Growth, spectra and continuous-wave 2.1 μm laser operation of a Ho³⁺-doped bismuth silicate crystal

Single crystals of Ho3+-doped bismuth silicate (BSO) were grown by a modified Bridgman method. The spectroscopic properties for 2 μm laser operation via the 5I7 → 5I8 transition were investigated based on the absorption, emission and gain cross-section spectra of the as-grown crystal. Judd-Ofelt (J-O) theory was applied to calculate the J-O intensity parameters Ω2,4,6, spontaneous emission probabilities, branching ratios and radiative lifetimes. The Ho3+:BSO crystal was in-band pumped at 1940.3 nm by a Tm fiber laser to produce an efficient continuous wave (CW) laser output at 2109.8 nm. The maximum output power obtained was 0.73 W, and the corresponding slope efficiency with respect to the absorbed power was 38.8% and the beam propagation factor M2 of the Ho3+:BSO laser in horizontal and vertical directions was estimated to be 1.09 and 1.06, respectively.