A system (1) for generating a signal from a surface (22) having a speed V in a direction U, comprising: a light source (2) emitting a Gaussian beam of light along a first optical path (11); a sensor (3) able to evaluate the effects of the electromagnetic interference of the first beam; an optical splitter (4) located upstream of the sensor (3), generating, from the first beam of light, a second beam of light along a second optical path (12); a focusing lens (5, 6) located on the first and/or the second optical path (11, 12), focusing the beam of light at a distance f and defining an upstream optical path (11′, 12′), and a means (7) for routing the second beam, comprising a mirror redirecting the second path such that the lengths of the first (11′) and second (12′) paths are different.