Six-fold residual birefringence in radially-polarized high-power Nd:YAG laser rods

Radial polarization is used in high-power solid-state lasers in order to overcome the thermally induced bifocusing that is the primary optical aberration when using the rod geometry. Maintaining radial polarization purity is crucial for beam-quality preservation in high-power amplifiers. Working at a very high heat load of 1500 W (100 W/cm) in a Nd:YAG rod-based laser amplifier with radially-symmetric pumping, a six-fold deviation from radial symmetry was measured in the polarization of a probe beam after propagation through the amplifier. The six-fold birefringence is a residual effect superimposed on the dominant radially symmetric birefringence and can, therefore, only be measured using a radially polarized beam.