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

Abstract

The continuous wave laser performance of Yb-doped Ca₃(NbGa)₅O₁₂-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 σ_ABS(λ = 971.7nm) = 1.97 ± 0.32 × 10⁻²⁰ cm². σ_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 ≈95% pump absorption show large laser slope efficiency along with maximized output power.