High stability microfiber Sagnac loop liquid refractive index sensor with FBG mode selecting

Abstract As a high-resolution sensor, it is very important to improve the spectral stability of microfiber Sagnac loop interferometer. A fiber Bragg grating (FBG) mode selecting method is proposed and demonstrated. The method can filter out some cladding modes by FBG in the Sagnac loop and make working modes energy more concentrated, which shows that the interference spectrum becomes more uniform. At the same time, the sensitivity and linearity of the sensor can be improved slightly by this method. In our experiment, the resolution of sensor can get 1.41 × 10 -5 ∼ 7.49 × 10 -6 in the liquid refractive index range of 1.3330∼1.38481. Moreover, this method can effectively improve the spectral stability of the interferometer, which shows that the dip wavelength absolute error of time fluctuation is improved from ±0.358 nm to 0.021 nm and from ±0.116 nm to 0.000 nm at 1536.6 nm and 1559.9 nm initial dip wavelength, respectively. This method also has advantages of compact structure, good mechanical properties, and easy preparation. On this basis, the FBG also can be replaced by various modes filters to develop more multifunctional high stability interferometers according to the detection needs.