Frontiers of Optoelectronics

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The smallest nanowire spectrometers
Jianji DONG
Front. Optoelectron.    2019, 12 (4): 341-341.
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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.
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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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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.
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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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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.
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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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Heuristic polling sequence to enhance sleep count of EPON
Bhargav Ram RAYAPATI, Nakkeeran RANGASWAMY
Front. Optoelectron.    2019, 12 (4): 422-432.
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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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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.
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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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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.
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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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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.
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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.
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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.
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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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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.
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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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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.
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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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Review on partially coherent vortex beams
Jun ZENG, Rong LIN, Xianlong LIU, Chengliang ZHAO, Yangjian CAI
Front. Optoelectron.    2019, 12 (3): 229-248.
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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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Robot visual guide with Fourier-Mellin based visual tracking
Chao PENG, Danhua CAO, Yubin WU, Qun YANG
Front. Optoelectron.    2019, 12 (4): 413-421.
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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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Airy-like field under high numerical aperture optical system
Yong LIU, Zhifeng ZHANG, Cuifang KUANG
Front. Optoelectron.    2019, 12 (4): 397-404.
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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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Topological photonic crystals: a review
Hongfei WANG, Samit Kumar GUPTA, Biye XIE, Minghui LU
Front. Optoelectron.    2020, 13 (1): 50-72.
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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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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.
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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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Reflectometric and interferometric fiber optic sensor’s principles and applications
Muhammad Noaman ZAHID, Jianliang JIANG, Saad RIZVI
Front. Optoelectron.    2019, 12 (2): 215-226.
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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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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.
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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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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.
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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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Analysis on multiple optical line terminal passive optical network based open access network
Love KUMAR, Amarpal SINGH, Vishal SHARMA
Front. Optoelectron.    2019, 12 (2): 208-214.
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Passive optical networks (PONs) offer sufficient bandwidth to transfer huge amount having different packet sizes and data rates being generated by fusion of various networks. Additionally, multiple optical line terminals (OLTs) PONs reduce the computational complexity of data processing for nonuniform traffic. However,in order to improve the bandwidth allocation efficiency of a mixture of service providers, dynamic bandwidth algorithm (DBA) is needed for uplink communication. In this paper, a PON based open access network (OAN) is analyzed for bi-directional communication at various data rates. Multiple wavelengths are used to modulate the data of various service providers to evade the complicated DBA for uplink data broadcasting. The performance of the network is reported in terms of bandwidth exploitation, uplink effectiveness, overhead-to-data ratio and time cycle duration. The network is analyzed at various data rates to reveal the data accommodation capacity.

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A novel modulation format identification based on amplitude histogram space
Tianliang WANG, Xiaoying LIU
Front. Optoelectron.    2019, 12 (2): 190-196.
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In this paper, we proposed a novel modulation format identification method for square M-quadrature amplitude modulation (M-QAM) signals which is based on amplitude histogram space of the incoming data after analog-to-digital conversion, chromatic dispersion compensation at the receiver. We demonstrated the identification of quadrature phase-shift keying (QPSK), 16-QAM, 64-QAM formats with an amplitude histogram space. Simulation results show that it achieve 100% identification accuracy when the incoming signal OSNR is 14 dB to identify the modulation format of QPSK, 16-QAM, and 64-QAM signals in digital coherent systems. The method has low complexity and small delay.

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Saturation in wavelength-division multiplexing free-space optical communication systems
Jeremiah O. BANDELE, Malcolm WOOLFSON, Andrew J. PHILLIPS
Front. Optoelectron.    2019, 12 (2): 197-207.
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The performance of a wavelength-division multiplexing (WDM) free-space optical (FSO) communication system in a turbulent atmosphere employing optical amplifiers to improve capacity is investigated, in the presence of amplified spontaneous emission noise, scintillation, beam spreading, atmospheric attenuation and interchannel crosstalk. Using on-off keying modulation, Monte Carlo simulation techniques are used to obtain the average bit error rate and system capability due to scintillation and the effect of introducing a power control algorithm (PCA) to the system is investigated. The PCA ensures that at any receiving instant, the same turbulence-free powers are received by all the receiving lenses. The performance of various WDM FSO communication system configurations such as non-amplified systems with an adaptive decision threshold (NOAADT), non-amplified systems with a non-adaptive decision threshold, fixed gain amplified systems with an adaptive decision threshold, fixed gain amplified systems with a non-adaptive decision threshold and saturated gain amplified systems with a non-adaptive decision threshold (SOANADT) are investigated. Results obtained show that the SOANADT is superior to the NOAADT and the PCA is only beneficial in amplified systems.

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Shape reconstruction of large optical surface with high-order terms in fringe reflection technique
Xiaoli JING, Haobo CHENG, Yongfu WEN
Front. Optoelectron.    2019, 12 (2): 180-189.
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A fast and effective shape reconstruction method of large aspheric specular surfaces with high order terms is proposed in fringe reflection technique, which combines modal estimation with high-order finite-difference algorithm. The iterative equation with high-order truncation errors is derived for calculating the specular surface with large aperture based on high-order finite-difference algorithm. To achieve the wavefront estimation and improve convergence speed, the numerical orthogonal transformation method based on Zernike polynomials is implemented to obtain the initial iteration value. The reconstruction results of simulated surface identified the advantages of the proposed method. Furthermore, a freeform in illuminating system has been used to demonstrate the validity of the improved method in practical measurement. The results show that the proposed method has the advantages of making the reconstruction of different shape apertures accurate and rapid. In general, this method performs well in measuring large complex objects with high frequency information in practical measurement.

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Photonic crystals and topological photonics
Front. Optoelectron.    2020, 13 (1): 2-3.
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Universal numerical calculation method for the Berry curvature and Chern numbers of typical topological photonic crystals
Chenyang WANG, Hongyu ZHANG, Hongyi YUAN, Jinrui ZHONG, Cuicui LU
Front. Optoelectron.    2020, 13 (1): 73-88.
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Chern number is one of the most important criteria by which the existence of a topological photonic state among various photonic crystals can be judged; however, few reports have presented a universal numerical calculation method to directly calculate the Chern numbers of different topological photonic crystals and have denoted the influence of different structural parameters. Herein, we demonstrate a direct and universal method based on the finite element method to calculate the Chern number of the typical topological photonic crystals by dividing the Brillouin zone into small zones, establishing new properties to obtain the discrete Chern number, and simultaneously drawing the Berry curvature of the first Brillouin zone. We also explore the manner in which the topological properties are influenced by the different structure types, air duty ratios, and rotating operations of the unit cells; meanwhile, we obtain large Chern numbers from −2 to 4. Furthermore, we can tune the topological phase change via different rotation operations of triangular dielectric pillars. This study provides a highly efficient and simple method for calculating the Chern numbers and plays a major role in the prediction of novel topological photonic states.

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Design and fabrication of compact Ge-on-SOI coupling structure
Jianfeng GAO, Junqiang SUN, Heng ZHOU, Jialin JIANG, Yang ZHOU
Front. Optoelectron.    2019, 12 (3): 276-285.
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In this paper, we have proposed and demonstrated a simple approach to fabricate vertical integrated structure for coupling between active germanium (Ge) waveguide and silicon-on-insulator (SOI) waveguide. The active Ge waveguide is sputtered after etching the underlying passive silicon (Si) waveguide. This method scuttles away from the difficulty involved in the waveguide fabrication by avoiding the etching process for the Ge waveguide, and thereby the waveguide quality is improved. The influences of the coupling structural parameters on the coupling loss are analyzed and discussed. The optimizing parameters are obtained for the fabrication. The minimal coupling loss is experimentally measured about 2.37 dB, and variation tendency of coupling loss against the structural parameters is consistent with the theoretical result. The proposed approach offers an effective path for vertical coupling between Ge and SOI optical components.

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Development of optical-thermal coupled model for phosphor-converted LEDs
Xinglu QIAN, Jun ZOU, Mingming SHI, Bobo YANG, Yang LI, Ziming WANG, Yiming LIU, Zizhuan LIU, Fei ZHENG
Front. Optoelectron.    2019, 12 (3): 249-267.
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In this review, first, we discussed the effect of phosphor features on optical properties by the software simulation in detail. A combination of these parameters: phosphor material, phosphor particle size and particle distribution, phosphor layer concentration, phosphor layer thickness, geometry, and location of the phosphor layer, will result in the final optical performance of the phosphor layer. Secondly, we introduced how to improve light extraction efficiency with various proposed methods. Thirdly, we summarized the thermal models to predict the phosphor temperature and the junction temperature. To stabilize the optical performance of phosphor-converted light emitting diodes (PC-LEDs), much effort has been made to reduce the junction temperature of the LED chips. The phosphor temperature, a critical reliability concern for PC-LEDs, should be attracted academic interest. Finally, we summed up optical-thermal coupled model for phosphors and summarized future optical- thermal issues exploring the light quality for LEDs. We foresee that optical-thermal coupled model for PC-LEDs should be paid more attention in the future.

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