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Photonics
Volume 8
Issue 4
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Photonics, Volume 8, Issue 4 (April 2021) – 47 articles

Cover Story (view full-size image): The cornea is a critical component of vision because it provides 2/3 of the total refracting power of the eye. Its inherent biomechanical properties are critical to maintaining its shape and function. Diseases such as keratoconus and therapies such as UV-A/riboflavin corneal collagen crosslinking can drastically alter corneal biomechanical properties and, consequently, vision. Therefore, the ability to measure corneal biomechanical properties could provide earlier disease detection and patient-specific therapies with superior outcomes. However, measuring corneal biomechanical properties is difficult due to its geometry. In this work, we present a compression-based optical coherence elastography technique to measure corneal stiffness. We validated our technique in situ and demonstrated its feasibility in vivo in a rabbit model. View this paper
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15 pages, 2236 KiB  
Article
A New Type of Shape-Invariant Beams with Structured Coherence: Laguerre-Christoffel-Darboux Beams
by Rosario Martínez-Herrero, Massimo Santarsiero, Gemma Piquero and Juan Carlos González de Sande
Photonics 2021, 8(4), 134; https://doi.org/10.3390/photonics8040134 - 20 Apr 2021
Cited by 11 | Viewed by 2825
Abstract
A new class of sources presenting structured coherence properties is introduced and analyzed. They are obtained as the incoherent superposition of coherent Laguerre-Gaussian modes with suitable coefficients. This ensures that the shape of the intensity profile and the spatial coherence features of the [...] Read more.
A new class of sources presenting structured coherence properties is introduced and analyzed. They are obtained as the incoherent superposition of coherent Laguerre-Gaussian modes with suitable coefficients. This ensures that the shape of the intensity profile and the spatial coherence features of the propagated beams are invariant during paraxial approximation. A simple analytical expression is obtained for the cross-spectral density of the sources of this class, regardless of the number of superposed modes. Properties of these sources are analyzed and described by several examples. Full article
(This article belongs to the Special Issue Structured Light Coherence)
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21 pages, 7371 KiB  
Review
Nonreciprocal and Topological Plasmonics
by Kunal Shastri, Mohamed Ismail Abdelrahman and Francesco Monticone
Photonics 2021, 8(4), 133; https://doi.org/10.3390/photonics8040133 - 20 Apr 2021
Cited by 21 | Viewed by 5020
Abstract
Metals, semiconductors, metamaterials, and various two-dimensional materials with plasmonic dispersion exhibit numerous exotic physical effects in the presence of an external bias, for example an external static magnetic field or electric current. These physical phenomena range from Faraday rotation of light propagating in [...] Read more.
Metals, semiconductors, metamaterials, and various two-dimensional materials with plasmonic dispersion exhibit numerous exotic physical effects in the presence of an external bias, for example an external static magnetic field or electric current. These physical phenomena range from Faraday rotation of light propagating in the bulk to strong confinement and directionality of guided modes on the surface and are a consequence of the breaking of Lorentz reciprocity in these systems. The recent introduction of relevant concepts of topological physics, translated from condensed-matter systems to photonics, has not only given a new perspective on some of these topics by relating certain bulk properties of plasmonic media to the surface phenomena, but has also led to the discovery of new regimes of truly unidirectional, backscattering-immune, surface-wave propagation. In this article, we briefly review the concepts of nonreciprocity and topology and describe their manifestation in plasmonic materials. Furthermore, we use these concepts to classify and discuss the different classes of guided surface modes existing on the interfaces of various plasmonic systems. Full article
(This article belongs to the Special Issue Plasmonic Metasurfaces)
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9 pages, 2879 KiB  
Communication
Fast Dark Signal Measurements of SVOM VT CCDs Using the Vertical Gradient of Dark Field Images
by Yue Pan, Xuewu Fan, Hui Zhao, Yulei Qiu, Wei Gao and Jian Zhang
Photonics 2021, 8(4), 132; https://doi.org/10.3390/photonics8040132 - 20 Apr 2021
Cited by 2 | Viewed by 2083
Abstract
This paper describes a fast technique for estimating the dark signals of the charge coupled devices (CCDs) of the visible telescope (VT) onboard the space multi-band variable object monitor (SVOM). It is based on the vertical gradient in the dark field images of [...] Read more.
This paper describes a fast technique for estimating the dark signals of the charge coupled devices (CCDs) of the visible telescope (VT) onboard the space multi-band variable object monitor (SVOM). It is based on the vertical gradient in the dark field images of the frame transfer CCDs. During the process of frame clear, exposure, frame transfer and readout, the characteristic of dark signal accumulation is analyzed firstly. Next, the linear fitting method is used to fit the signal level of the dark field image in the vertical direction, and the slope of the fitting line represents the dark signal factor. This technique only needs one dark field image and can be used for simple and efficient dark signal measurements of frame transfer CCDs. Besides, an experiment of detecting dark signals as a function of temperature based on the fast technique has been carried out. Making use of the Shockley-Hall-Read theory, two curve fitting formulas are adopted to the experimental results for VT Advanced Inverted Mode Operation (AIMO) CCD and VT Non-Inverted Mode Operation (NIMO) CCD respectively. The experimental results and the formulas are used to determine the optimal on-orbit cooling temperature of VT CCDs. Full article
(This article belongs to the Section Optoelectronics and Optical Materials)
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19 pages, 1306 KiB  
Review
Integrated Silicon Photonics for Enabling Next-Generation Space Systems
by George N. Tzintzarov, Sunil G. Rao and John D. Cressler
Photonics 2021, 8(4), 131; https://doi.org/10.3390/photonics8040131 - 20 Apr 2021
Cited by 31 | Viewed by 5673
Abstract
A review of silicon photonics for space applications is presented. The benefits and advantages of size, weight, power, and cost (SWaP-C) metrics inherent to silicon photonics are summarized. Motivation for their use in optical communications systems and microwave photonics is addressed. The current [...] Read more.
A review of silicon photonics for space applications is presented. The benefits and advantages of size, weight, power, and cost (SWaP-C) metrics inherent to silicon photonics are summarized. Motivation for their use in optical communications systems and microwave photonics is addressed. The current state of our understanding of radiation effects in silicon photonics is included in this discussion. Total-ionizing dose, displacement damage, and single-event transient effects are discussed in detail for germanium-integrated photodiodes, silicon waveguides, and Mach-Zehnder modulators. Areas needing further study are suggested. Full article
(This article belongs to the Special Issue Emerging Photonic Devices, Circuits and Systems)
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9 pages, 2911 KiB  
Communication
Evaluation of Performance Enhancement of Optical Multi-Level Modulation Based on Direct Modulation of Optically Injection-Locked Semiconductor Lasers
by Hyo-Sang Jeong, Jun-Hyung Cho and Hyuk-Kee Sung
Photonics 2021, 8(4), 130; https://doi.org/10.3390/photonics8040130 - 19 Apr 2021
Cited by 4 | Viewed by 2383
Abstract
The performance of optical M-level (multi-level) amplitude shift keying (ASK) modulation is improved by directly using modulated optically injection-locked (OIL) semiconductor lasers. The direct modulation performance of free-running and OIL semiconductor lasers is evaluated and compared theoretically based on coupled-rate equation. We have [...] Read more.
The performance of optical M-level (multi-level) amplitude shift keying (ASK) modulation is improved by directly using modulated optically injection-locked (OIL) semiconductor lasers. The direct modulation performance of free-running and OIL semiconductor lasers is evaluated and compared theoretically based on coupled-rate equation. We have found that OIL semiconductor lasers can significantly improve the modulation performance in terms of the signal eye opening and Q-factor. Additionally, we found that the Q-factor increases even more in the negative frequency detuning range due to its dependence on the locking parameters. Full article
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13 pages, 2335 KiB  
Article
ML-Based Identification of Structured Light Schemes under Free Space Jamming Threats for Secure FSO-Based Applications
by Amr M. Ragheb, Waddah S. Saif and Saleh A. Alshebeili
Photonics 2021, 8(4), 129; https://doi.org/10.3390/photonics8040129 - 19 Apr 2021
Cited by 8 | Viewed by 2581
Abstract
This paper exploits for the first time the use of machine learning (ML) based techniques to identify complex structured light patterns under free space optics (FSO) jamming attacks for secure FSO-based applications. Five M-ary modulation schemes, construed using Laguerre and Hermite Gaussian [...] Read more.
This paper exploits for the first time the use of machine learning (ML) based techniques to identify complex structured light patterns under free space optics (FSO) jamming attacks for secure FSO-based applications. Five M-ary modulation schemes, construed using Laguerre and Hermite Gaussian (LG and HG) mode families, were used in this investigation. These include 8-ary LG, 8-ary superposition-LG, 16-ary HG, 16-ary LG and superposition-LG, and 32-ary LG and superposition-LG and HG formats. The work was conducted using experimental demonstrations for two different jammer positions. The convolutional neural network (CNN)-based ML method was utilized to differentiate between the stressed mode patterns. The experimental results show a 100% recognition accuracy for 8-ary LG, 8-ary superposition-LG, and 16-ary HG at 1, −2, and −2 dB signal-to-jammer ratios (SJR), respectively. For SJR values < 0 dB, the standard LG modes are the most affected by jamming and are not recommended for data transmission in such an environment. Besides, the accuracy of determining the jammer direction of arrival was investigated using CNN and a simpler classifier based on linear discriminant analysis (LDA). The results show that advanced networks (e.g., CNN) are required to achieve reliable performance of 100% direction determination accuracy, at −5 dB SJR, as opposed to 97%, at 2 dB SJR, for a simple LDA classifier. Full article
(This article belongs to the Special Issue Optical Wireless Communication (OWC) Systems)
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13 pages, 11010 KiB  
Review
Recent Advancement of Anti-Resonant Hollow-Core Fibers for Sensing Applications
by Wenjun Ni, Chunyong Yang, Yiyang Luo, Ran Xia, Ping Lu, Dora Juan Juan Hu, Sylvain Danto, Perry Ping Shum and Lei Wei
Photonics 2021, 8(4), 128; https://doi.org/10.3390/photonics8040128 - 19 Apr 2021
Cited by 37 | Viewed by 8190
Abstract
Specialty fibers have enabled a wide range of sensing applications. Particularly, with the recent advancement of anti-resonant effects, specialty fibers with hollow structures offer a unique sensing platform to achieve highly accurate and ultra-compact fiber optic sensors with large measurement ranges. This review [...] Read more.
Specialty fibers have enabled a wide range of sensing applications. Particularly, with the recent advancement of anti-resonant effects, specialty fibers with hollow structures offer a unique sensing platform to achieve highly accurate and ultra-compact fiber optic sensors with large measurement ranges. This review presents an overview of recent progress in anti-resonant hollow-core fibers for sensing applications. Both regular and irregular-shaped fibers and their performance in various sensing scenarios are summarized. Finally, the challenges and possible solutions are briefly presented with some perspectives toward the future development of anti-resonant hollow-core fibers for advanced sensing. Full article
(This article belongs to the Special Issue Novel Specialty Optical Fibers and Applications)
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13 pages, 9494 KiB  
Article
High-Sensitivity Optical-Resolution Photoacoustic Microscopy with an Optical-Acoustic Combiner Based on an Off-Axis Parabolic Acoustic Mirror
by Xiang Zhang, Yang Liu, Chao Tao, Jie Yin, Zizhong Hu, Songtao Yuan, Qinghuai Liu and Xiaojun Liu
Photonics 2021, 8(4), 127; https://doi.org/10.3390/photonics8040127 - 18 Apr 2021
Cited by 10 | Viewed by 3367
Abstract
Optical-resolution photoacoustic microscopy (OR-PAM) is a promising noninvasive biomedical imaging technology with label-free optical absorption contrasts. Performance of OR-PAM is usually closely related to the optical-acoustic combiner. In this study, we propose an optical-acoustic combiner based on a flat acoustic reflector and an [...] Read more.
Optical-resolution photoacoustic microscopy (OR-PAM) is a promising noninvasive biomedical imaging technology with label-free optical absorption contrasts. Performance of OR-PAM is usually closely related to the optical-acoustic combiner. In this study, we propose an optical-acoustic combiner based on a flat acoustic reflector and an off-axis parabolic acoustic mirror with a conical bore. Quantitative simulation and experiments demonstrated that this combiner can provide better acoustic focusing performance and detection sensitivity. Moreover, OR-PAM is based on the combiner suffer low optical disorders, which guarantees the good resolution. In vivo experiments of the mouse brain and the iris were also conducted to show the practicability of the combiner in biomedicine. This proposed optical-acoustic combiner realizes a high-quality optical-acoustic confocal alignment with minimal optical disorders and acoustic insertion loss, strong acoustic focusing, and easy implementation. These characteristics might be useful for improving the performance of OR-PAM. Full article
(This article belongs to the Special Issue Photoacoustic Imaging for Biomedical Applications)
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8 pages, 2607 KiB  
Communication
Phase Compensation Method in OPA System Based on the Linear Electro-Optic Effect
by Shuaishuai Yang and Dean Liu
Photonics 2021, 8(4), 126; https://doi.org/10.3390/photonics8040126 - 17 Apr 2021
Cited by 2 | Viewed by 1999
Abstract
Factors such as mechanical deformation and temperature changes lead to phase mismatch in optical parametric amplification systems, impacting energy stability. A phase compensation method via the linear electro-optic effect can overcome this limitation. Phase mismatch compensation characteristics were simulated via the linear electro-optic [...] Read more.
Factors such as mechanical deformation and temperature changes lead to phase mismatch in optical parametric amplification systems, impacting energy stability. A phase compensation method via the linear electro-optic effect can overcome this limitation. Phase mismatch compensation characteristics were simulated via the linear electro-optic effect in 70%-deuterated DKDP and 95%-deuterated DKDP. This method was applied to OPA systems to verify its feasibility. The results show that the temperature acceptance bandwidth of 70%-deuterated DKDP and 95%-deuterated DKDP can be ~1.75 and ~2 times larger, respectively, than that of the OPA without compensation. Moreover, the angle acceptance bandwidth of 70%-deuterated DKDP and 95%-deuterated DKDP can be ~2 times larger than that of the OPA without compensation. The abovementioned method can facilitate the compensation of phase mismatch within a range and can be widely used in OPA and optical parametric chirped pulse amplification systems to improve laser stability. Full article
(This article belongs to the Special Issue Laser Amplifiers)
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11 pages, 2502 KiB  
Article
Study on the Nanosensor Based on a MIM Waveguide with a Stub Coupled with a Horizontal B-Type Cavity
by Shubin Yan, Haoran Shi, Xiaoyu Yang, Jing Guo, Wenchang Wu and Ertian Hua
Photonics 2021, 8(4), 125; https://doi.org/10.3390/photonics8040125 - 16 Apr 2021
Cited by 4 | Viewed by 2230
Abstract
Due to their compact size and high sensitivity, plasmonic sensors have become a hot topic in the sensing field. A nanosensor structure, comprising the metal–insulator–metal (MIM) waveguide with a stub and a horizontal B-Type cavity, is designed as a refractive index sensor. The [...] Read more.
Due to their compact size and high sensitivity, plasmonic sensors have become a hot topic in the sensing field. A nanosensor structure, comprising the metal–insulator–metal (MIM) waveguide with a stub and a horizontal B-Type cavity, is designed as a refractive index sensor. The spectral characteristics of proposed structure are analyzed via the finite element method (FEM). The results show that there is a sharp Fano resonance profile, which is excited by a coupling between the MIM waveguide and the horizontal B-Type cavity. The normalized HZ field is affected by the difference value between the outer radii R1 and R2 of the semi-circle of the horizontal B-Type cavity greatly. The influence of every element of the whole system on sensing properties is discussed in depth. The sensitivity of the proposed structure can obtain 1548 nm/RIU (refractive index unit) with a figure of merit of 59. The proposed structure has potential in nanophotonic sensing applications. Full article
(This article belongs to the Special Issue Optical Sensing)
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8 pages, 1666 KiB  
Article
Effect of Carrier-Transporting Layer on Blue Phosphorescent Organic Light-Emitting Diodes
by Bo-Yen Lin, Chia-Hsun Chen, Tzu-Chan Lin, Jiun-Haw Lee and Tien-Lung Chiu
Photonics 2021, 8(4), 124; https://doi.org/10.3390/photonics8040124 - 15 Apr 2021
Cited by 2 | Viewed by 3949
Abstract
This study presented the effects of carrier-transporting layer (CTL) on electroluminescence (EL) performance of a blue phosphorescent organic light-emitting diodes (PHOLEDs) with electron transporting host based on three kinds of electron-transporting layers (ETLs) including 3-(4-biphenyl-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), diphenyl-bis[4-(pyridin-3-yl)phenyl]silane (DPPS) and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) and two [...] Read more.
This study presented the effects of carrier-transporting layer (CTL) on electroluminescence (EL) performance of a blue phosphorescent organic light-emitting diodes (PHOLEDs) with electron transporting host based on three kinds of electron-transporting layers (ETLs) including 3-(4-biphenyl-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), diphenyl-bis[4-(pyridin-3-yl)phenyl]silane (DPPS) and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) and two kinds of hole-transporting layers (HTLs) such as 4,4′-bis[N-1-naphthyl-N-phenyl-amino]biphenyl (NPB), 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC). The carrier recombination and exciton formation zones in blue PHOLEDs strongly depend on the carrier mobility of CTLs and the layer thickness, especially the carrier mobility. Between ETLs and HTLs, the high electron mobility of ETL results in a lower driving voltage in blue PHOLEDs than the high hole mobility of HTL did. In addition, layer thickness modulation is an effective approach to precisely control carriers and restrict carriers within the EML and avoid a leakage emission of CTL. For CTL pairs in OLEDs using the electron transporting host system, ETLs with low mobility and also HTLs with high hole mobility are key points to confine the charge in EML for efficient photon emission. These findings show that appropriate CTL pairs and good layer thickness are essential for efficient OLEDs. Full article
(This article belongs to the Special Issue Optoelectronic Materials and Their Applications)
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13 pages, 865 KiB  
Article
Novel Broadband Slot-Spiral Antenna for Terahertz Applications
by Zhen Huang, Zhaofeng Li, Hui Dong, Fuhua Yang, Wei Yan and Xiaodong Wang
Photonics 2021, 8(4), 123; https://doi.org/10.3390/photonics8040123 - 14 Apr 2021
Cited by 10 | Viewed by 3686
Abstract
We report a novel broadband slot-spiral antenna that can be integrated with high-electron-mobility transistor (HEMT) terahertz (THz) detectors. The effect of various antenna parameters on the transmission efficiency of the slot-spiral structure at 150–450 GHz is investigated systematically. The performances of the slot-spiral [...] Read more.
We report a novel broadband slot-spiral antenna that can be integrated with high-electron-mobility transistor (HEMT) terahertz (THz) detectors. The effect of various antenna parameters on the transmission efficiency of the slot-spiral structure at 150–450 GHz is investigated systematically. The performances of the slot-spiral antenna and the spiral antenna both integrated with HEMTs are compared. The results show that the slot-spiral structure has a better transmission and miniaturization capability than the spiral structure. A formula for the responsivity is derived based on the transmission line principle and antenna theory, and results show that the detector responsivity is correlated with the antenna absorptivity. Additionally, guidelines for HEMT THz detector design are proposed. The results of this study indicate the excellent application prospects of the slot-spiral antenna in THz detection and imaging. Full article
(This article belongs to the Special Issue Frontiers in Terahertz Technology and Applications)
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15 pages, 1371 KiB  
Article
Ring-Core Photonic Crystal Fiber of Terahertz Orbital Angular Momentum Modes with Excellence Guiding Properties in Optical Fiber Communication
by Fahad Ahmed Al-Zahrani and Md. Anowar Kabir
Photonics 2021, 8(4), 122; https://doi.org/10.3390/photonics8040122 - 14 Apr 2021
Cited by 16 | Viewed by 3511
Abstract
The orbital angular momentum (OAM) of light is used for increasing the optical communication capacity in the mode division multiplexing (MDM) technique. A novel and simple structure of ring-core photonic crystal fiber (RC-PCF) is proposed in this paper. The ring core is doped [...] Read more.
The orbital angular momentum (OAM) of light is used for increasing the optical communication capacity in the mode division multiplexing (MDM) technique. A novel and simple structure of ring-core photonic crystal fiber (RC-PCF) is proposed in this paper. The ring core is doped by the Schott sulfur difluoride material and the cladding region is composed of fused silica with one layer of well-patterned air-holes. The guiding of Terahertz (THz) OAM beams with 58 OAM modes over 0.70 THz (0.20 THz–0.90 THz) frequency is supported by this proposed RC-PCF. The OAM modes are well-separated for their large refractive index difference above 10−4. The dispersion profile of each mode is varied in the range of 0.23–7.77 ps/THz/cm. The ultra-low confinement loss around 10−9 dB/cm and better mode purity up to 0.932 is achieved by this RC-PCF. For these good properties, the proposed fiber is a promising candidate to be applied in the THz OAM transmission systems with high feasibility and high capacity. Full article
(This article belongs to the Section Optical Communication and Network)
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9 pages, 1810 KiB  
Communication
Femtosecond Laser Fabrication of Hybrid Metal-Dielectric Structures with Nonlinear Photoluminescence
by Ekaterina Ponkratova, Eduard Ageev, Filipp Komissarenko, Sergei Koromyslov, Dmitry Kudryashov, Ivan Mukhin, Vadim Veiko, Aleksandr Kuchmizhak and Dmitry Zuev
Photonics 2021, 8(4), 121; https://doi.org/10.3390/photonics8040121 - 13 Apr 2021
Cited by 5 | Viewed by 2927
Abstract
Fabrication of hybrid micro- and nanostructures with a strong nonlinear response is challenging and represents a great interest due to a wide range of photonic applications. Usually, such structures are produced by quite complicated and time-consuming techniques. This work demonstrates laser-induced hybrid metal-dielectric [...] Read more.
Fabrication of hybrid micro- and nanostructures with a strong nonlinear response is challenging and represents a great interest due to a wide range of photonic applications. Usually, such structures are produced by quite complicated and time-consuming techniques. This work demonstrates laser-induced hybrid metal-dielectric structures with strong nonlinear properties obtained by a single-step fabrication process. We determine the influence of several incident femtosecond pulses on the Au/Si bi-layer film on produced structure morphology. The created hybrid systems represent isolated nanoparticles with a height of 250–500 nm exceeding the total thickness of the Au-Si bi-layer. It is shown that fabricated hybrid nanostructures demonstrate enhancement of the SHG signal (up to two orders of magnitude) compared to the initial planar sample and a broadband photoluminescence signal (more than 200 nm in width) in the visible spectral region. We establish the correlation between nonlinear signal and phase composition provided by Raman scattering measurements. Such laser-induced structures have significant potential in optical sensing applications and can be used as components for different nanophotonic devices. Full article
(This article belongs to the Special Issue Advanced/Novel Photonics Nanostructures)
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13 pages, 3042 KiB  
Article
Frequency Dependence of a Piezo-Resistive Method for Pressure Measurements of Laser-Induced Shock Waves in Solids
by Ricardo Gonzalez-Romero, Marija Strojnik, Guillermo Garcia-Torales and Gilberto Gomez-Rosas
Photonics 2021, 8(4), 120; https://doi.org/10.3390/photonics8040120 - 12 Apr 2021
Cited by 4 | Viewed by 2820
Abstract
A shock wave is a mechanical high-pressure pulse that travels inside a medium with a full width at half-maximum of a few nanoseconds that may be induced with a high-power laser pulse. A piezo-resistive measurement method to determine the shock wave pressure has [...] Read more.
A shock wave is a mechanical high-pressure pulse that travels inside a medium with a full width at half-maximum of a few nanoseconds that may be induced with a high-power laser pulse. A piezo-resistive measurement method to determine the shock wave pressure has been widely employed even though there is inner inaccuracy in the calibration process. We are interested in developing a precise theoretical model of laser material processing for applications in material sciences that includes the frequency dependence of the electronic post processing. We show an approach to determine the correction factor to frequency response at a high frequency of a piezo-resistive experimental setup and the results of the pressure measurements obtained in this experimental setup. The theoretical and experimental work demonstrates the feasibility of piezo-resistive methods to measure a laser-induced shock wave pressure in the nanosecond range. The correction factor of the frequency dependence calibration allows the technique to be applied in different shock wave experiments. Full article
(This article belongs to the Special Issue Advances in 3OM: Opto-Mechatronics, Opto-Mechanics, and Optical Metrology)
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14 pages, 4274 KiB  
Article
FEM Simulation of Frequency-Selective Surface Based on Thermoelectric Bi-Sb Thin Films for THz Detection
by Anastasiia Tukmakova, Ivan Tkhorzhevskiy, Artyom Sedinin, Aleksei Asach, Anna Novotelnova, Natallya Kablukova, Petr Demchenko, Anton Zaitsev, Dmitry Zykov and Mikhail Khodzitsky
Photonics 2021, 8(4), 119; https://doi.org/10.3390/photonics8040119 - 11 Apr 2021
Cited by 5 | Viewed by 2845
Abstract
Terahertz (THz) filters and detectors can find a wide application in such fields as: sensing, imaging, security systems, medicine, wireless connection, and detection of substances. Thermoelectric materials are promising basis for THz detectors’ development due to their sensitivity to the THz radiation, possibility [...] Read more.
Terahertz (THz) filters and detectors can find a wide application in such fields as: sensing, imaging, security systems, medicine, wireless connection, and detection of substances. Thermoelectric materials are promising basis for THz detectors’ development due to their sensitivity to the THz radiation, possibility to be heated under the THz radiation and produce voltage due to Seebeck effect. Thermoelectric thin films of Bi-Sb solid solutions are semimetals/semiconductors with the band gap comparable with THz energy and with high thermoelectric conversion efficiency at room temperature. Detecting film surface can be transformed into a periodic frequency selective surface (FSS) that can operate as a frequency filter and increases the absorption of THz radiation. We report for the first time about the simulation of THz detector based on thermoelectric Bi-Sb thin-filmed frequency-selective surface. We show that such structure can be both detector and frequency filter. Moreover, it was shown that FSS design increases not only a heating due to absorption but a temperature gradient in Bi-Sb film by two orders of magnitude in comparison with continuous films. Local temperature gradients can reach the values of the order of 100 K·mm−1. That opens new perspectives for thin-filmed thermoelectric detectors’ efficiency increase. Temperature difference formed due to THz radiation absorption can reach values on the order of 1 degree. Frequency-transient calculations show the power dependence of film temperature on time with characteristic saturation at times around several ms. That points to the perspective of reaching fast response times on such structures. Full article
(This article belongs to the Section Optoelectronics and Optical Materials)
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14 pages, 2902 KiB  
Article
Digital Image Processing and Development of Machine Learning Models for the Discrimination of Corneal Pathology: An Experimental Model
by Andres Bustamante-Arias, Abbas Cheddad, Julio Cesar Jimenez-Perez and Alejandro Rodriguez-Garcia
Photonics 2021, 8(4), 118; https://doi.org/10.3390/photonics8040118 - 10 Apr 2021
Cited by 6 | Viewed by 3895
Abstract
Machine learning (ML) has an impressive capacity to learn and analyze a large volume of data. This study aimed to train different algorithms to discriminate between healthy and pathologic corneal images by evaluating digitally processed spectral-domain optical coherence tomography (SD-OCT) corneal images. A [...] Read more.
Machine learning (ML) has an impressive capacity to learn and analyze a large volume of data. This study aimed to train different algorithms to discriminate between healthy and pathologic corneal images by evaluating digitally processed spectral-domain optical coherence tomography (SD-OCT) corneal images. A set of 22 SD-OCT images belonging to a random set of corneal pathologies was compared to 71 healthy corneas (control group). A binary classification method was applied where three approaches of ML were explored. Once all images were analyzed, representative areas from every digital image were also extracted, processed and analyzed for a statistical feature comparison between healthy and pathologic corneas. The best performance was obtained from transfer learning—support vector machine (TL-SVM) (AUC = 0.94, SPE 88%, SEN 100%) and transfer learning—random forest (TL- RF) method (AUC = 0.92, SPE 84%, SEN 100%), followed by convolutional neural network (CNN) (AUC = 0.84, SPE 77%, SEN 91%) and random forest (AUC = 0.77, SPE 60%, SEN 95%). The highest diagnostic accuracy in classifying corneal images was achieved with the TL-SVM and the TL-RF models. In image classification, CNN was a strong predictor. This pilot experimental study developed a systematic mechanized system to discern pathologic from healthy corneas using a small sample. Full article
(This article belongs to the Special Issue Ocular Imaging for Eye Care)
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12 pages, 4319 KiB  
Communication
Excitation of Multi-Beam Interference and Whispering-Gallery Mode in Silica Taper-Assisted Polymer Microspheres for Refractometric Sensing
by Huibo Fan, Dawei Zhou, Li Fan, Yuanyan Wu, Hao Tao and Junbin Gong
Photonics 2021, 8(4), 117; https://doi.org/10.3390/photonics8040117 - 10 Apr 2021
Viewed by 2166
Abstract
We propose and numerically analyze a fiber-optic sensor based on a silica taper-assisted multiple polymer microspheres to realize high-sensitivity refractometric sensing due to the excitation of multi-beam interference and whispering-gallery modes (WGMs) in each microsphere. Up to 5 UV-cured adhesive microspheres are dripped [...] Read more.
We propose and numerically analyze a fiber-optic sensor based on a silica taper-assisted multiple polymer microspheres to realize high-sensitivity refractometric sensing due to the excitation of multi-beam interference and whispering-gallery modes (WGMs) in each microsphere. Up to 5 UV-cured adhesive microspheres are dripped in sequence on the 2 µm-diameter silica taper with the certain distances in between. Scattering and reflection of light in each microsphere enhance the multi-beam interference because of the emergence of high-order modes. Moreover, WGMs with certain resonant wavelengths are excited in the microspheres, further enhancing the transmission spectrum with high contrast and quality factor. Furthermore, irregular transmission spectrum corresponds to the large wavelength tuning range. Propagating light between the microspheres could be strongly affected by the environmental factor with the large transmission spectrum shift, therefore realizing the refractometric sensing with high sensitivity of 846 nm/RIU based on the taper-assisted 2-spheres sensor. Furthermore, another two microspheres are placed on both sides of 2-spheres sensor to form 4-spheres-loop sensor, re-collecting those large-angle scattering light with the enhancement of multi-beam interference and WGMs. Therefore, the sensitivity of 4-shperes-loop sensor is further improved to the maximum of 1296.3 nm/RIU. Full article
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7 pages, 2313 KiB  
Communication
Plasmonic Fishnet Structures for Dual Band THz Left-Handed Metamaterials
by Shaohua Zhang, Zhifu Wei, Ling Xu, Jianwei Xu, Shoujian Ouyang and Yun Shen
Photonics 2021, 8(4), 116; https://doi.org/10.3390/photonics8040116 - 9 Apr 2021
Cited by 4 | Viewed by 2129
Abstract
Plasmonic fishnet structures are proposed to realize dual-band terahertz (THz) left-handed metamaterials (LHMs). The calculated permittivity ε and permeability μ of single-layer LHMs show that ε < 0 and μ < 0 can be simultaneously satisfied in two frequency bands, resulting in dual-band [...] Read more.
Plasmonic fishnet structures are proposed to realize dual-band terahertz (THz) left-handed metamaterials (LHMs). The calculated permittivity ε and permeability μ of single-layer LHMs show that ε < 0 and μ < 0 can be simultaneously satisfied in two frequency bands, resulting in dual-band LHMs. The electric and magnetic field distributions are consistent with the current distributions and confirm the physical mechanism of negative permeability. Furthermore, the existence of negative refraction is validated by a stacked LHMs prism with an angle of 1.79°. It is shown that at 2.04–2.42 THz and 3.12–3.28 THz bands, negative refractive indices of the prism can be obtained, facilitating the practical application of LHMs in THz image, sensor, detection, communication, and so on. Full article
(This article belongs to the Special Issue Plasmonic Metasurfaces)
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13 pages, 2104 KiB  
Article
Sulfophosphate Glass Doped with Er3+ and TiO2 Nanoparticles: Thermo-Optical Characterization by Photothermal Spectroscopy
by Zeinab Ebrahimpour, Humberto Cabrera, Fahimeh Ahmadi, Asghar Asgari and Joseph Niemela
Photonics 2021, 8(4), 115; https://doi.org/10.3390/photonics8040115 - 8 Apr 2021
Cited by 7 | Viewed by 2703
Abstract
In this work, time-resolved thermal lens and beam deflection methods were applied to determine the thermo-optical properties of Er3+ doped sulfophosphate glass in which different concentrations of Titanium dioxide (TiO2) nanoparticles (NPs) were embedded. Thermal diffusivity (D), thermal [...] Read more.
In this work, time-resolved thermal lens and beam deflection methods were applied to determine the thermo-optical properties of Er3+ doped sulfophosphate glass in which different concentrations of Titanium dioxide (TiO2) nanoparticles (NPs) were embedded. Thermal diffusivity (D), thermal conductivity (κ), and the temperature coefficient of the optical path length (ds/dT) were determined as a function of NPs concentrations. Moreover, the growth of TiO2 NPs inside the amorphous glass matrix was evidenced by Transmission Electron Microscopy (TEM) images as well as through optical effects such as refractive index change of the glass. The outcomes indicated relatively high values for D and κ as well as a low ds/dT as required for most optical components used for laser media. The addition of TiO2 NPs with concentration of dopants up to 0.6 mol% improved the optical properties of the glass samples but did not affect its thermal properties. The results indicate that the enhanced optical and thermal performance of the proposed co-doped glass fits the quality standards for materials used in photonic devices. Full article
(This article belongs to the Special Issue Photonics, Optics and Laser Technology)
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10 pages, 2306 KiB  
Communication
Broadband Absorption in Patterned Metal/Weakly-Absorbing-Spacer/Metal with Graded Photonic Super-Crystal
by Steve Kamau, Safaa Hassan, Khadijah Alnasser, Hualiang Zhang, Jingbiao Cui and Yuankun Lin
Photonics 2021, 8(4), 114; https://doi.org/10.3390/photonics8040114 - 8 Apr 2021
Cited by 2 | Viewed by 2170
Abstract
It is challenging to realize the complete broadband absorption of near-infrared in thin optical devices. In this paper, we studied high light absorption in two devices: a stack of Au-pattern/insulator/Au-film and a stack of Au-pattern/weakly-absorbing-material/Au-film where the Au-pattern was structured in graded photonic [...] Read more.
It is challenging to realize the complete broadband absorption of near-infrared in thin optical devices. In this paper, we studied high light absorption in two devices: a stack of Au-pattern/insulator/Au-film and a stack of Au-pattern/weakly-absorbing-material/Au-film where the Au-pattern was structured in graded photonic super-crystal. We observed multiple-band absorption, including one near 1500 nm, in a stack of Au-pattern/spacer/Au-film. The multiple-band absorption is due to the gap surface plasmon polariton when the spacer thickness is less than 30 nm. Broadband absorption appears in the near-infrared when the insulator spacer is replaced by a weakly absorbing material. E-field intensity was simulated and confirmed the formation of gap surface plasmon polaritons and their coupling with Fabry–Pérot resonance. Full article
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15 pages, 4483 KiB  
Article
Design and Dispersion Control of Microstructured Multicore Tellurite Glass Fibers with In-Phase and Out-of-Phase Supermodes
by Elena A. Anashkina and Alexey V. Andrianov
Photonics 2021, 8(4), 113; https://doi.org/10.3390/photonics8040113 - 8 Apr 2021
Cited by 12 | Viewed by 2274
Abstract
High nonlinearity and transparency in the 1–5 μm spectral range make tellurite glass fibers highly interesting for the development of nonlinear optical devices. For nonlinear optical fibers, group velocity dispersion that can be controlled by microstructuring is also of great importance. In this [...] Read more.
High nonlinearity and transparency in the 1–5 μm spectral range make tellurite glass fibers highly interesting for the development of nonlinear optical devices. For nonlinear optical fibers, group velocity dispersion that can be controlled by microstructuring is also of great importance. In this work, we present a comprehensive numerical analysis of dispersion and nonlinear properties of microstructured two-, four-, six-, and eight-core tellurite glass fibers for in-phase and out-of-phase supermodes and compare them with the results for one-core fibers in the near- and mid-infrared ranges. Out-of-phase supermodes in tellurite multicore fibers are studied for the first time, to the best of our knowledge. The dispersion curves for in-phase and out-of-phase supermodes are shifted from the dispersion curve for one-core fiber in opposite directions; the effect is stronger for large coupling between the fields in individual cores. The zero dispersion wavelengths of in-phase and out-of-phase supermodes shift to opposite sides with respect to the zero-dispersion wavelength of a one-core fiber. For out-of-phase supermodes, the dispersion can be anomalous even at 1.55 μm, corresponding to the operating wavelength of Er-doped fiber lasers. Full article
(This article belongs to the Special Issue Novel Materials and Technologies for Fiber Lasers)
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13 pages, 3697 KiB  
Article
Microshape Measurement Method Using Speckle Interferometry Based on Phase Analysis
by Yasuhiko Arai
Photonics 2021, 8(4), 112; https://doi.org/10.3390/photonics8040112 - 8 Apr 2021
Cited by 7 | Viewed by 3556
Abstract
A method for the measurement of the shape of a fine structure beyond the diffraction limit based on speckle interferometry has been reported. In this paper, the mechanism for measuring the shape of the fine structure in speckle interferometry using scattered light as [...] Read more.
A method for the measurement of the shape of a fine structure beyond the diffraction limit based on speckle interferometry has been reported. In this paper, the mechanism for measuring the shape of the fine structure in speckle interferometry using scattered light as the illumination light is discussed. Furthermore, by analyzing the phase distribution of the scattered light from the surface of the measured object, this method can be used to measure the shapes of periodic structures and single silica microspheres beyond the diffraction limit. Full article
(This article belongs to the Special Issue Advances in Optical Metrology)
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19 pages, 5572 KiB  
Article
Compressional Optical Coherence Elastography of the Cornea
by Manmohan Singh, Achuth Nair, Salavat R. Aglyamov and Kirill V. Larin
Photonics 2021, 8(4), 111; https://doi.org/10.3390/photonics8040111 - 7 Apr 2021
Cited by 27 | Viewed by 4369
Abstract
Assessing the biomechanical properties of the cornea is crucial for detecting the onset and progression of eye diseases. In this work, we demonstrate the application of compression-based optical coherence elastography (OCE) to measure the biomechanical properties of the cornea under various conditions, including [...] Read more.
Assessing the biomechanical properties of the cornea is crucial for detecting the onset and progression of eye diseases. In this work, we demonstrate the application of compression-based optical coherence elastography (OCE) to measure the biomechanical properties of the cornea under various conditions, including validation in an in situ rabbit model and a demonstration of feasibility for in vivo measurements. Our results show a stark increase in the stiffness of the corneas as IOP was increased. Moreover, UV-A/riboflavin corneal collagen crosslinking (CXL) also dramatically increased the stiffness of the corneas. The results were consistent across 4 different scenarios (whole CXL in situ, partial CXL in situ, whole CXL in vivo, and partial CXL in vivo), emphasizing the reliability of compression OCE to measure corneal biomechanical properties and its potential for clinical applications. Full article
(This article belongs to the Special Issue Tissue Optics)
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9 pages, 3760 KiB  
Communication
Femtosecond Laser Fabricated Apodized Fiber Bragg Gratings Based on Energy Regulation
by Qi Guo, Zhongming Zheng, Bo Wang, Xuepeng Pan, Shanren Liu, Zhennan Tian, Chao Chen and Yongsen Yu
Photonics 2021, 8(4), 110; https://doi.org/10.3390/photonics8040110 - 7 Apr 2021
Cited by 16 | Viewed by 3010
Abstract
In this paper, an energy regulation method based on the combination of a half-wave plate (HWP) and a polarization beam splitter (PBS) is proposed for the fabrication of apodized fiber gratings, which can effectively improve the side lobe suppression ratio of high-reflectivity fiber [...] Read more.
In this paper, an energy regulation method based on the combination of a half-wave plate (HWP) and a polarization beam splitter (PBS) is proposed for the fabrication of apodized fiber gratings, which can effectively improve the side lobe suppression ratio of high-reflectivity fiber Bragg gratings (FBGs) fabricated by femtosecond laser. The apodized FBGs prepared by this method has good repeatability and flexibility. By inputting different types of apodization functions through the program, the rotation speed of the stepping motor can be adjusted synchronously, and then the position of the HWP can be accurately controlled so that the laser energy can be distributed as an apodization function along the axial direction of the fiber. By using the energy apodization method, the gratings with a reflectivity of 75% and a side lobe suppression ratio of 25 and 32 dB are fabricated in the fiber with a core diameter of 9 and 4.4 μm, respectively. The temperature and strain sensitivities of the energy-apodized fiber gratings with a core diameter of 4.4 μm are 10.36 pm/°C and 0.9 pm/με, respectively. The high-reflectivity gratings fabricated by this energy apodization method are expected to be used in high-power narrow-linewidth lasers and wavelength division multiplexing (WDM) systems. Full article
(This article belongs to the Special Issue Advancements in Fiber Bragg Grating Research)
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11 pages, 9845 KiB  
Communication
Multipolar Lattice Resonances in Plasmonic Finite-Size Metasurfaces
by Artem S. Kostyukov, Ilia L. Rasskazov, Valeriy S. Gerasimov, Sergey P. Polyutov, Sergey V. Karpov and Alexander E. Ershov
Photonics 2021, 8(4), 109; https://doi.org/10.3390/photonics8040109 - 6 Apr 2021
Cited by 13 | Viewed by 3278
Abstract
Collective lattice resonances in regular arrays of plasmonic nanoparticles have attracted much attention due to a large number of applications in optics and photonics. Most of the research in this field is concentrated on the electric dipolar lattice resonances, leaving higher-order multipolar lattice [...] Read more.
Collective lattice resonances in regular arrays of plasmonic nanoparticles have attracted much attention due to a large number of applications in optics and photonics. Most of the research in this field is concentrated on the electric dipolar lattice resonances, leaving higher-order multipolar lattice resonances in plasmonic nanostructures relatively unexplored. Just a few works report exceptionally high-Q multipolar lattice resonances in plasmonic arrays, but only with infinite extent (i.e., perfectly periodic). In this work, we comprehensively study multipolar collective lattice resonances both in finite and in infinite arrays of Au and Al plasmonic nanoparticles using a rigorous theoretical treatment. It is shown that multipolar lattice resonances in the relatively large (up to 6400 nanoparticles) finite arrays exhibit broader full width at half maximum (FWHM) compared to similar resonances in the infinite arrays. We argue that our results are of particular importance for the practical implementation of multipolar lattice resonances in different photonics applications. Full article
(This article belongs to the Special Issue Plasmonic Metasurfaces)
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15 pages, 423 KiB  
Article
Performance Analysis of Hybrid Radio Frequency and Free Space Optical Communication Networks with Cooperative Spectrum Sharing
by Dong Qin, Yuhao Wang and Tianqing Zhou
Photonics 2021, 8(4), 108; https://doi.org/10.3390/photonics8040108 - 6 Apr 2021
Cited by 5 | Viewed by 3278
Abstract
This paper investigates the impact of cooperative spectrum sharing policy on the performance of hybrid radio frequency and free space optical wireless communication networks, where primary users and secondary users develop a band of the same spectrum resource. The radio frequency links obey [...] Read more.
This paper investigates the impact of cooperative spectrum sharing policy on the performance of hybrid radio frequency and free space optical wireless communication networks, where primary users and secondary users develop a band of the same spectrum resource. The radio frequency links obey Nakagami-m distribution with arbitrary fading parameter m, while the free space optical link follows gamma-gamma distributed atmospheric turbulence with nonzero pointing error. Because the secondary users access the spectrum band without payment, their behavior needs to be restricted. Specifically, the power of the secondary users is dominated by the tolerable threshold of the primary users. Considering both heterodyne and intensity modulation/direct detection strategies in optical receiver, the performance of optical relaying networks is completely different from that of traditional networks. With the help of bivariable Fox’s H function, new expressions for cumulative distribution function of equivalent signal to noise ratio at destination, probability density function, outage probability, ergodic capacity and symbol error probability are built in closed forms. Full article
(This article belongs to the Special Issue Optical Wireless Communications Systems)
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8 pages, 2515 KiB  
Communication
Meta-Deflectors Made of Dielectric Nanohole Arrays with Anti-Damage Potential
by Haichao Yu, Feng Tang, Jingjun Wu, Zao Yi, Xin Ye and Yiqun Wang
Photonics 2021, 8(4), 107; https://doi.org/10.3390/photonics8040107 - 6 Apr 2021
Cited by 3 | Viewed by 2331
Abstract
In intense-light systems, the traditional discrete optical components lead to high complexity and high cost. Metasurfaces, which have received increasing attention due to the ability to locally manipulate the amplitude, phase, and polarization of light, are promising for addressing this issue. In the [...] Read more.
In intense-light systems, the traditional discrete optical components lead to high complexity and high cost. Metasurfaces, which have received increasing attention due to the ability to locally manipulate the amplitude, phase, and polarization of light, are promising for addressing this issue. In the study, a metasurface-based reflective deflector is investigated which is composed of silicon nanohole arrays that confine the strongest electric field in the air zone. Subsequently, the in-air electric field does not interact with the silicon material directly, attenuating the optothermal effect that causes laser damage. The highest reflectance of nanoholes can be above 99% while the strongest electric fields are tuned into the air zone. One presentative deflector is designed based on these nanoholes with in-air-hole field confinement and anti-damage potential. The 1st order of the meta-deflector has the highest reflectance of 55.74%, and the reflectance sum of all the orders of the meta-deflector is 92.38%. The optothermal simulations show that the meta-deflector can theoretically handle a maximum laser density of 0.24 W/µm2. The study provides an approach to improving the anti-damage property of the reflective phase-control metasurfaces for intense-light systems, which can be exploited in many applications, such as laser scalpels, laser cutting devices, etc. Full article
(This article belongs to the Special Issue Advanced Metamaterials and Metadevices)
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29 pages, 7503 KiB  
Review
Advances on Mode-Coupling Theories, Fabrication Techniques, and Applications of the Helical Long-Period Fiber Gratings: A Review
by Hua Zhao and Hongpu Li
Photonics 2021, 8(4), 106; https://doi.org/10.3390/photonics8040106 - 6 Apr 2021
Cited by 30 | Viewed by 5082
Abstract
In this paper, we have briefly review the developing history and recent advances made with regard to helical long-period fiber gratings (HLPGs) in three aspects, i.e., the mode-coupling theories, the fabrication techniques, and the applications. It is shown that, due to the intrinsic [...] Read more.
In this paper, we have briefly review the developing history and recent advances made with regard to helical long-period fiber gratings (HLPGs) in three aspects, i.e., the mode-coupling theories, the fabrication techniques, and the applications. It is shown that, due to the intrinsic helicity characteristics, which are especially suitable to control the loss, polarization, and orbit-angular-momentum (OAM) states of the light in optical fiber, HLPGs have recently attracted great research interest and have found various applications, such as the mode-converters, the torsion sensors, the band-rejection filters, wave plates, linear- and circular-light polarizers, and OAM mode generators, etc. It is believed that HLPGs and the HLPGs-based devices would find further applications to not only the fields of optical sensors and optical communication, but also other fields such as ultrahigh precision measurement, quantum optics, and biochemistry, etc. Full article
(This article belongs to the Special Issue Advancements in Fiber Bragg Grating Research)
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9 pages, 4129 KiB  
Communication
Improving Low-Dispersion Bandwidth of the Silicon Photonic Crystal Waveguides for Ultrafast Integrated Photonics
by Jinghan Pan, Meicheng Fu, Wenjun Yi, Xiaochun Wang, Ju Liu, Mengjun Zhu, Junli Qi, Shaojie Yin, Guocheng Huang, Shuyue Zhu, Xin Chen, Wusheng Tang, Jiali Liao, Heng Yang and Xiujian Li
Photonics 2021, 8(4), 105; https://doi.org/10.3390/photonics8040105 - 6 Apr 2021
Cited by 4 | Viewed by 2451
Abstract
We design a novel slow-light silicon photonic crystal waveguide which can operate over an extremely wide flat band for ultrafast integrated nonlinear photonics. By conveniently adjusting the radii and positions of the second air-holes rows, a flat slow-light low-dispersion band of 50 nm [...] Read more.
We design a novel slow-light silicon photonic crystal waveguide which can operate over an extremely wide flat band for ultrafast integrated nonlinear photonics. By conveniently adjusting the radii and positions of the second air-holes rows, a flat slow-light low-dispersion band of 50 nm is achieved numerically. Such a slow-light photonic crystal waveguide with large flat low-dispersion wideband will pave the way for governing the femtosecond pulses in integrated nonlinear photonic platforms based on CMOS technology. Full article
(This article belongs to the Special Issue Silicon Photonics: Functional Enhancement by New Structures and Materials)
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