Abstract

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 × 3 directional coupler, we report a noise-equivalent phase shift of $2.5\times {10}^{-7}\text{rad}/\sqrt{\text{Hz}}$ at 10 kHz. The detectivity is believed to be limited by backscattered light.

© 1989 Optical Society of America

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