Turn-around point long-period fiber grating fabricated by CO2 laser for refractive index sensing

Turn-around point long period fiber gratings (TAP-LPFG) are designed by simulations and fabricated by CO2 laser point-to-point irradiations. It is found that the transmission power of a TAP-LPFG exhibits a large linear range with high sensitivity in response to the surrounding refractive index change. The linear power-refractive index relation enables an intensity based refractive index sensor with significantly simplified calibration and instrumentation. By choosing the grating period, the TAP-LPFG can be designed to operate in different refractive index ranges. The TAP-LPFG sensors are demonstrated for detection of sucrose solutions, refractive index matching liquids and low concentration glucose solutions. Compared with the traditional wavelength-shift based measurement method, intensity based TAP-LPFG sensors may provide a cost effective solution for fast and in situ detection of refractive index changes.