The polarization state is modulated by tilting birefringence component placed in the feedback external cavity. The variation of the polarization state in one period of modulation is found to be similar to sine wave. The periods become increasingly smaller. The maximum of variation in one period decreases against the rotated angle. The experimental phenomenon is subjected to the change of optical path and secondary reflection. The phenomenon is analyzed theoretically based on geometrical optics and crystal optics. High-accuracy measurements of absolute and relative angles can be realized based on the experimental phenomenon. The angle resolution is 0.1 arcsec in theory.