I recently chatted with Hal Roberts, Calix contributor to the latest advancements of NG-PON2 standards.
Teresa: The NG-PON2 G.989.2 amendment 2 was recently consented. What are the significant benefits for the industry?
Hal: It is no secret that NG-PON2 optics costs have been an impediment to wide scale adoption of the new technology. Amendment 2 is almost exclusively focused on tradeoffs that allow a wider set of optics technical solutions. Most significantly, changes were made to allow the use of the lower cost directly modulated lasers (DMLs) in the ONU. It was very difficult for DMLs to meet the original dispersion power penalty. A higher power penalty is now allowed if the laser compensates by transmitting at a higher power. Since a DML is can transmit at high power, this is generally a good trade-off.
Teresa: What other changes are in amendment 2?
Hal: DMLs suffer from ‘chirp’ which means that the color of the laser light changes as the laser is modulated. Different colors travel at different speeds through fiber and after 20 kilometers the 1’s and 0’s sent by the ONU can overlap at the OLT receiver. The dispersion power penalty from a DML can be reduced by reducing the Extinction Ratio (ER), the ratio of the power of a ‘1’ divided by the power of a ‘0’, but that causes its own power penalty. G.989.2 amendment 2 precisely defines how much additional power is needed in the DML and is shown as new operating regions in the diagram below.
Another cost-performance tradeoff was to allow a 10x increase in the turn-on/turn-off time of the ONU transmitters. Many manufacturers were finding it difficult to comply with the previous shorter times.
Teresa: When do you think we’ll see NG-PON2 amendment 2 optics?
Hal: Fortunately, optics vendors have been proceeding with the expectation that amendment 2 would be consented, which it was. I would say that from a technical standpoint, sometime in 2018 we could see large scale production. But this will depend heavily on operator demand.
Hal and I could talk all day about NG-PON2. Let us know in the comments what questions you’d like answered!