Abstract: The multiplexed classical and quantum transmission was one of the key application technologies of quantum secret communications, which could solve the problem that quantum information cannot transmit with classical information in a single fiber, it could reduce the application cost significantly. Aiming at the scheme that multiplex continuous variable quantum key with classical information based on WDM technology, this text quantitative analysis the characteristic of Raman scattering in the system ,under the condition that the classical information propagate in the forward mode or in the backward mode, and comparative study the vary of the secure key rate due to Raman scattering noise. The analysis shows that, the secure key rate in the forward classical transmission mode system is larger than it in the backward classical transmission mode system; if classical input power is a fixed value, the secure key rate variation induced by Ranman scattering in short distance is very small, the influence is more and more large when the distance increase; and the change triggered by the noise within a certain range of values could be ignored. The influence induced by noise in the range of 0.07N0 can negligible when the transmission distance is 50km.

Key words: Raman Scattering