Rigid piston approximation for computing the transfer function and angularresponse of a fiber-optic hydrophone

The transfer function of a fiber-optic hydrophone (FOH) is computed for var ious fiber core radii. The hydrophone is modeled as a rigid disk, with plan e waves impinging at normal or oblique incidence. The total sound field is written as the sum of the incident field and the field diffracted from the hydrophone. The diffracted field is approximated by the field generated by a vibrating planar piston in an infinite rigid baffle. For normal incidence and a pointlike Aber core, an analytical solution is presented. For finite fiber core radii, and for oblique incidence, the transfer functions are co mputed numerically. The calculated transfer functions exhibit an oscillator y frequency dependency that is most pronounced for small fiber cores. The s olution for a core radius of 2.5 mu m can be very well approximated by the analytical solution for a pointlike core at frequencies of up to 30 MHz. Th e results for normal incidence can be directly employed to deconvolute ultr asonic pressure signals measured with an FOH. From the transfer functions f or oblique incidence, the angular response of the hydrophone is calculated. The angular response obtained here differs significantly from the model co mmonly used for piezoelectric hydrophones. The effective hydrophone radius derived from the angular response shows a strong frequency dependency. For low frequencies, it is found to be larger than the outer fiber radius, wher eas it generally lies between the outer radius and the fiber core radius fo r frequencies above 10 MHz. (C) 2000 Acoustical Society of America. [S0001- 4966(00)01504-6].