The present invention proposes a method for modulating refractive indices of optical fiber gratings, wherein the UV exposure on a least one assigned location of an optical fiber grating is divided into to two UV shots, and the intensities or phases of the two UV shots, which expose the location sequentially, are controlled to make the total exposure intensity of one assigned location maintained at a fixed value, whereby the interference fringes, which are created by a superposition of the two UV shots, have fixed phases and adjustable intensities, and whereby the dc index of the optical fiber grating maintains fixed and the ac index independently adjustable.