H. Willebrand

Pattern formation in gas discharge systems with high impedance electrodes

Abstract A gas discharge is induced in a helium atmosphere using silicon electrodes and exhibits spatial structures of current filaments depending on the applied voltage, gas pressure and the geometry of the electrodes. These observations can be described phenomenologically by a reaction-diffusion equation. There is good qualitative agreement between solutions of the model equations and the experiment.

Observation of solitary filaments and spatially periodic patterns in a dc gas-discharge system

Abstract In a planar gas-discharge system consisting of a metal and a high ohmic semiconductor electrode we observe experimentally the generation of stationary current density filaments, a discontinuous device characteristic with hysteresis and the bifurcation from a homogeneous discharge into a spatially periodic discharge pattern.

An effective infrared-visible conversion technique for remote quantitative measurements of thermal fields

In this paper we study the applicability of a semiconductor gas discharge (SGD-) structure with a cryogenic discharge in neon for measuring spatially extended temperature distributions of heated bodies. The IR radiation from a heated body excites the photosensitive semiconductor cathode of the device thus controlling the current density and the visible light emission from the gas discharge layer. The infrared radiation distribution emitted by the heated body can be found from the visible discharge glow in the gas layer. Present experimental investigations show that the lower temperature limit of this technique is about 400 K while the spatial resolution is in the order of 50 to 100 μm.

Transition from pulsating to rocking localized structures in nonlinear dissipative distributive media with global inhibition

Abstract We consider localized structures generated in a certain of two component reaction-diffusion system. Four different electric systems, namely reverse-biased p-n diodes, p-n-p transistors, a gas-discharge system, and an electrical network are recapitulated and it is shown that, in spite of the existence of different underlying physical mechanisms, in all cases an activator-inhibitor system is realized. If these systems are driven by an external voltage source via a load resistor, the equation for the external circuit leads to the appearance of an integral term in the reaction terms causing a nonlocal additional inhibiting component. Numerical calculations of the equations system with the simplest reaction terms show exemplarily that there is a critical value of the load resistor in such systems: While below this critical value besides static only pulsating localized structures appear, for values larger than the critical one instead of pulsating only rocking localized structures can be stabilized. Near the critical resistor value we observe patterns composed of pulsating and rocking modes.

Medium access control protocols for robust wavelength division multiplexed local area networks

The wavelength-division-multiplexing (WDM) network architectures, in general, are based on transmitters and/or receivers that can be precisely, and speedily, tuned to predetermined, fixed wavelengths. These requirements are not easy to meet in practice. Robust WDM networks do not rely on fixed wavelength channels. Any node that requires a connection with another node dynamically selects a currently unused wavelength in the spectrum. Two medium access control protocols for robust WDM networks are presented and evaluated.

Robust wavelength division multiplexed local area networks

Abstract The wavelength division multiplexing (WDM) network architectures, in general, are based on transmitters and/or receivers that can be precisely and speedily tuned to predetermined, fixed wavelengths. These requirements are not easy to meet in practice. Robust WDM, the subject of this paper, has been introduced as a technique to implement WDM in the local area. The strength of this novel approach is that it does not rely on wavelength-sensitive devices that are required to perform with extreme stability. This paper describes the basis of this approach and presents two types of medium access control (MAC) protocols that are suitable for Robust WDM local area networks (LANs). The performance of networks based on these protocols is discussed in detail. It is shown that the new approach has considerable promise for WDM networks in the local area.

Adaptive robust optical fiber receiver/transmitter

In the next decade, data transmission with speeds at several tens of gigabits per second (Gb/s) and beyond in short-haul local area and metropolitan area computer networks, as well as long- haul telecommunications networks, will be necessary to satisfy the ever increasing demands on bandwidth. Time division multiplexing based transmission systems become increasingly difficult to implement at higher speeds due to the speed limitations of electronics. In addition, these networks based on single wavelength transmission, use the available bandwidth inefficiently. As a result, both timing synchronization and bandwidth sharing among large numbers of users become major challenges. Wavelength division multiplexing (WDM) eases most of these problems; but introduces wavelength synchronization as the primary technical hurdle. The adaptive robust WDM receiver adjusts dynamically to the sources and thus alleviates many of the most serious and costly disadvantages of WDM. The technical burdens of WDM transmission are shifted to the receiver which is designed to accommodate the manufacturing and operating imperfects of the transmitter sources. Consequently, the receiver has then to be more sophisticated, but the added complexity in the receiver is in VLSI, simple PIN diodes, and passive guided wave optics. These components are inherently among the least expensive components in an optoelectronic system. Commercial WDM systems, especially in local area computer network environments, can thus be produced at significantly lower cost.

Pattern Formation in Nonlinear Physical Systems with Characteristic Electric Properties

Systems are considered which consist of two layers, one of them characterized by a high ohmic, approximately linear and one by an S-shaped current-voltage characteristic. We describe such systems mathematically in terms of two-component reaction-diffusion equations. Analytical and numerical investigations of these equations reveal that the systems can self-organize different stable stationary inhomogeneous nonequilibrium structures as well as spatio-temporal irregular behaviour. These results are in good qualitative agreement with the current density patterns in two experimentally investigated systems, namely a one-dimensional npnp-semiconductor structure and a one-dimensional gas discharge system.

Excitation of wave patterns in an infrared-visible converter

By studying the non-stationary transport properties of a semiconductor gas discharge (SGD) structure used for the spatial and temporal conversion of infrared radiation into the visible wavelength range, we observed the phenomena of spontaneously excited ionization waves under strong light excitation of the converter structure. The waves propagate normal to the electric current flow and, by increasing the supply voltage, their velocity increases and can exceed that of acoustic waves in the gas filled region of the structure. Wave interaction is also found to be followed by interference effects. This dynamical instability of the homogeneous state can probably be explained by a nonlinear oscillator model.

Spatially and temporally resolved IR image detection with a semiconductor-gas-discharge device

We suggest a method for spatially and temporally resolved IR-image detection by using a semiconductor-gas discharge (SGD-) structure. The operation of the device is based on the conversion of IR-radiation into the visible spectral range. Especially, we discuss the influence of the system parameters on the operation of the device and propose a technical realization for a real time IR-image conversion camera based on the converter cell in connection with a standard CCD-camera. In combination with a gateable CCD-camera or an electron-optical recording technique (e.g., streak- or framing camera), the converter structure could be used for high speed IR-imaging in the microsecond range.