Polarization-based Optical Fiber Acoustic Sensor for Geological Applications

An interferometric fiber sensor was developed and used to detect polarization changes resulting from varying the amplitude and frequency of an acoustic signal. The sensor was designed to be suited to geological activities such as seismic tomography, detection of sink holes, and early warning earthquake detection. The fiber sensor and a commercial geophone were subjected to the same tests to compare their characteristic response to different vibrations. The average signal sensitivities were 9.15 a.u./mJ and 8.37 a.u./mJ for the fiber sensor and geophone, respectively. The ability of each sensor to distinguish between short, successive events showed that the fiber sensor has superior sensitivity and resolution. This is attributed to the short recovery time of the optical fiber sensor. The geophone is limited in this regard by its inherent Faraday magnet and coil damping mechanism. The bandwidth of the optical fiber sensor is shown to be 3.349 kHz, more than 20 times that of the commercial geophone.