Introduction

We considered in the previous sections a single solution of the paraxial wave equation, namely the fundamental Gaussian mode. Other solutions, mathematically forming an orthonormal and complete base, exist. Each oscillation in the resonator is a linear combination of those modes. Their transverse structures have a rectangular, cylindrical, or a mix of them symmetry : it is mainly defined by the mirrors shape (rectangular or circular). This structure is in general strongly affected by other perturbations and cannot be observed so easily.