23 Sep 2004
Liquid nitrogen cooling takes a Yb:YAG laser to a new level of performance.
By cooling a Yb:YAG crystal with liquid nitrogen, scientists from MIT Lincoln Laboratory in the US have eliminated the troublesome thermo-optic effects that hinder the power scaling of solid-state lasers. The result is a highly-efficient cw laser that emits up to 165 W in a near-diffraction-limited beam (M2 of 1.02) when pumped with 215 W from two laser diodes (Optics Letters 29 2154).
Although cryogenic cooling has been used before to improve the performance of solid-state sources, MIT's results are far superior to those reported to date. Previously, the best result for a low-temperature cooled Yb:YAG laser is a 40 W output beam with an M2 of 2 and a slope efficiency of 50%. In contrast, the MIT laser has a slope efficiency of 85% and an optical-to-optical efficiency of 76% at full power.
The design consists of a 15 mm long, anti-reflection coated Yb:YAG crystal that has a 4x4 mm square cross-section and is metallized. Indium solder is used to bond the rod to a heat-sink which is placed in a liquid nitrogen cryostat at a temperature of 77 K. The rod is end-pumped by two 940 nm laser diodes each capable of providing up to 110 W.
MIT's calculations suggest that even when pumped with up to 300 W of diode light, the temperature of the laser rod should remain around 100 K, a temperature that dramatically improves the rod's thermo-optic efficient, thermal conductivity and thermal expansion compared to room temperature.
"The pump power at which thermo-optic distortions and thermal birefringence become significant can be estimated to be 75 and 40 times, respectively, higher than for room temperature Nd:YAG lasers," reports the team in Optics Letters. "Furthermore, the low temperature Yb:YAG laser will be more efficient than its room temperature Nd:YAG counterpart and so will have a higher output power for a given pump power."