Infra-red and sub-mm wave modulation using free carrier absorption in p-n junction diodes

Abstract

An analysis is made of the operation of p-n junction radiation modulators of both the reverse- and the forward-biased configurations, radiation being incident in a direction either parallel or perpendicular to the junction. It is found that parallel incidence always gives somewhat better performance. Measures for matching at the free faces of the modulator have been suggested. It is shown that for obtaining good modulation index with reverse-biased operation either the diode or the beam must be extremely thin. Sandwiched diodes and TW action are suggested for obtaining improved performance. Distortion is found to be small and, in principle, reducible considerably through a squaring operation on the control voltage. Operation up to the microwave range appears to be attainable. A forward-biased modulator promises a better modulation efficiency and comparable modulation depth, but has a poorer frequency response. For low distortion a current rather than a voltage drive is essential.