Abstract: A mode-locking device for a laser operating at a defined wavelength, has a substrate, transparent to the defined wavelength, with a front side for receiving, incident radiation and a back side. An anti-reflection coating is formed on the front side to minimize reflection loss at the defined wavelength. A quantum well structure is arranged on the back side. A high reflectance structure is arranged behind the quantum well structure in such a way that an electric field peak in the incident radiation lies coincides with that of the quantum well structure. This device offers a more reliable and less costly approach for the production of non-linear mirrors since the layers of the dielectric stack do not have to be formed by epitaxial deposition.