Deutsche Bundespost

The approach to creation of electroclinic materials is proposed, based on mixtures from lamellar matrix and dopants of chiral molecules, having the inclination of long axes even in orthogonal phase of matrix. It is supposed to obtain the transition from collective switching mechanisms (e.g. soft or Goldstone modes) to molecular motions of chiral impurities. Short response times in submicro- and nanosecond range together with low driving voltage as well as the possibility to compensate the temperature dependences of the induced electro-optical angle could be the characteristics of the proposed electroclinic materials. Investigations of mixtures, prepared on the basis of proposed consideration, are described.

We investigated the low-field magnetoresistivity of a two-dimensional electron gas (2DEG) underneath microstructured ferromagnetic gratings. The strength and the direction of an externally applied magnetic field determines the strength and the shape of the micromagnet's stray field. Commensurability effects due to the imposed periodic magnetic field are in competition with similar effects resulting from a strain-induced periodic electrostatic potential, associated with the patterned ferromagnets on top of the heterojunction. The presence of both a periodic magnetic field and a periodic electrostatic potential gives rise to the interesting interference phenomena presented here.

A new concept for a highway switch, which can be used to connect different optical wavelength division multiplexing data highways for data exchange, is proposed and the system relevant properties are outlined. For the required add-drop filter elements we used ring resonators. Typical characteristics of channel bandwidth, channel spacing, free spectral range, amplification, and cross-talk behavior of a highway switch with double-cavity ring resonators are basically examined and to some extent compared with solutions that were obtained with standard single-ring resonators. A signal flow chart transformation for evaluating filter transfer functions is presented.

The deposition of ferromagnetic microstructures on top of a high-mobility two-dimensional electron gas (2DEG) allows the investigation of electron transport in a periodic magnetic field which alternates on a length scale small compared to the elastic mean free path of the electrons. The longitudinal resistance of the 2DEG displays, as a function of the externally applied field, the long-predicted magnetic commensurability oscillations which result from the interplay between the two characteristic length scales of the system, the classical cyclotron radius ${R}_{c}$ of the electrons and the period $a$ of the magnetic field modulation.

Iterative aggregation/disaggregation procedures are a convenient numerical solution method for computing the stationary distribution vector of an ergodic homogeneous Markov chain with a finite state space. We show the equivalence of this method and a two-level multigrid method. Based on error results of the A/D-method, we provide an error analysis of an efficient multigrid variant of the multiplicative Schwars-iteration method. Furthermore, we apply these results to a multigrid version of the replacement process approach developed by Sumita and Rieders.

Efficient power transfer between InP/InGaAsP-based optoelectronically integrated circuits (OEICs) and single-mode optical fibers can be realized by appropriate tapered waveguide- structures on the chip itself, so that the fiber can be directly butt-jointed to the chip. First, a simplified sample structure is considered for the purpose of testing a scalar and two different fully vectorial Finite Difference-Beam Propagation Methods (FD-BPM). The sample structure is excited with the eigenmode of the OEIC-chip waveguide and the optical waves propagating through the device are investigated by means of 3D BPM simulation. Starting from three already fabricated prototypes, scalar simulations with modified parameters of geometry are performed to predict performance of the devices in order to achieve an optimized design. Tapered structures with linearly and nonlinearly laterally tapered layers are investigated in order to minimize radiation loss by simultaneously maximizing coupling efficiency. Simulated data will be validated by measured results and numerically optimized structures will be presented.

Centroid depth determinations by angle-resolved self-ratio x-ray fluorescence spectrometry on ion-implanted depth profiles have been carried out at a synchrotron radiation source using ‘‘white’’ radiation. For ion implants of phosphorus in silicon wafers at the 1016 cm−2 level, the results are very satisfactory. The wafer temperature can be kept to a tolerable level if the low-energy component is preabsorbed in a suitable filter (foil) and if additionally the specimen is placed in an environment of helium at atmospheric pressure. The helium atmosphere is indispensable also for avoidance of the deposition of adventitious carbon on the specimen surface. Extrapolating from the results obtained so far, one can conclude that (with the specimen properly mounted in a helium atmosphere) centroid depth measurements should be possible also at a few times 1014 cm−2 dose density level.

Liquid water and water vapor are important components for many meteorological processes in the atmosphere. For radio communication links on Earth-satellite paths they may cause attenuations which can no longer be neglected for frequencies above 10 GHz and thus have a strong impact e.g. on low-margin systems. Unfortunately, there is a lack of information on quantities like cloud liquid and its temporal and spatial variability. In the past, it was shown that the liquid water content in clouds as well as the ubiquitous water vapor can be determined from radiometer measurements carried out simultaneously at two different frequencies at least. However, statistics are only available for short time periods. The authors discuss the capabilities of radiometer systems operating at 21.3 and 31.7 GHz. Data obtained over a 24 month period are also presented. >

Investigations of precipitation scattering at frequencies above 10 GHz have shown that the real extent of cochannel interference of the co-polar component can be underestimated considerably (up to 10 dB) by conventional measuring methods since some effects are not taken into account. This was previously proved theoretically and, inter alia, by measurements of the cumulative distribution of the transmission loss. An important consequence of this is that both the type and extent of the cochannel interference of the cross-polar component has to be assessed anew above 10 GHz, the more so as the polarization decoupling due to precipitation scattering is, on average, of the order of 10 dB. The present paper is therefore confined to two properties of the pure scattering signal: the form of the power density spectrum as a whole as well as the dynamic range of short-time fluctuations of the scattering power. In this context, the results of the cross-polar components are compared with those of the corresponding co-polar components.

A special procedure to fill the array of an array code (product code) is proposed that reduces the delay by about 35%, depending on the size of the array code. The method is based on an array code with diagonal readout, prefilling a region of approximately 30% of the array and then in parallel to the readout using diagonal, horizontal and vertical writing modes interchangeably. The mode changes are determined by a computer search, thereby minimising the number of prefilled components. >