Influence of double Rayleigh backscattering on Raman amplified higher order modulation transmissions

Abstract

State-of-the-art optical transponders use a coherent detection scheme with higher order modulation formats to achieve bitrates as high as 400 Gbit/s. However, the use of higher order modulation formats, like 16-QAM, increases the requirements on the optical signal-to-noise ratio (OSNR) [1]. To utilize even higher order modulation formats, e.g. 64-QAM, noise in the transmission system must be minimized. Distributed Raman amplifiers can be used to achieve low noise amplification [1], but introduce a new kind of noise: multi path interference (MPI) by double Rayleigh backscattering (DRB). This limits the practically useable gain [1]. To study the still unknown influence of DRB on higher order modulation we numerically simulate a contra directional pumped distributed Raman amplifier. As in [2] the Raman amplifier is modelled as a set of coupled differential equations. The resulting boundary value problem is solved with a fourth-order Runge-Kutta method [3]. The model provides the powers of all signal, pump and noise terms in the system. Hence, a system OSNR is calculated, covering forward amplified spontaneous emission (ASE) and DRB. Afterwards, a third term describing imperfect electrical components is introduced to the calculation. To compare the simulation results with experimental results the system OSNR is converted to an error vector magnitude (EVM).