On the design of NFT-based communication systems with lumped amplification

Optical signal transmitted in systems with lumped amplification (utilising erbium-doped fibre amplifiers, EDFA) experiences a distributed attenuation and a periodic pointaction gain. The lossless nonlinear Schrödinger equation (NLSE), formally speaking, is not directly applicable for the description of light evolution in fibre links with non-uniform loss and gain alternation. However, in this case the NLSE can still serve as a good approximation to describe the so-called path-averaged light dynamics. The difference between the NLSE-based path-average model and the true signal evolution in EDFA fibre lines results in the performance degradation for the transmission techniques that are inherently based on the properties of exact NLSE model. Thus, optical soliton-based and nonlinear Fourier transform (NFT) based communications, belonging to that type, suffer from additional errors emerging from models mismatch. In this work, we extend the theoretical approach (first proposed for solitons in EDFA systems) to the case of NFT-based systems to constructively diminish the aforementioned penalty: based on the quantitative analysis of distortions due to the use of path-average model, we optimise the signal launch and detection points to minimise the models mismatch.Without loss of generality, we demonstrate how the approach works for the NFT systems that use continuous NFT spectrum modulation (vanishing signals) and NFT main spectrum modulation (periodic signals). Through numerical modelling we quantify the corresponding improvements in system performance.