Anisotropy in the laser performance of NdP5O14

We report, for the first time in a biaxial laser material, on the distribution of fluorescence cross sections in the index ellipsoid. The largest effective fluorescence cross section of pure NdP5O14 at room temperature (2.0±0.1) ×10−19 cm2 for light at 1.0512 μ wavelength polarized parallel to the 2 axis which coincides with the crystallographic b axis. We predict from the cross sections the cw thresholds and polarizations of the four highest-gain laser transitions and verify the results quantitatively by operating one crystal in all major polarizations. We measured the major phase and group refractive indices and the thermal expansion coefficient of the optical lengths near 1.05 μ wavelength. We also discuss the twin domains which have different orientations of the crystallographic and optic axes. In imperfect crystals they can easily be created by thermal stresses which can drastically increase the laser threshold.