Gratings are optical components, which consist of a series of equidistant parallel lines that have been engraved on a substrate with reflective coating. The distance of adjacent lines and the angle of groove relative to the substrate will affect the dispersion and efficiency of gratings. If the incident wavelength is much longer than the distance between the lines, diffraction will not occur, while if much shorter, the carving surface of grating can only reflect the incident light, diffraction does not occur as well. Gratings are divided into two basic types according to the processing technology of grating groove, holographic grating and ruled grating. Ruled grating is formed by engraved on the reflective surface by an engraving machine with a diamond knife head. The gratings produced by laser forming interference fringes and photolithography are called interference grating or holographic gratings.