Microwave measurements by Fourier analysis of network pulse response

Measurements by Fourier analysis of time-domain reflectometry data are made, and errors caused by noise are evaluated.     

快响应光电探测器的瞬态特性测量

介绍了快响应光电探测器的发展动态，对其瞬态特性的测量原理和方法进行了综述，讨论此方面的发展趋势和限制因素。     

The influence of electron intervalley transfer on response pulse width of the MSM photodetector

This paper analyses the influence of nonstationary effects on response pulswidth of Metal-Semiconductor-Metal Photodetector (MSM-PD) made of a two-valley semiconductor. A phenomenological model is used for 2D numerical simulation of carrier transport and intervalley transfer. The analysis of the pulsewidth response of the MSM-PD has been made for various micron and submicron structures, bias voltages and incident light wavelengths.     

A fast diffusion-driven photodetector: theory and experiment

A high-speed photodetector has been demonstrated. Prototype silicon devices have detection bandwidths of 5 GHz and are limited by neither the saturation-velocity transit time nor the carrier lifetime. These detectors use a combination of the Dember effect and the east of generating fine photocarrier gratings, and scale to bandwidths of several hundred gigahertz     

Comparison of techniques for in vivo attenuation measurements

The attenuation of an ultrasound pulse within tissue can be estimated from either the amplitude decay or the frequency downshift of returning echoes. The authors compare the results of both analyses as applied to ultrasound B scan echoes from the livers of 49 individuals. The amplitude decay of the backscattered signal Fourier components with depth was used to calculate attenuation coefficients. In addition, the frequency downshift of the same backscattered signals was estimated using both zero-crossing and spectral centroid methods. The results show that the frequency-domain estimators yield consistently higher attenuation coefficients, with higher variability compared to the amplitude decay method. Explanations for the apparent bias and variability of the frequency-shift estimators include both the assumptions regarding tissue and signal which may not be met in practice and the effects of low-frequency electronic noise on spectral estimates     

A fast response impedance meter for acoustic reflex measurements

The response time of the instrument is a crucial parameter for the measurement of accurate morphology and latency of the acoustic reflex. An improved impedance meter accounting for this particular problem was designed for the measurement of the contralateral acoustic reflex. The temporal characteristic of the measurement apparatus is shown, as well as an example of an application in a normal hearing individual. The module can be incorporated into already existing instruments with some modifications     

Comparison of different fast modulation techniques for FIR laser stabilization

In the first part of this paper the characteristics of frequency deviation and modulation bandwidth required for phase-locking optically pumped far infrared lasers to reference synthesized signals are examined. Then different fast modulation techniques suitable for this purpose are discussed pointing out their particular features. Experimental results concerning laser frequency modulation by Stark effect, injection locking and electrically variable reactances (e.g Schottky diodes, plasma cell) are reported and compared.     

四论注入光敏器件的物理基础

叙述了从现象的发现到理论分析，然后用实验进行验证，最后利用这个现象研制成注入光敏器件的全过程，从而证明注入光敏器件有牢靠的物理基础。本文还分析了文献（１￣５，１０）中的几个主要不同论点及欠妥的讨论方法。     

Comparison of sinusoidal and pulse modulation techniques for generating optical frequency reference combs

The optical frequency combs generated by pulse and sinusoidal modulation of an LiNbO/sub 3/ Mach-Zehnder device are compared and contrasted. It is shown that the former operates with lower drive, although being restricted to narrower tooth spacings than offered by the sinusoidal technique.<>     

具有平顶陡边响应的级联微环结构光探测器

微环探测器具有高速、高量子效率、窄线宽、易于集成等显著优点,在未来的光通信系统和光电集成中有广阔的应用前景.提出了一种新型级联微环光探测器(CMRPD),从理论上推导出了其透射特性和量子效率.通过调节级联微环之间的耦合系数,使波长选择特性满足国际电信联盟远程通信标准化组(ITU-T)制定的密集波分复用系统(DWDM)中...     

Bipolar optical modulation and demodulation using dual-mode fiberand a fast diffusion-driven photodetector

A bipolar fiber-optic signal processing and photodetection concept which combines a dual-mode optical fiber with a fast diffusion-driven photodetector is demonstrated. A discussion of the concept is given, and the results of a simple experiment are given. Various permutations of the ideas described can permit different kinds of coherent and incoherent optical-signal processing and optical control of pulsed photodetection     

IC compatible optical coupling techniques for integration of ARROWwith photodetector

New structures for efficient coupling of light from an antiresonant reflecting optical waveguide (ARROW) to integrated photodiode are proposed and analyzed. Both end-fire and leaky-wave types of coupling have been considered and it is found that high-coupling efficiencies can be achieved by exploiting the intrinsic properties of the ARROW structure without requiring additional non-IC compatible materials. The coupling structures were simulated using a simple ray optics model as well as the beam propagation method (BPM). Fabrication process conditions and measurement results for various coupling structures are presented     

Comparative analysis of hematocrit measurements by dielectric and impedance techniques

In a previous paper, a new dielectric technique was used to estimate hematocrit (HTC) in extracorporeal blood circulation systems, independently of plasma conductivity or osmolarity. Although many impedance techniques have been formerly proposed in the literature, none has been evaluated against plasma conductivity and osmolarity. Herein, we estimate HTC based on permittivity changes and also with other four techniques found in the literature. Besides, the error incurred in each is also studied when plasma conductivity and osmolarity changed as much as 1 mS/cm and 50 mOsm/kg, respectively. The dielectric (permittivity) technique has an error close to 5.4%, while the others showed both tendencies, i.e., lower error (2.5%, two of them) and higher error (8.6% and 16.3%, the other two). The dielectric technique, even though did not produce the lowest error, provides a well-described physical model along with simple instrumentation.     

A novel resonant cavity enhanced photodetector with flat-top and steep-edge response

A novel resonant cavity enhanced (RCE) photodetector with flat-top and steep-edge response is presented. The response is obtained by designing a gradient-thickness P area in the absorption cavity. Simulation results show that the maximum and minimum values of the quantum efficiency in bandpass are 85.242% and 87.564% respectively, the ripple is about 3.6%, and 0.5 dB, 3 dB and 20 dB bandwidths are 0.3 nm, 0.4 nm and 1.2 nm, respectively. The mesa area is 10 μm × 10 μm and the frequency response bandwidth is 87 GHz. Compared with similar photodetectors, this photodetector has high quantum efficiency, narrow spectral response linewidth, good flat-top and steep-edge response and ideal high-speed characteristics.     

一种具有平顶陡边响应的长波长光探测器

在PIN型光探测器的基础上制备了一种适用于波分复用系统的具有平顶陡边响应的长波长光探测器。利用低压金属有机化学气相沉积(LP-MOCVD)设备在GaAs衬底上二次外延生长了具有台阶结构的GaAs/AlGaAs滤波腔和InP基PIN光探测器。高质量的GaAs/InP异质外延采用了低温缓冲层生长工艺;具有台阶结构的Fabry-Pérot(F-P)滤波腔采用了纳米量级台阶的制备方法。通过理论计算优化了实现平顶陡边光谱响应特性的器件结构;并通过实验成功制备出了具有平顶陡边响应性能的光探测器,器件的工作波长位于1 549nm,峰值量子效率大于25%,0.5dB光谱响应线宽为3.9nm,3dB光谱响应线宽为4.2nm,响应速率达到17GHz。     

Indium gallium arsenide antimonide photodetector grown by liquid phase epitaxy: Electrical characterization and optical response

Current–voltage (I–V) and R₀A curves and spectral response as a function of bias voltage and temperature of p–n indium gallium arsenide antimonide (In_0.14Ga_0.86As_0.13Sb_0.87)/n-GaSb photodiodes are presented. InGaAsSb quaternary alloys with a bandgap energy of about 653 meV were grown using the liquid phase epitaxy technique on top of (100) GaSb substrates. Device structure was fabricated using a process that includes passivation with sodium sulfide, thermal annealing and metallizations. The diode architecture was a back-illuminated (B-I) structure with a ring-shaped metallic contact in the GaSb substrate face. Photodiode spectral response showed good performance in the entire temperature range between 20 K and 300 K.     

IGCT在快脉冲条件下开通过程研究

集成门极换流晶闸管(IGCT)开关具有耐压高、通流能力强、工作重频高的特点,然而在纳秒级脉冲功率系统中应用较少。本文以株洲南车生产的非对称性IGCT做开关,通过搭建脉冲形成网络(PFN)纳秒级放电回路,初步研究了IGCT在快放电过程中的开关导通情况。通过理论分析、数值模拟和实验验证发现,目前工业领域的IGCT器件由于触发电流难以有效扩展导通,导致IGCT导通速度存在饱和值,很难在纳秒级别的脉冲下直接实现开关作用,但可以作为脉冲压缩的前级开关使用。     

Image denoising and quality measurements by using filtering and wavelet based techniques

An image is often corrupted by noise in its acquisition and transmission by various kinds of noises. Image denoising using thresholding methods find appropriate values (threshold) which separates noise values to actual image values without affecting the significant features of the image. Wavelet transform represents image energy in compact form and representation helps in determining threshold between noisy features and important image feature. In this paper we have worked with denoising of salt–pepper and Gaussian noise. The work is organized in four steps as follows: (1) image is denoised by filtering method, (2) image is denoised by wavelet based techniques using thresholding, (3) hard thresholding and filtering method applied simultaneously on noisy image, (4) results of PSNR (peak signal to noise ratio) and MSE (mean square error) are calculated by comparing all cases.     

Validation of pulse compression techniques for meteorologicalfunctions

A major technology barrier to the application of pulse compression techniques in meteorological radar is the presence of range sidelobes which mask and corrupt observations of weak weather phenomena in areas of strong extended meteorological scatterers or point target returns. Techniques for suppressing range sidelobes are well known, but without prior knowledge of the scattering medium's velocity distribution their performance degrades rapidly in the presence of doppler. Recent investigations have presented a “doppler tolerant” range sidelobe suppression technique. The thrust of the work described in this article is the extension of previous simulations to the transmission of dispersed/coded waveform pulses using the ELDORA X-Band weather research radar located at the National Center for Atmospheric Research (NCAR) Foothills Laboratory. This study shows that the use of Barker coded pulses along with pulse compression and doppler tolerant range sidelobe suppression provides: (1) increased sensitivity over a simple pulse of the same peak power, and nominal receive bandwidth by a factor equal to the time-bandwidth product, and (2) accurate spectral moment estimates