The 50 Gbit/s Passive Optical Network(PON)standard is basically mature,but the standards for the beyond-50 Gbit/s PON era are still blank.It is necessary to carry out relevant research to promote the entire PON industry chain to de-ploy in advance,from the aspects of system,optical module and chips.It is believed 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 has greater attraction for the operators.Intensity Modulation and Direct Detection(IM/DD)tech-nology has difficulty to continuously meet the Class C+power budget under 200 Gbit/s rate.Therefore,it is necessary to use coherent technology which is more sensitive than IM/DD technology.However,some key technologies need to be overcomed if coherent technology is applied to PON.As the PON system shows a typical Point-to-MultiPoint(P2MP)topology,the key technologies include the architecture reconstruction of PON Media Access Control(MAC)chip and system equipment,the Bi-Di technology and the burst-mode technology of coherent PON optical modules,and the wavelength management in PON sys-tem.Time Division Multiplexing(TDM)remains the best way to achieve P2MP transmission,and on this basis,new multi-plexing dimensions,such as Sub-Carrier Multiplexing(SCM)can be introduced.The introduction of new multiplexing dimen-sion brings flexibility to PON system,but it also increases the complexity and may change 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 to take the ability to switch instantly to avoid interference with other users,and coherent optical chips supporting burst-mode control need to be developed.Considering the uplink P2MP burst-mode coherent reception,it is necessary to achieve wavelength management for multiple lasers from the system level to avoid errors from frequency offset estimation,which is caused by rapid user wavelength switching.In summary,applying co-herent technology to PON is a completely new and complex task,which is difficult to be fulfilled by directly inherit the existing coherent system architecture.It needs to match the requirement of P2MP system application,achieving technological innova-tion from aspects of chips,modules,and equipment.
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