Multi-point fiber optic laser-ultrasonic transducer based on long-period fiber grating.

For ultrasonic non-destructive testing, the development of miniaturized, embeddable, and distributed ultrasonic transducers has always been a hot research topic. In light of this, a novel fiber optic laser-ultrasonic transducer, employing the long-period fiber grating (LPFG), was introduced in this study to facilitate the implementation of embedded and distributed applications. LPFG couples the light transmitted in the optical fiber into the photoacoustic conversion material. After the energy conversion of optic-heat-acoustic, ultrasonic waves can be excited. The finite element models of the mode coupling and photoacoustic conversion of LPFG were established. The transmission spectrum and optical power of LPFG were measured by experiments. The results show that by adjusting the structural parameters of LPFG to make it appear as different transmission depths, the quantitative control of the optical power distribution of the core and cladding can be realized. Two LPFGs, each with distinct coupling coefficients, were fused together and anchored onto aluminum plates to create an ultrasonic transducer with dual micro-ultrasonic sources. An ultrasonic excitation experiment was carried out, and the results revealed that both two LPFGs could excite ultrasonic waves with a peak-to-peak value of about 500 mV and a center frequency of about 3 MHz. The study validates the proposed fiber-optic ultrasonic transducer's steady performance and its potential for multi-point applications.