The spectral analysis of the device (instrument) operators, as an alternative approach to the dynamical polarization phenomena, is presented by means of an example of a classical time-varying optical device, the electrooptical modulator with longitudinal e ect in crystals of KDP type. The polarization-spectral structure of the modulated light is analysed on this basis, both in terms of spectral Jones vectors and of spectral polarization matrices. Some experimental results are presented. IntroductionIn the last few years dynamical polarization phenomena have been studied intensively in di erent branches of optics. Spontaneous pulsation and polarization multistability resulting from the nonlinear interaction of the standing waves with the active medium in gas lasers with weakly anisotropic cavities have been studied at di erent cavity anisotropies with and without an axial magnetic ®eld applied to the active medium [1,2]. Nonlinear dynamics of lasers at intracavity modulation of polarization was analysed [3]. Various polarization handling methods were elaborated, in connection with the needs of optical transmission of information [4]. The use of bistable elements based on polarization dynamics for optical storage devices and ultra-high bit-rate ®bre communication systems was proposed [5]. Various types of rotating element ellipsometers [6] have been described and the real-time ellipsometry was developed on this basis. Polarization-modulation ellipsometry has been developed, where the optical components remain ®xed and the state of polarization of the incident beam is modulated [7].Time-varying Jones and Stokes vectors were introduced in analysing the polarization dynamics phenomena [3, 8±10] and the time evolution of the state of optical polarization (SOP) in the output of anisotropic lasers [1,3,8,9] and optical modulators [10±13] was analysed in these terms.To complement this, a spectral analysis of the time-varying SOPs was performed. The polarization-spectral content of the anisotropic laser output [1,3,9], or of the emerging light from some electro-optical modulators [10] was discussed and the homodyne (beat-signal) spectrum of the output light was displayed [10±14].As a new theoretical approach to the topic of polarization dynamics, Fourier analysis was applied to time-varying Jones vectors and time-varying polarization