Heterodyne Interferometers Using Orthogonally Polarized and Two-Frequency Shifted Light Sources with Super-high Extinction Ratio

This paper describes heterodyne interferometers using orthogonally polarized and two-frequency shifted light sources of two types with super-high extinction ratio to reduce non-linearity of the interferometer due to polarization cross-talk. The acousto-optic modulators are used to shift light frequency. In the first interferometer the light source with Glan-Thomson prisms of very high extinction ratio (50 dB) is used to make the polarization cross-talk very small. In the second interferometer the light source of two-frequency shifted beams with small crossing angle (2.5 rnrad-10 mrad) is used to completely exclude non-linearity of the interferometer due to polarization cross-talk. By measuring the thickness of vacuum evaporation film, it was demonstrated that the interferometers are useful to measure thickness of a thin film in nanometer order.