Quantitative quasi-distributed vibration sensing by φ-OFDR for multiple events over spatially consecutive sensing spatial resolutions

We report on a quantitative quasi-distributed vibration sensing (DVS) system enabled by phase-sensitive optical frequency domain reflectometry (φ-OFDR), which allows for multiple vibration events over consecutive spatial resolutions. To achieve effective crosstalk suppression and mitigation of the instability during the phase extraction, fiber with embedded ultra-weak grating arrays has been adopted as the sensing fiber. It exhibits a particularly customized low spatial duty cycle, that is, high ratio between the size of the gratings and their spacing and the spacing is additionally designed to match the integer multiple of the theoretical spatial resolution. In combination with a rectified frequency-modulated continuous-wave optical probe enabled by the optical phase-locked loop, it allows to achieve quantitative quasi-DVS for multiple events over consecutive sensing spatial resolution as high as ∼2.5 cm along the distance over ∼2200 m. The ability to simultaneously retrieve arbitrary multi-point vibration events over spatially consecutive sensing spatial resolutions with consistently linear response and sensitivity up to a few nano-strain level even at long distances has shown great potentials for the application of φ-OFDR from a practical point of view.