3-dimensional displacements measurement with two-point source interference

An innovative double corner cube interferometer is proposed to detect the 3-dimensional linear movements as well as straightness of a linear stage. Based on the two-point source interference, two-point light sources superpose and generate an interference pattern on a specific observation plane. The interference pattern will alter accordingly when the relative position of the two light sources in space changes. This measurement system projects an expanded collimated light beam to two corner cube retroreflectors (CCR), one is fixed on the reference frame and the other is attached on the stage for measurement. The two reflected lights from the two CCRs are collected by a focal lens and produce an interference pattern on the focal plane. Then the period and direction of the interference pattern can be extract with Fast Fourier Transform (FFT) for off-axis (X-Y plane) motion measurement, and the four-step phase-shifting method is used for on-axis (Z-axis) motion measurement. The proposed measurement system can measure the three-axis displacement of the two reflectors in real-time. Different from traditional interferometry for 3D measurement which requires multiple sets of interferometers, the proposed system has the advantage to use one simple configuration and a measuring algorithm to achieve three degree-of-freedom measurements. The experiment demonstrates that the resolution for off-axis measurement is about 20 m, and on-axis measurement can reach 2 nm.