Enhanced Heterodyne Grating Interferometer for Simultaneously Measuring Tri-Axial Linear Motions

A novel heterodyne grating interferometer (HTGI) with a rotationally symmetric optical configuration is proposed to achieve the simultaneous measurement of tri-axial linear motions with a large Z-axis range and a large Z-axis rotation tolerance. In this design, the ±1st-order diffracted beams from one incident beam were purposely rotated around the central axis by an angle of 180° to interfere with them from the other incident beam. In this configuration, all beams can shift identical distances along the same direction to maintain interference, when the grating has large Z-axis motions and rotations. Experimental testing demonstrates high linearities comparable with commercial capacitive sensors, and sub-nanometric resolutions by identifying 0.25 nm motions via the spectrum analysis for all the tri-axial measurements. Moreover, a greatly extended Z-axis range of 7.6 mm and a Z-axis rotation tolerance of 0.75° were achieved, well demonstrating the superiority of the proposed design.