Influence of Frequency Allocations and Optical Filters on FDM Optical Fiber Communications

In long-haul frequency-division-multiplexing (FDM) lightwave transmission systems, transmission characteristics are degraded by four-wave mixing (FWM) generated in optical fibers. To date, equally spaced (ES), unequally spaced (US), and repeated US (RUS) allocations have been demonstrated in FDM lightwave transmission systems. It has already been theoretically and experimentally revealed that intensities of generated FWM lights with f_FWM=f_i for RUS are lower than those for ES, and a total bandwidth of signal lights for RUS is narrower than that for US, where f_FWM is a frequency of an FWM light, and f_i is a frequency of a signal light with a channel index i. Moreover, to overcome RUS, ES RUS (ERUS) and US RUS (URUS) have been proposed as modified RUSs and theoretically analyzed. In this paper, ES, RUS, ERUS, and URUS allocations are studied from the viewpoint of a transmission bandwidth of an optical filter. It is revealed that FWM noises are reduced in URUS more than in ES, RUS, and ERUS with a decrease in a transmission bandwidth of an optical filter. The upper limit of a filter transmission bandwidth to achieve a bit error rate of 10^-9 is also obtained.