Multimode approach to the physics of unstable laser resonators
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In this paper we present a new approach to the problem of unstable laser resonators. We consider a stable resonator in which we insert a thin diverging lens near one mirror. By varying the focal length of the lens, the lens-resonator system can go from stable to unstable. We expand the field in the lens-resonator system in terms of the normal modes of the original stable resonator. The polarization of the lens causes coupling of the stable modes. The present method allows one to introduce the active lasing medium in a natural way, as is shown. The normal modes of the unstable resonator are obtained numerically, along with analytic solutions in certain limiting cases. A physical discussion of the unstable resonator problem is given in terms of a pseudospin picture. 1975 The American Physical Society.