Method for designing diffraction lattices in optic fibre by varying the period of the lattice for its application as optic fibre.Method for designing diffraction Bragg lattices in fibre for its application to the filtering in the optical domain based on the variation of the diffraction lattice period along the fibre axis, according to the apodising function to maintain the Bragg condition constant throughout the entire structure. The lattices thus designed have an improvement in the filtering characteristics both in amplitude and phase. With respect to the former, the spectral characteristic approximates the ideal rectangular filter, with a high reduction of the secondary lobules that produce diaphony between the channels in the optical communications systems with wavelength multiplexing. As regards to the phase characteristic, it practically obtains an ideal constant delay and eliminates the possible effects due to dispersion.<IMAGE>