Ultrasensitive and Wide-Measuring-Range Refractive Index Sensor With Optical Path Length-Tunable Vernier Effect

The Vernier effect is used to enhance the sensitivity of interferometric fiber-optic sensors. However, the optical path length (OPL) matching condition of the Vernier effect is subjected to machining errors and large variations in the OPL sensing. Due to the former, achieving high sensitivity is challenging, while the latter renders high sensitivity to be ineffective for a wide measurement range. Thus, this study proposes to realize an ultrahigh refractive index (RI) sensitivity using an OPL-tunable Vernier effect, regardless of the interference of the machining errors. Furthermore, it can maintain consistency and high sensitivity for different measuring media in a wide detection range. The proposed sensor comprises a sensing Fabry–Pérot interferometer (SFPI) and a tunable reference FPI (RFPI). The OPL matching condition of the proposed sensor was controlled by adjusting the cavity length of the RPFI. RI sensitivities of 26 160, 25 985, and 26 260 nm/RIU were achieved using RIs of 1.33, 1.55, and 1.65, respectively. The proposed method overcomes the poor linearity of the RI sensitivity of the Vernier effect sensor in a wide detection range.