Thermal lensing in high-power end-pumped Nd:YLF lasers

Hardman, P.J., Pollnau, M., Clarkson, W.A. and Hanna, D.C. (1997) Thermal lensing in high-power end-pumped Nd:YLF lasers. Quantum Electronics Conference (QE13), , Cardiff, United Kingdom. 08 - 11 Sep 1997.

Abstract

One of the major limitations of scaling diode-end-pumped solid-state lasers to high powers is introduced by thermal effects. An attractive feature of Nd:YLF has been its superior thermo-optical properties compared to other laser crystals. This is due to a decrease of refractive index with increasing temperature, creating a negative thermal lens, which partially compensates for the positive lens due to bulging of the rod end faces. Other advantages of Nd:YLF include its natural birefringence and its long fluorescence lifetime. The latter feature is of interest for high-power Q-switched operation. Problems in realising the true potential of the laser, however, have often been encountered, for underlying spectroscopic reasons as indicated, e.g., in [1]. We investigated the thermal lensing under lasing and non-lasing conditions within a diode-bar-pumped system. Under non-lasing conditions the thermal lens was measured using a Nd:YAG probe laser which double-passed the Nd:YLF rod. The resulting change in beam divergence was measured. Under lasing conditions the laser-beam waist size on the output coupler was measured. Hence, using the ABCD-matrix formalism, focal-length values for the thermal lenses were determined. The results showed a significant difference in the thermal lens under lasing and non-lasing conditions. In the former case a weak thermal lens was observed which varied linearly with pump power. Under non-lasing conditions a much stronger thermal lens was measured, whose power increased non-linearly with pump power. With 11 W of pump power incident on the crystal, a factor of 6 difference between lasing and non-lasing values of focal length was determined (Pi-polarisation, plane perpendicular to c-axis).
These measurements demonstrate that significant additional heat is generated in the non-lasing case. A finite-element calculation, which considered the relevant processes including interionic upconversion, their contribution to thermal loading, as well as the temperature distribution in the Nd:YLF crystal, was performed. An experimentally observed fluorescence saturation at 1.05µm of more than 50 % under Ti:sapphire pumping was numerically reproduced, and the value of the published upconversion parameter [2] was thereby confirmed. With this information, the heat generation, spatial temperature distribution, and thermal lens under diode pumping were determined. The calculated thermal lens powers were in reasonable agreement with experimental results. Upconversion processes as well as the temperature dependencies of heat conductivity and thermo-optical parameters were found responsible for strong thermal lensing under non-lasing conditions and its non-linear behaviour with respect to absorbed pump power. Design improvement by a significant decrease of thermal lens power and spherical aberrations under Q-switched conditions can be achieved by increasing the pump-spot size, decreasing the dopant concentration and using a longer crystal, or detuning the pump wavelength from the absorption peak.

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