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

Sagnac interferometer-based sensors have shown high sensitivity, but limited measurement range due to the free spectral range (FSR) limitation especially the higher the sensitivity. In this paper, we designed an optical fiber Sagnac interferometer with a polarization maintaining fiber (PMF) and a fiber Bragg grating (FBG) in the loop. The strain sensing performance was experimentally conducted. The PMF in the Sagnac interferometer showed a high strain measurement sensitivity with a limited measurement range due to the FSR. On the contrary, FBG in the Sagnac interferometer showed a low strain sensitivity but a large measurement range. By combining the PMF and FBG in a Sagnac interferometer, 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. The proposed PMF and FBG constructed Sagnac interferometer shows the merits of simple structure, high sensitivity, and large measurement range, which make it a competitive candidate in strain monitoring.