Stable Resonators

Abstract

In this part the basic properties of both stable and unstable optical resonators with spherical mirrors are explained. We neglect the presence of an active medium inside the resonator and assume that both resonator mirrors exhibit 100% reflectance at the wavelengths considered. Resonators that do not provide amplification of the light are called passive resonators. Although the active medium is required to generate laser emission, the concept of the passive resonator is applicable to the investigation of the physics of laser radiation. The inclusion of the gain only modifies the resonator properties. The influence of the amplification upon the radiation characteristics of resonators will be discussed in Part IV. A stable optical resonator generally consists of two mirrors with radii of curvature p, and p2 separated by an optical distance L=nLo (Lo: geometrical mirror spacing, n: index of refraction inside the resonator). The range of L within which a resonator is stable is determined by the condition that a ray launched inside the resonator parallel to the optical axis remains inside the resonator after an infinite number of bounces. Equivalent to this definition is the lack of self-reproducing spherical waves inside the resonator (eigensolutions of the ABCD law, see Sec. 1.3). Fig. 5.1 The general optical resonator is determined by the g-parameters of the mirrors, the mirror spacing Lo and the radii a, of internal apertures.