Ultrasensitive Fabry–Perot Strain Sensor Based on Vernier Effect and Tapered FBG-in-Hollow Silica Tube

A dual Fabry-Perot interferometers (FPIs) sensor is demonstrated theoretically and experimentally for ultrasensitive strain sensing by using the Vernier effect and the tapered FBG-in-hollow silica tube (HST) structure. The proposed sensor consists of two parallel structured FPIs connected by a 3dB coupler to generate the Vernier effect, and the tapered FBG-in-HST structure has long active length to achieve strain sensitivity enhancement and plays the role as the sensing interferometer. Moreover, utilizing the characteristic of FBG not affected by strain, the temperature compensation has been achieved by means of a demodulating matrix. The fabricated strain sensor achieves an ultra-high strain sensitivity of 1.307 nm/με, which is the highest strain sensitivity of the FPI-based sensors reported so far. It is low-cost, robust, easy-fabrication, and expected to have great application prospects in several fields where large strain sensitivity is really required.