Highly Sensitive Ultrasonic Sensor Using Anti-Resonant Reflection Optical Waveguide Mechanism in a Hollow-Core Fiber

A compact fiber-optic ultrasonic sensor based on the anti-resonant reflecting optical waveguide (ARROW) in hollow core fiber is proposed and demonstrated experimentally. The proposed sensor consists of a section of hollow core fiber sandwiched by two single mode fibers. The hollow core fibers with different inner diameters are utilized to optimize the cladding thickness for detection. Moreover, the hollow core fiber's outer surface is coated with polymer materials that possess varying Young's modulus and refractive index. A sensitivity term, determined by the spectral slope and the material properties of ARROW, is proposed to evaluate the ultrasonic response of pre- and post-coating sensors. The results indicate that a thicker fiber cladding contributes to a higher sensitivity, and the polymer coatings also significantly improve the sensor response. The final sensor exhibits a −10 dB bandwidth of about 5.4 MHz and a temperature sensitivity of 220 pm/°C. By incorporating a waterproof aluminum layer, the acoustic pressure sensitivity is assessed, demonstrating its superiority compared to that of a fiber grating sensor. The proposed sensor introduces a novel high-performance ultrasonic probing approach relative to the conventional interference or grating methods.