Ultrasensitive RI Sensing Based on the Signal-Processing-Assisted Vernier Effect of a Single FFPI

Ultrasensitive refractive index (RI) sensing based on novel Vernier-effect demodulation through a single fiber Fabry–Perot interferometer (FFPI) is proposed and demonstrated. A core-offset fiber pair is used as an FFPI for RI sensing. An artificial reference spectrum (ARS) obtained by modulating and extracting a sensing spectrum twice is demonstrated. The Vernier effect is realized by superimposing the sensing spectrum with the ARS. By optimizing the cavity length of the FFPI and the free spectrum range (FSR) of the ARS, an ultrahigh sensitivity of 133948.33 nm/refractive index unit (RIU) with a magnification factor of 118.38 in the RI range of 1.3340~1.3354 is obtained. The magnification factor based on the ARS can be conveniently adjusted. Furthermore, the proposed scheme is suitable for various interferometric sensing applications.