[History and development of optical Ethernet] Google seeking consideration of Coherent-Lite method for ``800G-LR'' with a reach of 10 km [Net new technology]-INTERNET Watch

How can we achieve a 10km reach standard at 800G?

Continuing from last time, we will talk about the meeting in July 2021. The period during which we were free to make proposals was coming to an end, and even though we weren't in a hurry, various detailed proposals were starting to stand out.

First, "Coherent-Lite for beyond 400GbE" by Google's Cedric F. Lam, Xiang Zhou, and Hong Liu. He pointed out that although the specification itself of 800G with a reach of 10km has been accepted, how to achieve that 10km has not yet been considered.

10km should be a specification, the story was passed in Motion #6 at the April meeting

　Google points out that the traffic on its campus network has increased tenfold over the past four years, and as a bonus, the distance of the network on campus has expanded to 10km. In short, we want to establish a standard that can reach 10km at 800G for our own use.

I know this is "Inter-Campus", but since it's called "Intra-Campus", I wonder if there's any movement to reduce or shrink the Campus due to the impact of COVID-19? (Does it make sense to expand the distance between bases to avoid crowding?)

Now, we already know that CWDM4 will be difficult when it comes to 10km. Chromatic Dispersion Limit, if you dare to translate it into Japanese, it will be the chromatic dispersion limit, but in short, in the case of CWDM, the wavelengths of the four lights that are used are considerably wider. 800G Pluggable MSA plans to realize 500m FR4 with CWDM4, which is introduced in "800G Pluggable MSA" that extends the reach while realizing 1.6Tb/sec with PSM4 and CWDM4.

However, the "CWDM4 MSA" introduced in "Aggregating 25 Gbps x 4 into a single SMF to achieve 100 Gbps, with a reach of 2 km "CWDM4 MSA" and 10/20/40 km "4WDM MSA" has a slightly higher speed. CWDM4 consists of four wavelengths of 1271/1291/1311/1331nm and 20nm increments.

With this CWDM4 MSA, we are aiming for a 25 Gbps per wavelength and still a 2 km transmission distance. "4WDM MSA" is responsible for long distances of 10/20/40km, but although CWDM4 is used as is for 10km, DWDM is used for 20/40km.

As explained in "100G 4WDM-10/20/40" aiming for 10/20/40km reach at 100Gbps, the wavelength is 1295.56 to 1309.14nm, with an increment of around 4.5nm. It is reduced, and this is the appearance that suppresses the variation in characteristics due to the difference in wavelength.

　A similar story will naturally occur here as well, if it stays at 100G, it may be possible to go close to 5km, but at 200G, the limit is around 1km, and 10km is just another dream. .

If chirp control is performed properly, the reachable distance will increase, but this time the signal loss will become too large, making it difficult for the receiving side. In short, CWDM4 is a story to give up. Then DWDM? Instead, Google has proposed "Coherent-Lite".

The black line is the EML transmitter that is air-cooled, and the red and blue lines are the silicon MZM transmitter that incorporates a mechanism to suppress the chirp effect, which is a phenomenon in which the frequency changes over time. The red line is the theoretical limit and it actually becomes the blue line, but if it is 200G, it will be around whether it will reach 3km. "Beyond 1 Tb/s Intra-Data Center Interconnect Technology: IM-DD OR Coherent?", which examines performance and problems when implemented with two methods, IM-DD PAM and Coherent PM-QAM, is listed as a reference.