Abstract
We report on high power 870 mW continuous wave stable laser operation at 480 nm in a 1000 ppm wt. Thulium doped multimode ZBLAN up-conversion fiber laser. The fiber is pumped by a wavelength of 1064 nm generated from diode-pumped Nd:YAG laser. A threshold of 550 mW and slope efficiency of 14% with respect to the incident pump power has been obtained. The related problem of photo-degradation associated with formation of color centers is reported. The transparency of the darkened fiber prior to the lasing operation is restored by circulating 514 nm through the fiber core. The time dependant oscillatory behavior of the emitted laser is also addressed.