Methods and apparatus are provided for reducing Raman crosstalk in a wavelength-division-multiplexing (WDM) optical fiber transmission system that transmits a multiplex of channels. Idle data is sent over one or more of the channels of the WDM system in order to maintain the optical link when user data is not being sent. The idle data has an idle data pattern, which can be controlled such that a power spectral density of a signal carried by at least one channel is shifted in order to reduce Raman crosstalk between the channels. For example, the power spectral density may be shifted away from lower frequencies of the channel, and toward the higher frequencies, by controlling the idle data pattern. Alternatively, the power spectral density may be shifted towards unused frequencies of the channel, by controlling the idle data pattern.