In this Letter we describe a simple method for frequency doubling in high-power CW fiber lasers that offers the prospect of very high conversion efficiency and high power in the visible wavelength regime. Our approach is based on second harmonic generation in an enhancement resonator within the fiber laser cavity and does not require active cavity length stabilization. This technique has been applied to a cladding-pumped Yb-doped fiber laser to generate 19 W of linearly polarized CW green output at 540 nm, with excellent temporal stability for 90 W of absorbed diode pump power at 975 nm. The prospects for further improvement in performance with respect to conversion efficiency and output power are considered.

Abstract:This paper analyzes the second harmonic generation (SHG) efficiency of light with partial temporal coherence due to depolarization effects in birefringent media. It discusses relations between SHG efficiency fading, light source spectrum, crystal birefringence, and phase matching conditions. The efficiency of SHG pumped by the partially coherent light beam that may depolarize light in nonlinear birefringent crystal is also analyzed. The basic theory of SHG with its modification for partially coherent light with depolarization and some numerical calculations of the SHG process are described.

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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 LiB 3 O 5 (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. Fig. 1. Experimental set-up for internal frequency doubling in a fiber laserWith this setup, we obtained 15 W of CW second harmonic output (at 540 nm) in the forward direction (Fig. 1), corresponding to 19 W generated inside the LBO crystal, for 90 W of absorbed diode pump power (at 975 nm). The output power in the reverse direction was <100 mW. The internal conversion efficiency of the resonator with respect to the fundamental power entering the enhancement cavity was >63%. The output was linearly-polarized and its beam quality factor (M 2 ) was measured to be <1.25. The laser was tunable over the range of 540-560 nm (for 20 m long fiber) and over the range 520-550 nm (for 10 m long fiber) by adjusting the grating angle and the oven temperature to maintain phase matching. These preliminary results were obtained with a non-optimal set-up due to limited availability of components. The prospects for further improvement in performance in terms of output power and range of operating wavelengths will be discussed. Fig. 2. Generated second harmonic power at 540 nm as a function of absorbed pump power at 975 nm