In this work, an ultra-long single-span 10Gbps SDH optical communication system was built up and tested. The 10G signal transmitted from the bit error tester with an additional 10G XFP transponder module, and looped back to the bit error tester. All the conditions and results presented were verified as error-free for over 12h. The 10G XFP module was used to perform an optical-electric-optical (OEO) conversion, so that the modulation parameters can be adjusted for nonlinearity optimization, which will be discussed elsewhere. The total distance of the fiber is over 220km, and the maximum power of the signal injected into fiber is over 17dBm, indicating that the SBS threshold is suppressed over 17dBm with the help of the 10G XFP optimization. What is interesting is, a clear wide shoulder was observed in the output signal spectrum at the transmission fiber output, provided that the fiber input power exceeds 17dBm. After careful analysis and simulation, we attributed this shoulder generation to the intra-band FWM effect, which is caused due to the FWM interaction between the carrier wave and the modulation wave within the band. The error tester will identify the shoulder as the noise level, and the maximum input power is restrained by equivalent optical signal noise ratio (OSNR) due to the intra-band FWM effect. So that the maximum transmission distance will be restricted even if the stimulated Brillouine scattering (SBS) threshold can be further enhanced. Other nonlinear effects including self-phase modulation (SPM), and group velocity dispersion (GVD) are also analyzed.