Initial kinetics of formation of Bragg gratings in standard optical fibers

Results on experimental study on kinetics of fiber Bragg gratings formation in the standard telecom optical fiber SMF-28 at initial stage of irradiation are presented. For low fluence per pulse, it was found that the fast formation of color centers Ge(1) is a dominant mechanism of the refractive index change at initial stage for accumulated fluence of below 0.02 J/cm(2) at the wavelength of 266 nm. At low fluence per pulse, the induced change of the refractive index does not depend on UV light intensity and saturates at a level of similar to 2 x 10(-6). Dominance of the Ge(1) color center mechanism in the FBG formation at the initial stage makes possible a cost-effective recording of large arrays of weak FBGs for sensors utilizing lasers of low energy per pulse.