Double Rayleigh scattering noise in lumped and distributed Raman amplifiers

An improved analytical expression of the multiple path interference (MPI) induced by double Rayleigh scattered signal in fiber Raman amplifiers (FRAs), valid both for lumped and distributed FRAs, is derived with a small signal model. The expression is validated both by comparison with an exact solution and by MPI measurements on four different lumped and distributed Raman amplifiers.