Impact of cerium and lanthanum on the photo-darkening and photo-bleaching mechanisms in thulium-doped fibre

Abstract Photo-darkening prevents developing of new applications of rare-earth doped silica fibre lasers or amplifiers operating at wavelengths shorter than 1 μ m or at any wavelength at high power. The photo-darkening is characterized by a decrease of the laser intensity during amplification. In this article, we are interested in T m -doped fibres. This ion offers many potential optical transitions spanning from 0.45 to 1.9 μ m . Several interesting transitions can be excited via up-conversion, using one pump around 1 μ m . However, this scheme induces a particularly fast and intense photo-darkening, enough to prevent amplification in the short wavelengths region (0.45– 0.9 μ m ). To mitigate this effect, we co-dope the fibres with cerium or lanthanum ions. By characterizing the steady state values of photo-darkening and its characteristic times, we demonstrate the beneficial role of C e and L a on the bleaching mechanisms.