Grating measurement apparatus

Abstract

A grating measuring device includes: a light source module () for generating two light beams having different frequencies, one of which serves as a measuring beam and the other as a reference beam; a grating (); and a grating measuring probe () including a dual-frequency light reception module, a vertical measurement module, a vertical detection module and a reference detection module. The dual-frequency light reception module is configured to receive the measuring and reference beams, and the vertical measurement module is adapted to project the measuring beam onto the grating (), collect a zeroth-order diffracted beam resulting from double diffraction occurring at the grating, and feed the zeroth-order diffracted beam to the vertical detection module. The zeroth-order diffracted beam interferes with the reference beam in the vertical detection module, resulting in a vertical interference signal. In addition, the measuring and reference beams interfere with each other also in the reference detection module to result in a reference interference signal. The vertical and reference interference signals are received by a signal processing module and serve as a basis for calculating a vertical displacement of the grating (). This grating measuring device allows a great vertical displacement measurement range at any working distance.