Nonlinearities tolerant modulation format enabled Tb/s superchannel transmission over 420Â km of unrepeated Raman amplified link

In this work we investigate different nonlinearities compensation and mitigation techniques for an unrepeated Raman-amplified link over 420 km of ultra-low loss (ULL) optical fiber. For such long links (over 400 km) fiber nonlinearities become very significant as high forward pump power is needed to ensure sufficient optical-signal-to-noise-ratio (OSNR) to acquire a bit-error-rate (BER) not exceeding the forward-error-correction (FEC) threshold. Such nonlinearities will significantly limit the performance. Through numerical simulations, we show successful net 200 Gb/s single carrier and 1 Tb/s superchannel transmission using PM-QPSK and symbol-rate-optimization (SRO). First we implement single carrier 200 G transmission either using 28 Gbaud PM-16QAM or 56 Gbaud PM-QPSK. Nonlinearities are compensated by either using digital-back-propagation (DBP) or phase-conjugated twin waves (PCTWs). We compare the performance of DBP and PCTWs based single carrier 28 Gbaud PM-16QAM, at the same transmission distance and capacity, with single carrier PM-QPSK and find that PM-QPSK does not require any nonlinearity compensation to give better performance than 28 Gbaud PM-16QAM. Following this result, we show a successful unrepeated transmission of net 1 Tb/s PM-QPSK Nyquist-spaced superchannel with an intra-superchannel net spectral efficiency (SE) of ∼3.6 b/s/Hz, over 420 km of Raman amplified ULL fiber without using any nonlinearity compensation. To improve the performance of this superchannel we implement nonlinearity mitigation scheme based on SRO. We investigate 5 × 56 Gbaud, 10 × 28 Gbaud, 20 × 14 Gbaud and 40 × 7 Gbaud PM-QPSK channels and found that the best performance is shown by 20 × 14 Gbaud superchannel. For PM-16QAM, successful transmission is only possible either using DBP or PCTWs based transmission link. DBP has a very high computational complexity whereas PCTWs halves the overall link spectral efficiency. Even then both schemes do not outperform PM-QPSK for the same transmission distance and capacity. So for practical realization the nonlinearity robust modulation format PM-QPSK using SRO has been identified as the most promising approach for superchannel implementation of unrepeated Raman amplified links.