A finite element analysis of an interferometric optical fiber hydrophone with a concentric composite mandrel including a foaming layer

This paper outlines the design of an optical fiber hydrophone with a concentric composite mandrel that has a fundamental resonance frequency over 15 kHz and which demonstrates a good sensitivity in underwater conditions over 200 m. The composite mandrel consists of double layers (a thin foaming layer on top of a base layer) and a center hole. The foaming layer is coated on top of the base layer, made of a common material (aluminum), so as to radically improve the sensitivity of the sensor through its superior compliance. The optimal structure of the composite mandrel was determined using the finite element method (FEM) to analyze the influence of both the material properties of the foaming layer and the geometry of the composite mandrel on the performance of the hydrophone. The geometrical parameters included the thickness of the foaming layer, the inner and outer diameters, and the length of the mandrel. The analysis results indicated that for a higher sensitivity, a hydrophone needed as thick a foaming layer as possible, made from a material with a relatively low Young's modulus, if possible, less than 1 GPa. The sensitivity also increased with a smaller outer diameter and longer mandrel, and additional sensitivity was achieved by decreasing the ratio of the inner diameter to the outer diameter. The sensitivity of the optimal structure A1 composite mandrel was about - 82 dB in relation to 1 rad/μPa. This result is 14 dB higher than that of a simple concentric Al mandrel-type hydrophone of the same dimensions, which verifies the effectiveness of the foaming layer.