Fiber-Optic Ultrasound Transmitter Based on Multi-Mode Interference in Curved Adhesive Waveguide

Fiber-optic ultrasound transmitters can be used in structural health monitoring, material characterization, and biomedical imaging. However, the bandwidth of current fiber-optic ultrasound transmitters is tens of MHz which prevents the ultrasound image on cells with resolution of $\mu \text{m}$ level. Here, ultra-compact in-fiber ultrasound transmitter is proposed by using the multi-mode interference in the curved UV-cured adhesive waveguide, which is fabricated between two fiber endfaces over a sandwiched core-offset spliced fiber segment. The curved adhesive waveguide can improve the contrast of device reflection spectrum from 2.7 dB to 21.7 dB. The ultrasound is generated by optical pulse excitation via photoabsorption and thermal expansion of UV-cured adhesive, and then detected via the multi-mode interference of the same waveguide with hundreds of Hz to 306 MHz broad transverse acoustic waves. This ultra-compact and cost-effective device offers new opportunities to the advanced biomedical and ultrasound-based applications.