A laser measurement system for precise and fast positioning of an object has been developed. When the object speed is low, the movement of the object is measured by comparing the phase change of a light beam reflected by the object with the phase modulated by an electro-optic crystal (an active device by which the optical phase can be controlled by the application of a voltage). When the object speed is high, the movement is measured by a fringe counting technique. The system achieves an accuracy of 4 nm and a maximum allowable measurement speed of 1100 mm/s.
"Laser Measurement System For Precise And Fast Positioning," Optical Engineering 27(9), 279823 (1 September 1988). https://doi.org/10.1117/12.7976768