Erbium (Er) is a rare-earth metal with particularly useful energy dynamics for making telecoms wavelength optical sources. Erbium ions, Er3+, have a series of excited states which mean it can absorb at one wavelength and emit at another. This can form the basis of an ASE or laser source. Erbium ions can be pumped at 980nm, where they have a strong absorption band, and will exhibit amplified spontaneous emission, with a characteristic emission spectrum, having the majority of content between about 1520nm and 1570nm. This is associated with a corresponding absorption band in the same spectral region.
Erbium is modeled as a three-level system; with a signal band from about 1520 to 1570 nm. Er3+ ions can be pumped at several wavelengths including 510, 532, 665, 810, 980, and 1480 nm – though pumping at 980nm and 1480nm is favored as it avoids excited state absorption (ESA) of pump, where the Er3+ ions may be further excited by additional pump absorption. ESA briefly prevents these ions from providing useful gain and so is detrimental to efficient output.
At 1480 nm, pumping directly excites the upper sublevels of the 4I13/2 metastable manifold (the excited state is broadened into closely spaced sublevels due to Stark splitting), with ground to excited state transition
corresponding to both a 1520-1570nm signal band and 1460-1500nm pump band. When pumped in the 980 nm absorption band, Er3+ ions are moved from the ground state to the 4I11/2 excited state.