Improving the response of optical phase modulators in SOI bycomputer simulation

This paper reports on the simulation of a low-loss single-mode optical phase modulator fabricated in silicon-on-insulator (SOI) material. The device operates by injecting free carriers to change the refractive index in the guiding region, and has been modeled using the two-dimensional (2-D) device simulation package SILVACO. SILVACO has been employed to optimize the overlap between the injected free carriers in the intrinsic region and the propagating optical mode. Attention has been paid to both the steady state and transient properties of the device. In order to produce quantitative results, a particular p-i-n device geometry has been employed in the optimization, but the trends in the results are general enough to be of help in the design of many modulator geometries. The specific example device we have used is designed to support a single optical guided mode and is of multimicrometer dimensions thus simplifying fabrication and allowing efficient coupling to-from single-mode fibers (SMF's) or other single-mode devices. The modeling indicates that increased dc device performance results from an increase in the doping concentrations and the contact diffusions of the p⁺ and n⁺ regions. The transient performance of the device in terms of switching times depends on the separation of the p⁺ and n⁺ regions. The optimizations are applicable to large (multimicrometer size) modulators. Phase modulators with low driving currents (<8 mA) and modulators with transient rise times of 39 ns and fall times of 6 ns are predicted     

Single-channel blind estimation of arterial input function and tissue impulse response in DCE-MRI

Multipass dynamic MRI and pharmacokinetic modeling are used to estimate perfusion parameters of leaky capillaries. Curve fitting and nonblind deconvolution are the established methods to derive the perfusion estimates from the observed arterial input function (AIF) and tissue tracer concentration function. These nonblind methods are sensitive to errors in the AIF, measured in some nearby artery or estimated by multichannel blind deconvolution. Here, a single-channel blind deconvolution algorithm is presented, which only uses a single tissue tracer concentration function to estimate the corresponding AIF and tissue impulse response function. That way, many errors affecting these functions are reduced. The validity of the algorithm is supported by simulations and tests on real data from mouse. The corresponding nonblind and multichannel methods are also presented.     

Critical Layer Thickness in Exponentially Graded Heteroepitaxial Layers

Exponentially graded semiconductor layers are of interest for use as buffers in heteroepitaxial devices because of their tapered dislocation density and strain profiles. Here we have calculated the critical layer thickness for the onset of lattice relaxation in exponentially graded In _x Ga_1?x As/GaAs (001) heteroepitaxial layers. Upwardly convex grading with \( x = x_{\infty } \left( {1 - {\rm e}^{ - \gamma /y} } \right) \) was considered, where y is the distance from the GaAs interface, γ is a grading length constant, and x _∞ is the limiting mole fraction of In. For these structures the critical layer thickness was determined by an energy-minimization approach and also by consideration of force balance on grown-in dislocations. The force balance calculations underestimate the critical layer thickness unless one accounts for the fact that the first misfit dislocations are introduced at a finite distance above the interface. The critical layer thickness determined by energy minimization, or by a detailed force balance model, is approximately \( h_{\rm{c}} \approx <Exponentially graded semiconductor layers are of interest for use as buffers in heteroepitaxial devices because of their tapered dislocation density and strain profiles. Here we have calculated the critical layer thickness for the onset of lattice relaxation in exponentially graded In _x Ga_1−x As/GaAs (001) heteroepitaxial layers. Upwardly convex grading with x = x_￥ ( 1 - e ^{- g/y} ) x = x_{\infty } \left( {1 - {\rm e}^{ - \gamma /y} } \right) was considered, where y is the distance from the GaAs interface, γ is a grading length constant, and x _∞ is the limiting mole fraction of In. For these structures the critical layer thickness was determined by an energy-minimization approach and also by consideration of force balance on grown-in dislocations. The force balance calculations underestimate the critical layer thickness unless one accounts for the fact that the first misfit dislocations are introduced at a finite distance above the interface. The critical layer thickness determined by energy minimization, or by a detailed force balance model, is approximately h_c ? < h_{\rm{c}} \approx < Although these results were developed for exponentially graded In _x Ga_1−x As/GaAs (001), they may be generalized to other material systems for application to the design of exponentially graded buffer layers in metamorphic device structures such as modulation-doped field-effect transistors and light-emitting diodes.     

Analysis and Implementation of a Time-Interleaved ADC Array for a Software-Defined UWB Receiver

A software-defined radio (SDR) for ultrawideband (UWB) communication systems places several stringent requirements on the analog-to-digital converter (ADC). One alternative to using a single ADC is to sample the received signal with an array of lower speed ADCs that were driven by interleaved sampling clocks; however, mismatches among the ADCs will result in signal distortion. This paper makes three important contributions to overcoming this problem: 1) analytical quantification of the impact of ADC gain, offset, and timing mismatches on the performance of a time-interleaved sampling ADC array for UWB signals; 2) demonstration of the efficacy of using a pilot-based matched-filter architecture to mitigate the impact of timing mismatches in the presence of multipath; and 3) implementation of an 8-ADC time-interleaved UWB SDR testbed that operates at an effective sampling frequency of 6.4 GHz. In addition, our findings allow for the design specification of the number of pilots required to obtain a desired system performance. The simulation and measured performance results from this paper demonstrate that ADC mismatches can be controlled to within plusmn10%, yielding acceptable levels of distortion and bit-error-rate (BER) performance on the UWB SDR testbed. Both analytical and simulation results also demonstrate the efficacy of a pilot-based matched filter in mitigating the impact of timing mismatch errors, even in the presence of multipath.     

Algebraic decoding of the (32, 16, 8) quadratic residue code

An algebraic decoding algorithm for the 1/2-rate (32, 16, 8) quadratic residue (QR) code is found. The key idea of this algorithm is to find the error locator polynomial by a systematic use of the Newton identities associated with the code syndromes. The techniques developed extend the algebraic decoding algorithm found recently for the (32, 16, 8) QR code. It is expected that the algebraic approach developed here and by M. Elia (1987) applies also to longer QR codes and other BCH-type codes that are not fully decoded by the standard BCH decoding algorithm     

Standing up to the big squeeze

Consumer confidence is at its lowest ebb since 1980. Economic growth is at a standstill & unemployment & inflation are both rising. Here's a front-line report from more than three dozen operators who are fending off the recession's dual pressures of slow sales & rising costs. They are using discount pricing, partnering with vendors, setting pay for performance standards & a variety of other recession-beating practices.     

Automated method for L-carnitine determination

Because of renewed interest in a possible connection between carnitine, lipid disorders, and myopathy, an automated method of analysis is desirable. Deproteinization of serum by use of membrane filter cones and automated assay with a bichromatic analyzer (the ABA-100) substantially increases efficiency without sacrificing the specificity and accuracy of the original manual enzymatic method. The described procedure allows for analysis of 80 speciments a day and is thus suitable for screening of selected populations. Normal values found in blood sera of adults were in the range of 25.0-73.8 mu mol/liter and the method has sufficient sensitivity to accurately measure concentrations as small as 10 mu mol/liter.     

Decreased antidiuretic response in diabetes insipidus during prolonged treatment with carbamazepine presumably due to enzym-induction

Carbamazepine (CA)-induced antidiuresis and fluorescence in urinary cortisol assay was compared during short-term and prolonged treatment in 3 patients with pituitary diabetes insipidus. The fluorescence induced by excreted CA-metabolite(s) (probably CA-10-11 epoxide and its derivatives) was used for the rough clinical estimation of the rate of metabolic elimination of the drug. A marked decrease was observed both in the antidiuretic response induced by CA and excretion of CA metabolite(s) during prolonged treatment - with a fixed dose of CA. These findings suggest that diminishing antidiuretic effectiveness might be due to auto-induction of CA. Auto-induction of CA may explain the delay in improvement of renal concentrating operation in patients with DI during long-term treatment with fixed doses of CA. Another clinical consequence of this mechanism may be the prevention of the development of water intoxication in neuro-psychiatric patients undergoing chronic treatment with CA.     

Triglyceride lipases in acute hepatitis

Two triglyceride lipases in postheparin plasma, the lipoprotein lipase (LPL) and the hepatic triglyceride lipase (H-TGL) were separated by heparin-sepharose affinity chromatography and studied in controls and patients during the course of acute hepatitis. All three patients had increased content of triglycerides in the low density lipoproteins, and two of them had hypertriglyceridemia. Low activities of both lipases were found in the acute stage of the disease in all three patients. Concomitantly one of the patients had absolute low lecithin: cholesterol acyltransferase (LCAT) activity, and in the two other patients a relative LCAT deficiency was present. The increased content of triglycerides in LDL that may be found in liver disease, may not only be due to low H-TGL and LPL, but also LCAT deficiency.