When a light beam whose polarization and intensity are weakly modulated at a frequency ω<sub>m</sub> passes through a periodic analyzer of frequency ω<sub>a</sub>(<ω<sub>m</sub>) and the transmitted flux is linearly detected, the resulting total signal S<sub>t</sub> consists of two components: (i) a periodic baseband signal S<sub>bb</sub> with harmonics of frequencies nω<sub>a</sub> (n = 0,1,2,…) and (ii) an amplitude-modulated-carrier signal δS<sub>mc</sub> with center (carrier) frequency ω<sub>m</sub> and sideband frequencies at ω<sub>m</sub> ± nω<sub>a</sub>(n = 1,2,…). In this paper we show that the average polarization of the beam is determined by a limited spectral analysis of S<sub>bb</sub>, whereas the polarization and intensity modulation are determined by a limited spectral analysis of δS<sub>mc</sub>, or the associated envelope signal δS<sub>e</sub>, where δS<sub>mc</sub> = δS<sub>e</sub>cosω<sub>m</sub>t. The theory of this frequency-mixing detection (FMD) of polarization modulation is developed for an arbitrary periodic analyzer. The specific case of a rotating analyzer is considered as an example. Applications of FMD include the retrieval of information impressed on light beams as polarization modulation in optical communication systems, and the automation of modulated ellipsometry, AIDER (angle-of- incidence-derivative ellipsometry and reflectometry), and modulated generalized ellipsometry.
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