ABSTRACT
This chapter discusses the resonator design problems specific to very long cavities in terms of the basic properties of Gaussian beams within resonators, and the disturbances of the ideal “empty” resonator by the gain medium. It also discusses the problems raised by very long folded resonators, operating at high powers for both plane and curved mirror folds, and extends some of these ideas to the design of very compact folded resonators for annular gain media, using the Herriott cell concept. The problem with long cavities is that the large diameter, weakly focused beams can be significantly disturbed by refractive index gradients within the gain medium, or by poor surface figure in fold mirrors, either due to manufacturing tolerances or to thermal distortion in use. Curved-mirror folding thus allows much longer folded cavities to be contemplated, although it should be noted that smaller spot size relative to equivalent plane-mirror folded cavity means higher power densities and more problems from thermal mirror distortion.
