High-angular-sensitivity total-internal-reflection heterodyne interferometry for small displacement measurements

This paper proposes an optical method for measuring small displacements using high-angular-sensitivity total-internal-reflection (TIR) heterodyne interferometry. In the designed system, a half-wave plate and a quarter-wave plate that display specific optic-axis azimuths are combined to form a phase shifter. When an isosceles right-angle prism is placed between the phase shifter and an analyzer that displays suitable transmission-axis azimuth, it shifts and enhances the phase difference of the s- and p- polarization states at one TIR. The enhanced phase difference is a function of the prism incident angle, whose variation depends on the displacement of a target; therefore the displacement can be easily and precisely measured by estimating the phase-difference variation. The feasibility of our method was demonstrated with measurement resolution and sensitivity levels superior to 0.025 μm and 4.0°/μm, respectively in a measurement range of 10 μm.