Compensation for thermally induced birefringence in Nd-YAG lasers

Thermal gradients that occur ill optically pumped YAG lasers result in bifocusing and birefringence. Methods to compensate for these induced aberrations have been suggested by Osterink and Foster. We have employed one of these methods that uses 90° rotation of the polarization. A crystal quartz rotator is placed between two pumped YAG rods so that the part of the mode that is radially polarized in the first rod is tangentially polarized in the second. The experimental set-up consists of a 50-cm convex mirror, two flat ended 3 × 50 mm krypton-pumped Nd: YAG rods separated by the rotator and a 10-m concave output mirror. The dimensions of the cavity are such that the TEM00mode should be apertured by the rod diameters when their focal lengths are 50 cm each. The rod-lamp combinations have this focal length at a 1.5 kW input to each. The multimode output power at this input is 9 W with long-radius mirrors. With the above set-up the output is 6 W in the TEM00mode. Insertion of a Brewster plate results in a linearly polarized mode in a ratio of 30:1 with less than 10% loss of power. Variation of the power input to one of the lamps results in an increase in the crossed polarization component and shows that compensation is achieved by this technique. It is possible to obtain similar polarizations without loss of power for the multimode operation. In an attempt to reduce focusing effects, the pump radiation was filtered with a pass band from 0.66 to 1.33 microns, but no significant change in the observed focal length of the YAG rod was observed.