The present invention relates to a spectrally dispersive interferometric optical apparatus having a light source, generating a phase shift, measuring the intensity of the interference signals, selectively measuring the intensity of the interference signal and determining the phase angles and/or a relative phase shift of the intensity of the interference signals. In accordance with the invention, the generating of a phase shift between components of different polarization directions in at least one of the branches of the interferometer includes a diffraction grating. The selective determination of the intensity of the interference signal in dependence on the polarization moreover permits determination of the respective intensity for the TE components and for the TM components of the interference signal with respect to the coordinate system of the diffraction grating.