Evaluation of 100G DP-QPSK long-haul transmission performance using second order co-pumped Raman laser based amplification

Abstract

We present, for the first time, a detailed investigation of the impact of second order co-propagating Raman pumping on long-haul 100G WDM DP-QPSK coherent transmission of up to 7082 km using Raman fibre laser based configurations. Signal power and noise distributions along the fibre for each pumping scheme were characterised both numerically and experimentally. Based on these pumping schemes, the Q factor penalties versus co-pump power ratios were experimentally measured and quantified. A significant Q factor penalty of up to 4.15 dB was observed after 1666 km using symmetric bidirectional pumping, compared with counter-pumping only. Our results show that whilst using co-pumping minimises the intra-cavity signal power variation and amplification noise, the Q factor penalty with co-pumping was too great for any advantage to be seen. The relative intensity noise (RIN) characteristics of the induced fibre laser and the output signal, and the intra-cavity RF spectra of the fibre laser are also presented. We attribute the Q factor degradation to RIN induced penalty due to RIN being transferred from the first order fibre laser and second order co-pump to the signal. More importantly, there were two different fibre lasing regimes contributing to the amplification. It was random distributed feedback lasing when using counter-pumping only and conventional Fabry-Perot cavity lasing when using all bidirectional pumping schemes. This also results in significantly different performances due to different laser cavity lengths for these two classes of laser.

Nonlinear Inverse Synthesis for Optical Links With Distributed Raman Amplification

“…This random DFB laser acts as the first order pump, together with 2 nd order pump, to amplify the signal at 1550 nm. The lack of an FBG on the side of the forward pump significantly reduces the relative-intensity-noise transfer from the forward pump to the Stokes-shifted light at 1455 nm [35], which can seriously hinder coherent transmission [36]. We simulated the signal and noise power excursion for different pump power ratios in Raman amplifiers using the experimentally verified model [28] with an appropriate boundary conditions that shows a high degree match with the OTDR traces.…”

RIN Mitigation and Transmission Performance Enhancement With Forward Broadband Pump

“…As the RIN of the pump can be transferred to the signal resulting in significant long-haul transmission penalty [3][4][5][6], the RIN of the signal (at 1545.32 nm) at the span output was experimentally investigated, and the results are shown in Fig. 3(a) and 3(b).…”

“…In DRA, forward-propagated pumping can increase amplifier spacing, reduce signal power variation (SPV), and provide superior noise performance over backward-propagated pumping only [2]. However, the major challenge of forward (FW) pumping is relative intensity noise (RIN) related penalty, particularly for long-haul transmissions [3][4][5][6]. We previously reported a Raman amplification technique based on random distributed feedback fibre laser with bidirectional second order pumping which suppressed the RIN-related penalty and extended the maximum reach by ~12% [7][8][9], compared with backward (BW) pumping only.…”

Characterisation of random DFB Raman laser amplifier for WDM transmission

“…Optimised bidirectional Raman pumping can provide the most uniform gain within the transmission span [10] reducing the ASE noise accumulation that results in high received OSNR. However, the unavailability of low RIN high power pumps makes forward pumping in higher order Raman amplifiers problematic due to the high relative intensity noise (RIN) transfer from the noisy Raman laser pumps that counterbalance the benefits of distributed bidirectional amplification in transmission links [11].…”

“…Optimised bidirectional Raman pumping can provide the most uniform gain within the transmission span [10] reducing the ASE noise accumulation that results in high received OSNR. However, the unavailability of low RIN high power pumps makes forward pumping in higher order Raman amplifiers problematic due to the high relative intensity noise (RIN) transfer from the noisy Raman laser pumps that counterbalance the benefits of distributed bidirectional amplification in transmission links [11]. A novel amplification scheme that uses a single fibre Bragg grating (FBG) at the end of the transmission span, forming an open cavity with random distributed feedback (DFB) lasing [12] allows for a form of bi-directional pumping that can significantly reduce the growth of amplified spontaneous emmission (ASE) noise [13] without suffering from elevated RIN transfer from the forward pumps [14], becoming an efficient solution capable of extending the transmission distance for coherent data formats [4].…”