Lumentum’s CTO discusses photonic trends

CTO interviews part 2: Brandon Collings

• The importance of moving to parallel channels will only increase given the continual growth in bandwidth.
• Lumentum's integration of NeoPhotonics’ engineers and products has been completed.
• The use of coherent techniques continues to grow, which is why Lumentum acquired the telecom transmission product lines and staff of IPG Photonics.

Brandon Collings has been a CTO for over 13 years; first as CTO of the commercial optical products (CCOP) business within JDSU and then CTO of Lumentum when it spun out in 2015. In that time, the scope of his work has continued to grow.

Brandon Collings

"It has changed quite significantly given what Lumentum is engaging in," he says. "My role spans the entire company; I'm engaged in a lot of areas well beyond communications."

A decade ago, the main focus was telecom and datacom. Now Lumentum also addresses commercial lasers, 3D sensing, and, increasingly, automotive lidar.

Acquisitions

Lumentum was busy acquiring in 2022. The deal to buy NeoPhotonics closed last August. The month of August was also when Lumentum acquired IPG Photonics’ telecom transmission product lines, including its coherent digital signal processing (DSP) team.

NeoPhotonics’ narrow-linewidth tunable lasers complement Lumentum’s modulators and access tunable modules. Meanwhile, the two companies’ engineering teams and portfolios have now been merged.

NeoPhotonics was active in automotive lidar, but Lumentum stresses it has been tackling the market for several years.

“It’s an area with lots of nuances as to how it is going to be adopted: where, how fast and the cost dependences,” says Collings. “We have been supplying illuminators, VCSELs, narrow-linewidth lasers and other technologies into lidar solutions for several different companies.”

Lumentum gained a series of technological capabilities and some products with the IPG acquisition. “The big part was the DSP capability,” says Collings.

ROADMs

Telecom operators have been assessing IP-over-DWDM anew with the advent of coherent optical modules that plug directly into an IP router.

Cisco’s routed optical networking approach argues the economics of using routers and the IP layer for traffic steering rather than at the optical layer using reconfigurable optical add-drop multiplexers (ROADMs).

Is Lumentum, a leading ROADM technology supplier, seeing such a change?

“I don’t think there is a sea change on the horizon of moving from optical to electrical switching," says Collings. “The reason is still the same: transceivers are still more expensive than optical switches.”

That balance of when to switch traffic optically or electrically remains at play. Since IP traffic continues to grow, forcing a corresponding increase in signalling speed, savings remain using the optical domain.

“There will, of course, be IP routers in networks but will they take over ROADMs?” says Collings. “It doesn’t seem to be on the horizon because of this growth.”

Meanwhile, the transition to more flexible optical networking using colourless, directionless, contentionless (CDC) ROADMs, is essentially complete. 

Lumentum undertook four generations of switch platform design in the last decade to enable CDC-ROADM architectures that are now dominant, says Collings.

Lumentum moved from a simple add-drop to a route-and-select and a colourless, contentionless architecture.

A significant development was Lumentum’s adoption of liquid-crystal-on-silicon (LCOS) technology that enabled twin wavelength-selective switches (WSSes) per node that adds flexibility. LCOS also has enabled a flexible grid which Lumentum knew would be needed.

“We're increasingly using MEMS technology alongside LCOS to do more complex switching functions embedded in colourless, directionless and contentionless networks today,” says Collings.

Shannon’s limit

If the last decade has been about enabling multiplexing and demultiplexing flexibility, the next challenge will be dealing with Shannon’s limit.

“We can't stuff much more information into a single optical fibre - or that bit of the amplified spectrum of the optical fibre - and go the same distance,” says Collings. “We've sort of tapped out or reached that capacity.”

Adding more capacity requires amplified fibre bandwidth, such as using the L-band alongside the C-band or adding a second fibre.

Enabling such expansion in a cost- and power-efficient way will be fundamental, says Collings, and will define the next generation of optical networks.

Moreover, he expects consumer demand for bandwidth growth to continue. More sensing and more up-hauling of data to the cloud for processing will occur.

Accordingly, optical transceivers will continue to develop over the next decade.

“They are the complement requirement for scaling bandwidth, cost and power effectively,” he says.

Parallelism

Continual growth of bandwidth over the next decade will cause the industry to experience technological ceilings that will drive more parallelism in communications.

“If you look in data centres and datacom interconnects, they have long moved to parallel interface implementations because they felt that bandwidth ceiling from a technological, power dissipation or economic reason.”

Coherent systems have a symbol rate of 128 gigabaud (GBd), and the industry is working on 256GBd systems. Sooner or later, the consensus will be that the symbol rate is fast enough, and it is time to move to a parallel regime.

“In large-scale networks, parallelism is going to be the new thing over the next ten years,” says Collings.

Coherent technology

Collings segments the coherent optical market into three.

There are high-end coherent designs for long-haul transport developed by optical transport vendors such as Ciena, Cisco, Huawei, Infinera and Nokia.

Then there are designs such as 400ZR developed for data centre interconnect. Here a ‘pretty aggressive’ capability is needed but not full-scale performance.

At the lower end, there are application areas where direct-detect optics is reaching its limit. For example, inside the data centre, campus networks and access networks. Here the right solution is coherent or a ‘coherent-light’ technology that is a compromise between direct detection and full-scale coherence used for the long haul.

“So there is emerging this wide continuum of applications that need an equal continuum of coherent technology,” says Collings.

Now that Lumentum has a DSP capability with the IPG acquisition, it can engage with those applications that need solutions that use coherent but may not need the highest-end performance.

800 gigabits and 1.6 terabits

There is also an ongoing debate about the role of coherent for 800-gigabit and 1.6-terabit transceivers, and Collings says the issues remain unclear.

There's a range of application requirements: 500m, 2km, and 10km. A direct-detect design may meet the 500m application but struggle at 2k and break down at 10km. “There's a grey area, just in this simple example,” he says.

Also, the introduction of coherent should be nuanced; what is not needed is a long-haul 5,000km DSP. It is more a coherent-light solution or a borrowing from coherent technologies, says Collings: “You're still trying to solve a problem that you can almost do with direct detect but not quite.”

The aim is to use the minimum needed to accomplish the goal because the design must avoid paying the cost and power to implement the full complement coherent long-haul.

“So that's the other part of the grey area: how much you borrow?” he says. “And how much do you need to borrow if you're dealing with 10km versus 2km, or 800 gigabits versus 1.6 terabits.”

Data centres are already using parallel solutions, so there is always the option to double a design through parallelism.

“Eight hundred gigabit could be the baseline with twice as many lanes as whatever we're doing at 400 gigabits,” he says. “There is always this brute force approach that you need to best if you're going to bring in new technologies.”

Optical interconnect

Another area Lumentum is active is addressing the issues of artificial intelligence machine-learning clusters. The machine-learning architectures used must scale at an unprecedented rate and use parallelism in processors, multiple such processors per cluster, and multiple clusters.

Scaling processors requires the scaling of their interconnect. This is driving a shift from copper to optics due to the bandwidth growth involved and the distances: 100, 200 and 400 gigabits and lengths of 30-50 meters, respectively.

The transition to an integrated optical interconnect capability will include VCSELs, co-packaged optics, and much denser optical connectivity to connect the graphic processing units (GPUs) rather than architectures based on pluggables that the industry is so familiar with, says Collings.

Co-packaged optics address a power dissipation interconnect challenge and will likely first be used for proprietary interconnect in very high density GPU artificial intelligence clusters.

Meanwhile, pluggable optics will continue to be used with Ethernet switches. The technology is mature and addresses the needs for at least two more generations.

“There's an expectation that it's not if but when the switchover happens to co-packaged optics and the Ethernet switch,” says Collings.

Material systems

Lumentum has expertise in several material systems, including indium phosphide, silicon photonics and gallium arsenide.

All these materials have strengths and weaknesses, he says.

Indium phosphide has bandwidth advantages and is best for light generation. Silicon is largely athermal, highly parallelisable and scalable. Staff joining from NeoPhotonics and IPG have strengthened Lumentum’s silicon photonics expertise.

“The question isn't silicon photonics or indium phosphide. It's how you get the best out of both material systems, sometimes in the same device,” says Collings. “Sticking in one sandbox is not going to be as competitive as being agile and having the ability to bring those sandboxes together.”