Abstract: A control unit for controlling a laser unit comprising a laser gain medium and an external cavity having a reflecting dispersion device. The laser gain medium is adapted for providing a first beam towards the reflecting dispersion device, the reflecting dispersion device is adapted for receiving the first beam and reflecting a beam, having a reflection angle dependent on the wavelength, towards the laser gain medium, and the laser gain medium is adapted for providing a second beam in another direction than the first beam. The control unit comprises an angle unit adapted for providing an angular variation signal indicative of an angular variation of the second beam, and an analysis unit adapted for receiving the angular variation signal and controlling the reflection angle of the reflecting dispersion device dependent on the angular variation signal.