Capillary-encapsulated single-fiber optical tweezer for vibration sensing

In recent years, sensing structures based on optical trapping technology have found widespread application in measuring weak forces, vibrations, and temperature variations due to their exceptional detection sensitivity and resolution. However, the requirements for high integration, microsphere self-assembly, and sealed environments pose challenges to the practical engineering application of optical tweezer sensing structures. This study presents a novel optical tweezer sensing structure utilizing a capillary tube. A tapered hollow capillary coupled with a tapered optical fiber forms a trapping chamber, where a 5 μm silica microsphere is captured at the fiber tip. Vibrations are detected by monitoring variations in the reflected light intensity of the microsphere. Experimental results demonstrate that the sensing structure exhibits a strong response to vibrations within the frequency range of 10 Hz to 20 kHz and amplitudes at or above the sub-micrometer level. The confined space of the capillary tube restricts the diffusion of microspheres, enabling automatic resetting under the influence of optical trapping forces. This design not only protects the optical tweezer structure but also enhances the integration of the sensing unit, maximizing the flexibility of fiber-optic tweezer sensors and advancing their practical application.