RIN and transmission performance improvement using second order and broadband first order forward Raman pumping

Low RIN bidirectionally pumped Raman amplification is studied. We investigate the importance of the 1^st order forward pump generation technique and optimize the forward pump characteristics to achieve RIN comparable with backward only pumping and increased transmission reach.

I. Introduction

In optical fibre communications systems distributed Raman amplification is used to improve the signal to noise ratio compared with discrete amplification. Bidirectional dual order pumping further improves the optical to signal ratio (OSNR) by reducing the signal power variation along the span compared with conventional first order bidirectional and backward pumping [1], [2]. However the improvement in OSNR is counteracted by the impact of relative intensity noise (RIN) transfer from forward pump(s) to signal. Recently many methods have been applied to reduce this RIN transfer such as reducing the front end reflections [3], [4], using incoherent pumping [5].