Efficient 600-W-Laser Bars for Long-Pulse Pump Applications at 940 and 975 nm

We report the first demonstration of 1.2 J-emission into long pulses ( $\ge 2$ ms) from passively cooled laser bars. These efficient high-fill-factor bars were developed for long-pulse pumping of Yb:CaF 2 -laser media at a power level of 600 W. The spatial brightness inferred (23 MW/cm $^{2}{\times } \text {sr}$ ) exceeds the state of the art. The bars are based on an extreme-double-asymmetric and an extremely-low-vertical-divergence epitaxial design for pumping at 940 and 975 nm, respectively. While the first design is optimized for high power-conversion efficiency $\eta _{\mathrm{ E}} > 60$ % (at 600 W), the latter exhibits low vertical divergence to enable low-loss external spectral stabilization at the zero-phonon line of Yb:CaF 2 $(\eta _{\mathrm{ E}} = 50$ % at 600 W). The higher $\eta _{\mathrm{ E}}$ at 940 nm and lower thermal resistance due to its vertical structure improve the lateral farfield, increasing the brightness by nearly 50% over the 975 nm-structure. We varied the lateral layout comprising 186, 400, and 1200 $\mu \text{m}$ -stripes at 70% fill-factor. The smaller stripes revealed the highest brightness, while the degree of polarization increases by 5% using wider stripes.