Directivity and Frequency-Dependent Effective Sensitive Element Size of a Reflectance-Based Fiber-Optic Hydrophone: Predictions From Theoretical Models Compared With Measurements

Abstract

The goal of this work was to measure the directivity of a reflectance-based fiber-optic hydrophone at multiple frequencies and to compare it to four theoretical models: rigid baffle (RB), rigid piston (RP), unbaffled (UB), and soft baffle (SB). The fiber had a nominal 105- [Formula: see text] diameter core and a 125- [Formula: see text] overall diameter (core + cladding). Directivity measurements were performed at 2.25, 3.5, 5, 7.5, 10, and 15 MHz from ±90° in two orthogonal planes. Effective hydrophone sensitive element radius was estimated by least-squares fitting the four models to the directivity measurements using the sensitive element radius as an adjustable parameter. Over the range from 2.25 to 15 MHz, the average magnitudes of differences between the effective and nominal sensitive element radii were 59% ± 49% (RB), 10% ± 5% (RP), 46% ± 38% (UB), and 71% ± 19% (SB). Therefore, the directivity of a reflectance-based fiber-optic hydrophone may be best estimated by the RP model.

Fig. 1.

Directivity measurements at 6 frequencies.

Fig. 2.

Directivity measurements at 6 frequencies compared with 4 theoretical models: Rigid Baffle (RB), Rigid Piston (RP), Unbaffled (UB), and Soft Baffle (SB). Model functions are based on nominal or geometrical sensitive element size, a_g, for the fiber optic hydrophone.

Fig. 3.

Effective sensitive element radii based on best fit model functions with sensitive element size as an adjustable parameter. The nominal or geometric sensitive element radius, a_g, is shown in the horizontal black dotted line. The average standard deviation, obtained by comparing estimates of a_eff derived from directivity measurements in two orthogonal planes, is 2 μm.

Fig. 4.

Relative difference between effective and geometrical sensitive element sizes as a function of ka_g. A fit to the RB model is also shown. The average standard deviation, obtained by comparing estimates of (a_eff – a_g) / a_g derived from directivity measurements in two orthogonal planes, is 4%.