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.