Cavity-length controllable Fabry–Perot interferometer based on Vernier effect

As an effective and popular means of improving sensitivity, Vernier effect (VE) has been studied by many researchers to combine with optical fiber sensing method. In our paper, a novel, high-sensitive all-fiber temperature sensor based on VE of dual hollow core capillary (HCC) Fabry–Perot interferometers (FPIs) is demonstrated experimentally. The all-fiber sensor consists of two parallel FPIs fabricated by different methods to structure the VE. For the reference FPI with the free spectral range (FSR) of 3.51 nm, a 365 μm length HCC was fused with lead-in single mode fiber (SMF) under roughly length controlled. The sensing FPI with the FSR of 3.60 nm was made by introducing a 355 μm-length FPI cavity between two SMFs inside a HCC through using UV-curable resins. Noted that the FSR of sensing FPI can be easily adjusted by the length changing before the resins curing to achieve the sensibilization of VE. And due to the thermal sensitivity of resins, the all-fiber sensor has a high temperature sensitivity of 628 pm/°C, which is approximately 41.3 times higher than that of the sensor without combining VE. The high sensitivity, compact size, and easy fabrication, make the sensor widely used in many practical applications.