End-pumped thin-rod Yb:YAG amplifier for laser processing

The Yb (ytterbium) laser material is promising for high-power all-solid-state ultrashort pulse lasers because of its high quantum efficiency and wide gain spectrum. A didoe end-pumped technology is developed for high-average-power and efficient all-solid state ultrashort pulse lasers with high beam quality output. Two 100 W CW-LDs are used for pumping and the beams are focused on the end surfaces of a thin and long Yb:YAG rod. A pair of parallel side surfaces of the rod is placed in contact with cupper heat sinks to remove heat. A one-dimensional distribution of thermal stress induced birefringence was observed in the rod by this simple heat flow. The loss due to thermal birefringence was measured to be smaller than 3%, when the probe beam was linearly polarized in parallel or perpendicular to the direction of cooling. It can be expected theoretically that a moderate single-path gain of 4 for efficient amplification and the optical conversion efficiency of 50% are realized for multimode operation with a pump power of 200 W. In the experiment, output power was measured to be 55 W with beam quality factor of Mx2 x My2 = 5.5 x 6.0 and 27 W with Mx2 x My2 = 1.3 x 1.4 for simple plane-plane linear cavity configuration with no thermal lens compensations.