A Mach-Zehnder interferometric acoustic sensor using incomplete symmetry 3×3 coupler-based phase shifting demodulator

Abstract

A Mach-Zehnder interferometric (MZI) acoustic sensor and an incomplete symmetry 3×3 coupler-based phase shifting demodulator are presented. The demodulator is used to recover the acoustic signal detected by the proposed sensor. To eliminate interfering signals loaded on the transmission path, all couplers and interference arms constituting the sensing MZI are packaged into the sensor. Therefore, a path-matched interferometer (PMI) is used to separate the sensor from the demodulator and to compensate for the optical path difference in order to generate interference. The 3×3 coupler of the PMI is used to introduce phase differences of interferometric signals. Two constant large-amplitude carrier signals with a phase difference of π are loaded on the PMI to ensure normalization of the interferometric signals. By normalizing interferometric signals, fluctuations in both fringe visibility and received power are eliminated. The measurement errors caused by the incomplete symmetry of the 3×3 coupler are reduced through phase compensation. The sensor and PMI are designed to be insensitive to environmental disturbances. The phase shifting demodulation technique is then performed to recover the measurand. The proposed sensor is experimentally demonstrated to be able to detect acoustic signals with frequencies between 200 Hz and 20 kHz. This sensor exhibits a linear response to the sound pressure with a sensitivity of 9.101 nm/mPa. The minimum detectable pressure is 9.74 μPa/Hz1/2 at 20 kHz. The proposed sensor has high sensitivity, high robustness, low cost and anti-interference properties.