An improved phase generated carrier demodulation scheme for sinusoidal phase-modulating interferometer to detect air-solid interface acoustic waves

Abstract

When acoustic waves reach an air–solid interface, surface acoustic waves will be generated, and such waves may carry important acoustic information. In order to detect acoustic waves at air–solid interface, a non-contact detection method based on a partial-fiber sinusoidal phase-modulating interferometer (SPMI) is proposed in this paper. An improved phase generated carrier (PGC) demodulation scheme is also proposed to eliminate demodulation error caused by changes of interference signal parameters such as interference signal visibility, phase modulation depth and carrier phase delay. In our research, the improved PGC demodulation algorithm is first used to accurately calculate phase modulation depth and carrier phase delay, and then the in-phase and quadrature components of an interference signal are pre-normalized. Next, the visibility coefficient of the interference signal at each moment is derived by self-multiplying. Finally, the in-phase and quadrature components are completely normalized. The accuracy and effectiveness of the sinusoidal phase-modulating interferometer method and the improved PGC demodulation algorithm for detecting acoustic waves at air–solid interface are proven by numerical simulations and experiments. The air–solid interface acoustic waves excited by various acoustic sources are detected in experiments. The experiment results show that the detectable frequency range of the system is 100 Hz-3 kHz, the minimum detectable amplitude is 0.045 rad/√Hz, and the average signal-to-noise-and-distortion ratio (SINAD) is 28.98 dB.