Cross-Phase Modulation Induced By Dispersion Compensation Fibers In Coherent QPSK/OOK System

With the rapid development of broadband services, especially the explosive development of streaming media technology, high-definition television, cloud computing and peer to peer(P2P) service, it is imperative to improve the data transmission capacity of backbone network. Due to the direct upgrading costs a lot, reusing the existing intensity modulation/direct detection(IM/DD) optical fiber communication network, reducing the upgrading cost appears to have a strong practical significance. Hybrid modulation WDM system is the most attractive option to solve this problem, and the most typical case is coherent QPSK/OOK hybrid systems. Cross-phase modulation has been considered as the main impairment on QPSK channels in QPSK/OOK hybrid systems, but the previous XPM research ignores the contributions of the dispersion compensation fibers(DCF) in dispersion compensated fiber links. Increasing channel rate, channel number and transmission distance improve the input power and narrow the channel spacing, so XPM effect is more serious. Moreover, the cross-sectional area of DCF is more smaller and the nonlinear coefficient is more larger than those of single mode fiber(SMF). For these reasons, the XPM contribution of the DCFs can not be ignored. This paper studies the XPM effects in QPSK/OOK hybrid systems and does take into account the DCF nonlinearity, and analyses its influence on coherent QPSK channels.This thesis introduces the basic theory of coherent QPSK/OOK hybrid systems. Then the conversion between intensity and phase modulation in a nonlinear dispersion fiber is analyzed, the mechanism of the XPM effect is explained, and a common pump-probe model to study XPM effects is introduced.Secondly, the linear filter models are established to describe the intensity noise and the phase noise induced by XPM effects in dispersion compensated fiber links, respectively. Then the characteristics and impairments caused by DCF of the XPM intensity filter and the XPM phase filter are studied.Finally, the XPM-induced phase noise on coherent QPSK channel is analyzed and the variance of XPM-induced error is estimated in frequency domain in coherent QPSK/OOK hybrid systems. Meanwhile, the bit error rate(BER) is provided for coherent QPSK signal contaminated by both ASE and XPM. Then using VPItransmissionMaker constructed coherent QPSK/OOK hybrid systems to verify the theoretical analyses. The results show that the DCF nonlinearity does not affect the characteristics of the XPM filters, but impact the XPM filter amplitude. Moreover, the XPM contributions of the DCFs have a certain influence on the coherent QPSK channels. As a result, the DCF nonlinear effects in dispersion compensated fiber links have impacts on the performance of the coherent QPSK/OOK hybrid systems, they should be taken into consideration during the system design.