Cantilevered Microfiber Bragg Grating Sensor With High Ultrasonic Sensitivity and Designable Resonant Frequency

A cantilevered microfiber Bragg grating (CMFBG) sensor with high sensitivity and resonant frequency which can be designed as required, has been proposed and demonstrated using experimental, theoretical, and simulation methods. The CMFBG design can have a variable cross-section, and has been created to operate by receiving ultrasonic guided waves through the adhesive used. This allows for the formation of standing waves, when the wave is reflected from the end of the device, and also allows for concentrating the ultrasonic energy using the small fiber diameter that forms the device. Compared to a cantilevered conventional fiber Bragg grating, the CMFBG design proposed demonstrates a significant increase in signal amplitude of 227% for a chirp ultrasonic input and 204% for a pencil lead break input. The device exhibits a detuning phenomenon as its resonant frequency difference differs from that of the standing wave with the cantilevered fiber Bragg grating. In addition, the research carried out has shown that the geometric parameters of the CMFBG design, such as the diameter and the length ratio (between the waist and the taper regions), influence the ultrasonic sensitivity and the resonant frequencies observed.