Internally-frequency-doubled Yb fiber laser with 15 W CW green output

In this paper we present a high power continuous-wave (CW) green source using a novel approach for frequency doubling of high power CW fiber lasers based on an internal resonant enhancement cavity integrated within the fiber laser resonator. The experimental configuration (shown in Fig. 1) comprised a double-clad fiber with an Yb-doped core in a simple standing-wave resonator. Feedback for lasing was provided by a diffraction grating at one end of the fiber, and by an external cavity containing a resonant enhancement cavity at the opposite end of the fiber. The enhancement cavity comprised a Brewster-angled LiB₃O₅ (LBO) crystal placed in an oven and cut for type I non-critical phase-matching. In this scheme, the fiber laser automatically lases on axial modes that are resonant in the enhancement cavity avoiding the need for active stabilisation. The fundamental laser power is enhanced in the resonant cavity by virtue of its relatively low loss yielding high second harmonic conversion efficiency.