In long-haul frequency-division-multiplexing lightwave transmission systems, transmission characteristics are degraded by four-wave mixing (FWM) generated in optical fibers. To overcome this problem, modified repeated unequally spaced (RUS) allocations such as equally spaced RUS (ERUS) and unequally spaced RUS (URUS) allocations have already been examined. In addition, it has been shown that FWM noises decrease by separating signal frequencies and a zero-dispersion frequency in equally spaced (ES) allocations. In this paper, ES, ERUS, and URUS allocations are studied from the viewpoint of the position of the zero-dispersion frequency relative to the signal frequencies. It is revealed that FWM noises are reduced in ES, ERUS, and URUS, with an increase in the separation between the signal frequencies and the zero-dispersion frequency, and FWM noises in ERUS and URUS are much lower than FWM noises in ES.
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