Supercontinuum generation in thulium-doped fibres

Supercontinuum generation in thulium-doped fibres under pumping at 1.59 mu m is investigated. Amplification of supercontinuum in the range of 1.8-2.0 mu m is found for a fibre doped to a level of 2 x 10(19) cm(-3). For a fibre with an activator concentration of 2 x 10(20) cm(-3) amplification is also observed in the (2.1-2.45)-mu m band, which suggests the occurrence of the H-3(4) -> H-3(5) optical transition in the fibre. The occupation of the H-3(4) level can be explained by cooperative effects.