Multipoint All-Fiber Laser-Ultrasound Transducer Using Cascaded Fiber Waist-Enlarged Fusion Tapers

In this study, we propose and demonstrate a multipoint fiber-optic laser-ultrasound transducer based on fiber waist-enlarged tapers (WETs), which are fabricated by taper fusing two single-mode fibers. The WET structure can effectively extract some of the light energy from the fiber core and transfer it to the cladding to form a series of higher-order modes that can be used to excite ultrasound waves. The light energy in the cladding can be absorbed and converted into ultrasound waves by the photoacoustic composite coating on the fiber near WET. The coupling efficiency of the WET is dependent upon its taper length and, therefore, can be easily controlled by the splicer during fabrication. Thus, by controlling coupling efficiencies of the WETs, a multipoint laser-ultrasound actuator with a balanced signal strength can be obtained. We constructed a prototype of the actuator and successfully generated ultrasound waves with balanced strengths at four points by connecting four WETs with coupling ratios of 25.24%, 32.25%, 49.52%, and 90.62% in a fiber link. This multipoint laser-ultrasound actuator is simple to fabricate and provides a balanced ultrasonic signal strength, and thus, has various potential applications in fiber-optic active ultrasound testing technologies.