Interferometric nanocomparator for calibrating precision displacement sensors

Presented work deals with the description of a novel interferometric nanocomparator intended for calibrating displacement sensors with nanometer resolution used in precision engineering. The nanocomparator is based on a 633 nm laser homodyne interferometer with 2-pass measuring arm. Digital signal filtering increases the SNR and allows achieving sub-nanometer resolution of interferometric measurements. High dynamic range of the measuring mirror displacement is achieved using a two-stage positioning system formed of a linear guide way and piezoelectric actuators. A linear guide way is used for positioning over a 100 mm range with 50 nm resolution. Piezoelectric actuators linked in a closed loop locked to the interferometer value are used for fine positioning with better than 1 nm resolution over a 5 um range. Two alternative versions of the mechanical design of the coarse positioning stage were tested and compared: a design utilizing a linear guide way with ball carrier bearings and a positioning system formed of a parallelogram frame with flexible junctions. Wearing out of linear guide ways may cause angular deviations of the mirror from the ideally perpendicular position to the laser beam. Active stabilization of the mirror using piezoelectric actuators linked to a 4-quadrant light detector was developed to eliminate these deviations and other angular errors. A set of experimental calibrations of inductive and incremental rule precision displacement sensors was conducted.