Invasiveness of LiTaO/sub 3/ and GaAs probes in external E-O sampling

Disturbances of signals on a coplanar waveguide (CPW) induced by the presence of LiTaO/sub 3/ and GaAs electrooptic probes in external electrooptic (EO) sampling have been simulated and compared quantitatively. The finite-difference-time-domain method is used to simulate the full wave field around a coplanar waveguide on a GaAs substrate in an external EO sampling configuration. The results indicate that the induced signal disturbance, or invasiveness, of a LiTaO/sub 3/ probe is almost ten times that of a GaAs probe in terms of the magnitude of S/sub 11/, but that LiTaO/sub 3/ yields about two times the EO response for a given S/sub 11/ and optical probing wavelength. The transparency of LiTaO/sub 3/ to shorter wavelengths, however, allows an even higher sensitivity for this material relative to GaAs. The results suggest that these probes do not exhibit significant invasiveness (magnitude of S/sub 11/ smaller than -40 dB), if they are removed from contact by the distance of CPWs center conductor width.<>     

Hybrid cam mechanisms

This paper investigates the problem of driving a machine with several reciprocating heavy inertias. The drive should also realize some programmable flexibility of the motion, so that the operation of the machine, e,g., synchronization between different motions and duration of standstills, can easily be modified. The use of a single servomotor for every motion is not the best solution, since it does not allow for energy recuperation. This means that the installed servomotor power will be much higher than the required net peak power for the machine as a whole. The presented solution, however, reduces the servomotor peak torque and power by one order of magnitude, and makes energy transfer between the motions possible. The hybrid cam mechanism consists of a hybrid drive, which is a combination of a servomotor, a constant velocity motor and a cam follower mechanism. The operation principle exploits the nonlinear characteristics of the cam to add flexibility at low cost of energy. The hybrid solution is particularly successful for motions involving high peak acceleration. In addition, the concept can be used to reject disturbances. Simulations show the performance of the hybrid cam mechanism.     

GaInP/GaAs HBTs for high-speed integrated circuit applications

The use of GaInP/GaAs heterojunction bipolar transistors (HBTs) for integrated circuit applications is demonstrated. The discrete devices fabricated showed excellent DC characteristics with low V_ce offset voltage and very low temperature sensitivity of the current gain. For a non-self-aligned device with a 3-μm×1.4-μm emitter area, f_T was extrapolated to 45 GHz and f_max was extrapolated to 70 GHz. The measured 1/f noise level was 20 dB better than that of AlGaAs HBTs and comparable to that of low-noise silicon bipolar junction transistors, and the noise bump (Lorentzian component) was not observed. The fabricated gain block circuits showed 8.5 dB gain with a 3-dB bandwidth of 12 GHz, and static frequency dividers (divide by 4) were operable up to 8 GHz     

Absorptive corrections to structure functions at small-x

The role of corrections due to the interactions between different branches of parton (gluon) cascades is studied. It is shown that at HERA energies these interactions increase the absorptive (two-Pomeron cut) contributions by about 60 to 80% compared with the Gribov-Levin-Ryskin equation.This work was supported, in part, by a Soros Foundation Grant awarded by the American Physical Society     

Color quantitation through image processing in dermatology

Classical color models and their applications to computer vision are reviewed. The performances of color quantitation from digitized images are compared with those derived from a chromameter. The color quantitation obtained from either digitized color slides or directly digitized images is proved to be more efficient than the conventional visual assessment of observers. A methodology is proposed for determining the specific color indices which are needed in dermatology. An application of this methodology is developed for designing a blanching quantitation index in order to replace the visual assessment during McKenzie tests.     

Nonclassical states generated via four-wave mixing

The nonclassical properties of quantum fields generated by four- or three-wave mixing are investigated on the basis of assumptions of two-photon coherent (squeezed) input states and the approximation of strong pumping. The influence of losses is shown supposing the model of mutually independent phonon reservoirs. The possibility of producing squeezed and sub-Poisson light outputs is demonstrated in single or superposed modes using computer solutions.The author wishes to acknowledge helpful discussions with Dr. J. Peina and Dr. M. Kárská.     

Effect of multijunction grill length on its lower hybrid current drive efficiency

The distance between the junction of subsidiary waveguides in the main waveguide and the grill mouth has an important influence upon the spectrum of radiated waves, the total reflection coefficient, the power distribution among the separate waveguides and the actual phases of incident waves. This effect is nearly as important as that of the phase shift between the adjacent waveguides and it is necessary to carry out optimization with respect to this parameter in the grill design.