Stability aspects in the operation of a 2500-ppm thulium-doped ZBLAN fiber laser at 481 nm

We report on the stable operation of a 2500-ppm thulium-doped ZBLAN upconversion fiber laser at 481 nm and the related problem of photodegradation associated with the formation of color centers. The output coupling is observed to affect the level of induced absorption in the fiber, hence the output power level. The start-up laser threshold is shown to increase significantly over time after lasing as a result of a thermally driven relaxation of photobleached color centers. Three techniques are investigated for restoring the transparency of the darkened fiber prior to lasing operation: visible photobleaching at 514-nm, near-infrared photobleaching from the 780-nm laser transition of thulium, and annealing at temperatures above 100 degrees C. (C) 2000 Elsevier Science B.V. All rights reserved.