Ground-state and excited-state absorption in rare-earth doped optical fibres

The authors present a modelling of absorption in rare-earth doped optical fibres. The transmission through the fibre is derived from ground-state absorption (GSA) and excited-state absorption (ESA) parameters. Fitting to experimental data obtained with thulium at 676.4 nm (GSA only) and 647.1 nm (GSA+ESA) yields experimental values for the absorption parameters. At 647.1 nm, ESA gives rise to transmission decreasing with power, which modifies the usual behaviour of absorption saturation.