Abstract
A study has been made of the characteristics and applications of avalanche photodiodes with respect to the detection of optical carriers modulated at high frequencies. Several techniques and systems have been developed for measuring the circuit parameters and photodetection performance of these devices. Although only aimed at a limited range of avalanche photodiodes many of the techniques are applicable to other photo-detector types. Some of the circuit techniques developed are suitable for more general use in the microwave measurement area. A facility has been set up for measuring the high frequency (by which is meant 100 MHz to 2 GHz) photodetection performance of avalanche photodiodes (or other photodetectors). A key component of this system is a travelling-wave electrooptic modulator which is versatile, requires low drive power and is stable with time and temperature. Several other special components were designed and built. Using the techniques and systems described, the characteristics of a number of avalanche photodiodes have been measured. In particular the direct photodetection frequency response of the devices is reported. Several photodetection techniques have been examined. Particular attention has been paid to a technique in which the avalanche photodiode is used as an electronic mixer. Measurements performed demonstrate the viability of this technique, although only limited applications are envisaged. A theoretical model of the avalanche photodiode as a circuit element has been developed that takes account of the non-linear current-gain versus voltage and capacitance versus voltage characteristics of the device. Numerical techniques were used to analyse this model, the results of which agree well with the measurements. It was found possible to predict suitable operating conditions for, and performance of, the device.