Permanent Bragg gratings in monomode optical fibers are obtained by irradiation through an interference pattern of an ultraviolet laser. The main parameters of such gratings, in particular the index modulation amplitude and their length, are precisely determined by a fine adjustment between the measured reflectivity and a theoretical curve. Gratings have been formed in a standard monomode telecommunication fiber, with reflectivities above 90% and with bandwidths on the order of 1 to 3 nm. Their reflectivity spectra are symmetric and comparable with theoretical curves. Index modulations up to 1.2 10-3 have been determined, which is more than one order of magnitude greater than earlier reported. A precise knowledge of the maximal index change allows one to know the formation possibilities of gratings with regard to reflectivity and bandwidth