High sensitivity fiber optic strain sensor based on CFBG-FPI and vernier effect

A high-sensitivity fiber optic strain sensor based on chirped fiber Bragg grating-Fabry Perot interferometer (CFBG-FPI) and vernier effect is proposed and has been demonstrated to have a strain sensitivity of -123.8 pm/μϵ. With femtosecond laser direct writing technology, two pairs of CFBGs with the same reflectivity, length, chirp rate and center wavelength are inscribed inside the single-mode fiber core, forming two separated CFBG-FPIs. Combined with the vernier effect principle, the interferometric cavity lengths of the two CFBG-FPIs were designed at about 1.7 mm and 1.68 mm as the reference arm and sensing arm, respectively, to form the vernier effect system, and the spacing between the reference arm and the sensing arm was 1 m. Experimental results show that in this vernier effect system, the strain sensitivity gain due to the vernier effect reaches a staggering 91-fold relative to a single CFBG-FPI, resulting in an ultra-high sensitivity of -123.8 pm/μϵ. In addition to being compact and convenient to manufacture, the sensor exhibits a low temperature cross-sensitivity down to -0.14 μϵ/°C. The many advantages make this sensor extremely promising for high-precision strain measurement applications.