Ultrasound sensing based on an in-fiber dual-cavity Fabry–Perot interferometer

Highly sensitive and broadband ultrasound (tens of megahertz) detectors can extend the capabilities of photo-acoustic imaging in biomedical application and nondestructive test inspection. Here ultra-compact fiber-based multi-mode dual-cavity Fabry–Perot interferometer (DC-FPI) ultrasound sensors are proposed by splicing three sections of cleaved standard single-mode fibers with the fiber off-core cross section in the middle. The contrasts, reflectivity, and linewidths of the DC-FPIs have been comprehensively characterized. The broadband frequency responses, ranging from 5 kHz to 45.4 MHz, are demonstrated by using the high harmonics of a piezoelectric transducer centered at 3.7 MHz. In addition, the influences of different resonant modes in the DC-FPI on the ultrasound frequency response have been demonstrated. The high-frequency and broadband response of such a simple and cost-effective ultrasound device offers new opportunities to the industrial ultrasound-based and advanced biomedical applications.