Abstract: Firstly, three ways to improve the capacity of long-haul optical transmission system are analyzed, and then the research progress and standardization trends of the 3 technical hot-spots are introduced including long-haul 400G, multi-band transmission with wave-band extension and space division multiplexing (SDM). After that the application prospects of SDM and multi-band transmission technologies. Finally, a 400Gbit / s per wavelength real-time transmission experiment over C + L band is carried out to verify the improvement of channel power flatness and transmission penalty brought by optical channel power equalization technique. The research shows that compared with SDM, C + L band expansion is more practical for commercial deployment in the short term. For C + L band 400G transmission system, power equalization can improve the OSNR bottleneck of the system and reduce the nonlinear penalty of L band. 

Key words: large-capacity transmission, 400G C+L, wave-band extension