Low-threshold, highly efficient Gd3+, Lu3+co-doped KY(WO4)2:Yb3+planar waveguide lasers

High-quality crystalline KY(WO 4 ) 2 :Yb 3+ layers co-doped with large concentrations of optically inert Gd 3+ and Lu 3+ ions were grown by liquid phase epitaxy. Continuous-wave laser operation in a planar waveguide configuration was observed at 1025 nm. For 23% output coupling, a maximum output power of 195 mW was obtained; a slope efficiency of 82.3% was derived, which to the best of our knowledge is the highest value yet reported for a planar waveguide laser to date. For 1.7% output coupling, a laser threshold as low as 18 mW of absorbed pump power was achieved, which is largely due to the enhanced refractive index contrast of ∼ 7.5×10 -3 between the active layer and the undoped KY(WO 4 ) 2 substrate attained by the co-doping, leading to well-confined pump and laser modes.