Compact refractometer based on extrinsic-phase-shift fiber Bragg grating

A novel method to measure refractive index of liquids by using an extrinsic-phase-shift fiber Bragg grating (EPS-FBG) is proposed and experimentally demonstrated. The EPS-FBG is obtained by separating two identical FBG reflectors with a micrometer air gap as a phase-shift. This gap forms a phase-shift peak in the transmission spectrum which is dependent upon the refractive index (RI) of the liquid filled in the gap. A large RI measurement range from 1.33 to 1.47 u.r.i. (unit of refractive index) was obtained in the experiment. Experimental results agreed well with the numerical analysis, showing a linear response with the RI sensitivity of ∼1.69 nm/u.r.i. (RI resolution of ∼5.92 × 10−3 u.r.i.). Moreover, the micrometer interaction length in the proposed design makes it potential for microfluidic application.