I. Introduction
Distributed Raman amplifier (DRA) has many advantages in modern long-haul coherent dense wavelength division multiplexed (DWDM) transmission systems such as improved optical signal to noise ratio (OSNR) essential for higher order and spectrally efficient modulation formats [1], [2], flexible signal power profile along transmission span enabling efficient nonlinear compensation techniques i.e., optical phase conjugation (OPC) [3], [4] or nonlinear Fourier transform (NFT) [5] . Although DRA with bidirectional higher order Raman pumping [6], [7] is the best choice to maintain a quasi-lossless signal power profile with maximum reduction of amplified spontaneous emission (ASE) noise compared with conventional backward only pumping, RIN transfer [8], [9] from forward pump to the signal is a major drawback in these systems, which may counteract the benefit of improved OSNR. The stochastic intensity fluctuations in the Raman pump also induce relative phase noise through pump-signal cross phase modulation [10] and cross-polarization interactions in polarization division multiplexed coherent transmission systems. RIN transfer mainly depends on the amplitude noise level in the pump(s), pumping configurations and chromatic dispersion of the fibre which determine the walk-off between the pump and signal. There have already been many efforts in reducing the pump to signal RIN transfer such as: introducing high dispersion fibre [11], intensity modulation in dual order Raman pumping [9], forward pumping with incoherent broadband pump [12], [13] and dual order forward pumping with optimized low reflectivity fibre Bragg grating (FBG) at 1st order pump wavelength [14]. Recently we have proposed a RIN mitigation technique in a random distributed feedback (DFB) lasing based bidirectional, dual order DRA without any 1st order forward seed [15]. Although this technique shows improved transmission performance, it requires very high 2nd order forward pump power to transfer the gain efficiently to the signals which are two Stokes shift away from the pump. Using a low RIN, narrowband conventional 1st order forward pump seed (i.e., semiconductor laser diode) improves the overall pump efficiency but transmission performance may still be limited by the RIN transfer from high power 2nd order pump.