In order to meet the requirements of thousands of kilometers long-distance transmission in backbone network, coherent transmission technology is the mainstream transmission technology. But in the networking between cities, the transmission distance is usually less than 300 kilometers. In this case, how to avoid the use of coherent detection (complex system, high cost) to achieve good transmission and networking effect is a hot topic.

In order to realize the medium and short distance transmission, the current main technologies mainly consider direct modulation and direct detection. There are many modulation methods, including pam-4, DMT, SSB, etc. On the receiving side, a noncoherent method is used, with fewer optoelectronic devices. In order to simplify the system and reduce the cost.

In recent years, a number of direct debugging and direct inspection technology experiments have been carried out continuously. Through gradual research and continuous optimization, the pilot will start in the next three years. Inspection technology