Brandon Collings, CTO of Lumentum, talks CIG, 400ZR and 400ZR+, COBO, co-packaged optics and why silicon photonics is not going to change the world.
Lumentum has completed the sale of part of its datacom product lines to design and manufacturing company, Cambridge Industries Group.
The sale will lower the company's quarterly revenues by between $20 million to $25 million. Lumentum also said that it will stop selling datacom transceivers in the next year to 18 months.
Brandon CollingsThe move highlights how fierce competition and diminishing margins from the sale of client-side modules is causing optical component companies to rethink their strategies.
Lumentum’s focus is now to supply its photonic chips to the module makers, including CIG. “From a value-add point of view, there is a lot more value in selling those chips than the modules,” says Brandon Collings, CTO of Lumentum.
400ZR and ZR+
Lumentum will continue to design and sell line-side coherent optical modules, however.
“With coherent, there is a lot of complexity and challenge in the module’s design and manufacture,” says Collings. “We believe we can extract the value we need to continue in that business.”
The emerging 400ZR and 400ZR+ are examples of such challenging coherent interfaces.
The 400ZR specification, developed by the Optical Internetworking Forum (OIF), is a 400-gigabit coherent interface with an 80km reach. The 400 gigabit-per-second (Gbps) line rate will be achieved using a 64-gigabaud symbol rate and a 16-QAM modulation scheme.
[400ZR] is not client-side. Sixty-four gigabaud is very hard to do in such an extremely compact form factor.
Module makers will implement the 400ZR interface using client-side pluggable modules such as the QSFP-DD and the OSFP to enable data centre operators to add coherent interfaces directly to their switches.
But implementing 400ZR will be a challenge. “This is not client-side,” says Collings. “Sixty-four gigabaud is very hard to do in such an extremely compact form factor.”
First samples of 400ZR modules are expected by year-end.
The 400ZR+ interface, while not a specification, is a catch-all for a 400-gigabit coherent that exceeds the 400ZR specification. The 400ZR+ will be a multi-rate design that will support additional line rates of 300, 200 and 100Gbps. Such rates coupled with more advanced forward-error correction (FEC) schemes will enable the 400ZR+ to span much greater distances than 80km.
The 400ZR+ interface helps the developers of next-generation coherent DSP chips to recoup their investment by boosting the overall market their devices can address. “It is basically a way of saying I’m going to spend $50 million developing a coherent DSP, and the 400ZR market alone is not big enough for that investment,” says Collings.
Lumentum says there will be some additional functionality that will be possible to fit into a QSFP-DD such that at least one of the ZR+ modes will be supported. But given the QSFP-DD module’s compactness and power constraints, the ZR+ will also be implemented in the CFP2 form factor that has the headroom needed to fully exploit the coherent DSP’s capabilities to also address metro and regional networks.
400ZR+ modules are expected in volume by the end of 2020 or early 2021.
DSP economics
Lumentum will need to source a coherent DSP for its 400ZR/ ZR+ designs as it does not have its own coherent chip. At the recent OFC show held in San Diego, the talk was of new coherent DSP players entering the marketplace to take advantage of the 400ZR/ZR+ opportunity. Collings says he is aware of five DSP players but did not cite names.
NEL and Inphi are the two established suppliers of merchant coherent DSPs. Lumentum (Oclaro) has partnered with Acacia Communications to use its Meru DSP for Lumentum’s CFP2-DCO design, although it is questionable whether Acacia will license its DSP for 400ZR/ ZR+, at least initially.
Lumentum and Oclaro also partnered with Ciena to use its WaveLogic Ai for a long-haul module. That leaves room for at least one more provider of a coherent DSP that could be a new entrant or an established system vendor that will license an internal design.
God forbid if 10 or more players are doing this as no matter how you slice it, people will be losing [money]
Collings points out that it makes no sense economically to have more than five players. If it takes $50 million to tape out a 7nm CMOS coherent DSP, the five players will invest a total of $250 million. And if the investment cost for the module, photonics and everything else is a comparable amount, that equates to $500 million being spent on the 400-gigabit coherent generation.
As for the opportunity, Collings talks of about a total of up to 500,000 ports a year by 2020. That equates to an investment return in the first year of $1,000 per device sold. “God forbid if 10 or more players are doing this as no matter how you slice it, people will be losing [money].”
Beyond Pluggables
The evolution of optics beyond pluggables was another topic under discussion at OFC.
The Consortium of On-Board Optics (COBO), the developerof an interoperable optical solution that embeds optics on the line card, had a stand at the show and a demonstration of its technology. In turn, co-packaged optics, the stage after COBO in the evolution of optical interfaces that will integrate the optics with the silicon in one package, is also now also on companies' agenda.
Collings explains that COBO came about because the industry thought on-board optics would be needed given the challenge of 400-gigabit pluggables meeting the interface density needed for 12.8-terabit switches . “I shared that opinion four to five years ago,” he says, adding that Lumentum is a member of COBO.
But 400-gigabit optics has been engineered to meet the required faceplate density, including ZR for coherent. As a result, COBO is less applicable. “That need to break the paradigm is a lot less,” he says.
That problem is real. It is a matter of how far the current engineering can go before it becomes too painful.
That said, Collings says COBO has driven valuable industry discussion given that the data centre is heading in a direction where 32 ports of 800-gigabit interfaces will be needed to get data in and out of next-generation, 25-terabit switches.
“That problem is real,” says Collings. “It is a matter of how far the current engineering can go before it becomes too painful.” Scaling indefinitely what is done today is not an option, he says.
It is possible with the next generation of switch chip to simply use a two-rack-unit box with twice as many 400-gigabit modules. “That has already been done at the 100-gigabit generation that lasted longer because it doubled up the 100-gigabit port count,” he says.
“In the generation after that, you are now asking for stuff that looks very challenging with today’s technology,” he says. “And that is where co-packaging is focused, the 50-terabit switch generation.” Switches using such capacity silicon are expected in the next four years.
But this is where it gets tricky, as co-packaging not only presents significant technical challenges but also will change the supply chain and business models.
Collings points out that hyperscalars do not like making big pioneering investments in new technology, rather they favour buying commodity hardware. “They don’t like risk, they love competition, and they like a healthy ecosystem,” he says.
“There is a lot of talk from the technology direction of how we can solve this problem [using co-packaged optics] but I think on the business side, the riskside, the investment side is putting a lot of pressure on that actually happening,” says Collings. “Where it ends up I don’t honestly know.”
Silicon photonics
One trend evident at OFC was the growing adoption of silicon photonics by optical component companies.
Indeed, the market research firm, LightCounting, in a research note summarising OFC 2019, sees silicon photonics as a must-have technology given co-packaged optics is now clearly on the industry’s roadmap.
However, Collings stresses that Lumentum’s perspective remains unchanged regarding the technology.
“It’s a fabless exercise so we can participate in silicon photonics and, quite frankly, that is why a lot of other companies are participating because the barrier to entry is quite low,” says Collings. “Nevertheless, we look at silicon photonics as another tool in the toolbox: it has advantages in some areas, some significant disadvantages in others, and in some places, it is simply comparable.”
When looking at a design from a system perspective such as a module, other considerations come into play besides the cost of the silicon photonics chip itself. Collings cites the CFP2 coherent module. While the performance of its receiver is good using silicon photonics, the modulator is questionable. You also need a laser and a semiconductor optical amplifier to compensate for silicon photonics higher loss, he says,
The alternative is to use an indium phosphide-based design and that has its own design issues. “What we are finding when you look at the right level is that the two are the same or indium phosphide has the advantage,” says Collings. “And as we go faster, we are finding silicon is not really keeping up in bandwidth and performance.”
As a result, Lumentum is backing indium phosphide for coherent operating at 64 gigabaud.
“A lot of people are talking about silicon photonics because they can talk about it,” says Collings. “It’s not worthless, don’t get me wrong, but its success outside of Acacia has been niche, and Acacia is top notch at doing this stuff.”