Large-range refractive index measurement method for open cavity Fabry–Pérot interferometer

Abstract

A large-range refractive index (RI) measurement method for a dual-beam open cavity Fabry–Pérot interferometer (FPI) is studied and demonstrated. Through theoretical derivation and simulation, the relationship between the target free spectral range (FSR) corresponding to a specified wavelength and the RI of the medium in the open cavity is revealed. Accordingly, a novel RI measurement method for the open cavity FPI is proposed, which uses the target FSR to track the shift of the specified interference dip. This method allows the shift distance of the tracked dip to be larger than the initial FSR, thus it can achieve a super-large RI measurement range compared to the conventional method. In addition, it allows a smaller dip bandwidth while measuring a wide range of RIs, which reduces the detecting error of the dip wavelength and improves the accuracy of RI measurement. By using a Fabry–Pérot sensor with a 68 µm-long open cavity, the change regularity of the value and order of the target FSR with the variation of the RI is verified. The experimental results show that, by using this method, the sensor can achieve a large measurement range from 1.333 to 1.4 RIU (RI unit), which is seven times higher than the conventional method and exhibits a good linear response with a sensitivity of 1260.282 nm/RIU. This method expands the application field of open cavity FPI and promotes its practical application.