Precise measurement of length is one of the key technologies in precision machinery manufacturing industry and optical processing industry. Modern length measurement is mostly carried out by using the interference phenomenon of light waves, and its accuracy mainly depends on the monochromaticity of light. Laser is the most ideal light source, which is 100,000 times purer than the best monochromatic light source (krypton-86 lamp) in the past. Therefore, the laser length measurement range is large and the precision is high. According to the optical principle, the relationship between the maximum measurable length L of monochromatic light, the wavelength λ and the spectral line width δ is L=λ/δ. The maximum length that can be measured with a krypton-86 lamp is 38.5 cm. For longer objects, it needs to be measured in sections, which reduces the accuracy. If a helium-neon gas laser is used, it can measure up to tens of kilometers. Generally measure the length within a few meters, and its accuracy can reach 0.1 microns.