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A novel wavelength arrangement using C- and L-band-separated Raman preamplification is proposed for application to bidirectional unrepeatered transmission systems operating with multiple 43 Gb/s channels. The proposed wavelength allocation makes it possible to greatly mitigate Raman gain depletion by the counter-propagating signals. The authors have achieved bidirectional unrepeatered transmission of 32 × 43 Gb/s channels (= 1.28 Tb/s) over 200 km with Raman preamplifiers using the proposed technique. They found that the system performance of bidirectional transmission with C/L band-separated Raman preamplification is degraded by nonlinear interactions between the high power Raman pump lights and the WDM signals. The root cause can be described in terms of nondegenerate four-wave mixing induced by beating between the WDM signals and two longitudinal modes of the Raman pump light. A solution avoiding ND-FWM was demonstrated in a 32 × 43 Gb/s transmission experiment.