Combating Fiber Nonlinearity Using Dual-Order Raman Amplification and OPC

Abstract

We experimentally demonstrate, for the first time, a significant 34dB nonlinear product power compensation in a midlink optical phase conjugation (OPC) system using an optimised dual-order distributed Raman amplification (DRA) technique. The dual-order backward (BW)-pumping scheme shows a record signal power symmetry of 97% over 50.4km single mode fibre (SMF) spans. We also demonstrate that the required accuracy for span-to-span power alignment is within ±1dB in order to maintain 20dB nonlinear product compensation. For a 256Gb/s dualpolarisation-16QAM transmission over 100.8km (2x50.4km) SMF with mid-link OPC, the proposed Raman amplification scheme enables the OPC to improve the nonlinear threshold by ~7dB and the optimum signal launch power by ~5dB, compared to the system without OPC.

Enhancing the Signal Power Symmetry for Optical Phase Conjugation Using Erbium-Doped-Fibre-Assisted Raman Amplification

“…This scheme enables a signalpower symmetry of 93.4 % over two 50.4 km spans, giving the highest overall symmetry for a 100 km link in comparison with 65.8% achieved in in scheme 1 and 80.9 % in scheme 2. The symmetry level of 93.4% over two 50.4 km spans is also comparable to the best symmetry level of 97% over a single 50.4 km span using an optimised dual-order Raman amplification shown in [20]. The reason for the slightly degraded symmetry of two spans is simply the accumulation of 97% asymmetry over two spans.…”

“…The proposed scheme (scheme 3) used a 25 cm EDF to generate ~1.5 dB gain to compensate for the loss between spans, and therefore maintained excellent signal power symmetry simultaneously for both individual span and two spans link. This Raman scheme enabled the highest compensation (up to 37 dB) in nonlinear Kerr product power, which is comparable to the symmetry level of the single span in [20] whilst extending the result to a two-span configuration.…”

“…7(c) for EDF-assisted scheme was mainly because of a signal-power profile mismatch at high input signal power, as the pump depletion could lead to the changes in both the Raman and EDF gain. A precise power monitoring system would be required to help achieve more accurate results in particular for highly symmetrical link [20]. It has been established that this is due to the nearly perfect signal power symmetry (>93% symmetry) from the EDF-generated gain compensating the loss between two spans.…”

“…For scheme 1, the pump power at 1366 nm was ~330 mW, higher than ~100 mW at 1455 nm, which provided 97% intra-span symmetry (as defined in [18]) a significant improvement over first-order pumping alone [20]. The pump powers compensated only the loss from the SSMF.…”

“…Thus, the DRA not only provides a better performance without OPC, but also gives a large margin of performance (reach or data capacity) improvement when deploying an OPC to compensate a significant portion of the nonlinear interference accumulated across the long-haul transmission system. Previous studies showing significant benefit only focused on a single fibre span, where one may achieve signal power profile symmetry in different ways [18][19][20]. However, for a more practical multi-span link the attenuation from the passive components (mainly from the pump-signal combiners and the isolators) degrades the signal-power-profile symmetry.…”