Optical energy exchange in guides

Abstract

This chapter presents two basic classical methods of modelling mathematically the operation of semiconductor lasers and shows that they are, indeed, just different aspects of the same physics of energy conservation and are wholly compatible with one another. The first method applies the concepts of photon/electron particle exchange where one discusses the rate of absorption and emission of photons along with the rate of recombination of holes and electrons, ensuring at each stage that there is a detailed balance between photon generation and electron/hole recombination leading to particle conservation and energy conservation. This is the standard rate equation approach which is robust and well researched but can be difficult to apply when there are strong nonuniformities, and even more difficult when the phase of the electromagnetic field is important. For distributed-feedback lasers, both the phase of the field and nonuniformities are important and so one has to abandon the photon-rate equation in favour of an approach based on interactions between electromagnetic fields and the electric dipoles in an active optical medium. The electromagnetic field analysis is essential when the refractive index/permittivity changes periodically inside the laser. The chapter then concludes with an analysis of the coupled-mode equations which determine how a fraction of the forward-travelling field is coupled into the reverse-travelling field with a medium which has a periodic permittivity, i.e. the waveguide contains a Bragg grating.