Multipoint Laser Ultrasonic Transmitter With PDMS/CSNP Composite Based on Long-Period Fiber Gratings

This study proposes a cost-effective laser ultrasonic transmitter for multipoint ultrasonic excitation using long-period fiber gratings (LPFGs) and a photoacoustic composite material consisting of candle soot nanoparticles (CSNPs) and polydimethylsiloxane (PDMS). The LPFGs effectively mitigated fusion errors and inaccuracies in the coupling ratios during the preparation of the fiber microstructures. The photoacoustic PDMS–CSNP–PDMS configuration optimized the photoacoustic conversion efficiency via CSNP and PDMS, while enhancing the signal amplitudes by superposing forward and backward signals. Experimental comparisons indicated that the amplitude of the PDMS–CSNP–PDMS was five times that of the CSNP–PDMS configuration. In addition, simulated analyses of the LPFG transmission spectrum and energy coupling demonstrated that adjusting the LPFG length and modulation depth allowed for the manipulation of grating coupling efficiency and various coupling ratios for transmitter units. Experimentally, a four-point ultrasonic transmitter system was developed by fabricating four transmitter units with various coupling ratios, which generated stable and energy-balanced ultrasonic signals. The reliability and stability of the developed system demonstrate its potential use in nondestructive testing and distributed applications.