EDFAs have been extremely successful in the low-loss telecommunications win- dow. However, the insatiable demand for bandwidth has stretched and filled the Er~3+ window, creating the need for alter- Nate materials to provide amplification at Other wavelengths. Other rare-earth ions Capable of operating at these wavelengths Are more formidable and require ad- Vanced materials that are more difficult To process. Finally, rare-earth ions have Also been used to increase both photo- Sensitivity and nonlinearity for gratings And switches.
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