Abstract: To modify the longitudinal pitch distribution of a diffraction grating, longitudinal dimensional variation is imparted, over time, to a support along the entire length of a line on which said support carries, at least indirectly, a succession of diffraction elements of the grating. By way of example, the support is an auxiliary part on which an optical fiber is stuck, said fiber including said grating. An elongation coefficient dl/l represents the local relative value of dimensional variation. This value varies along said line. The invention is particularly applicable to making variable pitch light-refracting filters in optical fiber telecommunications systems.