Thulium-doped silica fibers with enhanced 3H4 level lifetime for fiber lasers and amplifiers

Thulium-doped fibers are renowned for their applications in high power fiber lasers at around 2 μm. It is despite the low quantum conversion efficiency of ³F₄ level in thulium doped silica fibers, which is about 10% compared to ~100% quantum conversion efficiency of the ytterbium- and erbium-doped fibers, at around 1 μm and 1.5 μm, respectively. The lower quantum conversion efficiency increases the 2 μm laser threshold but has almost no effect on the laser slope efficiency. Indeed, kW-class thulium-doped fiber lasers have been demonstrated recently. Quantum conversion efficiency of the ³H₄ level in non-modified silica fibers is much lower than that of ³F₄ level, only about 2% and therefore most of the applications of laser transitions originating from ³H₄ level are hindered by the lack of reliable low-phonon fiber host. We review of our contributions towards the comprehension and improvement of the spectroscopic properties of thulium ions doped into silica. We show potential of the developed thulium-doped fibers with enhanced ³H₄ level lifetime for applications in fiber lasers around 810 nm and in fiber amplifiers for communication S-band (1460-1530 nm).