Sagnac interferometer for underwater sound detection: noise properties

The use of a Sagnac interferometer as an acoustic sensor is discussed. Compared with a Mach-Zehnder interferometer it has the advantage of zero path-length difference. The sensitivity for low frequencies is shown to be proportional to frequency. With a passive homodyne-detection scheme using a 3 x 3 directional coupler, we report a noise-equivalent phase shift of 2.5 x 10(-7) rad/ radicalHz at 10 kHz. The detectivity is believed to be limited by backscattered light.