Spectral characterization and laser operation of Ho:SrF2 single-crystal fiber

An efficient Ho:SrF2 single-crystal fiber (SCF), lightly Ho-doped to a concentration of 0.5 at%, was successfully grown using the temperature gradient technique, and its laser characteristics were studied. Several optical parameters of the Ho:SrF2 SCF were determined using the Judd-Ofelt (J-O) theory, including the J-O intensity parameters, radiative lifetimes, and fluorescence branching ratios. In addition, the continuous-wave (CW), tunable, and passive Q-switched (PQS) laser operations of Ho:SrF2 SCF pumped by a thulium-doped fiber laser were demonstrated in this study. While the CW laser was operating, average output power greater than 1 W and a continuous tuning range of 62 nm were obtained. For the PQS operation, a minimum pulse duration of 779 ns with a repetition rate of 36.27 kHz was achieved using silver nanorods as saturable absorbers. These results indicate that the Ho:SrF2 SCF has potential application in tunable or ultrafast laser at 2.1 µm.