Specialty Optical Fibers Handbook

Stark levels and the first excited state has seven, so the emission around 1530 nm

results from an ensemble of 56 individual transitions. Although transitions

within the 4f shell are strictly forbidden in an electric dipole because the initial

and final states have the same parity, asymmetry from the local environment

causes some admixture of higher lying states with opposite parity and strongly

influences the oscillator strength. The local environmental asymmetry present

in glasses causes crystal field splitting of the energy levels and results in spectra

that are much broader than found in a crystalline host, but considerably more

narrow than that exhibited by other elements, such as transition metals in glass.

This is evident in the spectrum of ...