Ultra-high-speed

ultra-large-capacity and ultra-long-haul（3U）are the forever pursuit of optical communication or even the whole communication industry.As a new mode of communication 3U transmission can greatly promote the next generation of optical internet and broadband mobile communication network development and technological progress

therefore 3U has become the focus of international high-tech intellectual property and competition ground.This paper introduces the scientific problems

key technologies

and important achievements that have been made in 3U transmission.