Non-linear analysis of an S-I-S Josephson junction

The simplest model of an S-I-S Josephson junction is analysed by simulation in order to understand its switching behaviour. This model is assumed to have a parallel quasi-particle tunnelling current component, which is non.linear and ideal in the BCS sense. Fundamental properties, such as I _min and V _min, which specify hysteretic behaviour, as well as rise and fall times for switching, are calculated as functions of parameters β_c = 2ΠI _m0 R _n ² C/Φ₀ and γ = R _L R _n. How an AC Josephson current spoils the stability of a non-zero voltage state is discussed for the first time.     

共振混频过程的超快弛豫现象研究

本文报导利用共振混频过程,可在不利用超短脉冲激光情况下对皮秒甚至飞秒量级的超快弛豫进行测量。采用密度矩阵方法对该混频过程的二能级系统的非线性极化率XR进行了计算,结果表明过程的频率特性与样品的纵向弛豫时间T1和横向弛豫时间T2有关。实验测量得到孔雀绿水溶液的T1和T2分别为1.20ps和0.09ps。     

An analysis of a waveguide T junction with an inductive post

The authors analyze the T junction with an inductive post, taking its diameter into account for the case where the current distribution is assumed on the surface of the post. A single cylindrical post placed in a T junction improves the impedance matching and compensates the junction discontinuity in a wide frequency band. The effects of the design parameters, such as the diameter of the post and its location, are clarified. The measured return loss is accurately predicted. On the basis of this analysis, an effective design procedure for the T junction is proposed, and the reflection below -30 dB is realized over 4% bandwidth     

Transient analysis of junction transistors

The transient behavior of the surface barrier and diffused junction transistors may be represented by an equivalent network consisting of two diodes and a nonlinear base resistance. Expressions for the nonlinear resistance, the base-emitter voltage time response, and the collector current time response are derived and tested experimentally. The equivalent circuit allows us to predict hole storage time for direct-coupled transistor logic (DCTL) circuits.     

The potential of the superconductor-vacuum-superconductor junction as a mixing element at submillimetre wavelengths

By removing the solid insulator in the superconductor-insulator-superconductor junction and making use instead of the vacuum as the tunnelling medium it should be possible to reduce the junction capacitance by a factor of about ten. This would facilitate the development of tunnelling receivers for submillimetre wavelengths by making it possible to fabricate junctions of normal size with electrical properties appropriate for high-frequency operation.     

Spectral analysis of optical mixing measurements

A general rigorous theory of optical heterodyne and homodyne measurements is presented. The power spectrum of the photocurrent resulting from two uncorrelated optical beams mixing on a photodetector is derived. In particular, a rigorous analysis is presented for the delayed self-homodyne method which is used to characterize laser source linewidth by a Mach-Zehnder interferometer with a delay exceeding the source coherence length. Existing treatments are generalized to address non-Lorentzian laser sources of arbitrary lineshape. The analysis is further generalized to cover the case of modulated nonstationary sources. An example of the application of this theory is given. It is shown how the theory may be used to interpret an experimental result obtained using the gated delayed self-homodyne technique for characterizing the frequency chirp of laser sources under modulation     

金属-氧化物-半导体结构电容-电压特性的扫描非线性介电显微镜表征

扫描非线性介电显微镜(SNDM)利用非线性介电效应检测电容的变化情况,分辨率达到亚纳米量级,精度达到10-22F,主要应用于材料微区的电性能研究,目前以这项技术进行的研究主要集中在铁电材料和半导体材料方面,相关的报道也较少.本研究用扫描非线性介电显微镜对集成电路中具有n型与p型掺杂的60μm×60 μm区域进行二维表征,得到定点非线性电容与电压的关系曲线,并由积分得到对应的电容电压特性曲线,认为界面陷阱的作用是金属-氧化物-半导体非线性电容-电压特性曲线突变的影响因素.     

Analog performance of an ultrafast sampled-time all-optical fiber XPM wavelength converter

Analog performance of an all-optical ultrafast wavelength converter is measured and reported for the first time. The wavelength-conversion process is based on nonlinear cross-phase modulation in an optical fiber combined with an optical filter to convert phase modulation to amplitude modulation. The spurious-free dynamic range (SFDR) of the converter is measured to be 82 dB/spl middot/Hz/sup 2/3/. We define a new metric called the SFDR power penalty, which measures the degradation in SFDR relative to baseline the back-to-back analog optical link. The SFDR power penalty was measured to be 5 dB/spl middot/Hz/sup 2/3/ and is shown to be a function of the input optical power. This metric is used to characterize the linear region of the optical wavelength converter.     

Field analysis of rectangular waveguide open junction

The problem of a rectangular waveguide open junction is investigated using field theory and the relevant model of two normally intersected, infinite parallel-planes waveguides. Evanescent waveguide modes generated by waveguide wall edges and/or the discontinuity in dielectric are taken into account; an infinite set of equations is derived, where the mode coupling is given by the dielectric slab modes. Proper pole handling is discussed, and a solution for the system is given. Expressions are derived for the reflected, transmitted, and radiated power, which are shown to be sufficiently reliable in the domain of practical interest, regarding the width and the dielectric loading of the gap. The analysis shows that a substantial fraction of the microwave power leaks from the dielectric gap, confirming the absolute necessity of using a choke-flange at the waveguide junction     

Doping-dependent mobility analysis of junction field-effect transistors

An analysis of a junction field-effect transistor, taking into account the doping-dependent mobility, is presented for an assumed arbitrary impurity distribution in the channel. The values of transconductance and drain current obtained from the new relationships are found to deviate considerably from the theoretical values of these parameters estimated from the existing relationships.     

Propagation analysis of forward degenerate four-wave mixing

We discuss the propagation problem associated with the forward four-wave degenerate mixing induced by the third-order non-linear susceptibilitychi_{(-omega; omega, omega, -omega)}^{(3)} = chi_{1}^{(3)} + ichi_{2}^{(3)}. Exact analytical solutions are presented of two-photon resonances, evidencing the influence of the Kerr effect on the efficiency of the process. Around a two-photon resonance, we solve the problem of the nearly parallel propagation of two beams undergoing auto and induced two-photon absorption (TPA). The resonant interaction is studied in detail and analytical solutions are provided with operational accuracy for current experimental conditions. We show by the way that in the two-beam two-photon experiments, when the phase matching condition is fulfilled, although TPA is governed bychi_{2}^{(3)}, the real part ofchi^{(3)}must be considered in a substantial way. All the above solutions are put in a form simple enough that the numerical computations can be performed on a programmable calculator.     

A perturbation analysis of polarization-decoupledBrillouin-enhanced four-wave mixing

The technique of polarization-decoupled Brillouin-enhanced four-wave mixing is known for its high-reflectivity, high-quality phase conjugation. The treatment accounts for the transverse structure of the pump and signal beams within a waveguide in the paraxial approximation, assuming a steady-state acoustic wave and assuming that many transverse modes of the waveguide are excited by the four mixing waves. The zeroth-order equations are obtained for the polarization states of the signal and conjugate beams by dropping all phase-mismatch terms. Under the stated assumptions, the phase-mismatched terms are found to make only a small contribution. A first-order solution accounts for phase-mismatch terms and determines the loss of conjugation fidelity as a function of the angular spread of the pump laser beam, the pump-signal angle, the pump intensities, and the pump mutual fidelity     

Characteristics of all-optical ultrafast gate switches using cascade of second-harmonic generation and difference frequency mixing in quasi-phase-matched lithium niobate waveguides

In all-optical gate switches that employ the cascade of second-harmonic generation and difference frequency mixing in quasi-phase-matched (QPM) lithium niobate (LN) devices, walkoff between the fundamental and second-harmonic pulses causes crosstalk of the switch. We numerically calculate the switching performance of QPM-LN waveguide devices with consideration for the crosstalk effect, and discuss the favorable choice of the nonlinear optical tensor element for the LN crystal. In the analyses, the device length is maximized for a given bit rate under the condition that the crosstalk is maintained below a critical level that is determined by the allowable power penalty for the switched signal. It is shown that the time offset between the gate and signal pulses can compensate for the walkoff, and can significantly improve the switching efficiency. In ultrafast operation beyond 1 Tbps, the use of the nonlinear optical tensor element d/sub 31/ is found to be favorable when compared to the use of the commonly used maximum tensor element d/sub 33/.     

过采样扫描探测性能对比分析

分析了单采样和过采样两种扫描探测体制的成像原理,并建立了扫描图像仿真模型。在不同云层杂波场景下从目标能量收集、背景杂波抑制和弱小目标检测三个方面对单采样和过采样扫描图像进行了对比分析。理论分析和仿真结果表明：过采样扫描探测体制更有利于收集目标能量,杂波抑制能力较优,对弱小目标的探测能力更强。     

过采样扫描探测性能对比分析

分析了单采样和过采样两种扫描探测体制的成像原理,并建立了扫描图像仿真模型.在不同云层杂波场景下从目标能量收集、背景杂波抑制和弱小目标检测三个方面对单采样和过采样扫描图像进行了对比分析.理论分析和仿真结果表明:过采样扫描探测体制更有利于收集目标能量,杂波抑制能力较优,对弱小目标的探测能力更强.     

Real-time oscilloscope observation of an ultrafast photodiode response to mode-locked laser pulses

Pulses of 100 ps full width at half maximum (FWHM) have been displayed on a direct current to 5-GHz real-time oscilloscope. The 100-ps duration includes contributions from the oscilloscope, the photodetector, and the laser pulse,Maximum current in the linear regime of the photodiode is ≈3 amperes so that electric pulses of ≈70 ps FWHM and ≈150 volts can be obtained with the laser-detector combination. Results of a simple optical method for determining the exposure time of high-speed electronic cameras are also briefly given.     

Demonstration of the nonlinear fibre loop mirror as an ultrafast all-optical demultiplexer

The all fibre nonlinear optical loop mirror (NOLM) is used in the two wavelength version to show good contrast stable switching of selected pulses from a stream of short pulses. The polarisation properties of the device enable operation as a genuine four port demultiplexer.<>     

同步扫描三角测距成像系统的精确建模与分析

传统的同步扫描三角测量模型假设旋转镜厚度为零 ,从而建立了三维扫描点的 轨迹圆模型。本文将旋转镜厚度也作为系统参数进行考虑,提出一种同步扫描三角法在展开 光路中的几何 参数模型,通过14个参数精确地描述各个功能部件的位置姿态和几何光学参数,并建立了三 维空间点与系 统参数的关系。仿真分析表明:1mm厚的旋转镜会产生200mm的系统 误差。对接受透镜位置参 数的建模使得三维点具有动点旋转特性,与传统的轨迹圆模型相比,更精确地描述了旋转中 心的轨迹变化。 构建了原型系统,并通过实验验证了方法的正确性。     

二维扫描镜扫描特性理论分析

在航天遥感仪器中,为了缩小二维指向镜的大小可将45°镜偏转一定角度,如60°镜.在光学反射矢量理论基础上,对不同角度指向镜的摆动范围、像旋特性进行分析比较.从扫描特性角度分析了倾斜扫描镜使其减小尺寸的可行性.     

Performance Analysis of an All-Digital Acquisition Circuit

The performance analysis of a fully digital acquisition circuit for direct sequence spread-spectrum systems has been made. The disturbance is considered Gaussian but with a colored spectral density. The effects of the disturbance correlation show remarkable effects on the detection probability and on the mean acquisition time at low signal-to-noise ratios. An example of an application is reported.