Introduction

Fiber OPAs have a number of features that are potentially attractive for optical communication systems. In particular there is the prospect of using amplifiers with several hundred nanometers of gain bandwidth for the construction of wavelength-division multiplexing (WDM) systems with tens of terabits per second of capacity. However, before this potential can be realized a number of effects that can degrade the signal-to-noise ratio (SNR) must be addressed. Specifically these effects are: cross-gain modulation, mediated via pump depletion; four-wave mixing (FWM) between signals or between signals and a pump; cross-phase modulation (XPM) between signals. The physical origin of these effects is simple: efficient OPA operation generally requires that all the waves of interest be close to the zero-dispersion wavelength (ZDW) of the fiber, and this in turn implies that a number of other unwanted nonlinear interactions will also be well phase matched and so may generate large undesired effects. This origin indicates that these effects are fundamental in nature and may therefore be difficult to suppress.

Because of the importance of this issue, these effects have been investigated in depth by several research groups [1–12]. It has been found, through simulations and experimentally, that some of these effects can indeed be quite large under certain circumstances, to the point where they bring into question the viability of using fiber OPAs in WDM communication systems.