Experimental investigations of 10×118 Gbit/s DP-QPSK WDM transmission using three types of distributed Raman amplification techniques are presented. Novel ultra-long Raman fibre laser based amplification with second order counter-propagated pumping is compared with conventional first order and dual order counterpumped Raman amplification. We demonstrate that URFL based amplification can extend the transmission reach up to a distance of 7520 km in comparison with 5010 km and 6180 km using first order and dual order Raman amplification respectively. Keywords: coherent fibre optic communications, Raman amplification, Raman fibre laser.
INTRODUCTIONIn fibre optic communications, dual-polarisation quadrature-phase-shift keying (DP-QPSK) modulation with coherent detection is widely deployed due to its good balance between robustness against optical signal to noise ratio degradation (OSNR) and spectral efficiency [1,2]. In order to maximise the transmission distance, it is necessary to maintain an acceptable OSNR through the system. Distributed Raman amplification (DRA) reduces signal attenuation in the transmission span leading to a higher OSNR which can allow longer reach between the repeaters or longer total distance in unrepeatered systems [3]-[7]. Ultra-long Raman fibre laser (URFL) based amplification with fibre Bragg gratings (FBGs) can further reduce the signal power variation during transmission and has proved advantageous in comparison with EDFA based systems [8]- [10].In this paper, we experimentally investigate the performances of 10×118 Gbit/s DP-QPSK WDM transmissions with coherent detection in an 83.5 km SMF-28 recirculating loop using three types of DRA. These are first order counter-pumped, dual order (both first and second order) counter-pumped, and novel URFL based amplification with 2 nd order counter pumping. To the best of our knowledge, this is the first experimental transmission comparison between first order or dual order Raman amplification and URFL based amplification. In particular, we show an improved reach of 7520 km using URFL based amplification, compared with 5010 km and 6180 km using first order and dual order Raman amplification, respectively.