Spatially separated frequency-scanning heterodyne interferometer for absolute length measurement of opaque blocks

A double ended frequency-scanning heterodyne interferometer was established in our previous work for absolute length measurement of opaque blocks. We found that cyclic nonlinearity errors, caused by incorrect adjustments and defects of optical components, exist in this interferometer. These errors are significant and difficult to avoid. To address this issue, a novel, to the best of our knowledge, method based on a spatially separated frequency-scanning heterodyne interferometer is proposed and discussed in this paper. In this new method, beams with different frequencies are spatially separated, and the polarization states are never mixed, which reduces the cyclic nonlinearity errors in principle.