Impact of mandrel diameter and fiber winding layer number on system sensitivity in fiber bragg grating (FBG) based hydrophones

Fiber-based underwater acoustic sensors are rapidly replacing traditional piezoelectric sensors due to their high immunity to electromagnetic interference, compact structure, and superior sensitivity. These hydrophones are constructed by winding optical fibers around a cylindrical structure known as a "mandrel". In this study, the effects of mandrel diameter and the number of fiber winding layers on the sensitivity of fiber-optic hydrophones were investigated. Standard single-mode (SM) fibers with a diameter of 250 µm were wound on 31 mm mandrels in single and double layers, and on a 26 mm mandrel in a single layer. The influence of mandrel diameter on sensitivity was examined by comparing the single-layer mandrels with diameters of 26 mm and 31 mm, while the effect of the number of fiber layers was evaluated by comparing single- and double-layer windings on the 31 mm mandrels. The hydrophone sensitivities were measured using an optical setup based on the frequency diversity dual-pulse interrogation method, and controlled water tank experiments were conducted to assess their underwater detection performance. The results show that the sensitivity of the 31 mm single-layer mandrel was –139.72 dB re rad/μPa, while the double-layer version exhibited a sensitivity of –137.80 dB re rad/μPa, corresponding to an improvement of approximately 2 dB. The sensitivity of the 26 mm single-layer mandrel was measured as –142.53 dB re rad/μPa; therefore, the 31 mm single-layer mandrel demonstrated an approximately 3 dB higher sensitivity. These findings reveal that increasing the mandrel diameter and the number of fiber winding layers significantly enhances the hydrophone sensitivity, thereby enabling more accurate acoustic detection and more efficient performance under deep-water conditions.