Distributed fiber-optic sensor with a ring Mach-Zehnder interferometer

A novel distributed fiber-optic vibration sensing system is proposed and demonstrated. By employing a ring Mach-Zehnder interferometer structure as the sensing section, which consists of one 3dB fiber coupler, two optical circulators, two symmetric 3 x 3 fiber couplers, and two fibers, one interferometer operates as double ones. The first one is defined by the CW directional path and the second one is represented by the CCW directional path. When a vibration occurs at certain point, the two interferometers will produce the same phase modulation simultaneously. Both position and frequency of the vibration can be determined by combining two phase signals from the ring Mach-Zehnder interferometer. With a piezo-electronic cylinder contained in the sensing cable to simulate a vibration, we select multiple test positions along the sensing fiber to demonstrate the theoretical analysis. The experimental results exhibit a good repeatability and agreement with the theoretical response. Hence, we conclude that the sensor can measure both the frequency and position of vibration in real time, with a spatial positional resolution better than 40 m in a long sensing distance prototype system.