Fabrication and sensing characterization of thermally induced long period fiber gratings in few mode fibers

In this work, the long period fiber gratings (LPFGs) in few mode fibers are investigated systematically. A simple but effective method is employed to fabricate the thermally induced LPFG with asymmetric structure. When few mode fibers with LPFG are spliced between two single mode fibers, the obvious dip can be observed in the transmission spectrum. The mechanism of the transmission dip can be explained by the mode conversion process from LP01 to LP11, which is identified by both theoretical analysis and the mode field measurements in FMF. The resonance wavelength of FMF-based LPFG is sensitive to applied strain and temperature. The sensitivities of strain and temperature measurements are 5.4 pm/με and 58.9 pm°C respectively. Because cladding modes are not involved in the mode conversion process, the LPFGs fabricated in this method are not sensitive to the environmental refractive index. It is suitable for the measurements of temperature and strain in the environment with variable refractive index.