Highly precise and stable laser displacement measurement interferometer with differential optical passes in practical use

To achieve sub-nanometer precision of a laser displacement measurement interferometer, the optical systematic error should be small. This error depends on the complexity of the optical path, differential features between the reference beam and the measurement beam, the optical element materials and stray reflections at each optical surface. Using a 45 degree inclined quartz plate along the optical axis, a complete differential optical layout without any dead path was developed with fewer and smaller optical elements and fewer air boundaries. As a result, sub-nanometer stability and displacement measurement precision were achieved with the apparatus despite fluctuation of the local temperature.