Role of excited-impurity quasiparticles for amplification of radiation in an erbium-doped glass fiber amplifier

In this paper, I will show that excited-impurity quasiparticles may play an essential role for the amplification of radiation in an erbium-doped glass fiber amplifier. A quasiparticle consists of an excited state of an erbium ion interacting with other erbium ions and with the thermal bath via phonon-assisted energy transfer processes. The quasiparticles populate energy levels of a quasiband. Fermi’s statistics determines the average occupation numbers of the levels. The quasiparticle model leads to an analytical expression for the gain coefficient of an erbium-doped glass fiber amplifier, or generally, of a fiber amplifier or fiber laser.