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

Abstract

We report the first demonstration of 1.2 J-emission into long pulses ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\ge 2$ </tex-math></inline-formula> ms) from passively cooled laser bars. These efficient high-fill-factor bars were developed for long-pulse pumping of Yb:CaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -laser media at a power level of 600 W. The spatial brightness inferred (23 MW/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{2}{\times } \text {sr}$ </tex-math></inline-formula> ) 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\eta _{\mathrm{ E}} &gt; 60$ </tex-math></inline-formula> % (at 600 W), the latter exhibits low vertical divergence to enable low-loss external spectral stabilization at the zero-phonon line of Yb:CaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$(\eta _{\mathrm{ E}} = 50$ </tex-math></inline-formula> % at 600 W). The higher <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\eta _{\mathrm{ E}}$ </tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> -stripes at 70% fill-factor. The smaller stripes revealed the highest brightness, while the degree of polarization increases by 5% using wider stripes.