Fiber-optic detection of sound

The response of an optical fiber in water to low-frequency acoustic waves was investigated experimentally and compared with analytical results. A single-mode optical fiber was used to pass a laser beam through a tank in which an approximately plane acoustic wave was produced. A change in the optical index of refraction of the fiber creates an effective path-length change for the optical beam which results in a phase shift of the optical beam with respect to a reference beam unaltered by the acoustic field. By mixing the phase-shifted beam with a reference beam of constant phase in a photodetector the phase variation at the acoustic frequency is detected. Theoretically, the sensitivity of a fiber-optic interferometer should be independent of frequency. Experimental results confirm this.