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
at 10 kHz. The detectivity is believed to be limited by backscattered light.
© 1989 Optical Society of America
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