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

Photo-darkening prevents developing of new applications of rare-earth doped silica fibre lasers or amplifiers operating at wavelengths shorter than 1μm 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 Tm-doped fibres. This ion offers many potential optical transitions spanning from 0.45 to 1.9μm. Several interesting transitions can be excited via up-conversion, using one pump around 1μm. However, this scheme induces a particularly fast and intense photo-darkening, enough to prevent amplification in the short wavelengths region (0.45-0.9μm). 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 Ce and La on the bleaching mechanisms.