Simple theory of gain peaking in erbium-doped amplifier chains for long-haul communications

Abstract

We present a simple rule for predicting the peak gain wavelength of an Er-doped fiber amplifier. For a given fiber type, the peak gain wavelength is determined solely by the operating gain per unit length. Using this rule coupled with a simple Er-doped amplifier model and measured modeling parameters, the gain peak is predicted for a particular Er-doped fiber. The result is verified by direct measurement in a fiber loop, using polarization scrambling to eliminate instabilities. We demonstrate that the gain peak does not vary with pump power, pump wavelength, or signal power as long as the gain per unit fiber length is fixed. The theory is then extended to roughly estimate the pump power, signal power, compression level, and other design parameters. Finally, the inclusion of the wavelength dependences of other components in an amplifier chain is also discussed and demonstrated.