Abstract

Yb³⁺-doped potassium double tungstates exhibit an excellent gain performance [1]. Liquid-phase epitaxy of KGd_xLu_yYb_1-x-y(WO₄)₂ layers onto undoped KY(WO₄)₂ substrates allows one to independently design active doping level and refractive index [2,3]. The highest Yb³⁺ concentration enabling lattice matching between layer and substrate leads to the composition KGd_0.465Lu_0.000Yb_0.535(WO₄)₂. In Fig. 1(a), lifetime measurements (red) and correction [4] of radiation reabsorption (blue) show that energy migration among Yb³⁺ ions and subsequent energy transfer to impurities or cooperative upconversion diminishes the lifetime from ~270 μs to slightly less than 200 μs at 47.5% Yb³⁺ concentration. This small amount of quenching does not affect the gain significantly. We fabricated channel waveguides with Yb³⁺ concentrations of 15%, 20%, and 47.5%, and pump-probe measurements provided a gain of 280 dB/cm, 430 dB/cm, and 935 dB/cm, respectively, at the peak wavelength of 980.6 nm, see Fig. 1(b). Besides, we grew thin films with 47.5% and 57.5%, beyond the lattice-matching limit of 53.5%, and perpendicular gain measurements indicate a gain of 1028 dB/cm [1] and 1050 dB/cm [5], respectively. In all waveguides, a gain bandwidth of 55 nm from 977–1032 nm was measured, see Fig. 1(c), equaling 16.3 THz spectral bandwidth, providing 150 dB/cm gain for 47.5% Yb³⁺ concentration (red arrow).

© 2015 IEEE