Location of a time-varying disturbance using an array of identical fiber-optic interferometers interrogated by CW DFB laser

A novel technique for distributed detection and localization of vibrational disturbances is presented. It is based on a serial array of identical low-finesse Fabry-Perot interferometers interrogated at a fixed wavelength by DFB diode laser intensity modulated at 10 KHz. Interferometers are formed directly in the single mode SMF-28 fiber by pairs of fiber Bragg gratings with reflectivity < 0.05% each. A simple signal processing based on the Fourier transform of detected signals and evaluation of phases for different signal components enables localization of a perturbed interferometer with a high accuracy. In experiment, a localization accuracy of 10 meters has been demonstrated for 5 km long fiber after 1 s of averaging. The system has the capability of using well in excess of 100 interferometers in a single fiber channel. A simple sensor configuration and the use of low-frequency components make it potentially inexpensive and suitable for applications where a continuous monitoring of long structures has to be performed for appearance of vibrations.