Ho-doped silica fibre laser in-band pumped by a Tm-doped fibre laser

In this paper, we report preliminary results for a Ho silica fibre laser operating at 2090 nm in-band pumped by a cladding-pumped Tm doped fibre laser at 1980 nm. The Tm fibre pump laser was constructed in-house with wavelength selection provided by an external cavity containing a diffraction grating. This could produce over 20 W of output at 1980 nm (corresponding to the absorption peak in Ho-doped silica) in a beam with M2 ~ 1.5. The Ho-doped silica fibre was fabricated in-house and had a Ho-doped alumino-silicate core of 18 Icircfrac14m diameter and 0.08 NA, which was surrounded by a silica cladding with a diameter of 300 Icircfrac14m. Pump light at 1980 nm was launched into the core of the Ho fibre with a launching efficiency of -53 %. Approximately 85% of the launched pump light was absorbed in the 1.5 m length of Ho fibre used in our experiment. Feedback for lasing was provided by the Fresnel reflection from the perpendicularly cleaved facet at the output end of the fibre and by a mirror with high reflectivity (>99%) over the wavelength range 2050 to 2150 nm and high transmission (>90%) over the range 1900 - 2000 nm butted to the opposite end of the fibre. Under these operating conditions, the Ho fibre laser yielded a maximum output power of 4.3 W at 2090 nm for an absorbed pump power of-8.7 W.