STABILITY OF POLARIZED MODES IN GAS-LASERS

This tutorial paper is based on lectures delivered by G M Stephan at P ELS'97. The idea was to tell the students about the methodology used b y the authors to understand and to quantitatively compute the flips ob served between two polarization states in a single-frequency He-Ne las er. The study is limited to this phenomenon. Contents are: introductio n, quasi-isotropic lasers and polarization flips; the passive Fabry-Pe rot interferometer, an illustrative example; a vectorial description o f a two energy-level system with magnetic degeneracy; two useful tools : adiabatic elimination of fast variables and use of Lyapunov exponent s to study the stability of a solution; medium polarization in the adi abatic approximation; the laser equations for the field. These content s are organized for people coming into the field: rather than being to o general, we have preferred to focus on limited and well defined exam ples from which other cases can be studied.