A Frequency-Flat Sagnac Optical Fiber Microphone Based on a Composite Sensing Unit

A frequency-flat Sagnac interferometer (SI) optical fiber microphone (OFM) based on a composite sensing unit is proposed. Based on the proposed acoustic composite-type modulation response model related to the acoustic loss coefficient $\gamma \text {(} {f} \text {)}$ , a composite structure consisting of a single-mode optical fiber ring, and an aspartic polyurea resin (APR) as an acoustic sensing unit, a composite OFM (COFM) based on a composite sensing unit is prepared. The acoustic tests show that compared with the conventional SI, this COFM achieves a flatter frequency response in the range of 200–6400 Hz (an average enhancement of 21.55 dB in the frequency band of 200–1000 Hz), which solves the problem of the insensitivity of the conventional SI to low-frequency signals. Besides, this COFM performs well in terms of signal-to-noise ratio (SNR), linear response of the acoustic pressure, and the response of the respective incidence angles. These results indicate that this COFM has high sensitivity and fidelity and is suitable for acoustic detection of human sound waves in the air. In addition, the composite model can be used to tune the frequency-domain characteristics of the OFM by screening composites with corresponding $\gamma \text {(} {f} \text {)}$ curves according to the demand, which is promising for application.