  A non-destructive method for measuring the resonant absorption coefficient of
rare-earth-doped optical fibers is introduced. It can be applied to a broad
range of fiber designs and host materials. The method compares the
side-collected spontaneous emission at two arbitrary locations along the fiber
as a function of the pump wavelength to extract the absorption coefficient. It
provides an attractive and accurate alternative to other available techniques.
In particular, the proposed method is superior to the cut-back method, which
destroys the sample and is prone to inaccuracies due to the cladding mode
contamination. Moreover, because it does not involve any mechanical movement,
it can be used for fragile optical fibers.
