Abstract

Unstable resonators are an elegant solution to the problem of generating single transverse mode beams of large size. The size of variable reflectivity mirrors¹ as output couplers makes these resonators particularly attractive since these mirrors eliminate the diffraction lobes caused by hard apertures. In this paper we show that the insertion of nonlinear components in unstable resonators modifies significantly the spatial shape of the emitted laser beams. The nonlinear components under consideration are introduced to favor axial mode locking; typical examples are saturable absorbers and nonlinear interferometers.² Our analysis of the problem has revealed that a component with a nonlinear transmittance, such as a saturable absorber, can produce an effective variable reflectivity mirror controlled by the laser beam itself; experimental results obtained with a pulsed Nd:YAG laser have demonstrated a reduction in mode diameter by a factor of 1.6.

© 1989 Optical Society of America