DFB lasers can suppress the common mode jumps of ordinary cavity semiconductor lasers in a wide operating temperature and current range, and have a high side mode rejection ratio, especially under high-speed modulation, which still maintains single longitudinal mode characteristics, becoming a dynamic single Mode lasers are ideal light sources in optical long-distance, broadband and high-speed optical communication systems. Therefore, it has received great attention in the rapidly developing field of optoelectronics. So far, many methods have been reported to enable DFB lasers to achieve single-mode operation. Among them, the most realistic and easy to implement method is to coat the optical film on the cleavage end face of the laser, so that the reflection of the two end faces of the laser becomes an asymmetric structure, so that the probability of single-mode operation can be improved. In other words, the optical film coated on the end surface of the DFB laser is very important for single-mode operation. It is well known that the feedback required for DFB laser lasing is provided by the distributed reflection of the grating on the entire cavity. The phase of the grating at the cleavage end face has an important influence on the characteristics of the laser. This fact has been confirmed by the results obtained by using ion beam etching to engrave the end face of the laser to different depths, and by studying the threshold current and longitudinal mode characteristics. The method of coating the optical film on the end face of the cleavage can control the phase of the end face of the DFB laser. This article reports that in the conventional manufacturing process, one end of the laser is coated with a high-reflective film (HR) and the other end is coated with an anti-reflective film (AR). If there are still some lasers that work in dual-mode, an appropriate single The thickness of the layers makes them work in single mode. Experiments have proved that coating technology can be an auxiliary means to improve the single longitudinal mode operation of DFB lasers.