Encapsulated Microbubble Fiber-Tip Fabry-Perot Cavity With Tunable Bandwidth for Sensitive Acoustic Detection

A photothermally generated microbubble on a single-mode fiber tip with tunable frequency bandwidth was demonstrated for sensitive detection of acoustic waves. The microbubble with spherical gas-water interface formed a flexible Fabry-Pérot cavity at the fiber tip and detected the acoustic waves by interferometric readout of the bubble deformation. To work in different liquids besides water, the sensor was encapsulated into a water-filled tube and the effects of the encapsulation process on the optical and acoustic characteristics of the sensor were investigated. A 40 μm-diameter microbubble based sensor exhibited a noise-equivalent pressure level of ∼1.0 mPa/Hz ^1/2 at the frequency of ∼100 kHz. The bubble diameter was photothermally changed from 20 μm to 60 μm, which enabled online tuning of the sensor resonant frequency from 100 kHz to 300 kHz. This tunable feature is attractive to applications that requires the detection of acoustic waves covering different frequency bands. The encapsulated microbubble sensor with advantages of high sensitivity, tunable bandwidth and low-cost would be interesting for applications in biomedical and industrial areas involving various liquid environments.