Low Magnification Unstable Resonators Using Radially Varying Birefringent Elements

Abstract

It has been shown theoretically that a gaussian gain profile can confine an unstable resonator mode so that such a mode has a finite but large beam radius [1–4]. The stabilization of a large beam radius by a radially dependent gain profile has also been experimentally verified [5]. A gaussian loss element (i.e., a gaussian reflector) can also affect the beam profile in a similar manner [6–8]. More recently, Eggleston et al [9] have pointed out that a radially birefringent reflector can be approximated as a gaussian reflector, and have shown that such an element can support a large unstable resonator mode which does not suffer Fresnel fringing due to sharp changes in the reflectivity typical for an unstable resonator. This fringing can be a source of regions of high intensities which can cause optical damage. Giuliani et al [10] have shown that such a radial birefringent element (RBE) unstable resonator can be applied to a pulsed Nd:YAG laser. Ruby and alexandrite are usually oscillated at lower gain configuration than Nd:YAG so that low magnification unstable resonators are necessary to support an unstable resonator mode. Low magnification unstable resonators have higher internal standing wave power densities and have a greater problem with Fresnel fringing and, hence, are more susceptible to optical damage. Low magnification unstable resonators are less stable against perturbations in the unstable resonator mode which can also lead to damage [4,10]. All experiments with “Polka Dot” unstable resonators with alexandrite lasers have led to optical damage at relatively low output powers due to a stable mode oscillating from the central high reflectivity “Polka Dot” [11]. In this paper a low magnification unstable resonator using radially birefringent elements which is less susceptible to damage is described. This resonator is less susceptible to damage for two reasons. One reason is the radially birefringent element is a soft aperture without Fresnel fringing effect. Also, in the configuration used in our experiments, the Q for the unstable resonator mode was comparable to the Q for stable mode, so accidental lasing in lower order modes did not cause optical damage. In this paper we will also describe our results with this resonator for alexandrite and ruby.