Refractive Index Sensor Based on Fiber Ring Laser

In this letter, we investigate a novel refractive index sensor based on a simple fiber ring laser incorporating a bent fiber filter. The bent fiber structure acted as a fiber filter and a sensing element. The bent fiber filter is made of a section of semicircular bare standard single-mode fiber with a selected bending radius. The gain medium (erbium-doped fiber) is pumped by a 980 nm wavelength laser. Laser emission around 1557.3 nm with a low pumping threshold is obtained when a bent fiber filter with 4.5 mm bending radius is used. The 3 dB bandwidth of the laser is less than 0.3 nm, which is beneficial to high precision sensing. The medium refractive index varied from 1.3259 to 1.3730, and the relationship between the lasing wavelength and the medium refractive index exhibited linear behavior, where good linearity of about 60 nm/RIU was obtained. Finally, we fabricated sensor prototypes with different bending radii. When a 4 mm bending radius was selected, the maximum sensitivity is 124 nm/RIU. The fabrication of the proposed refractive index sensor is simple and cost-effective, which makes it a good candidate for the potential sensing applications.