All Fiber Distributed Vibration Sensing Using Modulated Time-Difference Pulses

An optical fiber distributed sensing system merging Mach-Zehnder interferometer (MZI) and phase-sensitive optical time domain reflectometer for vibration measurement with wide frequency response and spatial resolution is proposed and demonstrated. Two acoustic optical modulators are adopted to generate narrow and wide pulses, respectively, on both ends of sensing fiber with a time difference. Narrow pulses are used to generate Rayleigh backscattering light, which locates the vibration point, while wide pulses interfere with reference light as an MZI to obtain frequency response. To simulate the high frequency responses of crack in civil structures, the sudden break of pencils adjacent to the fiber loop has been measured. The experimental results show 5 m spatial resolution and up to 6.3 MHz frequency response with 50 ns pulse width are achieved in 1150 m sensing distance.