Virtual transparency in ϕ-OTDR using second order Raman amplification and pump modulation

10 pags., 6 figs.In distributed optical fibre sensors, distributed amplification schemes have been investigated in order to increase the measurement range while avoiding the limitation imposed by the fibre attenuation and the nonlinear effects. Recently, the use of Raman amplification with an engineered intensity modulation has been demonstrated as an efficient way to produce a virtually lossless trace employing a single-end configuration. In this paper, we propose the combination of this technique with a simultaneous second order Raman pumping scheme for increasing the measurement range. The optimal modulation profile has been numerically analyzed and we experimentally demonstrate a sensor able to detect perturbations along 70 km of fibre, with a minimal SNR penalty along the total length. Thanks to this new approach, the sensitivity in the worst point is considerably improved, and the ASD noise floor is also reduced. The measurement range is extended approximately 15 km compared with the equivalent first order pumping case.Comunidad de Madrid (SINFOTON2-CM: P2018/NMT-4326, TALENTO-CM 2017-T2/TIC-5227); European Research Council (OCEAN-DAS: ERC-2019-POC-875302); Ministerio de Ciencia, Innovación y Universidades (IJCI-2017-33856, RTI2018-097957-B-C31, RTI2018-097957-B-C33)