Parameter regions of an optical Resonator where the resonator is geometrically stable.

When a parameter of a Laser resonator such as an arm length or the dioptric power of the focusing element in the resonator is varied, the resonator may go through one or two stability zones. In a purely geometric analysis stability means that a Ray injected into the optical System will stay at a finite distance from the axis even after many round trips. In this regime, the resonator Modes are of much simpler structure than for an unstable resonator. The properties of the Resonator Modes can vary strongly within a stability zone and also between different zones.