Phase-bias-enhanced Sagnac interferometer for airborne audible sound detection based on 3×3 fiber coupler

Conventional detection method of airborne audible sound is predominantly performed using condenser microphones. However, fiber optic interferometers are gaining increasing attention due to their high sensitivity and immunity to electromagnetic interference. Membrane is eliminated in the principle to detect the refractive index modulation of air. Sagnac interferometer has drawn significant interest for its effectiveness in mitigating low-frequency disturbances. This paper investigates the application of a 3×3 fiber coupler to introduce a stable phase bias, significantly enhancing the acoustic sensitivity of the Sagnac interferometer. The theoretical framework underlying this enhancement is discussed, along with experimental results demonstrating the detection of audible sound in the range of 3-5 kHz using an 8 km fiber delay coil. High sensitivity and good signal-to-noise ratio were achieved.