A comparison of carrier-suppressed return-to-zero (CSRZ) and single sideband return-to-zero (SSB-RZ) formats is made in an attempt to find the optimum modulation format for high bit rate optical transmission systems. Our results show that CSRZ is superior to return-to-zero (RZ) and SSB-RZ with respect to signal degradation due to Kerr nonlinearities and chromatic dispersion in wavelength division multiplexing (WDM) as well as in single-channel 40-Gb/s systems over standard single-mode fibers (SSMF). It is shown that CSRZ enables a maximum spectral efficiency of approximately 0.7 (b/s)/Hz in a N × 40 Gb/s WDM system with equally polarized channels. Furthermore, the CSRZ format in N × 40 Gb/s WDM systems shows no further signal degradation compared to single-channel transmission.
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