Azimuth bearing estimation for a Mach–Zehnder interferometric acoustic sensor

A push-pull Mach–Zehnder interferometric acoustic sensor is composed of two parallel fiber optic line sensor elements separated by one half acoustic wavelength. The maximum signal response or optical path difference occurs for a plane acoustic wave with an acoustic wave angle of ±90° since one line sensor will be in a sound pressure peak while the other is in a sound pressure trough. The minimum signal occurs with an acoustic wave angle of 0° or 180° since both line sensors experience the same sound pressure variations. Here, we show how to estimate the acoustic wave arrival direction or azimuth bearing angle to within a few degrees of accuracy with acoustically short line sensor elements that are one-fifteenth of an acoustic wavelength long. This is accomplished by creating a coherent reference of the signal of interest and observing the phase shift as the interferometer baseline is rotated clockwise and counterclockwise a prescribed number of degrees.