Stochastic polarization switching dynamics in vertical-cavity surface-emitting lasers: theory and experiment

We present an analytical, numerical, and experimental study of the switching time and jitter of current-induced polarization switching in vertical-cavity surface-emitting lasers (VCSELs) as an example of switching in a nonlinear system in the presence of noise. Assuming that the switching is induced by changes in the dichroism, the problem can be reduced to the first-passage-time problem in gain-switched Class-A lasers. The theoretical results show excellent agreement both with numerical simulations based on the full-rate equations model and with experiments performed on oxide-confined VCSELs.     

Low-frequency fluctuations in vertical-cavity surface-emitting lasers with polarization selective feedback: experiment and theory

We present theory and measurements on a vertical-cavity surface-emitting laser (VCSEL) biased close to its solitary threshold and subject to polarization selective external optical feedback. The system shows low-frequency fluctuations (LFF) in the selected polarization mode (PM). Below solitary laser threshold, the orthogonal PM remains silent, while it only responds after a dropout event in the main mode above threshold. Our calculations show good agreement with the measurements and identify a type of synchronization between the low-frequency dynamics of the two PMs.     

Influence of temperature and optical confinement on threshold current of an InGaAs/InP quantum wire laser

In this paper, we investigate the influence of the temperature on gain and threshold current density of a V-groove quantum wire InGaAs/InP laser. The calculation shows that room-temperature operation can be achieved if the optical confinement is large enough (0.26% in our case), while its slight improvement above this limit (around 0.4%) can provide a significant reduction of the threshold current (more than 70%) and an improved temperature stability of the laser.     

Non-equilibrium quantum transport theory: current and gain in quantum cascade lasers

We have developed a self-consistent non-equilibrium Green’s function theory (NEGF) for charge transport and optical gain in THz quantum cascade lasers (QCL) and present quantitative results for the I-V characteristics, optical gain, as well as the temperature dependence of the current density for a concrete GaAs/Al_.15Ga_.85As QCL structure. Phonon scattering, impurity, Hartree electron-electron and interface roughness scattering within the self-consistent Born approximation are taken into account. We show that the characteristic QCL device properties can be successfully modeled by taking into account a single period of the structure, provided the system is consistently treated as open quantum system. In order to support this finding, we have developed two different numerically efficient contact models and compare single-period results with a quasi-periodic NEGF calculation. Both approaches show good agreement with experiment as well as with one another.     

Gain stabilization in a quantum‐dot semiconductor optical amplifier using tapered waveguide structure

In this paper, transient gain of a quantum‐dot semiconductor optical amplifier (QD‐SOA) is studied. Waveguide of the QD‐SOA is considered to have a tapered structure in which width of the waveguide increases along the QD‐SOA. It is observed that by employing tapered waveguide, gain as the key feature of the device acquires more stability, investigated by studying the impact of a powerful optical pulse on the gain as it passes through the amplifier. Thus, by gradually increasing the width of the waveguide along QD‐SOA active region, drop in the gain, caused by the strong pulse, decreases. Transient gain of the device is obtained for several outputs to input width ratios. It is demonstrated that as the width ratio increases, gain stability improves drastically; as for width ratio of 10, stability increases over 10 times compared with the generic QD‐SOA. In addition to the gain, cross‐gain modulation as a nonlinear process, which depends on the gain instability imposed by strong pulses, is studied. In this paper, the rate equations are employed for modeling tapered waveguide QD‐SOA. MATLAB ODE (MathWorks, MA, USA) along with the finite difference method is used for studying and simulating the device. Copyright © 2013 John Wiley & Sons, Ltd.     

Tight-binding and \vec k\cdot \vec p methods in carbon nanotubes: features, comparison and improvements

We use the tight-binding and the $\vec k\cdot \vec p$ methods to compute the energy dispersion relations and the density of states of carbon nanotubes, comparing the obtained results and pointing out the advantages and the drawbacks of the two techniques. We discuss a numerical method to easily obtain with the tight-binding technique only the most interesting energy bands, introducing an unusual but convenient choice of graphene unit vectors.     

Density-gradient theory: a macroscopic approach to quantum confinement and tunneling in semiconductor devices

Density-gradient theory provides a macroscopic approach to modeling quantum transport that is particularly well adapted to semiconductor device analysis and engineering. After some introductory observations, the basis of the theory in macroscopic and microscopic physics is summarized, and its scattering-dominated and scattering-free versions are introduced. Remarks are also given about the underlying mathematics and numerics. A variety of applications of the theory to both quantum confinement and quantum tunneling situations are then reviewed. In doing so, particular emphasis is put on understanding the range of validity of the theory and on its unexpected power as a phenomenology. The article closes with a few comments about the future.     

光学电流互感器在电力系统应用中出现的问题及处理方法

由于光学电流互感器OCT(Optical Current Transducer)出现扰动而使交流滤波器差动保护跳闸事件,对OCT在电力系统现场运行中使用出现的问题进行了研究,结合换流站OCT数据采集原理分析了事故发生时的设备故障原因,从而得出3种解决方案,即修改接口板、信号传送板;提高定值,躲过导致故障的尖脉冲;以及通过修改软件,在软件中判断过滤掉尖脉冲。并根据实践找到各种方法的可行性,最终提出可靠的处理方案。     

预防电缆沟起火的小电流接地系统单相接地故障处理

为避免小电流接地系统单相接地故障引发的电缆沟起火事故,采用火灾动态仿真分析方法得到了接地故障电弧导致电缆沟引燃的时间与空间阈值。在此基础上阐述了预防电缆沟起火的综合治理措施。论述了兼具相控消弧线圈和预调式消弧线圈优点的相控预调式消弧线圈的实现方式及其实验测试数据。阐述了跨线相继接地故障的处理方法,包含以暂态量法为主的连续检测过程及以稳态量法为主的增补处理过程。给出了单相接地保护跳闸延时的整定依据及间断性接地故障的处理方法。针对城区和郊区配电线路分别论述了集中智能容错单相接地故障处理方法和基于合闸速断的分布智能单相接地故障处理方法。介绍了在陕西全省172座110 kV变电站供电区域范围内配电网的整体应用成效,表明所论述的技术措施的有效性。     

光纤电流差动保护通道试验及研究

该文介绍了光纤电流差动保护的一些试验内容、试验方法,对专用及复用通道进行了试验性研究.对提高装置的可靠性、稳定性有一定意义.目前,该方法已成功应用于WXH-803数字式光纤电流差动保护装置.     

光纤电流差动保护通道试验及研究

该文介绍了光纤电流差动保护的一些试验内容、试验方法,对专用及复用通道进行了试验性研究。对提高装置的可靠性、稳定性有一定意义。目前,该方法已成功应用于WXH-803数字式光纤电流差动保护装置。     

Breakdown mechanism of a gap with a small arc current in SF6 gas

Using an experimental apparatus modeled for a gas-insulated switchgear (GIS), the influence of the arc duration on the breakdown voltage between the contact and the grounded tank of the GIS was investigated in the case that the arc was kept ignited in the contact area. From the result, characteristics of the breakdown voltage vs. the arc duration were found to be divided into six stages and the characteristics in each stage were estimated     

贵州电网小电流选线装置的运行现状及建议

目前贵州电网的小电流选线装置选线准确率低,误选拒选概率大,主要原因是大部分选线装置采用基于稳态工频信号的选线原理,该选线原理易受过渡电阻、互感器精度问题以及其它一些因素影响,导致故障线路与非故障线路之间的差别变小,影响选线效果。     

Effect of the small arc current generated by a poor contact on the breakdown voltage in SF6 gas

One of the failures related to gas-insulated switchgear is caused by the poor contact between conductors. This causes breakdown between the contact and the wall of chamber, initiated by the small discharge due to the poor contact. In this paper, the modeled experimental apparatus for the contact portion of the gas-insulated switchgear assumed a rod-to-plane gap with small arc current. The influence of the arc current on the breakdown into the chamber was investigated experimentally, and the breakdown mechanism between the contacts and the wall was studied. It was clarified that the breakdown voltage between the contact and the wall decreased by increasing the current, and the small arc current in the contact portion played an important role in the breakdown between the contact and the wall. The reduction ratio of the breakdown voltage increased with increasing gap length of the small gap. The breakdown voltage was different depending on the duration of the small arc current.     

基于锁定放大器的光学电流互感器的研究及应用

随着电力系统的发展,传统的直流互感器因为种种缺点已经难以适应超高压直流测量的需要.光学电流互感器自问世以来,以其无饱和、高绝缘、抗电磁干扰等诸多优点成为传统电磁式电流互感器的替代产品.但长期以来因种种不利因素的困扰OCT的实用化迟迟没有提上日程.本文针对提高光学电流互感器的输出信噪比展开研究,分析OCT的信噪特性,进行...     

Pulsed field losses and coupling current loop in a large‐current superconductor

Pulsed field losses in a short sample (445 mm length) of 46‐kA Nb₃Sn cable‐in‐conduit conductor of superconducting magnet for the fusion experimental reactor “ITER” are measured by a calorimetric method. The coupling time constant between strands is obtained as 30 ms through pulsed loss. Based on this time constant and interstrand conductance measurement, it is expected that there is a coupling current loop length of about 400 mm, which is longer than half the fabrication twist pitch in the fourth cabling stage. A geometrical calculation on several‐stage cabling shows this value to be reasonable. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 142(4): 27–33, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10088     

Photothermal-induced temperature changes in a model inner ear: acomparison of visible, infrared, and ultraviolet lasers

The temperature change in a model cylindrical vestibule (90 mm³) was measured following irradiation by argon (488-514 nm), CO₂ (10.6 μm), KTP (Nd:YAG) (532 nm), Er:YAG (2.9 μm), and XeCl (308 nm) lasers. Otic capsule bone was used to simulate the otosclerotic stapes footplate, and the thickness of each specimen was machined to variable thicknesses (0.20-0.90 mm). Thermocouples were used to measure the temperature below the air-bone surface at depths of 1.0, 3.0, and 5.0 mm. The time-dependent temperature change, thermocouple position, and bone thickness were measured following single pulse application from argon, CO₂, and KTP (Nd-YAG) lasers. The effect of infrared and ultraviolet lasers on vestibule fluid temperature changes were studied with several fluence and pulse sequences. The temperature change in the vestibule following pulsed laser irradiation decreased with increasing bone thickness and thermocouple depth. Laser irradiation from CO₂, argon, KTP-532, XeCl, and Er:YAG lasers produced minimal (less than 5°) vestibule temperature changes. Measured temperatures were in good agreement with an analytic model, based on a solution to the bio-heat equation in semi-infinite media. The results are discussed with relevance to ear surgery     

Er3+-doped channel optical waveguide amplifiers for WDMsystems: a comparison of tellurite, alumina and Al/P silicate materials

Er³⁺-doped tellurite and Er³⁺-doped alumina optical waveguide amplifiers are analyzed both as single amplifiers and as elements of 16-channel wavelength-division multiplexing (WDM) systems; their performances are compared with that from Er³⁺-doped Al/P silica waveguide amplifiers. The amplifier model is based on propagation and population-rate equations and includes both uniform and pair-induced up-conversion mechanisms. It is solved numerically by combining finite elements and the Runge-Kutta algorithm. The analysis predicts that Er³⁺-doped tellurite waveguides exhibit improved gain characteristics compared with alumina and Al/P silica waveguides. Using Er³⁺-doped tellurite waveguide amplifiers, it is suggested that 16 WDM channels may be transmitted to a maximum distance of 4250 km. By using in-line notch gain equalizing filters, the maximum transmission distance can be increased to 5250 km     

并联有源电力滤波器三角载波与滞环比较控制策略比较与仿真

随着大量非线性设备投入电力系统,其引起的谐波问题倍受关注,而有源电力滤波器被认为是抑制谐波污染的最有效设备之一,其性能很大程度上取决于电流控制部分,阐述了有源电力滤波器的工作原理,并针对并联有源电力滤波器的特点,着重分析了单相有源电力滤波器的工作过程,利用PSIM软件作为仿真平台,搭建了有源电力滤波器主电路及电流控制电路,电流控制部分采用三角载波与滞环比较2种控制策略,在基于瞬时无功理论谐波电流检测的基础上,采用2种控制方式分析了有源电力滤波器的性能并比较了三角载波与滞环比较控制策略的工作原理及优缺点,仿真过程与结果有效地为电流控制提供参考和依据.