Sagnac interferometer-based optical fiber strain sensor with exceeding free spectral measurement range and high sensitivity

•In this paper, we proposed a strain optical fiber sensor with exceeding free spectral measurement range and high sensitivity.•We designed an optical fiber Sagnac interferometer with a polarization maintaining fiber (PMF) and a fiber Bragg grating (FBG) in the loop.•A high strain sensitivity of about 17.08 pm/µε with a large measurement range of 0–5187 µε was experimentally achieved.•The method used in this paper can also be utilized to expand the sensing range in other interferometers.