Distributed fiber sensing system with wide frequency response and accurate location

• We propose a sensing system merging Mach–Zehnder interferometer and Φ-OTDR.
• We maintain the SNR with the help of FBG and heterodyne detection.
• The system has frequency response from 1 Hz to 50 MHz and 20 m locating accuracy.

A distributed fiber sensing system merging Mach–Zehnder interferometer and phase-sensitive optical time domain reflectometer (Φ-OTDR) is demonstrated for vibration measurement, which requires wide frequency response and accurate location. Two narrow line-width lasers with delicately different wavelengths are used to constitute the interferometer and reflectometer respectively. A narrow band Fiber Bragg Grating is responsible for separating the two wavelengths. In addition, heterodyne detection is applied to maintain the signal to noise rate of the locating signal. Experiment results show that the novel system has a wide frequency from 1 Hz to 50 MHz, limited by the sample frequency of data acquisition card, and a spatial resolution of 20 m, according to 200 ns pulse width, along 2.5 km fiber link.