Abstract
This paper presents an analytical study on the role of intensity profile of writing UV beam on fringe intensity distribution and its effect on the refractive index modulation of fiber Bragg gratings (FBGs) written by the phase mask technique. It is shown that during FBG inscription, the induced average refractive index and refractive index modulation profiles in the photosensitive fiber changes by the continuous exposure of UV fringes of non-uniform intensity distribution. This is explained on the basis of non-linear growth of refractive index and its saturation at different sections of the grating occurring at different irradiation times. The evolutions of refractive index modulation of FBGs written at different distances from the phase mask are also different. Effect of varying refractive index modulation profiles on evolution of reflection spectra of fiber Bragg gratings are discussed. Controlled UV fringe exposure times for FBG fabrication and fringeless UV exposure for post-fabrication processing are discussed in view of grating apodisation.
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