High-Sensitivity Wide-Range Refractive Index Fiber-Optic Sensor Based on Harmonic Vernier Effect and Fabry–Perot Interferometer

The Vernier effect has been successfully applied to various fiber-optic sensors as a way to increase sensitivity. The harmonic Vernier effect (HVE) serves as an extension of the Vernier effect. Although HVE can realize ultrahigh-sensitivity measurements, it has problems with ambiguous peak wavelength tracking and limited measurement range. In this work, we propose an HVE envelope peak wavelength tracking method to solve the problem of ambiguous peak wavelength tracking in wide-range high-sensitivity measurements, and a normalized free spectral range (FSR) method to solve the problem of limited measurement range. We designed a high-sensitivity fiber-optic refractive index (RI) sensor based on HVE. The sensor consists of two open Fabry–Perot interferometers (FPIs) connected in parallel. It is demonstrated that the sensor has a sensitivity of ~10743.34 nm/RIU over a wide RI range of 1.397–1.413 using the wavelength tracking method, and that a wide RI range of 1.352–1.437 can be measured using the normalized FSR method, and this result was in agreement with the theoretical analysis. In addition, the sensors used in this article are fabricated using the glass welding process, which has the advantages of high stability, corrosion resistance, high-temperature resistance, and resistance to electromagnetic interference. The sensor has great potential for application in the fields of marine environment monitoring and bioscience.