Discrete-EDFA-Free Amplification: EDF-Assisted Random DFB Raman Laser

This paper presents a simulation-based study of an erbium-doped-fibre-assisted random distributed-feedback (rDFB) Raman amplifier for long-haul wavelength-division multiplexed transmission. The proposed architecture combines distributed Raman amplification over a 50 km standard single-mode-fibre span with a 25 cm terminal erbium-doped fibre (EDF) section pumped only by the residual 1460 nm rDFB field. The terminal EDF is intended to compensate span-end connection losses without a dedicated 980 nm or 1480 nm pump source. The model is calibrated against an experimentally measured optical-time-domain-reflectometer (OTDR) power profile, and an additional 2 dB EDF-gain margin is introduced to represent inter-span losses. The study considers a single 2 dB net-gain operating point, selected by minimizing signal-power variation along the span. The optimized configuration is applied to a 50-channel C-band WDM system over an equivalent 150-span, 7500 km link with idealized per-span power reset. The simulated central-channel OSNR reaches approximately 18.9 dB after 7500 km, while the channel OSNRs remain between about 17.5 and 19 dB. However, central-channel optimization produces a strongly nonuniform gain profile, increasing from approximately 2 dB near the band centre to 6.5 dB at the long-wavelength edge. The Kerr nonlinear-product power also increases by about 30 dB between one span and 7500 km. These results demonstrate the potential of the proposed architecture for span-end loss compensation, while showing that gain flattening, multi-channel optimization, system-level nonlinear analysis, and experimental validation are required before practical deployment.