An optical component for modulation of polarization, a Mueller polarimeter and ellipsometer containing such an optical component. The optical component modulates a linearly polarized incident beam and returns a modulated beam. It includes a coupled phase modulator which modulates the incident beam twice in succession, the two modulations having the same frequency of &ohgr;/2&pgr;, and a coupling system modifying the polarization state of the light between the two modulations. The ellipsometer includes the means for detection of a measurement beam returned by a sample, which receives the modulated beam, in addition to a processing unit. The means of detection include a polarimeter producing n measured quantities representing the polarization states of the beam, and the processing unit produces m values for each of these quantities by Fourier transform, with n×m≧16 and m≧4, providing simultaneous access to the sixteen components of the Mueller matrix of the sample.