Introduction

We will describe here a simple approach based on geometric optics, before following a more exact description based on Maxwell equations (see the paragraph about gaussian beams).

For the sake of clarity, we will consider here only passive cavities : the real cavity could generally be considered as a passive one with some reasonable hypothesis (for example, the thermal effects caused by pump heating inside a laser crystal can be simulated with a simple lens). As usual in the context of geometric optics, the light propagation is described in terms of “rays”, defined at each point of a given wave as the direction normal to the wavefront. This is also the energy direction (Poynting vector). Finally, we will only consider centred systems with axial symmetry : the majority of the real cavities are of that kind, at least in a first order approach. We will work with paraxial rays, that is all the rays that are nearly parallel to the optical axis.