High resolution refractive index sensing with dual-wavelength fiber laser

Abstract

A novel refractive index (RI) sensor has been demonstrated based on a simple dual-wavelength fiber ring laser by incorporating two fiber Bragg grating (FBG) filters. The grating filters are made of two FBGs with different cladding thicknesses. By inserting the filters into the fiber ring cavity, a stable dual-wavelength fiber laser can realize at room temperature. The cladding-etched FBG (thinned) acted as the RI sensing elements, while the normal one as temperature reference. As the RI increases, the laser wavelength corresponding to the thinned FBG moves toward longer wavelength, while that corresponding to the normal FBG is unchanged. Then, RI changes can be determined by measuring the separation of the two lasing wavelengths. Moreover, compared with the conventional thinned FBG-based sensors, the fiber laser sensor has a narrow bandwidth, which is beneficial to high precision sensing. The RI measurement resolution of proposed sensor is about $1.44\times 10^{{ {-5}}}$ RIU.