It is shown that optical resonators may be broadly divided into two classes based on their mirror count and other properties, and that the two classes exhibit very different axis stability properties. When a resonator of one class is slightly misaligned, its optic axis (and hence its input axis for rotation sensing) remains close to its nominal position independently of the focussing action of the resonator optics. In contrast, when a resonator of the other class is slightly misaligned, the displacement and reorientation of its optic axis is restrained only by the focussing optics. Hence resonators of this second class are restricted in the range of mirror radii of curvatures they may employ. This has implications in noise and lockin performance for rotation sensors.
Irl W. Smith, Irl W. Smith,
"Optical Resonator Axis Stability And Instability From First Principles", Proc. SPIE 0412, Fiber Optic and Laser Sensors I, (19 September 1983); doi: 10.1117/12.935817; https://doi.org/10.1117/12.935817