Effects of internal scattering on X-ray microtomography imagereconstruction

A computer simulation is used to study the problem of internal scattering for targets that are imaged using X-ray microtomography. A strategy is outlined for selecting X-ray energy and image resolution based on properties of the material being imaged. The X-ray scanning process is simulated by applying a Monte Carlo technique to a modeled target that emulates the material properties of a microelectronic device. The X-ray photons are subject to photoelectric absorption, Rayleigh scattering, and Compton scattering. The simulation applies a method of high-resolution image reconstruction based on discrete Fourier transforms. Examples of reconstructed images that have 0.5-μm spatial resolution are shown for images of simulated lead and aluminum targets     

Duality transformations for general bi-isotropic (nonreciprocalchiral) media

The transformation is defined to leave a given bi-isotropic medium invariant, whence it is self-dual in this transformation. It is shown that duality transformations always exist in pairs, labeled as left-hand and right-hand transformations. Self-dual fields are seen to be generalizations of the wave fields E± applied in the analysis of reciprocal chiral media. It is demonstrated that plane wave propagation and reflection problems in bi-isotropic media can be solved easily in terms of self-dual field decompositions. Nonreciprocity is seen to give rise to effects like polarization rotation in reflection, which cannot be interpreted in terms of reciprocal chiral media     

Exactly integrable nonlinear Schrodinger equation models withvarying dispersion, nonlinearity and gain: application for solitondispersion

We show that the methodology based on the generalized inverse scattering transform (IST) concept provides a systematic way to discover the novel exactly integrable nonlinear Schrodinger equation models with varying dispersion, nonlinearity and gain or absorption. The fundamental innovation of the present approach is to notice that it is possible both to allow for a variable spectral parameter with new dependent variables and to apply of the famous "moving in time focuses" concept of the self-focusing theory to the IST formalism. We show that for nonlinear optics this algorithm is a useful tool to design novel dispersion managed fiber transmission lines and soliton lasers. Fundamental soliton management regimes are predicted     

Gain recovery of bulk semiconductor optical amplifiers

The gain recovery time of 1.55-μm bulk semiconductor optical amplifiers (SOA's) with lengths from 500 to 1500 μm has been measured with a continuous-wave (CW) probe in the time domain. It is shown to decrease with increasing length down to 60 ps for the longest SOA. This behavior is theoretically explained. A lower limit for the recovery time is observed and explained     

The combined use of ultrasound and densitometry in the prediction of vertebral fracture

Measurement of ultrasonographic parameters provides information concerning not only bone density but also bone architecture. We investigated the usefulness of ultrasonographic parameters and bone mineral density for evaluating the probability of vertebral fracture. 397 postmenopausal women (59.1 +/- 6.0 years) with (n = 178) or without (n = 219) atraumatic vertebral fractures were studied. In all women, bone mineral density (BMD) of the lumbar spine was evaluated by dual X-ray absorptiometry (DXA) and speed of sound (SOS); broadband ultrasound attenuation (BUA) and Stiffness in the calcaneus were evaluated by an Achilles unit (Lunar Corporation). Ultrasonographic parameters and BMD were compared by examining the magnitude of the odds ratios, to determine which produces the highest estimate of the probability of odds of fracture, and by examining widths of the respective confidence intervals (CI) to show which estimate of odd ratio is the most precise. The relative risk of vertebral fracture, after adjusting for potential confounders, was 3.5 (CI 2.6-4.8) for BUA; 4.5 (CI 3.2-6.2) for SOS; 5.8 (CI 4.0-8.4) for Stiffness and 7.5 (CI 4.8-11.5) for BMD. Ultrasound (US) parameters were still significant independent predictors of vertebral fracture, even after adjusting for BMD. The relative risk of fracture for a simultaneous decrease by 1 SD of BMD and by 1 SD of each ultrasound parameter was 17.3 (CI 9.4-39.6) for BMD and SOS; 18.3 (CI 8.4-30.6) for BMD and BUA and 22.1 (CI 8.9-52.7) for BMD and Stiffness. Our data suggest that US and BMD provide complementary information which can be combined to improve estimates of vertebral fracture risk.     

Nonlinear Kerr interactions between modulated waves propagating ina single-mode fiber

The interactions induced by the nonlinear refractive index in a single-mode optical fiber (Kerr effect) between a modulated wave and a carrier wave have been investigated for amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK). The theoretical analysis, based on a time-domain solution, exploits the well-known formalism of self-phase modulation to describe the propagation of the input electric field in the fiber. The theoretical results were validated experimentally for amplitude- and phase-modulated waves with heterodyne detection techniques. These experiments mimic a coherent transmission system and allow the measurement of the spectrum of each of the waves that participate in the nonlinear interaction     

High-reliability GaAs-AlGaAs HBTs by MBE with Be base doping andInGaAs emitter contacts

The authors have developed a modified MBE growth process to produce high-gain n-p-n GaAs-AlGaAs heterojunction bipolar transistors (HBTs) with a mean time to failure (MTTF) of 1.5×10⁸ h at 125°C. Beryllium incorporation and diffusion are controlled through a combination of reduced substrate temperature and increased As/Ga flux ratio during MBE growth, resulting in extremely stable HBT profiles. The authors also demonstrate graded InGaAs surface layers with nonalloyed refractory metal contacts that significantly improve ohmic reliability compared to alloyed AuGe contacts. The ability to produce robust HBTs by MBE is critically important to this technology     

Chemometrics in surface spectroscopy

During the past 15 years the advent of laboratory microcomputers has had a dramatic effect on the information we can extract from spectroscopic data. In this paper we take a brief look at the historical development of this field, consider some of the techniques being currently researched, and comment on possible future trends.