A quasi-common-path heterodyne grating interferometer for six-degree-of-freedom measurement

The six degree-of-freedom (DOF) measurement is of significant importance in precision engineering and scientific research, especially in fields such as aerospace, manufacturing, and robotics. Among various measurement techniques, heterodyne grating interferometry stands out due to its high precision, strong anti-interference capabilities, and ease of multi DOF expansion. However, existing methods have some issues: firstly, polarization and frequency aliasing can lead to high periodic non-linear errors, affecting measurement accuracy. Secondly, it is difficult to achieve six DOF measurement while ensuring miniaturization and high precision. To address these issues, we presents a heterodyne-based six DOF measurement grating interferometer, achieving sub-nanometer precision. By employing a heterodyne (dual-frequency) grating interferometry technique, the measurement accuracy is significantly enhanced. The system offers comprehensive spatial measurement capabilities, with 3-DOF displacement measured using interference signals from two-dimensional gratings and 3-DOF angular measurements derived from spot displacement on a quadrant photodetector. To address error reduction, a quasi-common optical path design minimizes polarization/frequency mixing-induced nonlinear periodic errors and optical dead zone effects, reducing measurement error to sub-nanometer levels. Additionally, a compact system design is realized. These innovations enhance the performance and applicability of precision measurement technologies, offering strong support for advancements in the field. Overall, the proposed system achieves sub-nanometer precision, 6-DOF measurement, low error levels, and compact design.