Non-linear interactions with backscattered light

A system 300 for measuring vibration in a structure comprises first and second lasers 302, 308 that each launch pulses of light of first and second frequencies into a first end of an optical fibre. Backscattering from within the fibre is processed to determine the vibration in the structure. The system may be a heterodyne distributed vibration sensing (hDVS) system using coherent optical time domain reflectometry (OTDR). The first and second lasers may act as pump and probe lasers, wherein the Rayleigh backscatter produced by the probe laser is amplified or attenuated by the pump light by Brillouin amplification or attenuation; this occurs when the frequency offset between the lasers is equal to the Brillouin frequency shift of the fibre, which is a function of fibre temperature and strain. Therefore, by monitoring the intensity of the Rayleigh backscatter, the value the Brillouin shift and thus fibre properties can be inferred. The sensor may be applied in a hydrocarbon production wellbore for measuring seismic signals or noise from flow events.