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We analyze the sideband multiwave mixing process of an audio-frequency-modulated signal in an erbium-doped-fiber amplifier. The nearly resonant interaction of an audio-frequency-modulated signal with the doped erbium ions results in multiwave mixing, which induces the generation of new sideband frequencies. We show that the input signal is distorted by such a multiwave mixing process. The condition to reduce the distortion of an input modulated signal in an erbium-doped-fiber amplifier is evaluated.

We present a theory that enables us to account for the pump-induced change of refractive index in a rare-earth-doped fiber. Experimental results carried out on an erbium-doped fiber with cw pump and cw signal operation confirm the accuracy of this theory at the steady state. Both theoretical and experimental results show that significant index changes can be obtained for the order of 1–2 mW of cw optical pump or signal power. In addition we studied the transient dynamics of the resonance-enhanced refractive index in a wide range of pulse widths from millisecond to tens of femtoseconds. The result shows that a fast response of refractive-index changes is achievable since the effects of spontaneous emission and nonradiative decay are negligible in the short-pulse case.