Supercontinuum generation in thulium-doped fibres

Supercontinuum generation in thulium-doped fibres under pumping at 1.59 μm is investigated. Amplification of supercontinuum in the range of 1.8--2.0 μm is found for a fibre doped to a level of 2 × 10¹⁹ cm^-3. For a fibre with an activator concentration of 2 × 10²⁰ cm^-3 amplification is also observed in the (2.1 — 2.45)-μm band, which suggests the occurrence of the ³H₄ → ³H₅ optical transition in the fibre. The occupation of the ³H₄ level can be explained by cooperative effects.