A novel long period fiber grating (LPFG) is inscribed in few-mode fibers(FMFs) by CO<sub>2</sub> laser-irradiation method. The experiment results and theoretical calculation demonstrate that the LPFGs are originated from the strong coupling between the high-order core mode LP<sub>02</sub> and the LP<sub>04</sub> cladding mode. The resonant wavelengths of the LPFGs is found to shift toward the lower wavelength with refractive index(RI) of surrounding or curvature increasing. For comparision, LPFGs that coupling fundamental core mode to the same cladding mode are inscribed in single-mode fibers(SMFs) by the same method. The LPFGs inscribed in FMFs have higher RI sensitivity and slight lower curvature sensitivity comparing with LPFGs inscribed in SMFs.
A LPFG with a period of 145μm has been successfully inscribed in the few modes fiber (FMF) by CO<sub>2</sub> laser-irradition method. The mode coupling theory indicates that the first two resonant wavelengths of the LPFG attribute to the mode coupling between the core mode LP<sub>01</sub> and the cladding modes LP<sub>0,21</sub> and LP<sub>0,22</sub>. The third one is the result of the coupling between the higher-order core mode LP<sub>02</sub> and cladding mode LP<sub>0,29</sub> or LP<sub>0,30</sub>. These resonant wavelengths shift in different direction with the increasing temperature, and the experimental results show their temperature sensitivity are 66.54pm/°C,90.36pm/°,115.1pm/°, respectively