Tunable cryogenic Tm:CaF2-SrF2 laser

We present temperature influence (from 78 to 300,K) on tuning and laser properties of thulium doped solid solution CaF2-SrF2 crystal (Tm:CaF2SrF2). The sample was 8.5 mm thick Tm:CaF2SrF2 block cut and face polished parallel to growing axis without any AR coating. The composition of sample was 60 mol% CaF2, 38 mol% SrF2 doped with 2 mol% TmF3. The sample was mounted in temperature controlled copper holder of the liquid nitrogen cryostat. The 148mm long semi-hemispherical laser resonator consisted of flat pumping mirror (HR @ 1.80 - 2.10 μ m, HT @ 0.78 μm) placed inside cryostat, and curved output coupler (r=150mm, R = 98:0% @ 2 μm) placed outside. For longitudinal pumping a fiber coupled laser diode was used. The diode was operating in the pulse mode (10 ms pulse length, 10 Hz repetition rate) at wavelength 763 nm. The 2mm thick MgF2 birefringent filter was placed at Brewster angle inside the resonator for laser wavelength tuning. With decreasing temperature the output oscillation wavelength shifted to shorter wavelength and range of tunability decreased from 117nm at 300K to 89nm at 100K following the fluorescence spectrum narrowing. The overall tunability was from 1795nm at 79K to 1944nm at 300 K. The highest output pulse energy 6.8mJ with slope efficiency 10% was obtained at 1814nm for T = 78 K. The temperature of Tm:CaF2SrF2 was found to have significant influence on laser output wavelength and tunability.