Measurement of small wavelength shift using diffraction grating and high-angular-sensitivity total-internal-reflection heterodyne interferometer

This study presents a method for detecting small wavelength shifts using grating diffraction effect and high-angular-sensitivity total-internal-reflection (TIR) heterodyne interferometry. In the proposed interferometer, a half-wave plate and a quarter-wave plate that exhibit 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 exhibits suitable transmission-axis azimuth, it shifts and enhances the phase difference of the s- and p- polarization states of the first-order diffraction beam at one TIR. The enhanced phase difference depends on the diffraction angle, which is a function of the beam wavelength; thus the wavelength shift can be easily and accurately measured by estimating the phase-difference variation. The feasibility of our method was demonstrated with a measurement resolution of approximately 0.007 nm and a sensitivity of 4.3°/nm in a measurement range of 5 nm. This method has the merits of both common-path interferometry and heterodyne interferometry.