Bent long-period fiber gratings for sensor applications

The transmission characteristics of bent long-period fiber gratings (LPFG's) and their applications as sensors are studied experimentally. We demonstrate that, by bending a LPFG, two major rejection bands in the wavelength region of interest can be produced, whose relative strength can be controlled by the amount of bending. The wavelength separation between the two bands increases linearly with the lateral displacement of the bent LPFG and a linear tuning range over 40 nm is demonstrated. All these characteristics show no significant polarization dependence. We also find that, by mounting a bent LPFG on a proper material, the thermally induced bending effect can enhance or cancel the direct thermal effect. With this technique, we obtain temperature sensitivities of 0.01 nm/°C and -0.35 nm/°C, which are, respectively, about 1/5 and 7 times of that of a straight bare LPFG. A temperature-insensitive LPFG-based sensor for the measurement of the concentration of NaCl in water is demonstrated with a sensitivity of -0.35 nm/molarity by keeping the temperature sensitivity at 0.01 nm/°C from 15 to 70 °C. By using the same technique, a strain sensitivity of -49 nm/%ε is obtained, which is about 15 times of that of a straight bare LPFG.