Reduction of beam divergence of a thermal lensing insensitive Nd:YAG laser

In a previous paper we proposed an active resonator designed especially for Nd:YAG lasers. The beam generated by this resonator had a flat-top beam profile and a divergence that was insensitive to the thermal lensing effect. The beam generated by this resonator was, however, too divergent to be focused into a fibre. With the help of numerical simulations, we have found a way to reduce this divergence, and to render the far-field of the laser beam fine enough to be coupled into a fibre with 600 μm core and NA = 0.2.