A miniature and quasi-distributed ultrasonic transmitter based on double-cladding fiber for ultrasonic testing

Generating stable ultrasonic waves is a prerequisite for ultrasonic testing. Conventional ultrasonic transmitters use piezoelectric ceramics as actuation elements. However, the centimeter-sized piezoelectric ceramics pose challenges for embedding within structures without damage. Piezoelectric ceramics require circuits for actuation, which limits their applications in high-humidity environments. A micron-sized fiber-optic ultrasonic transmitter was developed to tackle the challenges above. A section of double-cladding fiber (DCF) was fusion spliced into a single-mode fiber. The DCF couples the laser from the core to the cladding. The photoacoustic material outside the cladding absorbs the laser energy and generates a transient thermal expansion, which excites ultrasonic waves. The optical coupling rate refers to the proportion of light energy transferred from the core into the cladding. Finite element analysis and experiments demonstrated that the DCF length can regulate the optical coupling rate. Three ultrasonic transmitters were fusion spliced into a single optical fiber with optical coupling rates of 34.37 %, 51.08 %, and 99.92 %, respectively. A pulsed laser was injected into the fiber, and all three transmitters generated ultrasonic waves with peak-to-peak values around 600 mV, demonstrating energy-balanced multipoint ultrasonic excitation. Furthermore, the ultrasonic energy increased by 65 % through optimizing the photoacoustic transducing structure. Ultrasonic testing was performed using the developed ultrasonic transmitter. By analyzing the time and frequency domain indicators of ultrasonic waves, the variation in the metal plate thickness was inferred. This study details the principles and fabrication of a fiber-optic ultrasonic transmitter and offers preliminary verification of its capability for ultrasonic testing.