Detailed spatial-spectral numerical characterization of axially symmetric broadband ultrasonic resonances in standard optical fibers

Standard single mode optical fibers (SMFs) have been widely employed to generate and measure ultrasonic signals in remarkable applications. In particular, optoacoustic fiber sensors provide unique features for microscale high resolution ultrasound imaging in biomedicine. However, at specific resonance frequencies, SMFs work as acoustic filters inducing relevant geometrical attenuation bands higher than 10 dB, which limit the sensors’ sensitivity and frequency operation, causing image distortions and artifacts. We have numerically demonstrated high frequency axially symmetric ultrasonic resonances inside an optical fiber for the first time. The propagation of resonant axially symmetric acoustic modes along 1 cm fiber is investigated by means of 2D and 3D finite element techniques up to 80 MHz. The dispersion of the modes and induced beatlengths are characterized from the complex multimode interference with the 2D Fourier transform. The simulated spectra are validated with the renowned Pochhammer-Chree analytical equations. The frequency response of the acoustically induced strains in the fiber core is evaluated, and important acoustic parameters relevant for the modulation of phase, wavelength and power in optical fibers and diffractive gratings are derived and discussed. The results show that these resonances are strongly dependent on the modal beatlengths. Solutions to improve the operation of fiber-based devices are proposed, pointing out new alternatives to advance broadband optoacoustic sensors and monolithic acousto-optic modulators.