The site distributions for a given Stark-level have been determined for Nd" and Er" dooed silicate glasses, preforms and fibers from fluorescence line narrowing experiments near liquid helium temperature. The total energy 5j,read increases from bulk to fibers, from 17Z ctn to 24 cn for Nd doped materials, and from 45 cs to 193 ctnt for aluminosilicate Er3' and Nd doped preforms respectively. In that case, we conclude that inhomogeneous broadening of the 300 K luminescence band is more effective for the Nd doped fiber than for the Er3' one.
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