WDM transmission over multiple long spans with bidirectional Raman pumping

We investigate the feasibility of WDM transmission over multiple spans that are twice as long as those typically used in commercial systems. Using longer spans reduces the number of repeater sites, potentially lowering the operating costs of medium- and long-haul terrestrial systems. We show that bidirectional Raman pumping is a key enabling technology. In particular, we demonstrate the benefits of high levels of codirectional Raman gain, up to 10-dB more than is used in transmission experiments with standard span lengths. Then, we transmitted 40 × 10.66 Gb/s with uncorrected bit-error ratios better than 10^-9 using 200-km spans of nonzero dispersion fiber with 14.5-dB of Raman co-gain over a reach of 2400 km. We also measured the maximum amount of span loss that was tolerable for a given system reach for 200-km span transmission at 10 Gb/s. After optimization at twelve distances (800 to 3000 km), we found that the tolerable span loss increased by 1 dB for each 200-km reduction in reach.