Distributed multiple vibrations detection with high frequency response assisted by dual NLL sources

Distributed multiple vibrations detection with high frequency response is still a challenging task in distributed optical fiber vibration sensors. In terms of optical interferometer based vibration sensors, the location information can not be directly demodulated. Moreover, the detectable frequency response in the optical reflectometer based vibration sensors is limited by the repetition frequency of the modulated optical pulse. In this paper, an integrated vibration sensing scheme based on two dual narrow line-width laser (NLL) sources with different wavelengths has been theoretically proposed and experimentally demonstrated. In the scheme, one NLL source is used for constructing the phase sensitive optical time domain reflectometry (phi-OTDR), and the other one is used to build the Michelson interferometer. These two vibration sensors are integrated by using a dense wavelength division multiplexer. Based on the sensing mechanism of the integrated vibration sensor, the multiple simultaneous vibration points can be located by the phi-OTDR sensing part, meanwhile its corresponding frequencies information can be fully acquired by the Michelson part. Experiments on detection of the piezol-electric transducer (PZT) show that a spatial resolution of similar to 20 m is achieved in a 4.95 km sensing fiber link based on the width of the modulated optical pulse in phi-OTDR part was set as 200 ns. And a maximum frequency response can reach 45 kHz restricted by the vibration source of the PZT. Compared to conventional vibration structure, the proposed vibration sensing scheme can offer a series of unique advantages, such as simple signal demodulation process, high signal to noise ratio of the demodulated frequency spectrum, no high order harmonics in the frequency spectrum, no dead zone in the detectable frequency range and it can be achieved with a common sensing fiber.