Effect of stochastic dead space on noise in avalanche photodiodes

A stochastic dead-space model for impact ionization is developed and used to study the effect of the soft nature of the ionization capability of carriers on the excess noise factor of avalanche photodiodes. The proposed model is based on the rationale that the gradual, or soft, transition in the probability density function (PDF) for the distance from birth to impact ionization can be viewed as that resulting from uncertainty in the dead space itself. The resulting soft PDF, which is parameterized by a tunable softness parameter, is used to establish the limitations of the existing hard-threshold ionization models in ultrathin multiplication layers. Calculations show that for a fixed operational gain and fixed average dead space, the excess noise factor tends to increase as a result of the softness in the PDF in very thin multiplication layers (viz, <70 nm), or equivalently, under high applied electric fields (viz., >800 kV/cm). A method is proposed for extracting the softness parameter from noise versus multiplication measurements.     

The role of lattice matching in improving the performance of PbSnTe IR photodiodes

The role of lattice matching in determining the quantum efficiency of heterostructure PbSnTe photodiodes has been investigated. It has been established that a mismatch of less than 2 × 10^-4results in a photodiode with quantum efficiency of ∼ 43%, very close to the maximum 50% possible without an anti-reflection coating. This compares with a value of 28% reported in the case of a heterojunction with a mismatch of 4 × 10^-3. The increase in the quantum efficiency is attributed to the high crystalline quality at the heteroface and its vicinity in the lattice-matched system, in contrast to a high defect density found in the mismatched system.     

Numerical study of currents and fields in a photoconductive detector

A numerical study of the current, field, and carrier density distributions within a photoconductive detector is presented. The photodetector, an interdigitated Schottky barrier diode, is made with metallic fingers of alternating voltage bias on a thin semiconductor layer grown on a transparent dielectric substrate. The Poisson and continuity equations for electrons and holes are treated in two dimensions. A modified successive line overrelaxation method, faster than the capacitance matrix method, is developed as the Poisson solver. A simple alternative to the Scharfetter-Gummel treatment of current density is also introduced. We investigate steady-state cases with and without optical illumination, and transient responses to picosecond optical pulses. The steady-state current Shows near saturation with increasing voltage, as observed experimentally. The calculated typical response of a silicon detector to a picosecond optical pulse is a current pulse lasting on the order of 10 ps.     

Singularity expansion representations of fields and currents in transient scattering

A unified treatment of the natural mode representations for induced currents and scattered fields is obtained by use of fundamental concepts regarding causality and superposition. The transient scattered field response is shown to have the form of a constant coefficient complex exponential sum only in the "late-time," after the last driven reponse is received from the object. Prior to this, the "early.time" response is found to be due to direct physical optics fields as well as a sum of temporally modulated natural modes produced by the progressive illumination of the incident wavefront. Alternate representations ands-plane behaviors are considered. The implications of these results on natural resonance target identification schemes are discussed.     

Relative tower currents and fields in an AM directional array

With the help of method-of-moments techniques, the antenna monitor parameters that give the correct pattern can be calculated from the field parameters. The base voltages can be computed from the linear relationship between the relative fields and the base voltages. The computed voltages can then be used as sources for MININEC III. The base currents computed by MININEC III for those base voltages are used to determine the antenna monitor parameters that result in the correct directional antenna (DA) pattern. Calculating the antenna monitor parameters that give the correct DA pattern means that trial and error field adjustment is not necessary in those situations where reradiating objects are not present. Thin reradiating objects, such as AC power transmission towers, can be included in the computation when they can be accurately located and described. Detuning impedances from the floated towers to ground are easily calculated from the output of the MININEC III program. Examples of the use of this technique are presented     

On thermal and excess currents in GaSb tunnel diodes

Experimental results presented here show that excess and thermal currents coexist independent of one another. Excess current behaves according to an analytical expression based on linear temperature dependence of built-in voltage. Thermal current changes from recombination-generation at low voltage to normal injection current at higher voltages.     

红外热像仪在水平空投系统的应用研究

针对目前亟待解决的在恶劣气候条件下各种型号飞机进行空投瞄准,尤其是在夜间无法自主空投瞄准的问题,提出了将加载红外热像仪的光电吊舱与水平空投原理相结合的理论,该理论不仅能够很好地解决以上问题,还可以使飞机驾驶员以及领航员通过使用监视器从而摆脱目视光学系统带来的不便.建立了基于红外测量系统的空投空降的数学模型;并对该数学模型进行了误差分析,通过理论计算该系统的空投瞄准精度较以往的空投系统的瞄准精度更高,可以满足目前空投的要求,为提升夜间空投、空降的能力提供了理论依据,具有很强的实用价值和应用前景.     

Calculation of tunneling currents in (Hg,Cd)Te photodiodes using a two-sided junction potential

It has been shown previously that tunneling current can become the dominant dark current and hence the performance-limiting factor in diodes formed in narrow bandgap semiconductors, such as Hg1_xCdxTe. In this paper, we calculate the tunneling current using a Kane approximation for the nonparabolic conduction band and a more realistic junction potential than has been used previously. The potential used here is characteristic of a linearly graded n-type region intersecting a uniformly doped p-type region and is a better approximation to the actual potential in a diode formed by ion implantation into a p-type substrate. We show that significant errors sometimes arise when the abrupt junction model is used to calculate tunneling current in these structures. The effect of changes in base carrier concentration and n-side donor gradient is shown forxbetween 0.196 and 0.400, which corresponds to a photodiode spectral cutoff in the important 3- to 14-µm region.     

INSAR imagery of surface currents, wave fields, and fronts

The authors demonstrate the ability of interferometric radar imagery to determine both relative and absolute surface velocities in the open ocean. Absolute phase calibration is accomplished by noting the azimuthal displacement of range-travelling targets-demonstrating for the first time that under favourable circumstances phase calibration can be achieved in open-ocean in the absence of ground truth. The high resolution of radar imagery permits observation of sharp velocity discontinuities, e.g. the Gulf Stream boundary and the wave field. The recent SIR-C/X-SAR shuttle missions dramatically emphasize the experimental and observational aspects of space-based radar. The combination of absolute velocities, high spatial resolution, and wide-area coverage suggest that interferometric radar imagery can provide a unique and powerful aid both for studies of global circulation patterns and detailed analysis of slope/shelf water interactions with ocean currents. In particular, the authors employ this measurement of the surface currents and wave field near a velocity front to help refine and bound results of their modeling of calculated radar images of the front. The results of this paper are compared with available ground truth     

非制冷红外热成像技术的发展与现状

非制冷红外焦平面技术在过去的几年内飞速发展,非制冷焦平面由原来的小规模,发展到中、大规模320×240和640×480阵列,在未来的几年内有望获得超大规模的1024×1024非制冷焦平面阵列。像素尺寸也由50μm减小到25μm,提高了焦平面的灵敏度,使非制冷红外热成像系统在军事领域得到了成功应用,部分型号已经装备于部队,并受到好评。今后,随着焦平面阵列规模的不断增大、像素尺寸的进一步减小,非制冷热成像系统在军事领域的应用将越来越广泛,尤其在轻武器瞄具、驾驶员视力增强器、手持式便携热像仪等轻武器方面,非制冷热成像系统在近年内有望逐步取代价格高、可靠性差、体积大等笨重的制冷型热成像系统。     

Modelling and IR measurement of the electronic substrate thermal conductivity

The temperature distribution on a ceramic substrate with a small heating element in the middle has been measured by infrared thermography. By comparing the experimental data with a theoretical analysis, the thermal conductivity could be easily obtained.     

Induced electric currents in models of man and rodents from 60 Hzmagnetic fields

Induced electric currents in models of man, rat and mouse from 60 Hz magnetic fields are computed using the impedance method. The models all have realistic shapes, and in the case of rodents, a homogeneous average tissue conductivity is assumed. The model of man is analyzed for two cases, a homogeneous average tissue conductivity and a heterogeneous model, both consisting of 1.3 cm cubical tissue cells whose conductivities are representative of the tissue within the cube. The results for various models and species, as well as different orientations of the magnetic field, are compared. The data presented are useful as the first step in dosimetry for 60 Hz magnetic fields, and for interspecies scaling of biological interactions related to the tissue induced electric currents     

高超声速流场中CO2的红外辐射研究

文中针对高超声速绕流场中驻点线上的CO2气体展开了辐射光谱的相关研究,分析了CO2 对 整个高超声速目标的红外辐射产生的影响。研究了以球体作为飞行本体,通过计算流体力学的方法,描述高超声速流场中驻点线的物理状态以及CO2的含量变化。以此为基础,通过逐线法计算了不同 飞行条件下绕流场中驻点线上CO2的辐射光谱。估算了发生烧蚀现象时,CO2的辐射光谱。通过理论 计算,发现在典型的高超声速飞行状态下,无烧蚀的纯空气高超声速流场中驻点线上CO2 的红外辐 射远小于飞行器本体的红外辐射。而在有烧蚀的情况下,激波层中的CO2的红外辐射将不可忽略。     

The eigenfunction solution for scattered fields and surface currents of a vertex

The surface current and the scattered field of a vertex are studied by employing the exact eigenfunction solution. The first few contributing eigenvalues are presented in easy-to-use graphical form. Results will be shown for both the current on a plane angular sector and also for the scattered field.     

Comparison of electric fields induced in humans and rodents by 60-Hz contact currents

Contact currents flow when a conducting object such as an animal touches conductive surfaces at different potentials. This completes a path for current flow through the body. These currents provide an additional coupling mechanism between the human body and low-frequency external fields to that due to direct induction effects. Recent research indicates that childhood exposure to residential contact currents may play a role in explaining any possible association between residential magnetic fields and childhood leukemia. To verify this hypothesis, laboratory experiments with rodents are planned. Thus, it is important to understand the relationship between fields induced in rodents and humans. Results from numerical computations are reported here. They are based on high-resolution anatomically based inhomogeneous models of adult and child male humans and male and female rats and mice, for a variety of 60-Hz contact current scenarios. It is hoped that this work will aid in the design of experiments involving rodents and in the interpretation of results as applied to humans. It is found that for geometrically similar models, the induced electric-field scales in an anticipated inverse-square manner with the geometric scaling factor. For dissimilar models, scaling can provide a crude estimate for translating induced field results between species. However, numerical modeling provides the most suitable analysis tool for more accurate estimates.     

机载高分辨率连续变焦红外热像仪设计

针对中波640×512元红外焦平面探测器设计了一套1:20小型高分辨率连续变焦热像仪,对热像仪的光学系统和系统的灵敏度等进行了设计和计算。设计结果表明,连续变焦红外热像仪具有变倍比大、分辨率高、灵敏度高、体积小、像质好等特点,满足设计要求,可用于机载光电探测和跟踪系统。     

Losses in a high-temperature superconductor exposed to AC and DCtransport currents and magnetic fields

Summary form only given, as follows. The development of high-temperature superconductors (HTSs) is for the moment intensive and it is likely that HTSs will be used in different electric devices in a few years. In some of these devices that conductor will be exposed to both AC and DC transport currents and magnetic fields. In the design of electric devices, the power loss is indeed one of the most important parameters. Therefore there is a need of models that predict the power loss in an HTS under application-like conditions. In this paper, the authors present a semi-empirical model including a transport current and an applied magnetic field with both AC and DC components. The semi-empirical model is mainly based on the critical state theory. The results of the model are compared and found to be in good agreement with experimental results obtained with a calorimetric experimental set-up. The power loss was measured on a multi-filamentary Bi-2223 tape as function of transport current and applied magnetic field at a fixed temperature and frequency     

红外热像仪的空间噪声和时间噪声分析

噪声特性是衡量红外热像仪的一个重要指标。本文对热像仪的噪声来源和影响因素进行了分析,并通过实验分别计算了实测图像的空间噪声和时间噪声。实验结果表明了在实际观测过程中,图像的空间噪声通常要大于时间噪声,而且空间噪声在热像仪经过非均匀性校正之后的一段时间内会逐渐增大,而时间噪声不会有明显变化。基于时间噪声小于空间噪声且不随时间变化的特点,为红外图像中的目标检测提出了新的思路。     

基于灰度级共享的红外热像显示与增强

随着大动态范围热成像器件的广泛使用,有必要研究适合人类视觉特性的红外热像终端显示系统。讨论了目前常用的基于自动增益和偏移和基于直方图均衡的压缩技术,并对它们应用于热像的动态范围压缩显示的效果进行了分析比较。提出了一种基于灰度级共享的热像显示技术,将目标和背景各自压缩到全部灰度空间,再进行融合显示。算法尽量保留了热像的细节,符合人类视觉响应特性。通过将16位热像转换为8位灰度图显示,仿真结果验证了该算法具一定的工程实用性。     

单色光石英退偏器的近红外退偏性能测试研究

在７６０￣１０６０ｎｍ的光谱范围，对不同结构角的单色光石英退偏器的退偏性能进行了测试，并测试了入射线偏振光的截面大小对退偏性能的影响，就退偏器的平均退偏性能而言，大结构角退偏器优于小结构角退偏器，大光束截面优于小光束截面。