We propose and demonstrate a high-resolution and high-sensitivity tunable liquid refractive index (RI) sensor interrogated using a microwave photonic filter (MPF) with two taps implemented based on optical polarization orthogonality. In the structure, a fiber laser with its wavelength determined by a microfiber Bragg grating (mFBG) is employed as a light source. A chirped fiber Bragg grating (CFBG) is incorporated in one tap, and an optical tunable delay line (TDL) is included in the other tap. The sensing information is encoded in the laser wavelength caused by the RI change, which will cause a change in the time delay difference between the two taps due to the wavelength-dependent group delay of the CFBG, which would result in a change in the MPF frequency response. By monitoring the spectral response, the sensing information is detected. The spectral response of the MPF is tuned by controlling the length of the TDL, which would lead to a change in the sensor sensitivity. An experiment is performed. The results show, for three TDL time delays of 300 ps, 400 ps and 500 ps, the liquid RI sensitivities are 50.621 GHz/ RIU, 37.293 GHz/RIU and 26.015 GHz/RIU, respectively, and the corresponding resolutions are 8.0253×10-5 RIU, 1.0893×10-4 RIU and 1.5616×10-4 RIU.
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