“…4(b) shows that FWHM of 4 I 13/2 / 4 I 15/2 emission band increases gradually from 59 to 74 nm when Er 2 O 3 concentration varies from 0.5 to 3 mol%. These values are larger than that in other glass hosts used for optical ber ampliers, such as 38 nm in lead borosilicate glasses, 28 47 nm in lead phosphate glasses, 29 46 nm in niobium germanate glasses 30 and 48 nm in uorotellurite glasses. 5 The larger FWHM is ascribed to not only high refractive index ($2.1) which strengthens the ligand elds around Er 3+ but also to the existence of multiple structural units such as TeO 4 trigonal bipyramid, TeO 3 trigonal pyramid, TeO 3+1 polyhedra, WO 6 octahedra and WO 4 tetrahedra, which creates diverse coordination environments.…”