Multipoint laser ultrasound transmitter using single-multi-single mode fiber structures

Laser ultrasound technology is a promising alternative to piezoelectric ceramics in the field of structural health monitoring, especially the distributed applications. A novel multipoint laser ultrasound transmitter using single-multi-single mode fiber structure was proposed in this study. Both theoretical simulations and experimental results indicate transmitters with controllable coupling ratio can be fabricated by directly changing the length of multimode fiber. To validate the capability of distributed application, a four-point energy-balanced laser ultrasound transmitter system has been achieved by connecting transmitters with different coupling ratio in increasing order. The four transmitters exhibited similar characteristics both in time domain and frequency domain, with a peak-to-peak amplitude of 1018 mV and a center frequency closed to 4 MHz. The transmitters exhibited a relatively high signal-to-noise ratio and an elevated energy conversion efficiency with over 5 times that of previous researches. The proposed transmitter has a potential for distributed applications. However, further analysis of the mode of the ultrasonic waves or packaging of the transmitter is needed before it can be used in structural health monitoring.