Using a combination of two-probe-beam phase modulated polarimetry and a waveform extraction technique, we show it is possible to determine all the optical parameters of an anisotropic medium. Particularly, we have measured the dynamic changes of refractive index ellipsoid of the disperse red 19-doped poly(methyl methacrylate) under the irradiation of linear polarized lights. The long axis of the trans molecule can be aligned to the propagation direction of P-polarized light while it is irritated by two crossed polarized lights. The refractive indices and orientation angles can be determined in one cycle of modulation (20 μs) with sufficient precision of 10-6 and 0.1°, respectively. This work shows a new way to simultaneously monitor and control the photo-alignment process of azobenzene molecules in a polymer matrix.