On the Evolution of Noise in Multiple-Span Transmission With Forward Pumped Raman Amplifiers

The evolution of noise in transmission systems with multiple spans featuring forward pumped distributed Raman fiber amplifiers has been investigated experimentally and by numerical simulations. We found evidence that the transfer of relative intensity noise from the pump to the signal reveals characteristics of a nonlinear process, which cannot fully be captured by a Gaussian random variable added to the signal power. Power fluctuations of the pump radiation result in an intensity modulation of the amplified signal by stimulated Raman scattering. We have observed that a concatenation of multiple forward pumped amplifiers can lead to an increase of the bandwidth of the noise added to the signal compared to a single amplifier. A process acting like modulation instability of the pump radiation during propagation through the fiber was identified as a potential source of the bandwidth increase from span to span.