Research on Key Technologies of Coherent PON System Above 100 Gbit/s

The standards for 50 Gbit/s Passive Optical Network (PON) are nearly complete, but the standards for the post-50 Gbit/s PON era are still blank. It is necessary to carry out relevant research to guide the industry’s strategic planning in system, optical module and chips. Experts predict that single-wavelength 200 Gbit/s rate and coherent technology will be the two key features of the next-generation PON system after 50 Gbit/s PON. Firstly, the single-wavelength 200 Gbit/s rate is particularly attractive to operators. However, Intensity Modulation and Direct Detection (IM/DD) technology struggles to meet the Class C+ power budget requirements at this speed. Therefore, it is necessary to use a coherent technology which is more sensitive than IM/DD technology. However, some technical challenges need to be overcome if coherent technology is applied to PON. As the PON system shows a typical Point-to-MultiPoint (P2MP) topology, key challenges include the architecture reconstruction of PON Media Access Control (MAC) chips and system equipment, the Bi-Di technology and the burst-mode technology of coherent PON optical modules, and the wavelength management in PON system. Time Division Multiplexing (TDM) remains the best way to achieve P2MP transmission. New multiplexing dimensions, such as Sub-Carrier Multiplexing (SCM) can enhance this method. The introduction of new multiplexing dimension adds flexibility but also complicates the design, potentially overhauling the entire PON system architecture. With SCM, digital interfaces are no longer used to connect optical modules. Instead, optical modules will need to use highly linear driver and modulator. The user-side optical modules need the ability to switch instantly to avoid interference with other users, necessitating the development of new coherent optical chips with burst-mode control functionality. Considering the uplink P2MP burst-mode coherent reception, it is necessary to control the wavelength of multiple lasers at the system level to avoid errors from frequency offset estimation, caused by rapid wavelength switching among users. In summary, applying coherent technology to PON is a completely new and complex task, which cannot directly inherit the existing coherent system architecture. It needs to match the requirement of P2MP system application, through technological innovation in chips, modules, and equipment.