Sinusoidal-Core Long Period Fiber Grating for Refractive Index Measurement

Optic fiber sensors for refractive index (RI) measurement have brought about widespread attention in an extensive range owing to their unique properties. In this paper, we theoretically analyzed and experimentally verified a brand-new type of long period fiber grating with two resonant peaks. This structure is obtained by side-polishing method and melting tapered method. Firstly, a single-mode fiber (SMF) is produced into staggered V-shaped grooves on symmetrical sides by high-frequency CO2 laser. It is then heated and stretched by optic fiber melting tapered system for bending the core towards two-sides grooves. The sinusoidal core is closer to external environment and exhibits a stronger refractive index modulation, which prompts this structure more sensitive to RI. The structure possesses two resonant peaks of 1382.6 nm (dip-1) and 1473.4 nm (dip-2), which can be used to measure temperature and RI simultaneously. The experimental results present that the maximum RI sensitivity of the proposed sensor exhibits 620 nm/RIU (RI unit) in dip-2. Furthermore, compact sensing area, low cost and strong mechanical property of the proposed sensor make itself more suitable for high-accuracy RI measurement.