Frontiers of Optoelectronics

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A review of multiple optical vortices generation: methods and applications
Long ZHU, Jian WANG
Front. Optoelectron.    2019, 12 (1): 52-68.   https://doi.org/10.1007/s12200-019-0910-9
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Optical vortices carrying orbital angular momentum (OAM) have attracted increasing interest in recent years. Optical vortices have seen a variety of emerging applications in optical manipulation, optical trapping, optical tweezers, optical vortex knots, imaging, microscopy, sensing, metrology, quantum information processing, and optical communications. In various optical vortices enabled applications, the generation of multiple optical vortices is of great importance. In this review article, we focus on the methods of multiple optical vortices generation and its applications. We review the methods for generating multiple optical vortices in three cases, i.e., 1-to-N collinear OAM modes, 1-to-N OAM mode array and N-to-N collinear OAM modes. Diverse applications of multiple OAM modes in optical communications and non-communication areas are presented. Future trends, perspectives and opportunities are also discussed.

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The smallest nanowire spectrometers
Jianji DONG
Front. Optoelectron.    2019, 12 (4): 341-341.   https://doi.org/10.1007/s12200-019-0983-5
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Antimony doped Cs2SnCl6 with bright and stable emission
Jinghui LI, Zhifang TAN, Manchen HU, Chao CHEN, Jiajun LUO, Shunran LI, Liang GAO, Zewen XIAO, Guangda NIU, Jiang TANG
Front. Optoelectron.    2019, 12 (4): 352-364.   https://doi.org/10.1007/s12200-019-0907-4
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Lead halide perovskites, with high photoluminescence efficiency and narrow-band emission, are promising materials for display and lighting. However, the lead toxicity and environmental sensitivity hinder their potential applications. Herein, a new antimony-doped lead-free inorganic perovskites variant Cs2SnCl6:xSb is designed and synthesized. The perovskite variant Cs2SnCl6:xSb exhibits a broadband orange-red emission, with a photoluminescence quantum yield (PLQY) of 37%. The photoluminescence of Cs2SnCl6:xSb is caused by the ionoluminescence of Sb3+ within Cs2SnCl6 matrix, which is verified by temperature dependent photoluminescence (PL) and PL decay measurements. In addition, the all inorganic structure renders Cs2SnCl6:xSb with excellent thermal and water stability. Finally, a white light-emitting diode (white-LED) is fabricated by assembling Cs2SnCl6:0.59%Sb, Cs2SnCl6:2.75%Bi and Ba2Sr2SiO4:Eu2+ onto the commercial UV LED chips, and the color rendering index (CRI) reaches 81.

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Ether chain functionalized fullerene derivatives as cathode interface materials for efficient organic solar cells
Jikang LIU, Junli LI, Guoli TU
Front. Optoelectron.    2018, 11 (4): 348-359.   https://doi.org/10.1007/s12200-018-0842-9
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The electron transport layer (ETL) plays a crucial role on the electron injection and extraction, resulting in balanced charge transporting and reducing the interfacial energy barrier. The interface compatibility and electrical contact via employing appropriate buffer layer at the surface of hydrophobic organic active layer and hydrophilic inorganic electrode are also essential for charge collections. Herein, an ether chain functionalized fullerene derivatives [6,6]-phenyl-C61-butyricacid-(3,5-bis(2-(2-ethoxyethoxy)-ethoxy)-phenyl)-methyl ester (C60-2EPM) was developed to modify zinc oxide (ZnO) in inverted structure organic solar cells (OSCs). The composited ZnO/C60-2EPM interface layer can help to overcome the low interface compatibility between ZnO and organic active layer. By introducing the C60-2EPM layer, the composited fullerene derivatives tune energy alignment and accelerated the electronic transfer, leading to increased photocurrent and power conversion efficiency (PCE) in the inverted OSCs. The PCE based on PTB7-Th:PC71BM was enhance from 8.11% on bare ZnO to 8.38% and 8.65% with increasing concentrations of 2.0 and 4.0 mg/mL, respectively. The fullerene derivatives C60-2EPM was also used as a third compound in P3HT:PC61BM blend to form ternary system, the devices with addition of C60-2EPM exhibited better values than the control device.

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Screen printing process control for coating high throughput titanium dioxide films toward printable mesoscopic perovskite solar cells
Zhining WAN, Mi XU, Zhengyang FU, Da LI, Anyi MEI, Yue HU, Yaoguang RONG, Hongwei HAN
Front. Optoelectron.    2019, 12 (4): 344-351.   https://doi.org/10.1007/s12200-019-0904-7
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Screen printing technique has been widely applied for the manufacturing of both traditional silicon solar cells and emerging photovoltaics such as dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs). Particularly, we have developed a printable mesoscopic PSC based on a triple layer scaffold of TiO2/ZrO2/carbon. The deposition of the scaffold is entirely based on screen printing process, which provides a promising prospect for low-cost photovoltaics. However, the optimal thickness of the TiO2 layer for fabricating efficient printable PSCs is much smaller than the typical thickness of screen printed films. Here, we tune the concentration of the pastes and the printing parameters for coating TiO2 films, and successfully print TiO2 films with the thickness of 500−550 nm. The correlation between the thickness of the films and printing parameters such as the solid content and viscosity of the pastes, the printing speed and pressure, and the temperature has been investigated. Besides, the edge effect that the edge of the TiO2 films possesses a much larger thickness and printing positional accuracy have been studied. This work will significantly benefit the further development of printable mesoscopic PSCs.

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Heuristic polling sequence to enhance sleep count of EPON
Bhargav Ram RAYAPATI, Nakkeeran RANGASWAMY
Front. Optoelectron.    2019, 12 (4): 422-432.   https://doi.org/10.1007/s12200-019-0906-5
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Next-generation passive optical networks (PONs) demand power conservation to create a green environment. A reduction in power consumption of the traditional Ethernet passive optical network (EPON) can be achieved by increasing the sleep count in optical network units (ONUs). In this paper, this is accomplished by introducing a first-in-last-out (FILO) polling sequence in the place of a fixed polling sequence to increase the number of ONUs entering sleep mode (sleep count). In a fixed polling sequence, the optical line terminal (OLT) allocates idle time to the ONUs based on the overall load of the ONUs. This leads to a situation that whenever the idle time does not meet the wakeup time threshold of sleep mode, the ONUs are put into doze/active mode, which consumes more power. In the FILO polling sequence, the first polled ONU in the current cycle is made to be polled last in the following cycle. Polling continues in this way, and by this rearrangement, the idle time of delayed poll ONUs increases; hence, it helps to reduce the power consumption. Additionally, a modified load adaptive sequence arrangement (MLASA) method is suggested, where the ONUs are categorized into doze ONUs and sleep ONUs. A numerical simulation of the FILO polling sequence with a vertical cavity surface emitting laser (VCSEL) ONU shows a maximum reduction in power consumption of 15.5 W and a 20% improvement in energy savings compared with the traditional fixed polling sequence. The MLASA method results in better power consumption with minimum delay than that of the proposed FILO and existing LASA methods.

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Transmission characteristics of linearly polarized light in reflection-type one-dimensional magnetophotonic crystals
Chunxiang ZENG, Zeqing WANG, Yingmao XIE
Front. Optoelectron.    2019, 12 (4): 365-371.   https://doi.org/10.1007/s12200-019-0870-0
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The propagation properties of linearly polarized light in reflection-type one-dimensional magnetophotonic crystals are studied by using the 4×4 transmission matrix method. The structure models of reflection-type one-dimensional magnetophotonic crystals are designed, the magnetic field direction control characteristics of reflection spectrum and Kerr rotation angle are discussed, and the effect of applied magnetic field direction and strength on reflection spectrum and Kerr rotation angle are analyzed. The results show that the non-diagonal elements in the dielectric constant of magneto optical materials change when the angle ϕ between applied magnetic field and optical path changes, the reflectivity and Kerr rotation angle decrease when the angle ϕ increases; when the applied magnetic field strength changes, the reflectivity and Kerr rotation angle increase when the applied magnetic field strength increases; by adjusting the angle ϕ and strength of the applied magnetic field, the rotation angle of Kerr can be adjusted to 45°, and a more flat reflection spectrum can be obtained by designing the appropriate structure.

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Design of compensation pixel circuit with In-Zn-O thin film transistor for active-matrix organic light-emitting diode 3D display
Xiao HU,Xingheng XIA,Lei ZHOU,Lirong ZHANG,Weijing WU
Front. Optoelectron.    2017, 10 (1): 45-50.   https://doi.org/10.1007/s12200-016-0562-y
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This paper presents a new compensation pixel circuit suitable for active-matrix organic light-emitting diode (AMOLED) stereoscopic three dimensional (3D) displays with shutter glasses. The simultaneous emission method was used to solve the crosstalk problem, in which the periods of initialization and threshold voltage detection occur for each pixel of whole panel simultaneously. Furthermore, there was no need of the periods of initialization and threshold voltage detection from the second frame beginning by employing threshold voltage one-time detection method. The non-uniformity of the proposed pixel circuit was considerably low with an average value of 8.6% measured from 20 discrete proposed pixel circuits integrated by In-Zn-O thin film transistors (IZO TFTs). It was shown that the OLED current almost remains constant for the number of frames up to 70 even the threshold voltage detection period only exists in the first frame.

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Terahertz wave generation from ring-Airy beam induced plasmas and remote detection by terahertz-radiation-enhanced-emission-of-fluorescence: a review
Kang LIU, Pingjie HUANG, Xi-Cheng ZHANG
Front. Optoelectron.    2019, 12 (2): 117-147.   https://doi.org/10.1007/s12200-018-0860-7
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With the increasing demands for remote spectroscopy in many fields ranging from homeland security to environmental monitoring, terahertz (THz) spectroscopy has drawn a significant amount of attention because of its capability to acquire chemical spectral signatures non-invasively. However, advanced THz remote sensing techniques are obstructed by quite a few factors, such as THz waves being strongly absorbed by water vapor in the ambient air, difficulty to generate intense broadband coherent THz source remotely, and hard to transmit THz waveform information remotely without losing the signal to noise ratio, etc. In this review, after introducing different THz air-photonics techniques to overcome the difficulties of THz remote sensing, we focus mainly on theoretical and experimental methods to improve THz generation and detection performance for the purpose of remote sensing through tailoring the generation and detection media, air-plasma.

For the THz generation part, auto-focusing ring-Airy beam was introduced to enhance the THz wave generation yield from two-color laser induced air plasma. By artificially modulated exotic wave packets, it is exhibited that abruptly auto-focusing beam induced air-plasma can give an up to 5.3-time-enhanced THz wave pulse energy compared to normal Gaussian beam induced plasma under the same conditions. At the same time, a red shift on the THz emission spectrum is also observed. A simulation using an interference model to qualitatively describe these behaviors has be developed.

For the THz detection part, the results of THz remote sensing at 30 m using THz-radiation-enhanced-emission-of-fluorescence (THz-REEF) technique are demonstrated, which greatly improved from the 10 m demonstration last reported. The THz-REEF technique in the counter-propagation geometry was explored, which is proved to be more practical for stand-off detections than co-propagation geometry. We found that in the counter-propagating geometry the maximum amplitude of the REEF signal is comparable to that in the co-propagating case, whereas the time resolved REEF trace significantly changes. By performing the study with different plasmas, we observed that in the counter-propagating geometry the shape of the REEF trace depends strongly on the plasma length and electron density. A new theoretical model suggesting that the densest volume of the plasma does not contribute to the fluorescence enhancement is proposed to reproduce the experimental measurements.

Our results further the understanding of the THz-plasma interaction and highlight the potential of THz-REEF technique in the plasma detection applications.

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Spectrally efficient single carrier 400G optical signal transmission
Jianjun YU
Front. Optoelectron.    2019, 12 (1): 15-23.   https://doi.org/10.1007/s12200-018-0833-x
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In this paper, the recent progress on spectrally efficient single carrier (SC) 400G optical signal transmission was summarized. By using quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation (16QAM) and 64QAM, we can realize transmission distance over 10000, 6000 and 3000 km, respectively, with large area fiber and all-Raman amplification. To improve the system performance and generate high-order QAM, advanced digital signal processing algorithms such as probabilistic shaping and look-up table pre-distortion are employed to improve the transmission performance.

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On-chip programmable pulse processor employing cascaded MZI-MRR structure
Yuhe ZHAO, Xu WANG, Dingshan GAO, Jianji DONG, Xinliang ZHANG
Front. Optoelectron.    2019, 12 (2): 148-156.   https://doi.org/10.1007/s12200-018-0846-5
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Optical pulse processor meets the urgent demand for high-speed, ultra wideband devices, which can avoid electrical confinements in various fields, e.g., all-optical communication, optical computing technology, coherent control and microwave fields. To date, great efforts have been made particularly in on-chip programmable pulse processing. Here, we experimentally demonstrate a programmable pulse processor employing 16-cascaded Mach-Zehnder interferometer coupled microring resonator (MZI-MRR) structure based on silicon-on-insulator wafer. With micro-heaters loaded to the device, both amplitude and frequency tunings can be realized in each MZI-MRR unit. Thanks to its reconfigurability and integration ability, the pulse processor has exhibited versatile functions. First, it can serve as a fractional differentiator whose tuning range is 0.51−2.23 with deviation no more than 7%. Second, the device can be tuned into a programmable optical filter whose bandwidth varies from 0.15 to 0.97 nm. The optical filter is also shape tunable. Especially, 15-channel wavelength selective switches are generated.

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Ultra-flat broadband microwave frequency comb generation based on optical frequency comb with a multiple-quantum-well electro-absorption modulator in critical state
Cong SHEN, Peili LI, Xinyuan ZHU, Yuanfang ZHANG, Yaqiao HAN
Front. Optoelectron.    2019, 12 (4): 382-391.   https://doi.org/10.1007/s12200-019-0915-4
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In this paper, we proposed a novel ultra-flat broadband microwave frequency comb (MFC) generation based on optical frequency comb (OFC) with a multiple-quantum-well electro-absorption modulator (MQW-EAM) in critical state. The scheme is simple and easy to adjust. The performance of the MFC generation scheme is investigated using software Optisystem. The results show that the comb spacing of MFC can be adjusted from 5 to 20 GHz by changing RF signal’s frequency and the MFC is almost independent on the linewidth of the tunable laser diode. The performance of the MFC can be improved by reasonably increasing the voltage of the RF, the small-signal gain of the Erbium-doped fiber amplifier (EDFA) and the responsivity of the photodetector. The MFC generated by this scheme has 300 GHz effective bandwidth with 15 comb lines, whose power variation is 0.02 dB, when the components’ parameters in the Optisystem are set as follows: the power of tunable laser diode (TLD) is 0 dBm, the wavelength is 1552.52 nm, and linewidth is 1 MHz; RF signal’s frequency is 20 GHz and the voltage is 10 V; the reverse bias voltage of MQW-EAM is 6.92 V; the small-signal gain of the EDFA is 40 dB; the responsivity of the photodetector (PD) is 1 A/W.

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Experimental and simulation assessments of underwater light propagation
Fatah ALMABOUADA, Manuel Adler ABREU, João M. P. COELHO, Kamal Eddine AIADI
Front. Optoelectron.    2019, 12 (4): 405-412.   https://doi.org/10.1007/s12200-019-0865-x
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This paper investigates the light propagation through several types of water by experimental and simulation. The Zemax-ray tracing software allowed to simulate the propagation of light in water and to observe the receiver response by reproducing the real conditions of propagation. The underwater environment has been reproduced by a 1.2 m long water tube and 20 cm in diameter with a glass window fitted on one side. The use of tap water with different amounts of sand leads toward three types of water with different attenuation coefficients (0.133, 0.343, 0.580 m1). The light transmission in the three types of water was experimentally evaluated using a doubled Nd:YAG laser with energy of 4.3 mJ and a pulse width of 20 ns. Comparisons were done between simulation and experimental results.

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Improved offline multi-objective routing and wavelength assignment in optical networks
Harpreet KAUR, Munish RATTAN
Front. Optoelectron.    2019, 12 (4): 433-444.   https://doi.org/10.1007/s12200-019-0850-4
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Optical networks act as a backbone for coming generation high speed applications. These applications demand a very high bandwidth which can be exploited with the use of wavelength division multiplexing (WDM) technology. The issue of setting light paths for the traffic demands is routing and wavelength assignment (RWA) problem. Based on the type of traffic patterns, it can be categorized as offline or online RWA. In this paper, an effective solution to offline (static) routing and wavelength assignment is presented considering multiple objectives simultaneously. Initially, the flower pollination (FP) technique is utilized. Then the problem is extended with the parallel hybrid technique with flower pollination and intelligent water drop algorithm (FPIWDA). Further, FPIWD is hybrid in parallel with simulated annealing (SA) algorithm to propose a parallel hybrid algorithm FPIWDSA. The results obtained through extensive simulation show the superiority of FPIWD as compared to FP. Moreover, the results in terms of blocking probability with respect to wavelengths and load of FPIWDSA are more propitious than FP and FPIWD.

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Near-infrared carbon-implanted waveguides in Tb3+-doped aluminum borosilicate glasses
Yue WANG, Jiaxin ZHAO, Qifeng ZHU, Jianping SHEN, Zhongyue WANG, Haitao GUO, Chunxiao LIU
Front. Optoelectron.    2019, 12 (4): 392-396.   https://doi.org/10.1007/s12200-019-0869-6
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Ion implantation has played a unique role in the fabrication of optical waveguide devices. Tb3+-doped aluminum borosilicate (TDAB) glass has been considered as an important magneto-optical material. In this work, near-infrared waveguides have been manufactured by the (5.5+ 6.0) MeV C3+ ion implantation with doses of (4.0+ 8.0) × 1013 ions·cm2 in the TDAB glass. The modes propagated in the TDAB glass waveguide were recorded by a prism-coupling system. The finite-difference beam propagation method (FD-BPM) was carried out to simulate the guiding characteristics of the TDAB glass waveguide. The TDAB glass waveguide allows the light propagation with a single-mode at 1.539 mm and can serve as a potential candidate for future waveguide isolators.

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Chemical sensing through photonic crystal fiber: sulfuric acid detection
Etu PODDER, Md. Bellal HOSSAIN, Rayhan Habib JIBON, Abdullah Al-Mamun BULBUL, Himadri Shekhar MONDAL
Front. Optoelectron.    2019, 12 (4): 372-381.   https://doi.org/10.1007/s12200-019-0903-8
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A photonic crystal fiber (PCF) for sensing of sulfuric acid is designed and analyzed using Comsol Multiphysics. To analyze the sensor performance, 0%, 10%, 20%, 30%, 40% H2SO4 solution is placed into the fiber separately and then relative sensitivity, confinement loss, birefringence, effective area etc. are investigated for each solution over wavelength ranging from 0.8 to 1.8 mm. The sensor structure affords moderately high relative sensitivity and around 63.4% sensitivity is achieved for the highest concentration of H2SO4 at the wavelength 1.5 mm in x polarization direction. This PCF model also shows zero confinement loss for all solutions of H2SO4 over wavelength ranging from 1 to 1.35 mm and later on approximately 1.422 × 1017 dB/km confinement loss is found for the highest concentration of H2SO4 at 1.5 mm wavelength. Besides, higher birefringence is attained when the concentration of sulfuric acid is lower and it is achieved 7.5 × 104 at 1.5 mm wavelength. Moreover, higher sensing area is achieved at high concentration of sulfuric acid.

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Fiber-based optical trapping and manipulation
Hongbao XIN, Baojun LI
Front. Optoelectron.    2019, 12 (1): 97-110.   https://doi.org/10.1007/s12200-017-0755-z
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An optical fiber serves as a versatile tool for optical trapping and manipulation owing to its many advantages over conventional optical tweezers, including ease of fabrication, compact configurations, flexible manipulation capabilities, ease of integration, and wide applicability. Here, we review recent progress in fiber-based optical trapping and manipulation, which includes mainly photothermal-based and optical-force-based trapping and manipulation. We focus on five topics in our review of progress in this area: massive photothermal trapping and manipulation, evanescent-field-based trapping and manipulation, dual-fiber tweezers for single-nanoparticle trapping and manipulation, single-fiber tweezers for single-particle trapping and manipulation, and single-fiber tweezers for multiple-particle/cell trapping and assembly.

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Robot visual guide with Fourier-Mellin based visual tracking
Chao PENG, Danhua CAO, Yubin WU, Qun YANG
Front. Optoelectron.    2019, 12 (4): 413-421.   https://doi.org/10.1007/s12200-019-0862-0
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Robot vision guide is an important research area in industrial automation, and image-based target pose estimation is one of the most challenging problems. We focus on target pose estimation and present a solution based on the binocular stereo vision in this paper. To improve the robustness and speed of pose estimation, we propose a novel visual tracking algorithm based on Fourier-Mellin transform to extract the target region. We evaluate the proposed tracking algorithm on online tracking benchmark-50 (OTB-50) and the results show that it outperforms other lightweight trackers, especially when the target is rotated or scaled. The final experiment proves that the improved pose estimation approach can achieve a position accuracy of 1.84 mm and a speed of 7 FPS (frames per second). Besides, this approach is robust to the variances of illumination and can work well in the range of 250-700 lux.

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Review on partially coherent vortex beams
Jun ZENG, Rong LIN, Xianlong LIU, Chengliang ZHAO, Yangjian CAI
Front. Optoelectron.    2019, 12 (3): 229-248.   https://doi.org/10.1007/s12200-019-0901-x
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Ever since vortex beams were proposed, they are known for owning phase singularity and carrying orbital angular momentum (OAM). In the past decades, coherent optics developed rapidly. Vortex beams have been extended from fully coherent light to partially coherent light, from scalar light to vector light, from integral topological charge (TC) to fractional TC. Partially coherent vortex beams have attracted tremendous interest due to their hidden correlation singularity and unique propagation properties (e.g., beam shaping, beam rotation and self-reconstruction). Based on the sufficient condition for devising a genuine correlation function of partially coherent beam, partially coherent vortex beams with nonconventional correlation functions (i.e., non-Gaussian correlated Schell-model functions) were introduced recently. This timely review summarizes basic concepts, theoretical models, generation and propagation of partially coherent vortex beams.

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Airy-like field under high numerical aperture optical system
Yong LIU, Zhifeng ZHANG, Cuifang KUANG
Front. Optoelectron.    2019, 12 (4): 397-404.   https://doi.org/10.1007/s12200-019-0866-9
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The tightly focused field of an incident light beam through cubic phase modulation has been investigated by vectorial diffraction theory. For different modulation index of cubic phase and polarization states of the incident light, the focused fields have been presented. The results show that the Airy-like field can be produced by cubic phase modulation under high numerical aperture (NA) optical system. Intensity pattern and length of the main lobe are depended on modulation index for the spatial uniform polarization, and the Airy-like field is affected by polarization state for the spatial nonuniform polarization. It is helpful to structure new optical fields in optical manipulation, optical imaging, and surface plasma controlling.

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Detection of photonic orbital angular momentum with micro- and nano-optical structures
Chenhao WAN, Guanghao RUI, Jian CHEN, Qiwen ZHAN
Front. Optoelectron.    2019, 12 (1): 88-96.   https://doi.org/10.1007/s12200-017-0730-8
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Light with an optical orbital angular momentum (OAM) has attracted an increasing amount of interest and has found its way into many disciplines ranging from optical trapping, edge-enhanced microscopy, high-speed optical communication, and secure quantum teleportation to spin-orbital coupling. In a variety of OAM-involved applications, it is crucial to discern different OAM states with high fidelity. In the current paper, we review the latest research progress on OAM detection with micro- and nano-optical structures that are based on plasmonics, photonic integrated circuits (PICs), and liquid crystal devices. These innovative OAM sorters are promising to ultimately achieve the miniaturization and integration of high-fidelity OAM detectors and inspire numerous applications that harness the intriguing properties of the twisted light.

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Topological photonic crystals: a review
Hongfei WANG, Samit Kumar GUPTA, Biye XIE, Minghui LU
Front. Optoelectron.    2020, 13 (1): 50-72.   https://doi.org/10.1007/s12200-019-0949-7
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The field of topological photonic crystals has attracted growing interest since the inception of optical analog of quantum Hall effect proposed in 2008. Photonic band structures embraced topological phases of matter, have spawned a novel platform for studying topological phase transitions and designing topological optical devices. Here, we present a brief review of topological photonic crystals based on different material platforms, including all-dielectric systems, metallic materials, optical resonators, coupled waveguide systems, and other platforms. Furthermore, this review summarizes recent progress on topological photonic crystals, such as higher-order topological photonic crystals, non-Hermitian photonic crystals, and nonlinear photonic crystals. These studies indicate that topological photonic crystals as versatile platforms have enormous potential applications in maneuvering the flow of light.

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Reflectometric and interferometric fiber optic sensor’s principles and applications
Muhammad Noaman ZAHID, Jianliang JIANG, Saad RIZVI
Front. Optoelectron.    2019, 12 (2): 215-226.   https://doi.org/10.1007/s12200-019-0824-6
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Fiber optic sensors have been widely used and studied in recent times. This paper presents operating principles and applications of fiber optic sensors namely reflectometric and interferometric fiber optic sensors. Majority of optical fiber sensors fall under these two broad categories. Both interferometric and reflectometric fiber optic sensors are becoming popular for their ease of use, flexibility, long distance sensing, and potentially noise free detection. Also, these sensors can easily be used in various applications such as structural health monitoring, perimeter intrusion detection, temperature monitoring, and other numerous applications. This paper broadly classifies fiber optic sensors into two subtypes. The paper further highlights different sensors based on their sensing resolution, range, spatial advantages, and applications.

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Generation and detection of pulsed terahertz waves in gas: from elongated plasmas to microplasmas
Fabrizio BUCCHERI, Pingjie HUANG, Xi-Cheng ZHANG
Front. Optoelectron.    2018, 11 (3): 209-244.   https://doi.org/10.1007/s12200-018-0819-8
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The past two decades have seen an exponential growth of interest in one of the least explored region of the electromagnetic spectrum, the terahertz (THz) frequency band, ranging from to 0.1 to 10 THz. Once only the realm of astrophysicists studying the background radiation of the universe, THz waves have become little by little relevant in the most diverse fields, such as medical imaging, industrial inspection, remote sensing, fundamental science, and so on. Remarkably, THz wave radiation can be generated and detected by using ambient air as the source and the sensor. This is accomplished by creating plasma under the illumination of intense femtosecond laser fields. The integration of such a plasma source and sensor in THz time-domain techniques allows spectral measurements covering the whole THz gap (0.1 to 10 THz), further increasing the impact of this scientific tool in the study of the four states of matter.

In this review, the authors introduce a new paradigm for implementing THz plasma techniques. Specifically, we replaced the use of elongated plasmas, ranging from few mm to several cm, with sub-mm plasmas, which will be referred to as microplasmas, obtained by focusing ultrafast laser pulses with high numerical aperture optics (NA from 0.1 to 0.9).

The experimental study of the THz emission and detection from laser-induced plasmas of submillimeter size are presented. Regarding the microplasma source, one of the interesting phenomena is that the main direction of THz wave emission is almost orthogonal to the laser propagation direction, unlike that of elongated plasmas. Perhaps the most important achievement is the demonstration that laser pulse energies lower than 1 mJ are sufficient to generate measurable THz pulses from ambient air, thus reducing the required laser energy requirement of two orders of magnitude compared to the state of art. This significant decrease in the required laser energy will make plasma-based THz techniques more accessible to the scientific community, as well as opening new potential industrial applications.

Finally, experimental observations of THz radiation detection with microplasmas are also presented. As fully coherent detection was not achieved in this work, the results presented herein are to be considered a first step to understand the peculiarities involved in using the microplasma as a THz sensor.

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Modulation of orbital angular momentum on the propagation dynamics of light fields
Peng LI, Sheng LIU, Yi ZHANG, Lei HAN, Dongjing WU, Huachao CHENG, Shuxia QI, Xuyue GUO, Jianlin ZHAO
Front. Optoelectron.    2019, 12 (1): 69-87.   https://doi.org/10.1007/s12200-017-0743-3
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Optical vortices carrying orbital angular momentum (OAM) have attracted extensive attention in recent decades because of their interesting applications in optical trapping, optical machining, optical communication, quantum information, and optical microscopy. Intriguing effects induced by OAMs, such as angular momentum conversion, spin Hall effect of light (SHEL), and spin–orbital interaction, have also gained increasing interest. In this article, we provide an overview of the modulations of OAMs on the propagation dynamics of scalar and vector fields in free space. First, we introduce the evolution of canonical and noncanonical optical vortices and analyze the modulations by means of local spatial frequency. Second, we review the Pancharatnam–Berry (PB) phases arising from spin–orbital interaction and reveal the control of beam evolution referring to novel behavior such as spin-dependent splitting and polarization singularity conversion. Finally, we discuss the propagation and focusing properties of azimuthally broken vector vortex beams.

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Single polarization photonic crystal fiber filter based on surface plasmon resonance
Md. Nazmul HOSSEN, Md. FERDOUS, Kawsar AHMED, Md. Abdul KHALEK, Sujan CHAKMA, Bikash Kumar PAUL
Front. Optoelectron.    2019, 12 (2): 157-164.   https://doi.org/10.1007/s12200-018-0843-8
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In this paper, we propose a photonic crystal fiber (PCF) polarization filter based on surface plasmon resonance (SPR) characteristics. Gold nanowire is used as the active plasmonic material. Light into silica core becomes coupled to gold nanowire stimulating SPR. It splits light into two orthogonal (x-polarization and y-polarization) polarization in the second order of surface plasmon polarization. Numerical investigations of the proposed PCF filter is finite element method (FEM). By tuning the diameter of gold nanowire and shifting their position, the performance of the proposed PCF filter is inspected rigorously. Filtering of any polarization can be obtained by properly placing the metal wires. The maximum confinement loss of x-polarization is 692.25 dB/cm and y-polarization is 1.13 dB/cm offers at resonance position 1.42 µm. Such a confinement loss difference between two orthogonal polarizations makes PCF a talented candidate to filter devices. Consequently, the recommended PCF structure is useful for polarization device.

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Design and analysis of high birefringence and nonlinearity with small confinement loss photonic crystal fiber
Rekha SAHA, Md. Mahbub HOSSAIN, Md. Ekhlasur RAHAMAN, Himadri Shekhar MONDAL
Front. Optoelectron.    2019, 12 (2): 165-173.   https://doi.org/10.1007/s12200-018-0837-6
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High birefringence with low confinement loss photonic crystal fiber (PCF) has significant advantages in the field of sensing, dispersion compensation devices, nonlinear applications, and polarization filter. In this report, two different models of PCFs are presented and compared. Both the models contain five air holes rings with combination of circular and elliptical air holes arrangement. Moreover, the elliptical shaped air holes polarization and the third ring air holes rotational angle are varied. To examine different guiding characteristics, finite element method (FEM) with perfectly matched layer (PML) absorbing boundary condition is applied from 1.2 to 1.8 µm wavelength range. High birefringence, low confinement loss, high nonlinearity, and moderate dispersion values are successfully achieved in both the PCFs models. Numeric analysis shows that model-1 gives higher birefringence (2.75 × 102) and negative dispersion (−540.67 ps/(nm·km)) at 1.55 µm wavelength. However, model-2 gives more small confinement loss than model-1 at the same wavelength. In addition, the proposed design demonstrates the variation of rotation angle has great impact to enhance guiding properties especially the birefringence.

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Origin of peculiar inerratic diffraction patterns recorded by charge-coupled device cameras
Kuanhong XU, Xiaonong ZHU, Peng HUANG, Zhiqiang Yu, Nan ZHANG
Front. Optoelectron.    2019, 12 (2): 174-179.   https://doi.org/10.1007/s12200-018-0840-y
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A peculiar and regular diffraction pattern is recorded while using either a color or a monochrome charge-coupled device (CCD) camera to capture the image of the micro air plasma produced by femtosecond laser pulses. The diffraction pattern strongly disturbs the observation of the air plasma, so the origin and eliminating method of these diffraction patterns must be investigated. It is found that the Fourier transform of the periodic surface structure of either the mask mosaic of the color CCD or the pixel array of the monochrome CCD is responsible for the formation of the observed pattern. The residual surface reflection from the protection window of a CCD camera plays the essential role in forming the interesting two-dimensional diffraction spots on the same CCD sensor. Both experimental data and theoretical analyses confirm our understanding of this phenomenon. Therefore removing the protection window of the CCD camera can eliminate these diffraction patterns.

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