Span-Length Optimization of Random DFB Raman Amplifiers for Long-Haul WDM Transmission up to 7500 km

We present a simulation study of bidirectionally pumped distributed Raman amplification (DRA) noise performance for two recirculating-loop span configurations—50 km and 75 km—evaluated over a 50-channel C-band grid (191,200–196,100 GHz, 100 GHz spacing) representative of dense WDM transmission. For each span length, the forward pump power was first optimized on a central channel (193,600 GHz) to minimize signal power variation (SPV), yielding optimum values of 1.2 W for the 50 km span and 2 W for the 75 km span; the backward pump power was then set to achieve 0 dB net gain on the same central channel. These optimized pump conditions were applied uniformly across all 50 channels to characterize on–off gain variation for each single-span configuration. Using an input OSNR of 30 dB, recirculating-loop simulations were then performed for both spans across 1 to 60 (50 km) and 1 to 40 (75 km) round trips, covering a common reach of up to 7500 km. The OSNR degrades more slowly with distance for the 50 km than for the 75 km span up to 7500 km. Under the conditions studied, shorter DRA spans give better noise performance, even though more recirculations are required for the same reach.