Characterization of Yb3+:Sr5-xBax(PO4)3 F crystals for diode-pumped lasers

Ytterbium-doped Sr₅(PO₄)₃F (S-FAP) has been shown to be a useful material for diode pumping, since it displays high gain, low loss, and a long radiative lifetime. One of the issues with S-FAP is that it has a relatively narrow absorption bandwidth (~5 nm) at 900 nm, the diode-pumping wavelength, while the diode´s output bandwidth can be large (~10 nm). By changing the host slightly, the absorption feature can be broadened to better match the pump bandwidth. Four mixed crystal boules of Yb³⁺:Sr_5-x Ba_x(PO₄)₃F were grown by the Czochralski method with x=0.25: 0.5, 1, and 2. The bandwidth of the 900-nm absorption feature was found to grow with increasing barium concentration from 4.7 nm to a maximum of 15.9 nm. Emission spectra showed a similar bandwidth increase with barium content from 4.9 nm to a maximum of 10 nm. Emission cross sections for these materials were deduced by the methods of reciprocity, the Einstein method, and small-signal gain. The absorption feature´s homogeneity was probed using a tunable pump source which qualitatively showed that the barium broadened lines were at least partly inhomogeneous. Each of these materials lased with a variety of output couplers. This family of materials was found to provide suitable laser hosts where a broader absorption and/or emission bandwidth is desired