“…This effect, which is in proportion with Er 3 + dopant concentration, has been carefully described by Ehrmann et al [38]. According to this hypothesis, with an increase in the erbium content, the effect of selfabsorption may affect the emitted fluorescence mainly around the peak, consequently the measured emission intensity around it will be lower than their intrinsic values and then one can observe a bandwidth broadening [37]. On the other hand, changes in the chemical bonding of the ligands surrounding the Er 3 + ion have been discarded since, as has been already mentioned, no appreciable changes have been observed in the absorption spectra as a function of erbium content.…”