Existence of intrinsic ´background loss´ in rare-earth doped fibres: drawbacks and usefulness

Estimates are given of the intrinsic multiphonon absorption of trivalent rare earth (RE) dopants in glasses. The orders of magnitude of 1.5 dB/km in ZBLAN glass and 3.5 dB/km in silica glass as far as 2000 cm^-1 from an electronic transition can contribute to what is generally called the ´background loss´ and could explain why fibre lasers and amplifiers can be pumped outside usual electronic RE transitions.