core is formed to provide feedback for the first Stokes order, whereas, for the second order, both a linear cavity consisting of two FBGs and a half-open cavity with one FBG and random distributed feedback (RDFB) via Rayleigh backscattering along the fiber are explored. LDs with different wavelengths (915 and 940 nm) are used for pumping enabling Raman lasing at different wavelengths of the first (950, 954 and 976 nm), second (976, 996 and 1019 nm) and third (1065 nm) Stokes orders. Output power and efficiency, spectral line shapes and widths, beam quality and shapes are compared for different configurations. It is shown that the RDFB cavity provides higher slope efficiency of the second Stokes generation (up to 70% as that for the first Stokes wave) with output power up to ~30 W, limited by the third Stokes generation. The best beam quality parameter of the second Stokes beam is close to the diffraction limit (M²~1.3) in both linear and half-open cavities, whereas the line is narrower (<0.2 nm) and more stable in the case of the linear cavity with two FBGs. However, an optimization of the FBG reflection spectrum used in the half-open cavity allows this linewidth value to be approached. The measured beam profiles show the dip formation in the output pump beam profile, whereas the first and second Stokes beams are Gaussian-shaped and almost unchanged with increasing power. A qualitative explanation of such behavior in connection with the power evolution for the transmitted pump and generated first, second and third Stokes beams is given. The potential for wavelength tuning of the cascaded Raman lasers based on LD-pumped multimode fibers is discussed. Full article

(This article belongs to the Special Issue Specialty Optical Fibers, Fiber Lasers and Their Applications)

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13 pages, 4289 KiB

Open AccessCommunication

Tracing Evolution of Angle-Wavelength Spectrum along the 40-m Postfilament in Corridor Air

by Daria V. Mokrousova, Dmitrii V. Pushkarev, Nikolay A. Panov, Irina A. Nikolaeva, Daniil E. Shipilo, Nikita A. Zhidovtsev, Georgy E. Rizaev, Daria S. Uryupina, Arnaud Couairon, Aurélien Houard, Dmitry V. Skryabin, Andrey B. Savel’ev, Olga G. Kosareva, Leonid V. Seleznev and Andrey A. Ionin

Photonics 2021, 8(10), 446; https://doi.org/10.3390/photonics8100446 - 15 Oct 2021

Cited by 6 | Viewed by 1785

Abstract

Postfilamentation channel resulting from filamentation of freely propagating 744-nm, 5-mJ, 110-fs pulse in the corridor air is examined experimentally and in simulations. The longitudinal extension of postfilament was determined to be 55–95 m from the compressor output. Using single-shot angle-wavelength spectra measurements, we [...] Read more.

Postfilamentation channel resulting from filamentation of freely propagating 744-nm, 5-mJ, 110-fs pulse in the corridor air is examined experimentally and in simulations. The longitudinal extension of postfilament was determined to be 55–95 m from the compressor output. Using single-shot angle-wavelength spectra measurements, we observed a series of red-shifted maxima in the spectrum, localized on the beam axis with the divergence below 0.5 mrad. In the range 55–70 m, the number of maxima and their red-shift increase with the distance reaching 1 μm, while the pulse duration measured by the autocorrelation technique is approximately constant. Further on, for distances larger than 70 m and up to 95 m, the propagation is characterized by the suppressed beam divergence and unchanged pulse spectrum. The pulse duration increases due to the normal air dispersion. Full article

(This article belongs to the Special Issue Optical Solitons: Current Status)

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9 pages, 1275 KiB

Open AccessArticle

Topological Charge and Asymptotic Phase Invariants of Vortex Laser Beams

by Alexey A. Kovalev, Victor V. Kotlyar and Anton G. Nalimov

Photonics 2021, 8(10), 445; https://doi.org/10.3390/photonics8100445 - 14 Oct 2021

Cited by 8 | Viewed by 2104

Abstract

It is well known that the orbital angular momentum (OAM) of a light field is conserved on propagation. In this work, in contrast to the OAM, we analytically study conservation of the topological charge (TC), which is often confused with OAM, but has [...] Read more.

It is well known that the orbital angular momentum (OAM) of a light field is conserved on propagation. In this work, in contrast to the OAM, we analytically study conservation of the topological charge (TC), which is often confused with OAM, but has quite different physical meaning. To this end, we propose a huge-ring approximation of the Huygens–Fresnel principle, when the observation point is located on an infinite-radius ring. Based on this approximation, our proof of TC conservation reveals that there exist other quantities that are also propagation-invariant, and the number of these invariants is theoretically infinite. Numerical simulation confirms the conservation of two such invariants for two light fields. The results of this work can find applications in optical data transmission to identify optical signals. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Optical and Wireless Communication Networks)

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23 pages, 1079 KiB

Open AccessArticle

Proposal for a Skin Layer-Wise Decomposition Model of Spatially-Resolved Diffuse Reflectance Spectra Based on Maximum Depth Photon Distributions: A Numerical Study

by Victor Colas, Walter Blondel, Grégoire Khairallah, Christian Daul and Marine Amouroux

Photonics 2021, 8(10), 444; https://doi.org/10.3390/photonics8100444 - 14 Oct 2021

Cited by 10 | Viewed by 3226

Abstract

In the context of cutaneous carcinoma diagnosis based on in vivo optical biopsy, Diffuse Reflectance (DR) spectra, acquired using a Spatially Resolved (SR) sensor configuration, can be analyzed to distinguish healthy from pathological tissues. The present contribution aims at studying the depth distribution [...] Read more.

In the context of cutaneous carcinoma diagnosis based on in vivo optical biopsy, Diffuse Reflectance (DR) spectra, acquired using a Spatially Resolved (SR) sensor configuration, can be analyzed to distinguish healthy from pathological tissues. The present contribution aims at studying the depth distribution of SR-DR-detected photons in skin from the perspective of analyzing how these photons contribute to acquired spectra carrying local physiological and morphological information. Simulations based on modified Cuda Monte Carlo Modeling of Light transport were performed on a five-layer human skin optical model with epidermal thickness, phototype and dermal blood content as variable parameters using (i) wavelength-resolved scattering and absorption properties and (ii) the geometrical configuration of a multi-optical fiber probe implemented on an SR-DR spectroscopic device currently used in clinics. Through histograms of the maximum probed depth and their exploitation, we provide numerical evidence linking the characteristic penetration depth of the detected photons to their wavelengths and four source–sensor distances, which made it possible to propose a decomposition of the DR signals related to skin layer contributions. Full article

(This article belongs to the Special Issue Tissue Optics)

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10 pages, 1135 KiB

Open AccessCommunication

Interferometric Technique for the Spectral Characterization of High Frequency Current-Modulated Mid-Infrared Semiconductor Lasers

by Marc-Simon Bahr and Marcus Wolff

Photonics 2021, 8(10), 443; https://doi.org/10.3390/photonics8100443 - 14 Oct 2021

Cited by 1 | Viewed by 2240

Abstract

We describe two methods, based on Michelson interferometery, that enable the determination of the absolute wavelength of current-modulated semiconductor lasers. By non-linear regression of the instantaneous frequency of the interference signal, the rate of change of the wavelength of the radiation can be [...] Read more.

We describe two methods, based on Michelson interferometery, that enable the determination of the absolute wavelength of current-modulated semiconductor lasers. By non-linear regression of the instantaneous frequency of the interference signal, the rate of change of the wavelength of the radiation can be inferred. Alternatively, the absolute wavelength can be directly calculated from the maxima and minima of the interference signal. In both cases a reference absorption line enables the determination of the absolute wavelength. Both methods offer respective advantages. The methods allow a highly resolved wavelength measurement under lower kilohertz range current-modulation with relatively little effort. As a result, we present the rates of wavelength change and absolute wavelengths exemplarily for a specific interband cascade laser. It is furthermore shown that the spectral dynamic range of the laser decreases with increasing modulation frequency. Full article

(This article belongs to the Special Issue Near- and Mid-Infrared Photonics Technologies)

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12 pages, 5406 KiB

Open AccessArticle

Pulse Length Monitor for Breakdown Diagnostics in THz and Mm-Wave Accelerators

by Sergey V. Kutsaev, Vladimir Goncharik, Alex Murokh, Ilya Rezanov, Dmitry Shchegolkov and Alexander Y. Smirnov

Photonics 2021, 8(10), 442; https://doi.org/10.3390/photonics8100442 - 14 Oct 2021

Viewed by 2156

Abstract

The development of novel high-gradient accelerating structures operating at THz frequencies is critical for future free-electron lasers and TeV scale linear colliders. To reach high energies with reasonable length requires high accelerating gradients of ~100 MV/m. The main limitation to reaching these high-energy [...] Read more.

The development of novel high-gradient accelerating structures operating at THz frequencies is critical for future free-electron lasers and TeV scale linear colliders. To reach high energies with reasonable length requires high accelerating gradients of ~100 MV/m. The main limitation to reaching these high-energy gradients is the vacuum RF breakdown phenomenon, which disrupts normal accelerator operations. For stable operations and to understand the breakdown microscopic dynamics, a new device capable of detecting the breakdown occurrences is required. In this paper, we provide the design of a pulse length monitor based on an analog to digital converter for fast signal digitization without the need to use high-speed digitizers to be used in a commercial mm-wave heterodyne spectrometer. Full article

(This article belongs to the Special Issue Frontiers in Terahertz Technology and Applications)

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23 pages, 5668 KiB

Open AccessArticle

Research on X-ray Fluorescence Enhanced Fluoroscopy Imaging Technology

by Zhenyao Yan, Liang Li, Rui Qiu and Zhiqiang Chen

Photonics 2021, 8(10), 441; https://doi.org/10.3390/photonics8100441 - 14 Oct 2021

Cited by 3 | Viewed by 4753

Abstract

Chest X-ray fluoroscopy is a commonly used medical imaging method, which has a wide range of applications in the diagnosis of lung diseases and other fields. However, due to low contrast and relatively close linear attenuation coefficients, some early small lesions are difficult [...] Read more.

Chest X-ray fluoroscopy is a commonly used medical imaging method, which has a wide range of applications in the diagnosis of lung diseases and other fields. However, due to low contrast and relatively close linear attenuation coefficients, some early small lesions are difficult to detect in time. Using the X-ray fluorescent effect of high atomic number metal elements and metal atom-containing agents that can be enriched in the lesion, the fluoroscopy signal and the fluorescent signal emitted by the metal atoms can be detected at the same time during the fluoroscopy, and the images of the two can be integrated, which can theoretically enhance the contrast between the lesion and the surrounding tissue. Based on GEANT4, this paper conducts Monte Carlo simulations to explore the feasibility and enhancement effects of three enhancement schemes: the pencil beam spot scanning method, cone-beam collimation method, and slit scanning method, and discusses the specific geometric structure and material selection. Full article

(This article belongs to the Special Issue X-ray Luminescence and Fluorescence)

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9 pages, 3560 KiB

Open AccessCommunication

Flexible Broadband Metamaterial Perfect Absorber Based on Graphene-Conductive Inks

by Le Van Long, Nguyen Sy Khiem, Bui Son Tung, Nguyen Thanh Tung, Trinh Thi Giang, Pham Thanh Son, Bui Xuan Khuyen, Vu Dinh Lam, Liangyao Chen, Haiyu Zheng and Youngpak Lee

Photonics 2021, 8(10), 440; https://doi.org/10.3390/photonics8100440 - 13 Oct 2021

Cited by 17 | Viewed by 2750

Abstract

In this work, we proposed a flexible broadband metamaterial perfect absorber (FBMPA) by exploiting a pasted conductive-graphene ink on a polyimide substrate. For the flat FBMPA, an absorption over 90% was found to cover a wide frequency range (from 7.88 to 18.01 GHz). [...] Read more.

In this work, we proposed a flexible broadband metamaterial perfect absorber (FBMPA) by exploiting a pasted conductive-graphene ink on a polyimide substrate. For the flat FBMPA, an absorption over 90% was found to cover a wide frequency range (from 7.88 to 18.01 GHz). The high-absorption feature was polarization-insensitive and regarded as stable with respect to the oblique incidence up to 30 degrees of electromagnetic wave. The high absorption was maintained well even when the absorber was wrapped. That is, the FBMPA was attached to cylindrical surfaces (with the varying radius from 4 to 50 cm). For both flat and curved states, the absorption mechanism was explained by the perfect impedance matching and the dielectric loss of the proposed absorber. Our work provides the groundwork for the commercialization of future meta-devices such as sensors, optical filters/switchers, photodetectors, and energy converters. Full article

(This article belongs to the Special Issue Metamaterials, Photonic Crystals and Plasmonics - Selected Papers from META 2021)

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13 pages, 378 KiB

Open AccessArticle

Full-Vectorial Fiber Mode Solver Based on a Discrete Hankel Transform

by Michael Steinke

Photonics 2021, 8(10), 439; https://doi.org/10.3390/photonics8100439 - 13 Oct 2021

Viewed by 2193

Abstract

It is crucial to be time and resource-efficient when enabling and optimizing novel applications and functionalities of optical fibers, as well as accurate computation of the vectorial field components and the corresponding propagation constants of the guided modes in optical fibers. To address [...] Read more.

It is crucial to be time and resource-efficient when enabling and optimizing novel applications and functionalities of optical fibers, as well as accurate computation of the vectorial field components and the corresponding propagation constants of the guided modes in optical fibers. To address these needs, a novel full-vectorial fiber mode solver based on a discrete Hankel transform is introduced and validated here for the first time for rotationally symmetric fiber designs. It is shown that the effective refractive indices of the guided modes are computed with an absolute error of less than 10

^{- 4}

with respect to analytical solutions of step-index and graded-index fiber designs. Computational speeds in the order of a few seconds allow to efficiently compute the relevant parameters, e.g., propagation constants and corresponding dispersion profiles, and to optimize fiber designs. Full article

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10 pages, 1623 KiB

Open AccessCommunication

Highly Sensitive Biosensor Based on Partially Immobilized Silver Nanopillars in the Terahertz Band

by Shuo Liu, Lin Li and Zhenxu Bai

Photonics 2021, 8(10), 438; https://doi.org/10.3390/photonics8100438 - 13 Oct 2021

Cited by 7 | Viewed by 1903

Abstract

In this paper, a highly sensitive biosensor based on partially immobilized silver nanopillars is proposed. The working frequency of this sensor is in the terahertz band, and the range of the detected refractive index is 1.33 to 1.38. We set air holes of [...] Read more.

In this paper, a highly sensitive biosensor based on partially immobilized silver nanopillars is proposed. The working frequency of this sensor is in the terahertz band, and the range of the detected refractive index is 1.33 to 1.38. We set air holes of two different sizes on the cross-section of the optical fiber and arranged them into a hexagon. In order to improve the sensitivity, silver nanopillars were immobilized on part of the surface of the fiber cladding. The method for detecting the change of refractive index of the bio-analyte was based on local surface plasmon resonance properties of noble metal. The research recorded valuable data about the values of loss peak and full width at half maximum as well as resonance frequency shift under different setting conditions. The data present the biosensor’s final sensitivity as 1.749 THz/RIU. Full article

(This article belongs to the Special Issue Optical Sensing)

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11 pages, 2147 KiB

Open AccessArticle

An Ultra-Compact Design of Plasmonic Memristor with Low Loss and High Extinction Efficiency Based on Enhanced Interaction between Filament and Concentrated Plasmon

by Ye Tian, Saiwen Zhang and Weishi Tan

Photonics 2021, 8(10), 437; https://doi.org/10.3390/photonics8100437 - 12 Oct 2021

Cited by 8 | Viewed by 2442

Abstract

We present a numerical design of the plasmonic memristive switching device operated at the telecommunication wavelength of 1.55 μm, which consists of a triangle-shaped metal taper mounted on top of a Si waveguide, with rational doping in the area below the apex of [...] Read more.

We present a numerical design of the plasmonic memristive switching device operated at the telecommunication wavelength of 1.55 μm, which consists of a triangle-shaped metal taper mounted on top of a Si waveguide, with rational doping in the area below the apex of the taper. This device can achieve optimal vertical coupling of light energy from the Si waveguide to the plasmonic region and, at the same time, focus the plasmon into the apex of the metal taper. Moreover, the area with concentrated plasmon is overlapped with that where the memristive switching occurs, due to the formation/removal of the metallic nano-filament. As a result, the highly distinct transmission induced by the switching of the plasmonic memristor can be produced because of the maximized interactions between the filament and the plasmon. Our numerical simulation shows that the device hasa compact size (610 nm), low insertion loss (~1 dB), and high extinction efficiency (4.6 dB/μm). Additionally, we point out that stabilizing the size of the filament is critical to improve the operation repeatability of the plasmonic memristive switching device. Full article

(This article belongs to the Special Issue Silicon Photonics: Functional Enhancement by New Structures and Materials)

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8 pages, 12679 KiB

Open AccessArticle

Double-Frequency-Shift Acousto-Optic Modulator with Controllable Pulse Pair Frequency Difference

by Yuhe Wang, Yudong Lian, Shiwei Han, Yang Yu, Xuan Qi, Zhenxu Bai, Yulei Wang and Zhiwei Lu

Photonics 2021, 8(10), 436; https://doi.org/10.3390/photonics8100436 - 12 Oct 2021

Viewed by 2220

Abstract

A scheme for controlling the frequency difference of output pulse pair with double frequency shift loops is proposed. The frequency shift system includes two loop elements of 20 and 200 MHz. The first one carries out a single selective positive frequency shift of [...] Read more.

A scheme for controlling the frequency difference of output pulse pair with double frequency shift loops is proposed. The frequency shift system includes two loop elements of 20 and 200 MHz. The first one carries out a single selective positive frequency shift of 1–20 MHz, and the second one can satisfy a single fixed positive frequency shift of 200 MHz. The reverse cascade technology of two acousto-optic crystals is introduced to solve the limitation of the small frequency shift of crystal size. A multichannel synchronization signal completes the time domain control of each acousto-optic modulator. Finally, the frequency shift difference of the output pulse pair ranges of 0–2 GHz, and the frequency shift accuracy is 5 MHz. Full article

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15 pages, 2430 KiB

Open AccessArticle

Dual-Modality Imaging Microfluidic Cytometer for Onsite Detection of Phytoplankton

by Bo Xiong, Tianqi Hong, Herbert Schellhorn and Qiyin Fang

Photonics 2021, 8(10), 435; https://doi.org/10.3390/photonics8100435 - 12 Oct 2021

Cited by 1 | Viewed by 2669

Abstract

Phytoplankton monitoring is essential for better understanding and mitigation of phytoplankton bloom formation. We present a microfluidic cytometer with two imaging modalities for onsite detection and identification of phytoplankton: a lensless imaging mode for morphological features, and a fluorescence imaging mode for autofluorescence [...] Read more.

Phytoplankton monitoring is essential for better understanding and mitigation of phytoplankton bloom formation. We present a microfluidic cytometer with two imaging modalities for onsite detection and identification of phytoplankton: a lensless imaging mode for morphological features, and a fluorescence imaging mode for autofluorescence signal of phytoplankton. Both imaging modes are integrated in a microfluidic device with a field of view (FoV) of 3.7 mm × 2.4 mm and a depth of field (DoF) of 0.8 mm. The particles in the water flow channel can be detected and classified with automated image processing algorithms and machine learning models using their morphology and fluorescence features. The performance of the device was demonstrated by measuring Chlamydomonas, Euglena, and non-fluorescent beads in both separate and mixed flow samples. The recall rates for Chlamydomonas and Euglena ware 93.6% and 94.4%. The dual-modality imaging approach enabled observing both morphology and fluorescence features with a large DoF and FoV which contribute to high-throughput analysis. Moreover, this imaging flow cytometer platform is portable, low-cost, and shows potential in the onsite phytoplankton monitoring. Full article

(This article belongs to the Special Issue Optical Sensing)

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22 pages, 5252 KiB

Open AccessReview

Optical Trapping, Sensing, and Imaging by Photonic Nanojets

by Heng Li, Wanying Song, Yanan Zhao, Qin Cao and Ahao Wen

Photonics 2021, 8(10), 434; https://doi.org/10.3390/photonics8100434 - 11 Oct 2021

Cited by 21 | Viewed by 4908

Abstract

The optical trapping, sensing, and imaging of nanostructures and biological samples are research hotspots in the fields of biomedicine and nanophotonics. However, because of the diffraction limit of light, traditional optical tweezers and microscopy are difficult to use to trap and observe objects [...] Read more.

The optical trapping, sensing, and imaging of nanostructures and biological samples are research hotspots in the fields of biomedicine and nanophotonics. However, because of the diffraction limit of light, traditional optical tweezers and microscopy are difficult to use to trap and observe objects smaller than 200 nm. Near-field scanning probes, metamaterial superlenses, and photonic crystals have been designed to overcome the diffraction limit, and thus are used for nanoscale optical trapping, sensing, and imaging. Additionally, photonic nanojets that are simply generated by dielectric microspheres can break the diffraction limit and enhance optical forces, detection signals, and imaging resolution. In this review, we summarize the current types of microsphere lenses, as well as their principles and applications in nano-optical trapping, signal enhancement, and super-resolution imaging, with particular attention paid to research progress in photonic nanojets for the trapping, sensing, and imaging of biological cells and tissues. Full article

(This article belongs to the Special Issue Photonic Jet: Science and Application)

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18 pages, 4709 KiB

Open AccessArticle

Study of an Integration Platform Based on an Adiabatic Active-Layer Waveguide Connection for InP Photonic Device Integration Mirroring That of Heterogeneous Integration on Silicon

by Changming Chen, Junyu Li, Chunxue Wang, Yingyan Huang, Daming Zhang, Zuosen Shi, Zhanchen Cui, Fei Yi and Seng-Tiong Ho

Photonics 2021, 8(10), 433; https://doi.org/10.3390/photonics8100433 - 9 Oct 2021

Cited by 1 | Viewed by 3431

Abstract

In this work, a photonic device integration platform capable of integration of active-passive InP-based photonic devices without the use of material regrowth is introduced. The platform makes use of an adiabatic active-layer waveguide connection (ALWC) to move an optical beam between active and [...] Read more.

In this work, a photonic device integration platform capable of integration of active-passive InP-based photonic devices without the use of material regrowth is introduced. The platform makes use of an adiabatic active-layer waveguide connection (ALWC) to move an optical beam between active and passive devices. The performance of this platform is analyzed using an example made up of four main sections: (1) a fiber coupling section for enabling vertical beam coupling from optical fiber into the photonic chip using a mode-matched surface grating with apodized duty cycles; (2) a transparent waveguide section for realizing passive photonic devices; (3) an adiabatic mode connection structure for moving the optical beam between passive and active device sections; and (4) an active device section for realizing active photonic devices. It is shown that the coupled surface grating, when added with a bottom gold reflector, can achieve a high chip-to-fiber coupling efficiency (CE) of 88.3% at 1550 nm. The adiabatic active-layer mode connection structure has an optical loss of lower than 1% (CE > 99%). The active device section can achieve an optical gain of 20 dB/mm with the use of only 3 quantum wells. The optimized structural parameters of the entire waveguide module are analyzed and discussed. Full article

(This article belongs to the Special Issue Silicon Based Integrated Optics)

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17 pages, 13563 KiB

Open AccessArticle

Case Study on the Fitting Method of Typical Objects

by Liu Zhang, Jiakun Zhang, Hongzhen Song, Wen Zhang and Wenhua Wang

Photonics 2021, 8(10), 432; https://doi.org/10.3390/photonics8100432 - 9 Oct 2021

Cited by 5 | Viewed by 1777

Abstract

This study proposes different fitting methods for different types of targets in the 400–900 nm wavelength range, based on convex optimization algorithms, to achieve the effect of high-precision spectral reconstruction for small space-borne spectrometers. This article first expounds on the mathematical model in [...] Read more.

This study proposes different fitting methods for different types of targets in the 400–900 nm wavelength range, based on convex optimization algorithms, to achieve the effect of high-precision spectral reconstruction for small space-borne spectrometers. This article first expounds on the mathematical model in the imaging process of the small spectrometer and discretizes it into an

A X = B

matrix equation. Second, the design basis of the filter transmittance curve is explained. Furthermore, a convex optimization algorithm is used, based on 50 filters, and appropriate constraints are added to solve the target spectrum. First, in terms of spectrum fitting, six different ground object spectra are selected, and Gaussian fitting, polynomial fitting, and Fourier fitting are used to fit the original data and analyze the best fit of each target spectrum. Then the transmittance curve of the filter is equally divided, and the corresponding

A X = B

discrete equation set is obtained for the specific object target, and a random error of 1% is applied to the equation set to obtain the discrete spectral value. The fitting is performed for each case to determine the best fitting method with errors. Subsequently, the transmittance curve of the filter with the detector characteristics is equally divided, and the corresponding

A X = B

discrete equation set is obtained for the specific object target. A random error of 1% is applied to the equation set to obtain the error. After the discrete spectral values are obtained, the fitting is performed again, and the best fitting method is determined. In order to evaluate the fitting accuracy of the original spectral data and the reconstruction accuracy of the calculated discrete spectrum, the three evaluation indicators MSE, ARE, and RE are used for evaluation. To measure the stability and accuracy of the spectral reconstruction of the fitting method more accurately, it is necessary to perform 500 cycles of calculations to determine the corresponding MSE value and further analyze the influence of the fitting method on the reconstruction accuracy. The results show that different fitting methods should be adopted for different ground targets under the error conditions. The three indicators, MSE, ARE, and RE, have reached high accuracy and strong stability. The effect of high-precision reconstruction of the target spectrum is achieved. This article provides new ideas for related scholars engaged in hyperspectral reconstruction work and promotes the development of hyperspectral technology. Full article

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14 pages, 5491 KiB

Open AccessArticle

Target Detection Method for Low-Resolution Remote Sensing Image Based on ESRGAN and ReDet

by Yuwu Wang, Guobing Sun and Shengwei Guo

Photonics 2021, 8(10), 431; https://doi.org/10.3390/photonics8100431 - 8 Oct 2021

Cited by 6 | Viewed by 2562

Abstract

With the widespread use of remote sensing images, low-resolution target detection in remote sensing images has become a hot research topic in the field of computer vision. In this paper, we propose a Target Detection on Super-Resolution Reconstruction (TDoSR) method to solve the [...] Read more.

With the widespread use of remote sensing images, low-resolution target detection in remote sensing images has become a hot research topic in the field of computer vision. In this paper, we propose a Target Detection on Super-Resolution Reconstruction (TDoSR) method to solve the problem of low target recognition rates in low-resolution remote sensing images under foggy conditions. The TDoSR method uses the Enhanced Super-Resolution Generative Adversarial Network (ESRGAN) to perform defogging and super-resolution reconstruction of foggy low-resolution remote sensing images. In the target detection part, the Rotation Equivariant Detector (ReDet) algorithm, which has a higher recognition rate at this stage, is used to identify and classify various types of targets. While a large number of experiments have been carried out on the remote sensing image dataset DOTA-v1.5, the results of this paper suggest that the proposed method achieves good results in the target detection of low-resolution foggy remote sensing images. The principal result of this paper demonstrates that the recognition rate of the TDoSR method increases by roughly 20% when compared with low-resolution foggy remote sensing images. Full article

(This article belongs to the Special Issue Smart Pixels and Imaging)

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34 pages, 6278 KiB

Open AccessReview

III-Nitride Light-Emitting Devices

by Md Zunaid Baten, Shamiul Alam, Bejoy Sikder and Ahmedullah Aziz

Photonics 2021, 8(10), 430; https://doi.org/10.3390/photonics8100430 - 7 Oct 2021

Cited by 25 | Viewed by 7227

Abstract

III-nitride light-emitting devices have been subjects of intense research for the last several decades owing to the versatility of their applications for fundamental research, as well as their widespread commercial utilization. Nitride light-emitters in the form of light-emitting diodes (LEDs) and lasers have [...] Read more.

III-nitride light-emitting devices have been subjects of intense research for the last several decades owing to the versatility of their applications for fundamental research, as well as their widespread commercial utilization. Nitride light-emitters in the form of light-emitting diodes (LEDs) and lasers have made remarkable progress in recent years, especially in the form of blue LEDs and lasers. However, to further extend the scope of these devices, both below and above the blue emission region of the electromagnetic spectrum, and also to expand their range of practical applications, a number of issues and challenges related to the growth of materials, device design, and fabrication need to be overcome. This review provides a detailed overview of nitride-based LEDs and lasers, starting from their early days of development to the present state-of-the-art light-emitting devices. Besides delineating the scientific and engineering milestones achieved in the path towards the development of the highly matured blue LEDs and lasers, this review provides a sketch of the prevailing challenges associated with the development of long-wavelength, as well as ultraviolet nitride LEDs and lasers. In addition to these, recent progress and future challenges related to the development of next-generation nitride emitters, which include exciton-polariton lasers, spin-LEDs and lasers, and nanostructured emitters based on nanowires and quantum dots, have also been elucidated in this review. The review concludes by touching on the more recent topic of hexagonal boron nitride-based light-emitting devices, which have already shown significant promise as deep ultraviolet and single-photon emitters. Full article

(This article belongs to the Special Issue Emerging Photonic Devices, Circuits and Systems)

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16 pages, 7938 KiB

Open AccessArticle

An Improved Calibration Method to Determine the Strain Coefficient for Optical Fibre Sensing Cables

by Pengju An, Chaoqun Wei, Huiming Tang, Qinglu Deng, Bofan Yu and Kun Fang

Photonics 2021, 8(10), 429; https://doi.org/10.3390/photonics8100429 - 7 Oct 2021

Viewed by 2533

Abstract

The strain coefficient of an optical fibre sensing cable is a critical parameter for a distributed optical fibre sensing system. The conventional tensile load test method tends to underestimate the strain coefficient of sensing cables due to slippage or strain transfer loss at [...] Read more.

The strain coefficient of an optical fibre sensing cable is a critical parameter for a distributed optical fibre sensing system. The conventional tensile load test method tends to underestimate the strain coefficient of sensing cables due to slippage or strain transfer loss at the fixing points during the calibration procedure. By optimizing the conventional tensile load test setup, the true strain of a sensing cable can be determined by using two sets of displacement measuring equipment. Thus, the strain calculation error induced by slippage or strain transfer loss between a micrometre linear stage and sensing cable can be avoided. The performance of the improved calibration method was verified by using three types of sensing cables with different structures. In comparison to the conventional tensile load test method, the strain coefficients obtained by the improved calibration method for sensing cables A, B, and C increase by 1.52%, 2.06%, and 1.86%, respectively. Additionally, the calibration errors for the improved calibration method are discussed. The test results indicate that the improved calibration method has good practicability and enables inexperienced experimenters or facilities with limited equipment to perform precise strain coefficient calibration for optical fibre sensing cables. Full article

(This article belongs to the Special Issue Fiber-Optic Sensors)

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17 pages, 1909 KiB

Open AccessArticle

Intelligent Matching of the Control Voltage of Delay Line Interferometers for Differential Phase Shift Keying-Modulated Optical Signals

by Jing Zhou and Duandan Liang

Photonics 2021, 8(10), 428; https://doi.org/10.3390/photonics8100428 - 6 Oct 2021

Viewed by 2140

Abstract

In optical communications, differential phase shift keying (DPSK) provides a desired modulation format that offers high tolerance to nonlinear effects in high-speed transmissions. A DPSK demodulator converts the phase-coded signal into an intensity-coded signal at receivers. One demodulation scheme is called balanced detection [...] Read more.

In optical communications, differential phase shift keying (DPSK) provides a desired modulation format that offers high tolerance to nonlinear effects in high-speed transmissions. A DPSK demodulator converts the phase-coded signal into an intensity-coded signal at receivers. One demodulation scheme is called balanced detection and is based on a tunable delay line interferometer (DLI). Demodulation performances are determined by the phase delay generated by the DLI, while the phase delay is controlled by a tunable driving voltage on the DLI device. However, a problem in the dynamic adjustment of the control voltage prevents the application of DPSK demodulators. The receivers need to scan the whole control voltage range of the DLI and find the control voltage that maximizes the demodulation performance, but the scan-based method needs to undergo a very long searching time. In our work, we found that the relation between DLI control voltages and demodulation performance can be predicted rapidly by a feedforward neural network (FNN). In this paper, we propose a new method to quickly locate the best DLI control voltage based on an FNN. We also verify the proposed method in simulations and telecommunication systems, and the results show that the proposed method can significantly improve the efficiency of resolving the best demodulation voltages. Full article

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

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