Use of polycrystalline Nd:YAG rods to achieve pure radially or azimuthally polarized beams from high-average-power lasers.

We report maintenance of perfect radial-polarization purity in high-power, cw pump chambers using strengthened, polycrystalline Nd:YAG laser rods. Although the cubic symmetry of single-crystal rods caused threefold symmetric birefringence due to shear stresses at the ends of the pumped zone, polycrystalline rods macroscopically behaved as isotropic material and enabled polarization preservation. Elimination of this source of depolarization prevents the major source of bifocusing aberrations in a chain of amplifiers.

Radiative and non-radiative transitions of excited Ti3+ cations in sapphire.

We have measured the fluorescence quantum efficiency in Ti3+:sapphire single crystals between 150 K and 550 K. Using literature-given effective fluorescence lifetime temperature dependence, we show that the zero temperature radiative lifetime is (4.44 ± 0.04) µs, compared to the 3.85 µs of the fluorescence lifetime. Fluorescence lifetime thermal shortening resolves into two parallel effects: radiative lifetime shortening, and non-radiative transition rate enhancement. The first is due to thermally enhanced occupation of a ΔE = 1,700 cm-1 higher (top) electronic state of the upper multiplet, exhibiting a transition oscillator strength of f = 0.62, compared to only 0.013 of the bottom electronic state of the same multiplet. The non-radiative rate relates to multi-phonon decay transitions stimulated by the thermal phonon occupation. Thermal enhancement of the configuration potential anharmonicity is also observed. An empiric expression for the figure-of-anharmonicity temperature dependence is given as [Formula: see text] (T) = [Formula: see text] (0)(1 + ß exp(-ℏωco /kBT )), where [Formula: see text] (0) = 0.276, ß = 5.2, ℏωco = 908 cm-1, and kB is the Boltzmann constant.