Richard A. Kiehl

Behavior and dynamics of optically controlled TRAPATT oscillators

The experimental behavior of optically controlled TRAPATT oscillators is described and results of a detailed numerical analysis of the dynamics of this optical control are presented. Wide ranges in illumination level and circuit tuning conditions are considered in order to investigate the extent of the optical control and to bring out various features of this control. It is shown that illumination of the TRAPATT diode results in increased electrical activity and serves to produce substantial shifts in oscillation frequency and power. A salient feature of this optical control is that the variations in frequency and power are dependent on the frequency to which the oscillator circuit is tuned, making it possible to adjust the characteristics of the frequency and power control. The observed behavior is shown to be the result of an optical enhancement of the carrier avalanche process which acts to control the dynamics of plasma formation and, in turn, the final density of the trapped plasma. Results of the calculations are found to be in good agreement with experiment and to indicate that further improvement in the degree of control can be obtained with optimized device structures.

Performance of Optically Microwave Switching Devices Coupled

The performance of optically coupled microwave switching devices for pulse generation or other applications is detailed. The bias dependence of the RF power transfer is presented for a range of operating frequencies, thereby establishing the bias conditions required for a given ON/OFF ratio and insertion loss. Limits on peak RF power level and pulse repetition rate, as well as limitations arising from harmonic distortion and shot noise, are also examined.

Narrow microwave pulse generation by optical enhancement of TRAPATT oscillations

The generation of narrow, high-power microwave pulses has been achieved by illuminating a TRAPATT diode with narrow optical pulses from a GaAlAs DH laser. Tuning the oscillator circuit to a frequency substantially above the usual operation frequency of the diode was found to result in low-level oscillation during the unilluminated portions of the TRAPATT bias pulse and a strong enhancement (up to 20 dB) of the oscillation during the optical pulse. Experimental results on the generation of 35 W pulses as narrow as 25 ns (25 RF cycles) at 1 GHz, with a rise- and fall-time of 2 ns (2 RF cycles) and excellent pulse-to-pulse stability are presented. The physical mechanism of optical enhancement is explained with results from detailed TRAPATT numerical calculations.

Improvement of TRAPATT performance with optically generated carriers

It is demonstrated that a number of the difficulties which have thus far limited the application of the TRAPATT oscillator may be overcome by optically generating carriers within the devices active region. Illumination with a short light pulse at the beginning of the bias pulse extends the ranges of (i) bias, (ii) circuit tuning, and (iii) temperature over which the duration of leading edge jitter is short. Illumination during the application of the bias pulse significantly shifts the oscillation frequency. It is shown that this effect may be used to eliminate undesirable intra-pulse frequency drift with shaped light pulses. Also, frequency switching in time-frames approaching one RF period are accomplished, which opens up new applications for the TRAPATT oscillator.

An Optically Coupled Microwave Switch

A new microwave switch that uses lightwaves to couple microwave energy between its ports is shown to offer outstanding performance in terms of on/off ratio and reverse isolation while exhibiting a state independent input impedance.

Optical control of IMPATT oscillator dynamics

The results of detailed computer simulations of the effect of optically generated carriers on the dynamics of an IMPATT diode oscillator are presented. Both small- and large-amplitude oscillations are examined over wide ranges in operating conditions and optical power level. Agreement is obtained between the theoretical results and experimental observations. Key results of the calculations are used to establish the extent to which the amplitude and frequency of a conventional IMPATT oscillator can be optically modulated as well as the operating conditions under which this control is optimized.

Optically induced AM and FM in IMPATT diode oscillators

Optical modulation of an IMPATT oscillator is investigated both theoretically and experimentally. Detailed computer simulations of the oscillatory dynamics are used to determine the basic mechanisms responsible for optical modulation and to examine the extent of and means for optimizing the optical response. It is shown that optically induced changes in the conduction current minimum and in the levels of carrier slowdown or depletion-region modulation are the primary mechanisms responsible for optical modulation. Amplitude modulation via an optical quenching of the oscillations is found to be the dominant response, although optical enhancement is also possible under certain conditions. It is demonstrated that substantial levels of FM can also be produced, despite the fact that the IMPATT susceptance is insensitive to illumination. The experimental results are found to be in general agreement with the predictions of the theory. It is concluded that the optical response of the IMPATT is sufficient to make optical modulation a useful technique.

WA-B4 avalanching optoelectronic microwave gate

applications. We now have obtained AM1 efficiency values approaching 25 percent at 200 suns. The basic material’s considerations going into high efficiency 1-sun cell design have been presented previously.2 In this paper, we will describe the design criteria and results on high concentration cells. In concentrator cells, the emphasis is on achieving minimal effective series resistance. The ohmic contact has two major effects on the performance of the solar cell; it reduces the photocurrent because of the shadowing effect of the contacts on the incident light, and it introduces a series resistance because the cell current has to flow transversely through the sheet resistance of the surface layer, the contact resistance of the ohmic contact, and the sheet resistance of the metal grid before it is delivered to the load. These two effects represent opposing requirements on the grid design. A detailed analysis investigating the distributed nature of the series resistance is currently underway and the results of this study will be presented. Measurements of cell performance were made in natural sunlight at the Jet Propulsion Laboratory’s facility at Table Mountain, CA, using a Cassegrain reflecting concentrator3 on an equational mount in the tracking mode. The average insolation during the time of the measurements was %95 mW/cm2. The primary data consisted of IV curves taken at various concentrations. Simultaneously, the cell temperature, controlled by active cooling, was monitored. For the best of these cells, AM1 values of 24.6 percent at 180 suns, and 20.5 percent at 440 suns were obtained.