Low threshold operation of the holmium upconversion-pumped green fiber laser

The advent of high power red laser diode together with low threshold operation pump powers, has made diode laser laser-pumping of the holmium laser feasible. Experiments were performed using ZBLAN fiber with a Ho³⁺ ion concentration in the fiber core of 1000 ppm molar. The core and cladding diameters were 3 μm and 125 μm, respectively. The dependence of incident pump power on the pump wavelengths for fibers 12.5 and 27.5 cm in length are presented. Results showed that the lowest incident threshold pump power are less than 15 mW in the 641-645 nm region for 12.5 cm fiber, this the lowest pump power threshold observed for any upconversion pumped laser. The average green output power for the 27.5 cm fiber is 5 mW with 96% output coupling is also presented.