5.5 kW 1050 nm Bidirectional Output All-Fiber Laser Oscillator Employing Composite Cavity

We proposed and demonstrated a high-power bidirectional output all-fiber laser oscillator emitting at 1050 nm wavelength employing a novel composite cavity architecture. The laser incorporates two 10%-reflectivity output-coupler fiber Bragg gratings (FBGs) at both ends, and an additional FBG of 50%-reflectivity inserted within the resonant cavity. Compared to traditional bidirectional output fiber lasers with single resonant cavity, this composite cavity laser can significantly enhance the threshold of transverse mode instability (TMI), exhibiting <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula>19% and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula>36% improvement in A-end pump and B-end pump schemes, respectively. Moreover, under bidirectional pump scheme, a total output power of 5.5 kW with no signs of TMI is achieved for the first time. The optical-to-optical conversion efficiency is 85.7% at the full output power, and the beam quality (M<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> factor) at respective outputs is 1.52 and 1.34. This work provides significant guidance for developing TMI-free high-power bidirectional output fiber lasers.