Using pseudo-random sequence phase modulation method to control the effective coherence length in fiber optical hydrophone: model and application

Interferometric Fiber Optic Hydrophones (IFOHs) are highly sensitive for underwater acoustic detection but face challenges owing to the trade-off between laser monochromaticity and coherence length. In this study, we propose a pseudo-random binary sequence (PRBS) phase modulation method to control the laser coherence length. For the first time, we establish a clear theoretical model that quantitatively linked the PRBS modulation parameters to the laser coherence length and IFOH noise. Our findings indicate that interference visibility, which represents the coherence length, is approximately proportional to the PRBS bit period, with a coefficient of 0.14. Moreover, both theory and experiments demonstrate that the amplitude of the noise introduced into IFOH due to insufficient coherence length is inversely proportional to the coherence length. This study provides a comprehensive theoretical and experimental solution to the long-standing contradiction between high laser monochromaticity, stability and appropriate coherence length. The proposed method is not only applicable to hydrophone fields but also to fiber gyroscopes, distributed acoustic sensing (DAS), and other related fields.