Investigation of phase noise in semiconductor lasers allows one to understand the physical processes that influence both the width and the shape of the emission line. Furthermore, the limitations associated with phase noise have to be taken into account when semiconductor lasers are employed in coherent optical communication systems. In this letter we report phase‐noise measurements on single‐mode semiconductor lasers in the presence of optical feedback. We are able to observe, besides the expected linewidth reduction, a peculiar behavior in the high‐frequency region not yet reported in the literature, strongly dependent on the external cavity length.