Travelling wave infra-red and sub-mm wave modulation using free carrier absorption in reverse-biased p- n junction diodes

Abstract

The paper presents a study of travelling wave light modulator using free carrier absorption in reverse biased junction diodes. Several configurations of these involving the use of a parallel plane transmission element are described and the problem of propagation through these studied. Condition of synchronous interaction between light and the modulating signal is given. The frequency response of the modulator is examined and is shown to consist of a number of bands. It is found that under ideal conditions the modulator can be designed to operate up to 1000 Mc/s. The possible effect of diode series resistance on the performance of the modulator is examined and amount of distortion estimated. Results indicate that a unit consisting of 10 diodes should give 30–50 per cent modulation depth with a few hundred Mc/s bandwidth, negligible distortion and an efficiency equal to that of a single distortionless diode giving the same depth of modulation. Special arrangements ensuring small radiation loss are also briefly mentioned.