In-line fiber Fabry–Perot refractive index tip sensor based on photonic crystal fiber and spectrum differential integration method

A novel and simple refractive index tip sensor based on filling-free Fabry–Perot interferometer is realized by simply splicing a section of endlessly single-mode photonic crystal fiber to a conventional single-mode fiber. The temperature and refractive index characteristics of the proposed device are investigated. Experimental results show that the device can provide temperature-independent measurement of refractive index with a fringe–contrast-sensitivity of ~52.4 dB/RIU. Meanwhile, it can also be used as a temperature sensor with a wavelength–temperature-sensitivity of ~7 pm/°C. In addition, we also propose that using a spectrum differential integration method to improve the performance of the device as compared to the method of measuring the fringe contrast.