Characterization of phasemask interference visibility and the evolution of grating visibility during grating formation

In this work, we proposed a simple mathematical model to explain the formation of grating structure in photosensitive germane-silica fibers written by KrF excimer laser and phasemask. In the observation, the growth of local index changes in the fiber in the bright fringes and dark fringes of the interference field share similar characteristics and can be represented by a single characteristic function. Through curve-matching technique, the interference visibility and the evolution of grating visibility during grating inscription can be determined.