Symmetric self-electrooptic effect device: optical set-reset latch,differential logic gate, and differential modulator/detector

The symmetric self-electrooptic-effect device (S-SEED), a structure consisting of two p-i-n diodes electrically connected in series and acting as an optically bistable set-reset latch, is discussed. Applications and extensions of this device are also discussed. The devices do not require the critical biasing that is common to most optically bistable devices and thus is more useful for system applications. They have been optically cascaded in a photonic ring counter and have been used to perform different NOR, OR, NAND, and AND logic functions. Using the same device, a differential modulator that generates a set of complementary output beams with a single voltage control lead and a differential detector that gives an output voltage dependent on the ratio of the two optical input powers have been demonstrated     

Triggering GaAs lock-on switches with laser diode arrays

Progress toward the triggering of high-power photoconductive semiconductor switches (PCSSs) with laser diode arrays, is reported. An 850-W optical pulse from a laser diode array was used to trigger a 1.5-cm-long switch that delivered 8.5 MW to a 38.3-Ω load. Using 166-W arrays, it was possible to trigger a 2.5-mm-long switch delivering 1.2 MW with 600-ps rise-times at pulse repetition frequencies of 1 kHz. These 2.5-mm-long switches survived 10⁵ pulses at 1.0 MW levels. In single-pulse operation, up to 600 A was switched with laser diode arrays. The goal is to switch up to 5 kA in a single-shot mode and up to 100 MW repetitively at up to 10 kHz. At electric fields below 3 kV/cm GaAs switches are activated by creation of one electron-hole pair per photon. This linear mode demands high laser power and, after the light pulse, the carriers recombine in nanoseconds. At higher electric fields GaAs acts as a light-activated Zener diode. The laser light generates carriers as before, but the field induces gain such that the amount of light required to trigger the switch is reduced by a factor of up to 500. The gain continues until the field across the sample drops to a material-dependent lock-on field. The gain in the switch allows for the use of laser diodes     

A resolution comparison of several time-frequency representations

Two signal components are considered resolved in a time-frequency representation when two distinct peaks can be observed. The time-frequency resolution limit of two Gaussian components, alike except for their time and frequency centers, is determined for the Wigner distribution, the pseudo-Wigner distribution, the smoother Wigner distribution, the squared magnitude of the short-time Fourier transform, and the Choi-Williams distribution. The relative performance of the various distributions depends on the signal. The pseudo-Wigner distribution is best for signals of this class with only one frequency component at any one time, the Choi-Williams distribution is most attractive for signals in which all components have constant frequency content, and the matched filter short-time Fourier transform is best for signal components with significant frequency modulation. A relationship between the short-time Fourier transform and the cross-Wigner distribution is used to argue that, with a properly chosen window, the short-time Fourier transform of the cross-Wigner distribution must provide better signal component separation that the Wigner distribution     

Fast power-efficient circuit-block switch-off scheme

A methodology is proposed for glitch-free power switching of unused circuit blocks in leakage dominated deep-submicrometre technologies. With respect to conventional non-glitch-free approaches, significantly faster settling time and lower power consumption during the activation of the block are obtained.     

An analytical model for charge pumping below strong inversion and accumulation

A compact analytical model for charge pumping (CP) is derived. It accounts for emission during the high and low gate bias levels and assumes instantaneous transition edges. This model, which does not apply on the top of the CP curves when using large gate voltage swings, where emission fully proceeds during the transitions edges of the gate signal, applies on all the other regions provided that the transition times of the gate signal are much shorter than the times at steady state biases. Therefore it holds at large bias swings on both edges of Elliot curves and at small bias swings on the whole Elliot curves provided that the interface traps are completely filled [1]. It is compared with the analytical model proposed by Wachnik and Lowney [2] in which emission is not accounted for at all. This model, which holds at Elliot curve maxima when small voltage swings are used, has been shown to be extremely useful for studying interface trap properties [1, 2, 3 and 4]. The CP model proposed primarily, that of Brugler and Jespers [5], is used as a reference. The model derived in this article very satisfactorily fits the experimental curves in the regions of large CP current where it holds. Discrepancies at low current levels are due to the well-known contribution of the transistor source and drain regions or could be due to edge effects. Comparing the different models and the experimental curves allows to evaluate emission and capture during different regions of the gate bias period. The three regions of CP response, depending on the gate voltage swing and involving or not emission and the full filling of the interface traps, are also evidenced.     

Variable time delay of microwave signals using high dispersion fibre

A programmable time delay of 0-5 ns has been demonstrated for a 3 GHz signal using a tunable semiconductor diode laser and 1 km of a new high-dispersion singlemode fibre with negative chromatic dispersion. Because it is fibre based and uses direct detection, this system is resistant to vibration effects.<>     

A note on the mixed potential representation of electric fields inlayered media

A mixed potential formulation is given for electrical fields in layered environments. Contributions to the field from charges are identified explicitly through a scalar Green's function for layered media. The outcome is a computationally expedient Sommerfeld integral representation     

Frequency-hopped ARQ for wireless network data services

The throughput efficiency and probability of undetected error of automatic-repeat-request (ARQ) protocols employing slow frequency hopping over Gilbert-Elliott (1960, 1963) fading channels are investigated. Throughput efficiency formulas for the three basic ARQ schemes, “stop-and-wait” (SW), “go-back-N” (GBN), and “selective-repeat” (SR), are obtained, and the improvement in throughput efficiency due to slow frequency hopping is examined. Results are also presented on the problem of determining the number of carriers used for frequency hopping so as to minimize the effect of fading channel memory and, therefore, optimize system performance. This work has application to TDMA architectures and packet data services for third generation wireless networks, where projected data service requirements call for error rates on the order of 10^-9 , and the multiple service, multiple access environment demands that the delay for any service be minimized