The present invention relates to a method and to an apparatus for measuring an optical property, such as group delay or chromatic dispersion, of a DUT, comprising: a first coupler connected to the DUT for coupling a first optical signal having a first wavelength and a second optical signal having a second wavelength into the DUT, the second wavelength being variable with respect to the first wavelength, a detector connected to the DUT for detecting the first and the second signal leaving the DUT, an evaluation unit connected to the detector, for determining a difference in runtime, if any, between the first and the second signal having traveled at least one time through the DUT and for deriving the optical property from the detected difference.