Fiber-optic grating hydrophones have a wide range of marine applications, such as underwater target detection, marine resource exploration, and ocean background noise measurements. As fiber grating hydrophones usually use weak reflectivity gratings with a reflectance below 5%, they can reach an order of magnitude comparable to that of Rayleigh scattering after multiple grating reflections and are therefore more sensitive to Rayleigh parasitic interference of the leading fiber. In this study, a leading fiber Rayleigh parasitic interference model is developed for fiber-grating hydrophone arrays, and the concept of highly sensitive Rayleigh scattering segments is proposed. Theoretical analyses show that the number and spatial distribution of high-sensitivity Rayleigh scattering segments in the leading fiber are related to the length of the leading fiber and the interrogation frequency and that the length of each high-sensitivity Rayleigh scattering segment is the same as the length of the fiber between the two gratings. The high-sensitivity Rayleigh scattering segments introduce interference in the leading fiber between them and the fiber grating hydrophone units. The parasitic interference introduced into the hydrophone phase demodulation results in an increase in the system background noise. The experiments reveal that Rayleigh parasitic interference in the leading fiber can cause a step change in the background noise level of the fiber grating hydrophone array, with a maximum difference in the response amplitude of approximately 10 dB before and after the jump. After the jump, the amplitude of the noise signal increases more slowly as the number of highly sensitive Rayleigh scattering segments increases, with the amplitude of the noise signal increasing by approximately 5 dB for each additional highly sensitive Rayleigh scattering segment without causing nonlinear effects, subject to the constraint that the distribution probability is no less than 90%.

In this study, a model of leading fiber Rayleigh parasitic interference is developed for fiber grating hydrophone arrays, and the precise spatial distribution of highly sensitive Rayleigh scattering segments is calculated. The various states of the leading fiber noise at different locations caused by parasitic interference are observed experimentally, and the stepwise growth patterns of the leading fiber background noise level caused by Rayleigh parasitic interference are summarized.

Theoretical derivations show that parasitic interference necessitates the demodulation of the superposition results of the main interference and multiple parasitic interferences at the dry end. When there is leading-fiber interference between the highly sensitive Rayleigh scattering segment and the primitive sensing grating, the parasitic interference increases the impact of the leading-fiber noise. In experiments, the presence of a highly sensitive Rayleigh scattering segment is the root cause of the step change. The same leading fiber interference before and after the first highly sensitive Rayleigh scattering point results in a step change in the background noise amplitude, and a large fluctuation is obtained in the background noise amplitude when demodulating after the jump. After the jump point, the noise signal amplitude of the fiber-grating hydrophone array increases slowly as the number of highly sensitive Rayleigh scattering segments increase.

Parasitic interference in leading fiber Rayleigh scattering, causes a step change in the background noise level of the fiber grating hydrophone array, with a maximum difference of approximately 10 dB in the response amplitude of the noise signal at the jump. After the jump point, the noise signal amplitude increases more slowly as the number of highly sensitive Rayleigh scattering segments increases. The noise signal amplitude increases by approximately 3 dB for each additional highly sensitive Rayleigh scattering segment without causing nonlinear effects, subject to a distribution probability of no less than 90%.