Distributed Sensors Assisted by Modulated First-Order Raman Amplification

In distributed optical fibre sensors, the measurement range is typically limited to a few tens of kilometers due to the fibre attenuation and the input pump power limits imposed by the emergence of nonlinear effects. To overcome these problems, distributed amplification schemes have been investigated, generally employing continuous-wave pumping schemes. In this article, we propose the use of Raman amplification with an engineered intensity modulation. We show that this kind of assistance may provide, in principle, a close to perfect loss compensation along the fibre length. To find the optimal modulation profile, an analytical model is presented, and numerically, its performance is analyzed. Using a close to optimal pump modulation profile, we experimentally demonstrate a flat optical trace along 50 km of fibre with a minimal SNR penalty along the length. The result is favorably compared with the continuous-wave pumping case injecting the same average power. Thanks to this new approach, the sensitivity in the worst point is considerably better and the ASD noise floor is also reduced.