Real-time validation of downstream 50G/25G and 50G/100G flexible rate PON based on Miller encoding, NRZ, and PAM4 modulation

Experimental demonstration is given of a real-time flexible downstream 50 Gb/s passive optical network (PON) mixed with 25 Gb/s or 100 Gb/s signals. 25 Gb/s transmission is enabled by using 25 Gb/s Miller encoding. For back-to-back a lower sensitivity for 25 Gb/s Miller of ∼2.2dB is found relative to 50 Gb/s non-return-to-zero (NRZ). An extended reach of 40 km and up to ∼5dB optical power budget improvement after worst-case 20 km standard single-mode fiber over 50 Gb/s NRZ was shown for 25 Gb/s Miller. It was also shown that optical network units (ONUs) that have 9 dB additional optical received power because they experience a lower optical loss can receive 100 Gb/s four-level pulse amplitude modulation (PAM4) instead of NRZ in a flexible rate PON, which will double the bit-rate to these ONUs. 50 Gb/s NRZ transmission under mixed modulation (50% NRZ and 50% Miller or 50% PAM4) can operate with very low penalty. A power penalty of <0.4dB was found for 50 Gb/s NRZ after worst-case total dispersion (as specified in ITU-T G.9804) when mixed with 25 Gb/s Miller or 100 Gb/s PAM4. For back-to-back there was a smaller penalty of <0.2dB for 50 Gb/s NRZ under mixed modulation. The 25 Gb/s Miller signal spectrum is similar to 50 Gb/s NRZ and 100 Gb/s PAM4, which enables good performance of the real-time clock and data recovery with an integrated adaptive feed-forward-equalizer based equalizer under mixed signal modulation. The increased optical power budget enabled by the 25 Gb/s Miller encoding effectively improves power margins of the high-speed PON and will enable reliable transmission for ONUs deployed with out-of-spec (higher) loss and/or (larger) fiber reach. The additional margin provided by 25 Gb/s Miller encoding will also enable a safe-mode for ONUs that experience out-of-spec scenarios because of faults in the PON fiber network.