Spectroscopy and laser gain measurements of CaF2: Nd, X3+, Z3+ (X, Z = Gd, La, Ce, Y, Lu, Sc) crystals for broadband lasers applications

An efficient approach to avoid Nd3+ clustering in CaF2:Nd3+ is the co-doping with non-optically active buffer (Bu) ions creating Nd-Bu emitting clusters for laser applications around 1050 nm (4F3/2 → 4I11/2) taking advantage of their unique large absorption and emission bands. A detailed spectroscopic investigation of buffer codoping with Gd3+, La3+, Ce3+, Y3+, Lu3+ and Sc3+ ions is presented. The codoping with large buffer ions compared to Nd3+ like Gd3+, La3+, Ce3+ leads to one type of dominant Nd-Bu cluster, while the codoping with smaller ions like Y3+, Lu3+ and Sc3+ involves the formation of two types of Nd-Bu clusters, which can be discriminated in absorption/excitation/luminescence spectra and which further exhibit two different lifetimes. The laser gain performance of these codoped crystals with one or two buffer ions is investigated using a dedicated double modulation pump-probe setup allowing the tuning of both pump and probe wavelengths. As an example, a laser gain of 35% is measured in a 5 mm long CaF2: 0.5%Nd, 5%Lu crystal at 1054 nm for 400 mW of absorbed pump power at 797 nm. This direct measurement of the laser gain is particularly interesting for crystals codoped with small buffer ions where the presence of two Nd emitting centers complicates the calculation of the laser gain. For large buffer ions, a direct calculation of the laser gain is shown to be in good agreement with the experimental gain values.