High-power Yb- and Tm-doped double tungstate channel waveguide lasers

The potassium double tungstates KGd(WO4)2, KY(WO4)2, and KLu(WO4)2 are excellent candidates for solid-state lasers because of their high refractive index of ~2.0-2.1, the large transition cross-sections of rare-earth (RE3+) ions doped into these hosts, and a reasonably large thermal conductivity of ~3.3 W m-1 K-1. Exploiting these advantages, Yb- and Tm-doped KY(WO4)2 planar waveguide lasers were demonstrated. Co-doping of KY(WO4)2:RE3+ thin films with Gd3+ and Lu3+ ions provides lattice matching and enhanced refractive index contrast of up to 7.5×10-3 with respect to the KY(WO4)2 substrate, thus thinner waveguides with better mode confinement, enabling highly efficient planar waveguide lasers and facilitating microstructuring. We grew KY1-x-yGdxLuy(WO4)2:RE3+ layers with various compositions onto undoped KY(WO4)2 by liquid phase epitaxy. Replacing Y3+ in the layer completely by Gd3+ and Lu3+ ions results in layers with a refractive-index contrast of >2 x 10-2. Channel waveguides were microstructured into the layers by Ar+ beam etching. The excellent pump and signal mode confinement in these channel waveguides, combined with the aforementioned attractive properties of the host material, resulted in highly efficient lasers. In KGd0.49Lu0.485Yb0.025(WO4)2 channel waveguides with ~0.34 dB/cm propagation loss at 1.0 µm, channel waveguide lasers with butt-coupled mirrors delivered 418 mW of output power at 1023 nm with a slope efficiency of 71%. By pumping at 973 nm and lasing at 980 nm, a record-low quantum defect of 0.7% was achieved. 4-µm-deep Bragg gratings were etched by focused ion beam (FIB) milling. An on-chip integrated laser cavity was formed by this distributed Bragg reflector and a FIB-polished waveguide end-facet and the first on-chip integrated double tungstate waveguide laser at 980 nm was demonstrated. In KY0.4Gd0.295Lu0.29Tm0.015(WO4)2 channel waveguides with ~0.11 dB/cm propagation loss at 1.9 µm, laser experiments with butt-coupled mirrors demonstrated an output power of 149 mW and slope efficiency of 31.5% when pumping at 794 nm in TM polarization. The lowest threshold was 7 mW. The laser wavelength shifted from 1930 nm via 1906 nm to 1846 nm with increasing outcoupling degree.