The optical nonlinearity in a single-mode fiber imposes a fundamental limitation on the capacity of optical frequency-division multiplexed (OFDM) systems. In particular, four-wave mixing (FWM) crosstalk may severely degrade the system performance when the fiber input powers are large and/or the channel spacing is too small. Theoretical and experimental results of the effects of FWM in OFDM systems are presented. The theoretical results demonstrate the dependence of FWM on various systems parameters. Also included is an analysis of FWM in both unidirectional and bidirectional transmission systems. The receiver sensitivity degradation from FWM crosstalk was measured in a 16-channel coherent system. A sensitivity penalty of 0.4 dB resulted when a signal power of -3 dBm/channel was transmitted through 12 km of dispersion-shifted fiber.

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