Finite element numerical investigation of multimode ultrasonic interference and beatlengths in high frequency fiber-optic devices: 3D design, modeling, and analysis

A novel numerical study based on the finite element method is developed to demonstrate the beatlengths induced by high frequency acoustic modes inside an optical fiber for the first time. A practical methodology to model, compute and analyze the multimode interaction in the fiber is exemplified with a detailed numerical experiment. The frequency response of 1 mm long standard fiber is evaluated from 30 to 60 MHz, corresponding to the highest attenuation band of experimental fiber optoacoustic devices. The 3D simulated complex ultrasonic fields are decomposed and characterized with the averaged peak-to-peak method and 2D Fourier transform. The resulting dispersion spectra are compared and theoretically validated by the recognized Pochhammer-Chree solutions. The acoustic parameters required to modulate optical fibers are derived from the simulations and discussed. A route to overcome the frequency-induced limitations of the current devices is provided, pointing out new research possibilities for the development of highly efficient and compact all-fiber acousto-optic modulators and fiber-optic ultrasonic sensors.