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

We report on the stable operation of a 2500-ppm thulium-doped ZBLAN upconve rsion fiber laser at 481 nm and the related problem of photodegradation ass ociated with the formation of color centers. The output coupling is observe d to affect the level of induced absorption in the fiber, hence the output power level. The start-up laser threshold is shown to increase significantl y over time after lasing as a result of a thermally driven relaxation of ph otobleached color centers. Three techniques are investigated for restoring the transparency of the darkened fiber prior to lasing operation: visible p hotobleaching 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.