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.