Characterization of the spatio-temporal response of optical fiber sensors to incident spherical waves

A theoretical framework is presented for calculating the opto-acoustic response of optical fiber ultrasound sensors with several layers of coating. A harmonic point source generating a spherical wavefront with arbitrary position and frequency is assumed. The fiber is acoustically modeled by a layered cylinder on which spherical waves are scattered. The principle strains on the fiber axis are calculated from the scattering of the acoustic waves and used in a strain-optic model to calculate the phase shift of the guided modes. The theoretical results are compared to experimental data obtained with a sensing element based on a π-phase-shifted fiber Bragg grating and with photoacoustically generated ultrasonic signals.