<title>Two-frequency laser interferometric path measuring system for extreme velocities with high accuracy</title>

Abstract

Atwofrequency laser interferometric path measuring system is introduced which has been developed especially for the precision rapid-positioning systems of micro lithographic large-scale instruments and for ultra-precision technology. The modulation frequency of 640 MHz, generated by a highly-stabilised He-Ne-laser allows-in combination with a novel HF signal processing method -measuring velocities up to 6,4 m/s without "loosing" even the least increment (10 nm). It is a further metrological merit that the least increment is not generated by an additional NF interpolation-as was usual so far-but arises from the parallel processing in the HF part.