Ultrasensitive parallel double-FPIs sensor based on Vernier effect and Type II fiber Bragg grating for simultaneous measurement of high temperature and strain

In this work, we demonstrated an ultrasensitive parallel double Fabry–Perot interferometers (FPIs) sensor based on Vernier effect and Type II fiber Bragg grating (FBG) for simultaneous measurement of temperature and strain in the temperature range of 26 °C–1000 °C. Two FPIs are manufactured from hollow silica tube (HST), single mode fiber (SMF) and multimode fiber (MMF). The superposition of both waves from the FPIs produces an interference spectrum with a large envelope that has a temperature sensitivity of 6.98 pm/°C and a strain sensitivity of 127.32 pm/μɛ at 800 °C. This strain sensitivity is ∼7.6 times higher than that of a single FPI structure (without Vernier Effect). In addition, an FBG is inscribed in one of the FPIs using a femtosecond laser to provide additional temperature reading (temperature sensitivity ∼15.14 pm/∘C) and to serve as thermal compensation for the measurement. The FBG is encapsulated in glass capillary tube so that it is unaffected by the applied strain. The proposed sensor is robust and highly sensitive, and it is suitable for the applications under extreme temperature condition.