CO2-Laser-Induced Long Period Fiber Gratings in Few Mode Fibers

A novel long period fiber grating (LPFG) is inscribed in few mode fibers by CO₂-laser-irradiation method. The experiment results and theoretical calculation demonstrate that the LPFGs are originated from the strong coupling between the fundamental core mode LP01 and the high-order core mode LP11. The resonant wavelength is found to shift toward the lower wavelength with temperature or force increasing, which is quite different from LPFGs fabricated in common single mode fibers. The gratings are quite sensitive to strain and slightly insensitive to temperature. Such structures are easy to be fabricate and can be promising candidates as strain sensors.