This paper describes the theoretical model and simplified model of Raman amplifiers

analyses and compares three simulation methods for forward pumped Raman amplifiers

gives analytical equation of the backward pumped Raman amplifiers when neglecting pump depletion. On the basis of the results

this paper gives the amplification distance of forward and backward pumped Raman amplifiers as well as the maximum unrepeatered length of dual pumped Raman amplifiers.