A nanonewton force sensor using a U-shape tapered microfiber interferometer.

Nanomechanical measurements, especially the detection of weak contact forces, play a vital role in many fields, such as material science, micromanipulation, and mechanobiology. However, it remains a challenging task to realize the measurement of ultraweak force levels as low as nanonewtons with a simple sensing configuration. In this work, an ultrasensitive all-fiber nanonewton force sensor structure based on a single-mode-tapered U-shape multimode-single-mode fiber probe is proposed and experimentally demonstrated with a limit of detection of ~5.4 nanonewtons. The use of the sensor is demonstrated by force measurement on a human hair sample to determine the spring constant of the hair. The results agree well with measurements using an atomic force microscope for the spring constant of the hair. Compared with other force sensors based on optical fiber in the literature, the proposed all-fiber force sensor provides a substantial advancement in the minimum detectable force possible, with the advantages of a simple configuration, ease of fabrication, and low cost.