Nd3+ and Yb3+ simultaneous laser operation coupled by energy transfer in Nd3+,Yb3+,Gd3+:CaF2 crystals

Co-doped laser materials with two active ions coupled by energy transfer is an interesting case where both ions can take part in the laser output intensity. Depending on the concentration of both co-dopants, the strength of the energy transfer coupling them or even the pumping rate, a competition between the two ions can take place which will affect the laser characteristics such as the laser wavelength. This competition between two emitting ions is illustrated here in the case of CaF₂ co-doped with neodymium and ytterbium ions. The advantage of Nd³⁺ is that its absorption cross-section around 791nm is higher than that of Yb³⁺ at ~980nm, enabling an efficient excitation of Yb³⁺ by pumping Nd³⁺ with a subsequent Nd³⁺-to-Yb³⁺ energy transfer (ET). Combining Nd³⁺ and Yb³⁺ with Gd³⁺ buffer ions further enables breaking up opticallyquenched Nd3+ clusters. Nd³⁺,Yb³⁺,Gd³⁺:CaF₂ crystals exhibit broadband emission, extending the Yb³⁺ spectrum via the Nd³⁺ contribution at longer wavelengths. Before addressing the laser gain competition between Nd³⁺ and Yb³⁺ ions, the Nd³⁺⟶Yb³⁺ ET efficiency was estimated using two approaches based on luminescence intensity and lifetimes, showing its dependency on Yb³⁺ doping. We achieved an ET efficiency of 80% in 0.5%Nd,3%Yb,2%Gd:CaF₂. CW laser action using different Nd,Yb,Gd:CaF₂ crystals resulted in a 17% slope efficiency versus absorbed pump power and a 200mW laser threshold. The expected laser gain competition between Nd³⁺ and Yb³⁺ ions leads to a change of the laser wavelength within the 1045-1067nm range depending on the Yb³⁺ concentration and output coupler transmission. These results are clearly explained by investigating the respective contribution of Nd³⁺ and Yb³⁺ to the gain cross-section and its dependence on Yb³⁺ doping concentration and Nd3+ population inversion. The gain cross-section profile is either dominated by one of the Nd³⁺ emission peaks (1065nm, 1048nm), or flat across 1045-1067nm, in which case the laser oscillates randomly within this spectral range. We show how a large Nd3+ inversion ratio leads to depletion of the Yb³⁺ 2F7/2 ground state through ET, resulting in two main effects: the saturation of the ET, as the ground-state Yb³⁺ acceptor concentration diminishes, and a shift of the laser line towards shorter wavelengths, due to weaker Yb³⁺ reabsorption.