13.1 W, high-beam-quality, narrow-linewidth continuous-wave fiber-based source at 970 nm.

We report an efficient fiber-laser-based source of high-power, continuous-wave (cw), linearly-polarized radiation at 970 nm in a simple, compact, and practical design. Using direct single-pass second-harmonic-generation (SP-SHG) of a cw thulium fiber laser at 1940 nm in a 40-mm-long periodically-poled LiNbO3 (PPLN) crystal, we have generated 13.1 W of output power at 970 nm for a fundamental power of 40 W at a conversion efficiency as high as 32.7%. The generated second-harmonic output exhibits a passive power stability better than 1.4% (1σ value) over 1 hour, has a linewidth better than 0.3 nm, and a TEM(00) spatial beam profile with M(2)<1.6. Relevant theoretical calculations for the characterization of SP-SHG in the crystal have also been performed.