Fiber optic acoustic sensor based on the Sagnac interferometer

Abstract

Advances in technology have reshaped the fiber optic acoustic sensing landscape. The Sagnac interferometer configuration can be used to sense environmental parameters other than rotation simply by creating a path length difference. The output of the acoustic sensor contains information about the amplitude and location of an acoustic disturbance. The Sagnac interferometer has the ability to generate polarization effects. These effects are used to generate nonreciprocal phase shifts between counterpropagating beams in the fiber coil, which combine with variations in the different polarization states of the counter propagating beams in the fiber coil to generate intensity fluctuations that are used to monitor acoustic signals. The operating wavelength of the acoustic sensor is 1300 nm. The primary purpose of the acoustic sensor is to sense acoustic signals with frequencies of 0 - 50 kHz. The following are methods for improving the sensitivity and linearity of the acoustic sensor. At the center of the Sagnac loop, sensitivity is minimal. Thus, the sensing region is placed closer to one end of the loop. Also, introducing `teeth' in the sensing region, using different fiber coatings and shielding the sensor achieves better sensitivity. Additionally, a piezoelectric cylinder wrapped with a length of fiber is included in the Sagnac loop to take care of phase modulation. Choosing a light source and a light detector with linear operation will improve linearity. Also, effective signal processing is employed in our system to improve overall performance.