| In recent years, the ultra-long distance optical signal transmission and wavelength division multiplexing (WDM) systems play significant roles. However, even though there are a lot of techniques to compensate the signal impairment caused by fiber loss, group velocity dispersion and nonlinear effects, this problem is still not be completely solved. By noting the features of Mid-span spectral inversion (MSSI), this thesis examines compensation of the signal impairments caused by fiber loss, group velocity dispersion and nonlinear effects. Firstly, this thesis theoretically proves that MSSI can compensate the impairments caused by dispersion and nonlinear effects by using nonlinear Schrodinger equation. In ideal case, the effects of even order dispersion and nonlinear effects in the first half link could be completely eliminated in the second half link after optical phase conjugation, which is the core technique in MSSI. However, the impairment caused by fiber loss and odd order dispersion cannot be compensated because of the lack of symmetry. Actually, higher odd order dispersion will have less effect on signal so we considered here only fiber loss. To solve this issue, the most effective method is to improve the symmetry of the loss gain cycle around the mid-point. Raman amplification is considered to be used instead of EDFAs in this thesis because it is distributed amplification and will improve the loss/gain symmetry. As an example, a computer model is used to simulate a 4,000km WDM fiber link, with MSSI in the middle. The bit rate is l0Gbit/s and there are 6 channels. Both Raman amplification and EDFA are examined in this model. We have discovered that Raman amplification performs better than EDFA not only in low dispersion, low power situations but also in standard situation. In a standard fiber link (power= ImW, beta2=- 20ps[2]/km), Raman amplification has a 3dB benefit than EDFA. And the penalty of Q value in Raman case is 1.9dB after 4,000km transmission in the absence of amplifier noise. That means by combining MSSI and Raman amplification, the impairment caused by fiber loss, group velocity dispersion and nonlinear effects can be compensated in WDM systems. |
