Verizon tips silicon photonics as a key systems enabler

Verizon's director of optical transport network architecture and design, Glenn Wellbrock, shares the operator’s thoughts regarding silicon photonics.

Part 3: An operator view

Glenn Wellbrock is upbeat about silicon photonics’ prospects. Challenges remain, he says, but the industry is making progress. “Fundamentally, we believe silicon photonics is a real enabler,” he says. “It is the only way to get to the densities that we want.”

Glenn Wellbrock

Wellbrock adds that indium phosphide-based photonic integrated circuits (PICs) can also achieve such densities.

But there are many potential silicon photonics suppliers because of its relatively low barrier to entry, unlike indium phosphide. "To date, Infinera has been the only real [indium phosphide] PIC company and they build only for their own platform,” says Wellbrock.

That an operator must delve into emerging photonics technologies may at first glance seem surprising. But Verizon needs to understand the issues and performance of such technologies. “If we understand what the component-level capabilities are, we can help drive that with requirements,” says Wellbrock. “We also have a better appreciation for what the system guys can and cannot do.”    

Verizon can’t be an expert in the subject, he says, but it can certainly be involved. “To the point where we understand the timelines, the cost points, the value-add and the risk factors,” he says. “There are risk factors that we also want to understand, independent of what the system suppliers might tell us.” 

The cost saving is real, but it is also the space savings and power saving that are just as important  

All the silicon photonics players must add a laser in one form or another to the silicon substrate since silicon itself cannot lase, but pretty much all the other optical functions can be done on the silicon substrate, says Wellbrock: “The cost saving is real, but it is also the space savings and power saving that are just as important.”  

The big achievement of silicon photonics, which Wellbrock describes as a breakthrough, is the getting rid of the gold boxes around the discrete optical components. “How do I get to the point where I don’t have fibre connecting all these discrete components, where the traces are built into the silicon, the modulator is built in, even the detector is built right in.” The resulting design is then easier to package. “Eventually I get to the point where the packaging is glass over the top of that.” 

So what has silicon photonics demonstrated that gives Verizon confidence about its prospects? 

Wellbrock points to several achievements, the first being Infinera’s PICs. Yes, he says, Infinera’s designs are indium phosphide-based and not silicon photonics, but the company makes really dense, low-power and highly reliable components.

He also cites Cisco’s silicon photonics-based CPAK 100 Gig optical modules, and Acacia, which is applying silicon photonics and its in-house DSP-ASICs to get a lower power consumption than other, high-end line-side transmitters.

Verizon believes the technology will also be used in CFP4 and QSFP28 optical modules, and at the next level of integration that avoids pluggable modules on the equipment's faceplate altogether.  

But challenges remain. Scale is one issue that concerns Verizon. What makes silicon chips cheap is the fact that they are made in high volumes. “It [silicon photonics] couldn’t survive on just the 100 gigabit modules that the telecom world are buying,” says Wellbrock. 

If these issues are not resolved, then indium phosphide continues to win for a long time because that is where the volumes are today

When Verizon asks the silicon photonics players about how such scale will be achieved, the response it gets is data centre interconnect. “Inside the data centre, the optics is growing so rapidly," says Wellbrock. "We can leverage that in telecom."

The other issue is device packaging, for silicon photonics and for indium phosphide. It is ok making a silicon-photonics die cheaply but unless the packaging costs can be reduced, the overall cost saving is lost. ”How to make it reliable and mainstream so that everyone is using the same packaging to get cost down,” says Wellbrock.

All these issues - volumes, packaging, increasing the number of applications a single part can be applied to -  need to be resolved and almost simultaneously. Otherwise, the technology will not realise its full potential and the start-ups will dwindle before the problems are fixed.

“If these issues are not resolved, then indium phosphide continues to win for a long time because that is where the volumes are today,” he says. 

Verizon, however, is optimistic. “We are making enough progress here to where it should all pan out,” says Wellbrock.