A single-beam 3DoF homodyne interferometer

In this paper, a camera-based 3DoF homodyne interferometer is proposed to measure displacements and rotations with precision below nanometre/microradian level. The interferometer uses a single beam, has a simple and compact layout and is easy to adjust. A spatial interference pattern is introduced by a tilted reference mirror and recorded by a camera chip. The variation of the spacing and orientation of the fringes is correlated with the yaw and pitch angular rotation of the moving mirror, while the phase correlates with its displacement. The noise level and resolution of displacement and angle measurements were determined to be below 5 pm/root Hz and 5 nrad/root Hz above 1 Hz. Periodic nonlinearities of the system were mainly caused by multi-reflections between optical surfaces in the system. By choosing particular components and improving their adjustment, the displacement nonlinearity was reduced to be below 0.1 nm, and angle nonlinearities achieved sub-microradian level.