Unstable resonators with excited converging wave

The properties of unstable resonators with an additional mirror inside or outside the resonator were investigated, both experimentally and theoretically. It was found that the additional mirror excites the converging wave so that output coupling is decreased without affecting beam quality. Experiments were performed with a pulsed Nd:YAG system; the theoretical model was based on the coupled Kirchhoff integrals and solved numerically by the Fox and Li algorithm. Agreement between theory and experiments indicates that this kind of resonator provides high focusability and maximum extraction efficiency simultaneously, even with low-gain media. This enables one to apply unstable resonators to solid-state lasers with low small-signal gain such as alexandrite or CW-pumped Nd:YAG