Raman cell optimisation for distributed amplification based transmission systems

Commercially available Raman fiber pumps display relatively high Relative Intensity Noise (RIN) that gets transferred to the output signal in systems relying on Raman distributed amplification. This has been shown to drastically hinder performance in Raman-based transmission systems [1], limiting the practically usable forward pump powers and setting constraints to the design of low signal power variation (SPV) bidirectional systems with optimised optical signal-to-noise ratios (OSNR) [2]. Recent works have highlighted a remarkable improvement in the resilience to RIN transfer of higher-order ultralong Raman fiber lasers (URFL) in a random Distributed Feedback (rDFB) laser amplifiers configurations with no fiber Bragg grating (FBG) at the front-end [3]. Moreover, we have shown [4] that a low-reflectivity front-end FBG can be more beneficial than no back- reflections at all, for enhancing pumping efficiency while still keeping RIN transfer under control. Here we extend previous analysis of the low-reflectivity URFL amplification scheme including experimental investigation of RIN transfer, OSNR and pump power requirements for different architectures. We perform a comparison with the rDFB approach and present a detailed optimisation of the main amplifier parameters for the effective design of Raman-based transmission systems