Efficient CW operation of a 2.9 mu m Ho³⁺-doped fluorozirconate fibre laser pumped at 640 nm

CW laser operation of a Ho³⁺-doped ZBLAN multimode fibre, emitting within the range 2.83-2.95 mu m, was achieved by optical pumping at 640 nm, as a result of excited state absorption of the pump. Thresholds were approximately 65 mW absorbed power for a 90% output coupler and approximately 290 mW with reflection of the fibre end only. Slope efficiencies and maximum output powers were 2.9% and 12.6 mW, and 4.4% and 10.2 mW, respectively.