Transmission Performance of 256-Gb/s PDM-16QAM With Different Amplification Schemes and Channel Spacings

We experimentally investigate the performance of a 256-Gb/s polarization-division-multiplexed 16-ary quadrature-amplitude-modulation (PDM-16QAM) transmission system in dispersion uncompensated TeraWave SLA+ fiber spans using erbium-doped-fiber amplifiers (EDFAs), hybrid EDFA/Raman amplifiers, and all backward-pumped Raman amplifiers. The effects of 37.5- and 50-GHz channel spacings and cascaded optical add/drop multiplexers (ROADMs) on the system performance are studied. We find that when there are no ROADMs, hybrid EDFA/Raman amplifiers and all-backward-pumped Raman amplifiers can increase the reach by about 70% and 100%, respectively, compared to EDFA-only amplifiers, from 2000 to about 3500 and 4200 km, respectively, assuming 20% soft-decision forward-error-correction (SD-FEC) for the system with both 37.5- and 50-GHz channel spacings. We show that cascaded ROADMs have a small impact on the system with 50-GHz channel spacing, but can severely degrade the performance of the system with 37.5-GHz channel spacing and virtually eliminate the reach gain from Raman amplifiers in such systems due to large penalties and higher error floors induced by ROADM filtering effects. We also show that for the same number of channels, the system with 37.5- and 50-GHz channel spacings has similar performance when there are no ROADMs.