Comparative spectroscopic and thermo-optic study of Tm: LiLnF4 (Ln = Y, Gd, and Lu) crystals for highly-efficient microchip lasers at ~2 μm

We report on a detailed comparative study of the spectroscopic and thermo-optic properties of tetragonal Tm:LiLnF4 (Ln = Y, Gd, and Lu) crystals indicating their suitability for highly-efficient microchip lasers diode-pumped at ~791 nm and operating at ~1.91 μm. An a-cut 8 at.% Tm:LiYF4 micro-laser generated 3.1 W of linearly polarized output at 1904 nm with a slope efficiency of η = 72% and a laser threshold of only 0.24 W. The internal loss for this crystal is as low as 0.0011 cm-1. For 8 at.% Tm:LiGdF4 and 12 at.% Tm:LiLuF4 lasers, the output power reached ~2 W and η was 65% and 52%, respectively. The thermal lens in all Tm:LiLnF4 crystals is weak, positive and low-astigmatic. The potential for the Tm:LiLnF4 lasers to operate beyond ~2 μm due to a vibronic coupling has been proved. The Tm:LiYF4 vibronic laser generated 375 mW at 2026-2044 nm with η = 31%. The Tm:LiLnF4 crystals are very promising for passively Q-switched microchip lasers.