High-Frequency Ultrasound Sensing From Multimode Coupling in CO₂-Written Long-Period Fiber Gratings

High-frequency (> 50 MHz) ultrasound sensing requires the detection of subtle, rapid perturbations, often a small fraction of the acoustic wavelength, which can be much smaller than the optical wavelength. This leads to the associated phase shift due to ultrasound modulation to being too small to be detected using a telecom fiber-based interferometer. Structured fiber-based sensors can overcome these challenges by detecting locally induced deformations in the fiber structure inside the core. We propose a novel approach using CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>-written long-period fiber gratings (LPFGs), where randomly distributed micro-deformities act as amplitude gratings, eliminating the need for phase detection. Unlike commonly used UV light inscribed LPFGs, where the inscription pitch is uniform in the fiber between the periods, the wavelength of the CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser falls in the absorption band of SiO2. This causes thermal stress-induced deformations in the fiber, leading to the formation of randomly spaced Fabry-Perot (FP) cavities in the micrometer range, as demonstrated by the spatial frequency spectrum (inverse fast Fourier transform (IFFT)). The higher-order modes in CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>-written LPFGs and tilted LPFGs enhance the sensitivity to high-frequency ultrasound waves. This sensitivity arises from the broadband frequency resonance condition spanning 1 to 80 MHz in randomly spaced deformation-formed FP cavities. By analyzing the transmission and spatial frequency spectra of CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>-and UV-written LPFGs, where the latter fails to respond to ultrasound signals beyond 10MHz, we establish a framework for practical high-sensitivity ultrasound sensing.