Next-Generation Ultrasound Propagation in Suspended Core Optical Fibers

We demonstrate the temporal propagation of high-frequency ultrasonic pulses inside of a suspended core fiber (SCF) for the first time. An SCF composed of a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5~\mu $ </tex-math></inline-formula>m core suspended by nano bridges and four air holes is analytically and numerically investigated using the 2D and 3D finite element method. We reveal the propagation of ultrasonic modes in the SCF and describe the variation of their properties with increasing frequency from 1 to 100 MHz. Ultrasonic Gaussian-modulated pulses traveling with distinct frequencies (25 and 72 MHz) and bandwidths (56 and 10 MHz) are evaluated along the SCF length and time. The induced strain, pressure, and sensing responsivity are reviewed and discussed. Analytical and numerical results show highly confined effective single-mode propagation in the SCF core, amplifying strains along the fiber (up to 18 times) and pressures at the fiber output (up to 3 times). Compared to previous studies, the demonstrated SCF provides the widest 86 MHz high-responsivity bandwidth (with a -218 dB average and -75 dB maximum responsivity at 93 MHz). These achievements are promising for developing smaller, faster, and more efficient acousto-optic modulators for fiber lasers, enhancing simultaneous light-sound guidance and spatial resolution of fiber-based optoacoustic imaging sensors and neurostimulators in biomedicine.