Laser mode control with holographic mirrors

摘要

We present experimental results showing that variable reflectivity gratings can be used to control the modal content, the spatial profile and the phase curvature of the beams emitted by solid-state lasers. The setup used to fabricate the apodized holographic gratings will be described; the gratings were recorded in photoresist materials with a Krypton ion laser operated at 413 nm. The results from the optical characterization of these elements will be presented. The holographic mirrors have been tested as output couplers in Littrow geometry for Nd : YAG lasers emitting at 1.064 mum. Experiments have been made with several configurations of laser resonators. The most interesting results were obtained with positive branch confocal unstable resonators, where nearly diffraction-limited beams with output energy of up to 100mJ were generated. To compensate for the astigmatic aberration due to the Littrow geometry, we developed an analytical model predicting the reflective properties of the holographic gratings; from that model we found the conditions for which, in principle, the effects of the astigmatic aberration are completely eliminated. The degradation of photoresist materials by the environment and by high laser fluences is a factor limiting the applications of the holographic mirrors; to resolve that problem we are developing a new fabrication procedure through the replication of the holograms in epoxy materials that can withstand high laser powers.