A roadmap for laser optimization of Yb:Ca3(NbGa)5O12-CNGG-type single crystal garnets

The continuous wave laser performance of Yb-doped Ca-3(NbGa)(5)O-12-CNGG-type disordered single crystal cubic garnets has been analyzed on the statistical basis of 26 crystals grown by the Czochralski method under different conditions. The low purity of the CaCO3 chemical used for phase synthesis is the major source of colour induced by non-correlated visible and red-near infrared optical absorptions, but the centres responsible for this colouration have a very minor impact on Yb3+ laser performance. The use of high purity (>= 99.99%) Yb2O3 is essential to preserve the radiative properties of the Yb3+ laser. The Yb segregation coefficient in CNGG amounts to S-Yb = 1.12 +/- 0.11. Up to 20% of Ca2+ in the dodecahedral garnet site can be substituted by Na+ and Yb3+ with S-Na = 0.39 +/- 0.05 and S-Yb = 1.34 +/- 0.09, respectively, i.e. Na promotes Yb incorporation. In contrast, tetrahedral Li+ reduces Yb incorporation to S-Yb = 0.88 +/- 0.07. The Yb3+ peak absorption cross section in CNGG amounts to sigma(ABS)(lambda = 971.7nm) = 1.97 +/- 0.32 x 10(-20) cm(2). sigma(ABS) is sensitive to Na and Li incorporation but no evidence of significant emission or gain cross section changes is found indicating that the majority Yb3+ centre is responsible for the observed fluorescence. Correspondingly, laser performance of crystals containing up to 15 at% of Yb shows only a slight improvement in Li-modified Yb:CLNGG crystals, associated with better crystalline quality. In them laser tuning extends in an unprecedented range up to 60 nm, even beyond 1080 nm in Li and Na co-modified crystals. 7-8 at% Yb doping of Li-modified CLNGG crystals with approximate to 95% pump absorption show large laser slope efficiency along with maximized output power.