An optical measuring unit for measuring an optical property of an incoming optical beam comprises a polarization converter receiving the incoming optical beam and providing a first output beam and a second output beam thereof, which are spatially separated and with substantially matching states of polarization. A polarization dependent analysis unit receives the first and the second output beams from the polarization converter, and measures the optical property of both the first and the second output beams. The polarization converter comprises a polarization dependent beam splitter splitting up an incoming optical beam into a first output beam of the polarization converter and into a second beam, each with a defined state of polarization but different from each other. polarization adapter receives the second beam and for provides the second output beam of the polarization converter, whereby the state of polarization of the second output beam substantially matches with the state of polarization of the first output beam. The first output beam and the second output beam are spatially separated from each other to avoid interference effects.