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All, I wanted to open a discussion on wavelength choices for the bidi optics.
The following are just my opinion, and I put them out there to help get the discussion going.
(Feel free to respond by exploder or by a contribution, both are fine.)
The DML 25G transmission dispersion penalty has already been evaluated in the 802.3cc project, where they use 1295-1325 for 10km links, and 1295-1310nm for 40km links. I think we should aim to stay within those
bands to enable the use of DMLs, because DMLs are cheaper, emit more power, and don’t require cooling in many cases. Therefore, I suggest that we use 1295 – 1310 nm as the downstream band for 25G, and by extension, 50G PAM4 as well.
I would freely admit that this only gives us 15 nm for guard band for the diplex filter. This is about as complicated as a CWDM filter, which is a cost increase relative to a 40 nm guard band diplex filter.
This cost delta has to do mostly with the packaging constraints of the different types of filters. The alternative is to use an EML in the downstream direction. The EML dispersion penalty at 1330 nm is quite OK. But EML’s cost more than DMLs. So, we are
left with a very direct relative cost comparison. 1295-1310nm: 2*DML + 2*[15nm diplex filter + packaging] + all the other parts
1320-1340nm: DML + EML + 2*[40nm diplex filter + packaging] + all the other parts I would like it if the optics vendors could comment on how they think this cost comparison works out.
[Disclaimer: We only talk about relative costs in our group. Please do not give any absolute costs.]
Sincerely, Frank Effenberger, VP fixed access networks lab, Futurewei Technologies Chair IEEE P802.3cp Bidirectional 10, 25, and 50 Gb/s Optical Access PHYs Task Force
Rapporteur ITU-T Q2/15 Optical Access Networks Question
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