Interrogation of a Fiber Bragg Grating Using a Mechanically Induced Long-Period Fiber Grating

A fiber-sensing scheme with controlled sensitivity comprising a fiber Bragg grating (FBG) and a mechanically induced long-period fiber grating (MLPFG) is presented. The FBG was written by exposing the fiber to 248-nm UV laser radiation such that the Bragg wavelength is localized on the slope of a resonant band of a mechanical grating, which was produced by winding a nylon string around a fiber/grooved tube set. The strength of that resonant band was altered by applying loads to the MLPFG. For different loads, the FBG was submitted to strain values of up to 2200 muepsiv, in steps of 200 muepsiv, during which the Bragg wavelength and the respective transmitted peak power through the MLPFG were recorded. It was demonstrated that by applying a weight with a value of 0.78 kg to the MLPFG, the sensitivity of the FBG interrogation technique to strain variations increased from 2.23 (without load) to 3.20 pW/muepsiv