In a process for obtaining a transparent substrate including a refractive index modulation pattern, a transparent substrate is irradiated with a laser radiation focused on the substrate in the form of at least one laser line, where the substrate at least partially absorbs the laser radiation, a relative movement is generated between the substrate and the laser line focused on the substrate, in a direction (X) transverse to the longitudinal direction (Y) of the laser line, and, in the course of this relative movement, the power of the laser line is temporally modulated as a function of the speed of the relative movement and as a function of the dimensions of the pattern in the direction (X) of the relative movement.