Thermal lensing effects in rod-based Tm³⁺: YLF amplifiers versus pump and cooling conditions

Abstract

We report on a comprehensive study of the thermal-lensing penalties in rod-based, end-pumped amplifiers made of thulium-doped YLF. Aiming to optimize the beam quality under optimized pump and cooling conditions, this applies to the definition of highly efficient laser designs with operation up to the saturation of the gain. Single-pass and double-pass pump schemes are benchmarked by means of an innovative modeling process, to determine the appropriate rod's length and the complete set of input data which determines the spatial transfer function of a given rod. This is done in the form of an equivalent, pump-dependent, thick GRIN lens. The characteristics of this highly astigmatic and basically divergent lens are computed thanks to complementary 3D-FEM thermo-mechanical modeling. To benchmark the different contributors to natural thermal-lensing phenomena, we refer to the situation of uniform sidecooling. The computational results are parameterized in a broad range of operating conditions. Then we suggest non-uniform side-cooling, as a possible option of interest for cancelling the astigmatism. The development of YLF-based amplifiers of a new generation taking advantage of a highly stable and easily controllable beam quality, either using rod-based or slab-based architectures, will be part of the potential applications of this fairly generic modeling approach.

202 W dual-end-pumped Tm:YLF laser with a VBG as an output coupler

“…In this paper, the thermal lensing effect in the Tm:YLF crystal is not negligible under high pump power [24,25] . As is well known, the crystal always operates as a divergent lens, resulting from the coupled effects of a negative dn/dT, and from side cooling.…”

Highly efficient mJ level laser amplifiers at 2 microns for frequency comb spectroscopy

“…The complete resolution of the rate equations has been discussed in our latest paper [7]. Thanks to the resolution of these coupled equations, we can determine the efficiency of a single pass in a Tm:doped amplifier.…”