In external cavity diode lasers, the filter serves as a frequency selector, playing a role in achieving stable single longitudinal mode output. The filter can filter out the broad-spectrum light emitted by the laser diode, select the light that resonates with the atomic transition frequency, and cause the longitudinal mode oscillation at a specific frequency, achieving the function of frequency selection, improving the monochromaticity and stability of the output laser. As described in Chap.  2 , traditional frequency selection devices include gratings, interference filters, and F-P (Fabry–Pérot) cavities. Common grating-type lasers mainly include Littrow and Littman structures that serve as both frequency selection and feedback. The wavelength selection of the grating can be expressed by the diffraction formula, and its center wavelength is related to the incident angle; the output wavelength of the interference filter-type laser is related to the angle between the incident light and the interference filter; the F-P cavity is composed of two parallel reflecting mirrors, which can select narrow spectral lines, but its loss is large, the fine tuning requirements are high, and it is very sensitive to environmental influences such as mechanical vibration. It can be seen that traditional diode lasers use macroscopic devices for frequency selection, and the output laser frequency usually depends on the angle of the frequency selection device. They cannot intrinsically correspond to the atomic transition spectrum line. In recent years, with the rapid development and application of atomic filters using the interaction characteristics of light and atoms, it has provided a new idea for external cavity lasers to achieve ultra-narrow band and specific quantum transition frequency selection. At present, atomic filters suitable for laser frequency selection include Faraday anomalous dispersion atomic filters, Voigt atomic filters, and birefringence-induced atomic filters.