Phase characteristic of 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. A phase characteristic of the sensor is obtained by evaluating the optical path difference equation at acoustic wave angles from 0° to 360°. At each acoustic wave angle, the phase of the peak optical path difference is plotted against the acoustic wave angle. The phase characteristic indicates from which side of the line sensors the acoustic wave arrives. Using the magnitude response alone it is not possible to determine which side of the line sensors from where the acoustic wave arrives.