IVb-8 optical injection locking of transistor oscillators

TLDR: In this article, it was shown that the second grounding pulse led to a buildup of space charge during the switching event which invariably resulted in an undesirable conductive transient lasting on the order of microseconds.

Abstract: grounding the conduction path. We found that this second grounding pulse led to a buildup of space charge during the switching event which invariably resulted in an undesirable conductive transient lasting on the order of microseconds. This transient requires a second backup gate if high isolation is required and thus diminishes the usefulness of the original switch design.' New design configurations that allow turn-on and off with two pulses at the same wavelength reduce but do not eliminate this problem to the desired degree. Such configurations were tested and the time r sponse measured. For . these configurations, the power handling capabilities can be significantly increased by using semiconducting films deposited on insulator substrates. Subsequent studies of ultrafast switching in GaAs using a single pulse were successful in eliminating these undesirable transients. Single optical pulse switching can be accomplished by using a semiconducting substrate in which the photoconductivity is rapidly quenched by the presence of a high concentration of deep traps. This approach was tested by fabricating microstrip switches on chromium doped GaAs ubstrates. The duration of the switching event can be measured by an optoelectronic sampling technique employing a silicon microstrip switch.* The results of this measurement indicate a switching time of 70 picoseconds. Lower mobilities in chromium doped GaAs require higher densities of photoinjected charge. The rapid radiationless recombination of this high density electron-hole plasma imparts a significant amount of thermal energy to the semiconducting substrate and is thought to account for the observable tendency for laser induced surface damage in this class of switch. It is therefore concluded that fast (-50 ps duration) switching will have to be performed at moderate repetition rates (<1 MHz) in order to avoid surface damage. Optoelectronic microstrip switches for slower switching do not suffer from the above mentioned problems. Microstrip switches deposited on composite CdSe-A1203 substrates have proven to be effective switches.' They may be driven directly by injection lasers and offer terminal to terminal isolation per stage of 20 dB or greater. The suitability of microstrip optoelectronic devices for microwave phase shifting applications will also be discussed.