Temperature-insensitive fiber-optic refractive index sensor based on cascaded in-line interferometer and microwave photonics interrogation system

A compact and high-resolution fiber-optic refractive index (RI) sensor based on a microwave photonic filter (MPF) is proposed and experimentally validated. The sensing head utilizes a cascaded in-line interferometer fabricated by an input single-mode fiber (SMF) tapered fusion with no-core fiber-thin-core fiber (TCF)-SMF. The surrounding RI (SRI) can be demodulated by tracing the passband's central frequency of the MPF, which is constructed by the cascaded in-line interferometer, electro-optic modulator, and a section of dispersion compensation fiber. The sensitivity of the sensor is tailorable through the use of different lengths of TCF. Experimental results reveal that with a 30 mm length of TCF, the sensor achieves a maximum theoretical sensitivity and resolution of -1.403 GHz/refractive index unit (RIU) and 1.425x10-7 RIU, respectively, which is at least 6.3 times higher than what has been reported previously. Furthermore, the sensor exhibits temperature-insensitive characteristics within the range of 25 degrees C-75 degrees C, with a temperature-induced frequency change of only +/- 1.5 MHz. This value is significantly lower than the frequency change induced by changes in the SRI. The proposed MPF-based cascaded in-line interferometer RI sensor possesses benefits such as easy manufacture, low cost, high resolution, and temperature insensitivity.