Simple method for determining quantum efficiency and background propagation loss in thulium-doped fibres

A method for determining the quantum efficiency and background propagation loss in rare earth-doped silica fibres is presented. These parameters can be determined using straightforward measurements of slope efficiency and emission spectra at various lengths of active fibre. This method has been applied to a thulium-doped silica fibre with a structured ‘nested-ring’ doping profile. The quantum efficiency of this fibre was 1.93 ± 0.01 and the background propagation loss was 0.15 ± 0.01 dB/m. This characterisation method also allows the relative impact of these parameters on the slope efficiency of an all fibre high-power system to be predicted. It was found that the non-ideal background propagation loss of this fibre results in a decrease of ∼ 18 percentage points on the predicted slope efficiency, whereas the quantum efficiency results in a decrease of ∼ 3 percentage points. The predicted slope efficiency with respect to absorbed pump power of 62 ± 2% for the all-fibre high-power system is consistent with the experimental result of 61.5%. This characterisation provides feedback to the fibre design and fabrication that for efficient power scaling, efforts to decrease the background propagation loss of this fibre can provide a greater increase in efficiency than efforts to further increase the quantum efficiency