Directional Fiber-Optic Ultrasonic Transceiver Using Tilted Fiber Bragg Gratings

Optical fiber ultrasonic transducers have exhibited excellent performance in medical diagnosis, material characterization, and structural health monitoring. The functional integration of transmission and reception of ultrasound through the side walls of optical fibers has been little studied and the corresponding devices have barely been explored. Here, we demonstrate a novel approach based on side-viewing ultrasonic transceiver fabricated in single-mode optical fiber. The transceiver consists of a tilted fiber Bragg grating (TFBG) whose cladding is coated with a composite functionalized carbon nanotube layer. The abundance of cladding modes in optical fibers enables the generation of ultrasound signals using any cladding mode whose wavelength matches the wavelength of light injected from a nanosecond pulse laser. We describe a core mode-based ultrasonic detection function that was able to detect a 5-MHz ultrasound signal with a signal-to-noise ratio (SNR) of similar to 51 dB. Using this device, we performed a side-viewing ultrasound transceiving experiment to successfully capture the variation in pulse-echo signals with target distance for the first time. This type of fiber-optic ultrasound transducer exhibits higher mechanical strength than other fiber-optic ultrasound transducers, making it a potential tool for quasi-distributed nondestructive monitoring in confined spaces and extreme environments.