Electrodynamic approach to the estimate of the pumping efficiency of double cladding active optical fibres

A method of increasing the pumping efficiency of active optical fibres doped with Er³⁺ ions by decreasing the diameter of the inner cladding is analysed. An active optical fibre is described by using the electrodynamic model of a three-layer dielectric waveguide. The dependence of the average pump power flux through the core on the ratio of the radii of the core and the inner cladding is obtained. It is shown that this dependence can also be interpreted as the dependence of the damping constant averaged over all the waves in the core and the cladding.