Gerald Soelkner

Experimental and theoretical laser-Doppler frequency spectra of a tissuelike model of a human head with capillaries

Laser-Doppler flow measurements with penetration depths from 10 to 30 mm have been performed on a layered model of the human head consisting of a regular array of capillaries with diameters of 340 microm embedded in an epoxy matrix with tissuelike scattering and absorption properties. Monte Carlo simulations and an analytical approach based on diffusing wave spectroscopy have corroborated the measurements and have led to a quantitative description of flow in deep-lying tissue layers with regard to layers near the surface also. The results indicate that it may be possible to measure changes in cortical blood flow even in the presence of a well-perfused scalp.

MONTE CARLO SIMULATIONS AND LASER DOPPLER FLOW MEASUREMENTS WITH HIGH PENETRATION DEPTH IN BIOLOGICAL TISSUELIKE HEAD PHANTOMS

Laser Doppler flow measurements on biological tissuelike phantoms have shown that penetration depths of 30 mm could be obtained, thus exceeding the penetration depth of commercial instruments for the measurement of skin perfusion by more than an order of magnitude. Monte Carlo simulations were performed and compared with measurement results obtained on a headlike tissue model to quantify the influence of perfusion of the scalp on the cortex perfusion results. We found Doppler frequency spectra to be independent of the mean scattering angle and could be fitted with a sum of Gaussian functions, using a simple analytical model.

On-chip reliability investigations on power modules actually working in inverter systems

The paper contains the results of temperature measurements for reliability investigations on IGBT modules actually working in inverters for industrial and traction applications. For this, the chip temperature and its transient behaviour have been monitored under different driving conditions of the inverters. Specially prepared IGBT-modules have been used to investigate the industrial and the traction inverters in their electrical and thermal behaviour. The measurement techniques were suitably adapted to the different configurations of the two inverters.

Back side optical beam induced current method for the localization of electric field enhancements in edge termination structures of power semiconductor devices

Abstract Planar edge termination structures of power semiconductor devices reduce the high electric potential close to the sawed physical edges. Careful design of field ring diffusions and metallized field plates are, however, required to avoid local electric field enhancements that could lead to premature breakdown. It has been shown that back side optical beam induced current (BS-OBIC) measurement offer a means to experimentally probe electric field distributions in termination structures that are optically inaccessible due to metallized field plates.

Time-resolved transillumination of turbid media

The suitability and limits of time-resolved transillumination to determine inner details of biological tissues are investigated by phantom experiments. The achievable improvement is demonstrated by using different phantoms (absorbing objects embedded in a turbid medium). By means of line-scans across a sharp edge the spatial resolution and its dependence on temporal resolution can be determined. To demonstrate the physical resolution according to the Rayleigh-criterion, measurements were performed on blackened bead pairs. Investigations with partially transparent beads demonstrate the high sensitivity of time-resolving techniques with respect to variations in scattering or absorption coefficients.

Laser probing system for the accurate detection of surface acoustic wave phase velocities

The present work demonstrates a high-resolution technique for the optical detection of the phase and amplitude of high frequency surface acoustic waves (SAWs). The test setup incorporates a mode-locked picosecond laser, harmonic mixing, and coherent detection, and it allows not only the measurement of the surface wave field but also the direct determination of the phase velocity. A measurement bandwidth in excess of 2 GHz is achieved. The maximum scan length was 4 cm. As a substrate, LiNbO3 has been used for the test measurements. Minimum detectable surface displacement and dynamic range were 1 pm/Hz1/2 and 40 dB, respectively. The method enables the determination of the phase velocity with a resolution of 1.5 (DOT) 10-5 in dependence of crystal cut, temperature, and frequency. An additional feature is that phase velocity values can be obtained with spatial resolution, i.e., velocity variation effects along the propagation path can be evaluated. We found that the assumption that the SAW velocity is constant across the whole device surface is not valid in general.