Reducing Noise Figure and Nonlinear Penalty in Distributed Raman Amplifier System Utilizing Low-noise Forward Pumping Technique

TL;DR

This study demonstrates that low-noise forward pumping in distributed Raman amplifiers improves noise figure and signal quality in WDM systems, outperforming backward pumping in noise reduction and enabling lower launch powers.

Contribution

The paper introduces a low-noise forward pumping technique for DRA systems, showing its advantages over backward pumping in noise figure and signal quality through experimental validation.

Findings

Fwd pumping yields higher Q-factors than Bwd pumping.

Implementing low-noise Fwd pumping reduces required signal launch power.

Fwd pumping with conventional sources shows more disadvantages than advantages.

Abstract

In this paper, we experimentally and theoretically show the improvement in noise characteristics in a distributed Raman amplifier (DRA) system for wavelength division multiplexing (WDM) transmission, utilizing our proposed pumping technique. We show that forward (Fwd) pumping is clearly superior to backward (Bwd) pumping in terms of noise figure (NF) defined by amplified spontaneous emission (ASE) noise and gain. We also show that bi-directional pumping is a more desirable configuration for NF improvement. However, it is known that Bwd pumping is preferable to Fwd pumping in suppressing signal quality degradation caused by nonlinear optical effects, especially the interaction between signal and pump light. To compare these advantages and disadvantages of Fwd pumping, we conducted WDM transmission experiments using a recirculating loop including DRA and erbium doped fiber amplifiers. We measured the Q-factors of 9-channel 131.6-GBaud polarization division multiplexed probabilistically shaped 32-quadrature amplitude modulation signals while changing the ratio of Fwd pumping to Bwd pumping in the DRA. By introducing the previously proposed low-noise Fwd pumping technique, a higher Q-factor could be achieved with a lower signal launch power, even when the total Raman gain remained constant. A Q-factor improvement of 0.4 dB and signal launch power reduction of more than 3.4 dB were simultaneously achieved. We also show that when Fwd pumping was performed with a conventional pump light source, the disadvantage of Fwd pumping was more noticeable than its advantage.