The in-line interferometric fiber Bragg grating (FGB) hydrophone array contains only FBGs at the wet end. The dramatically reduced number of undersea components and fusion points leads to high reliability, lightweight, and miniaturization, which makes the FBG hydrophone a promising candidate for future large-scale and highly-integrated applications. However, the special undersea structure brings technical challenges in solving the problems of polarization-induced signal fading and random phase fading, and the bidirectional reflections of the FBG may increase the channel crosstalk. Although the in-line interferometric FGB hydrophone array is considered to be the most promising next-generation hydrophone, the above-mentioned technical challenges must be solved before its application in harsh marine environments. Hence, this paper presents an in-line interferometric FGB hydrophone array to fully explore the system performance, providing experimental results before practical applications.

The presented in-line interferometric FGB hydrophone array contains 64 sensing channels. A simple undersea design structure with only 80 FBGs is adopted to form 8SDM (space division multiplexing)×2WDM (wavelength division multiplexing)×8TDM (time division multiplexing) multiplexing hydrophone channels. FBGs with reflectivity less than 1% are fabricated to suppress the multi-reflection-induced channel crosstalk. The undersea hydrophone unit has a mandrel structure with dimension of ?18mm×50mm, and the array has a diameter of 32mm. A hybrid processing method of the phase generated carrier (PGC) and polarization switching (PS) is used to solve the polarization-induced signal fading and random phase fading problems simultaneously, and the PGC modulation frequency and PS modulation frequency are 12.5 kHz and 80 kHz, respectively.

The measured acoustic sensitivity is about (-131.15±1.15) dB, and the frequency response from 20 Hz to 2 kHz is presented with a fluctuation of less than ±1.5 dB. The background phase noise is -95 dB/Hz@1 kHz, which indicates a low equivalent noise level of 36 dB@1 kHz. The SDM, WDM, and TDM channel crosstalks are less than -50 dB, -40 dB, and -40 dB, respectively.

We present an in-line interferometric FGB hydrophone array that contains only 80 FBGs at the wet end. The effectiveness of the PGC-PS hybrid processing method and low reflectivity design is validated, and they can achieve satisfactory system performance. The research results show that the in-line interferometric FGB hydrophone array is suitable for practical applications.