Effects of strain on the high speed modulation of GaAs- andInP-based quantum-well lasers

A small-signal numerical analysis of pseudomorphic GaAs- and InP-based Fabry-Perot quantum-well lasers using calculated optical gain spectra with strain effects included is reported. Examination of the effect of lifetime broadening shows that the resonance frequency increases at a rate of ~250-MHz/meV reduction in the lifetime broadening for a GaAs-based strained layer laser. The modulation speed is limited by either device heating or facet damage. If the limitation is imposed by the optical power then the modulation speed increases as the laser cavity becomes shorter and the number of quantum wells increases. If the limitation is imposed by the injection current density, however, then the modulation speed decreases for the laser with shorter cavity length. The highest modulation speed is given by an optimum well number. A resonance frequency of ~16 GHz is predicted for a pseudomorphic GaAs-based laser with 30% excess In and average output power of ~5 mW     

Shrunken estimators of Weibull shape parameter from type-IIcensored samples

A shrunken estimator of the Weibull shape parameter for failure censored samples is suggested. This shrunken estimator is compared with shrunken estimators previously given. Even for sample sizes of five and ten this shrunken estimator, based on failure data censored at only three and four, respectively, can be used with advantage when one has a reasonable guess for the shape parameter. This estimator has higher efficiency and nearness than other estimators     

Estimation of average life for negative exponential distribution

This paper deals with the problem of average life estimation when the life times follow negative exponential distribution. A minimum mean square error estimator belonging to the class of linear combination of complete sufficient statistics has been obtained. The estimator thus obtained requires knowledge of coefficient of variation. The effect of using a guessed value of coefficient of variation on the estimator has also been studied.     

Enumeration of minimal cutsets from matrix representation of directed/undirected networks

This paper describes an algorithm for finding the minimal cutsets between a specified source and terminal nodes of directed/undirected networks. Adjacent nodes of a source node/merged node are determined by the collection of node numbers of columns corresponding to non-zero elements of the first row of the adjacency matrix to form the reduced adjacency matrix (RAM) corresponding to merged nodes. Connectivity of RAM has been checked to obtain the minimal cutsets.     

Correction of T2 distortion in multi-excitation RARE sequence

Correction schemes have been implemented to correct for T2 distortions in a multiexcitation RARE (rapid acquisition with relaxation enhancement) sequence where data from multiple echoes and multiple excitations are combined. Computer simulation studies and human imaging studies have been conducted to develop and test the correction procedures. A direct method and an iterative technique have been investigated. The direct technique utilizes Hermitian symmetry of the T2 weighted data and is shown to reduce distortions in T2 weighted images. The iterative scheme begins with an estimation of T2, wherefrom k-space data are computed and compared to the true data to provide error images. The error images are then used to refine iteratively the reconstructed images at a specified echo time. The iterative procedure has been used to improve T1 weighted images acquired through a sequence based on acquisition of two half-plane Fourier samples. These correction techniques should enable a practical implementation of RARE for producing T1 and T2 weighted images comparable to standard spin echo images.     

A model for electroluminescence in SrS:Ce ACTFEL display devices

An analytical model for electroluminescence in SrS:Ce AC thin-film electroluminescent display devices is presented. The model incorporates an exact calculation of the electric field and the effect of activator ionization and bulk traps. Activator ionization is needed to explain several features of luminescence behavior in SrS:Ce devices. These features include the second luminescence peak at the trailing edge of the voltage pulse and the time lag between the luminescence and the applied voltage when the applied voltage consists of bipolar trapezoidal pulses and rectangular pulses of low voltage amplitudes. As a mechanism for the ionization of activators, field-assisted tunneling is shown to be more likely than impact ionization by hot electrons. Physical processes are described in terms of rate equations, and field, current, and luminescence waveforms are calculated for one set of device parameters. The calculated and experimental luminescence waveforms agree     

A fast-settling GaAs-enhanced frequency synthesizer

An indirect, phase-locked-loop (PLL), GaAs-enhanced frequency synthesizer with 700 ns loop settling time has been developed. The two-chip GaAs insertion reduced the size of an existing synthesizer from 90 in³ to only 30 in³. The 6.0 in×5.5 in.×0.9 in module contains a 400 gate GaAs programmable divider and a sample and hold (S/H) which improved the settling time by 77% and reduced the size by 67% over the current state-of-the-art synthesizer. Furthermore, the divider reduced power dissipation by 9.7 W and the S/H reduced power dissipation by 1.3 W     

Learning-based ventricle detection from cardiac MR and CT images

The objective of this work is to investigate the issue of automatically detecting regions of interest (ROI's) in medical images. It is assumed that the regions to be detected can be roughly segmented by a threshold based on a likelihood measure of the ROI, First, an analysis of the global histogram is used to compute a preliminary threshold that is likely near the optimal one. The histogram analysis is motivated by the analytical result of a bell image intensity model proposed in this work. Then, the preliminary threshold is used to segment the input image, resulting in an attention map, which contains an attention region that approximates the ROI as well as many spurious ones. Due to the nonoptimality of the preliminary threshold, it can happen that the attention region contains a part of, or more regions than, the ROI. Learning takes place in two stages: (1) learning for automatic selection of the preliminary threshold value and (2) learning for automatically selecting the ROI from the attention map while dynamically tuning the threshold according to the learned-likelihood function. Experiments have been conducted to approximately locate the endocardium boundaries of the left and right ventricles from gradient-echo magnetic resonance (MR) images. Cardiac computed tomography (CT) images have also been used for testing. The boundary of the segmented region provided by this algorithm is not very accurate and is meant to be used for further fine tuning based on other application-specific measures     

Ion beam-assisted planarization of chemically vapor deposited diamond thin films using electron cyclotron resonance plasma

The high inherent surface roughness of as-deposited polycrystalline diamond films has made effective planarization processing of these films essential for most industrial applications. We have investigated the efficacy of ion beam sources for planarization in an electron cyclotron resonance plasma system using both direct substrate biasing and an accelerating grid system. Rough polycrystalline diamond films were synthesized using hot filament chemical vapor deposition. Both the etching rates and the resultant surface roughnesses were found to decrease as the angle of incidence (relative to the substrate surface normal) of the ion beam was increased. In the case of direct biasing of the sample, acicular features were observed following processing at higher incident angles. The use of double ion-extraction grids in conjunction with concomitant sample rotation was found to produce more uniform planarization of the diamond films. The rate of surface roughness reduction was found to be nonlinear and decreased with time. For both ion extraction methods investigated, the average film roughness (R_a) was significantly reduced from 0.2 to 0.05–0.06 μm.     

Finite-difference time-domain analysis and experimental examinationof the performance of a coupled-cavity MQW laser/active waveguide at1.54 μm

The performance characteristics of a coupled cavity InGaAsP-InP MQW laser/active waveguide made by one-step epitaxy and well-controlled reactive ion etching (RIE) have been theoretically analyzed and experimentally determined. A theoretical model based on a finite-difference time-domain (FDTD) technique was used to simulate the propagation of an optical wave launched in the coupled system and determine the reflectivity of the facets created by RIE. The calculated effective reflectivity of the coupling region consisting of two facets and an air gap is between 0.45 and 0.55, which is in good agreement with the experimentally measured value of 0.5. The reflectivity of a single etched mirror derived from this value is estimated to be 0.3. A 120-μm-long monolithically integrated active waveguide when biased as a modulator and excited by the laser shows a maximum extinction ratio of 8 dB and a modulation bandwidth ⩾14 GHz at a dc bias of -0.5 V with a bias swing of 2 V