Thermal lensing mitigation for high-power optical dipole traps

Thermal lensing is an intensity-dependent phenomenon that changes the effective focal length of an optical component when illuminated by a high power beam. In experiments with cold atoms, high-power optical dipole traps are widely used to achieve quantum degeneracy via evaporative cooling. The use of high-powered lasers, up to hundreds of watts, inevitably introduces thermal lensing effects that can cause noticeable variations between the real system and the intended design. We discuss methods for mitigating thermal lensing in an optical dipole trap for lithium-6 atoms generated by a 200 W ytterbium single-mode fiber laser. In addition to choosing optimal materials to limit thermal effects, we show that careful choice of lens and telescope configuration can have significant benefits. A full characterization of thermal effects in our optical system is presented. We also discuss other results from the experiment, including optimized gray molasses cooling for lithium-6 and the production of high-intensity "light sheets" of few-micron thickness which serve as confining barriers for a rectangular optical box trap.