Theoretical Analysis of Frequency Allocations in FDM Lightwave Transmission Systems

In long-haul frequency-division-multiplexing (FDM) lightwave transmission systems, transmission characteristics are degraded by four-wave mixing (FWM) in optical fibers. To date, equally spaced (ES), unequally spaced (US), and repeated unequally spaced (RUS) allocations have been demonstrated in FDM lightwave transmission systems. It has been already revealed theoretically and experimentally that optical power of FWM light with f_FWM = fi for RUS is lower than that for ES, and a total bandwidth of signal light for RUS is comparable to that for US, where f_FWM is a frequency of FWM light and /< is a frequency of signal light with a channel index i. Moreover, it has been shown that equally spaced RUS (ERUS), unequally spaced RUS (URUS), and paired RUS with combined base units have lower FWM noises than RUS with a single base unit. In this paper, paired URUS is proposed as modified URUS and theoretically analyzed. Efficiency of FWM light and a total bandwidth of signal light for paired URUS are compared with those for URUS. As a result, it is found that the efficiency of FWM light with f_FWM = fi for paired URUS is lower than that for URUS, and the total bandwidth of signal light for paired URUS is slightly narrower than that for URUS. Bit error rate and power penalty for paired URUS are also lower than those for URUS at a modulation speed of 10 Gb/s. Therefore, it is concluded that paired URUS is superior to URUS in FDM lightwave transmission systems.