Demodulation method for underwater acoustic waves based on modulation and fourier series analysis in frequency-swept interferometry

Accurate measurement of low-frequency underwater acoustic signals remains one of the key challenges in underwater detection. Conventional demodulation techniques often suffer from complex hardware architectures and strict environmental requirements. In this study, the demodulation method for underwater acoustic waves based on modulation and Fourier series analysis in frequency-swept interferometry, is proposed. By multiplying the raw acoustic interference signal with a modulation signal and exploiting the inherent frequency periodicity of the frequency-swept light source, a modulated multi-period signal is generated. Suitable bandpass filter cutoff frequencies are determined through Fourier series expansion of the modulated multi-period signal. Subsequently, via filtering and fast Fourier transform analysis, the amplitude and frequency of the low-frequency acoustic signal can be accurately recovered. Underwater experimental results reveal that the acoustic amplitude is linearly proportional to signal intensity, with a coefficient of determination (R 2 ) greater than 0.9996, and shows good consistency with measurements from a digital electrical hydrophone. Furthermore, the proposed method can accurately demodulate acoustic signals in the range of 1–500 Hz, with relative errors below 0.5%. This method is applicable to rapid low-frequency signal measurement in complex marine environments, and can promote the development of detection technologies for stealth and semi-stealth underwater targets.