Holmium in ZBLAN: interplay of lasing, co-lasing and excited-state absorption

Trivalent holmium in ZBLAN can be easily pumped with an argon laser operating around 0.48 μm. Its lasing transitions at 0.55, 0.75, 1.2 and 2.0 μm are now well-known. However, excited-state absorption at pump and signal wavelengths from the 5I7 and 5I6 levels make the problem of maximum efficiency somewhat tricky. Fluorescence and lasing experiments are reported and details of the influence of excited-state absorption and co-lasing on laser operation are given.