Influences of writing-beam size on the performances of dispersion-free multi-channel fiber Bragg grating

Analysis for the influences of writing-beam size on the performances of a dispersion-free multi-channel fiber Bragg grating (FBG) is presented. It is found that the writing-beam size strongly affects the response of this kind of grating and would make the grating spectrum considerably distorted once the beam size of the UV light is too large to form the complex index-modulation pattern within the fiber core. A novel design method has been proposed to compensate this beam-size effect and a nine-channel dispersion-free FBG with identical spectrum response has been obtained. This compensation method could decrease the tolerance for the demand of small writing-beam size and can be easily extended to the design and fabrication for most of the other complex FBGs.