Improvement of Beam Ellipticity and Amplification Efficiency of Two Side Cooled Yb:YAG Thin Rod Using Atomic-Diffusion-Bonding

Yb:YAG gain media are widely used to amplify IR laser lights from CW to femtosecond region. We applied the atomic-diffusion-bonding (ADB) [1] to reduce the thermal resistance between Yb:YAG thin rod and heatsink, and demonstrated the improvement of amplification efficiency compared to the conventional amplifier using indium foil as thermal contact material [2]. Since the 2×2-mm squared section ADB Yb:YAG rod was cooled only two opposing faces, we obtained the amplified output with an elliptical Gaussian profile. In this paper, we report on improvement of beam ellipticity and amplification efficiency by optimizing the aspect ratio of cross section. One of the approaches supressing the ellipticity is to increase the bonding surfaces from two to four. To simplify the bonding process, however, we applied a 2×5-mm squared section Yb:YAG to the two side-cooled gain medium. Figure 1(a) shows the experimental setup of ADB Yb:YAG thin rod amplifier. A pair of 3.2-mm-thick aluminum block were bonded to the 5×30-mm surfaces of a 2 at.% Yb-doped YAG ceramic with dimensions of 2×5×30-mm via ADB. The ADB unit was mounted to a copper water cooled heatsink via 100-f.!m thick Indium foil. The pump beam from fiber coupled laser diode with a wavelength of 940 nm was split into two by a polarized beam splitter and focused on the ADB unit. The 250-mW output from Yb-fiber laser seed source, at 1030-nm wavelength with 30-ns pulse width at 6-kHz repetition rate, was coupled with pump beam by dichroic mirror and amplified by the bidirectional pumped ADB unit with double pass configuration. We also prepared a 2×2-mm squared section ADB Yb:YAG unit for comparison.