Suppression of noise induced by group velocity matching of signal and forward pump light in a distributed Raman amplifier

In the distributed Raman amplifier system with forward (Fwd) pumping, significantly large optical noise is induced when the group velocities of the pump and signal lights become close. In this paper, we show that this noise can be reduced by utilizing our previously proposed low-noise Fwd-pump unit. The results of a QAM signal transmission experiment conducted when group velocities were close demonstrate that the signal quality was improved when our Fwd-pump unit was used, while the signal quality was drastically degraded when using a conventional Fwd-pump unit.