Intel's 100-gigabit silicon photonics move

Intel has unveiled two 100-gigabit optical modules for the data centre made using silicon photonics technology.

Alexis Bjorlin

The PSM4 and CWDM4/CLR4 100-gigabit modules mark the first commercial application of a hybrid integration technique for silicon photonics, dubbed heterogeneous integration, that Intel has been developing for years.

Intel's 100-gigabit module announcement follows the news that Juniper Networks has entered into an agreement to acquire start-up, Aurrion, for $165 million. Aurrion is another silicon photonics player developing this hybrid integration technology for its products. 

Hybrid integration

With heterogeneous integration, materials such as indium phosphide and gallium arsenide can be bonded to the silicon substrate before the 300mm wafer is processed to produce the optical circuit. Not only can lasers be added to silicon but other active devices such as modulators and photo-detectors as well as passive functions such as isolators and circulators.   

There is no alignment needed; we align the laser with lithography

Intel is using the technique to integrate the laser as part of the 100-gigabit transceiver designs.

"Once we apply the light-emitting material down to the silicon base wafer, we define the laser in silicon," says Alexis Bjorlin, vice president and general manager, Intel Connectivity Group. “There is no alignment needed; we align the laser with lithography.”

Intel claims it gets the highest coupling efficiency between the laser and the optical waveguide and modulator because it is lithographically defined and requires no further alignment.

100-gigabit modules

Intel is already delivering the 100-gigabit PSM4 module. “First volume shipments are happening now,” says Bjorlin. Microsoft is one Internet content provider that is using Intel’s PSM4.

The chip company is also sampling a 100-gigabit CWDM4 module that also meets the more demanding CLR4 Alliance’s optical specification. The 100-gigabit CLR4 module can be used without forward-error correction hardware and is favoured for applications where latency is an issue such as high-performance computing.

Intel is not the first vendor to offer PSM4 modules, nor is it the first to use silicon photonics for such modules. Luxtera and Lumentum are shipping silicon photonics-based PSM4 modules, while STMicroelectronics is already supplying its PSM4 optical engine chip. 

We are right on the cusp of the real 100-gigabit connectivity deployments

“Other vendors have been shipping PSM4 modules for years, including large quantities at 40 gigabit,” says Dale Murray, principal analyst at LightCounting Market Research. “Luxtera has the clear lead in silicon photonics-based PSM4 modules but a number of others are shipping them based on conventional optics.”

The PSM4 is implemented using four independent 25-gigabit channels sent over a single-mode ribbon fibre. Four fibres are used for transmission and four fibres for receive. 

“The PSM4 configuration is an interesting design that allows one laser to be shared among four separate output fibres,” says Murray. “As Luxtera has shown, it is an effective and efficient way to make use of silicon photonics technology.”

The CWDM4 is also a 4x25-gigabit design but uses wavelength-division multiplexing and hence a single-mode fibre pair. The CWDM4 is a more complex design in that an optical multiplexer and demultiplexer are required and the four lasers operate at different wavelengths.

“While the PSM4 module does not break new ground, Intel’s implementation of WDM via silicon photonics in a CWDM4/CLR4 module could be more interesting in a low-cost QSFP28 module,” says Murray. WDM-based QSFP28 modules are shipping from a number of suppliers that are using conventional optics, he says. 

Intel is yet to detail when it will start shipping the CWDM4/CLR4 module.

Market demand

Bjorlin says the PSM4 and the CWDM4/CLR4 will play a role in the data centre. There are applications where being able to break out 100-gigabit into 25-gigabit signals as offered by the PSM4 is useful, while other data centre operators prefer a duplex design due to the efficient use of fibre.

“We are right on the cusp of the real 100-gigabit connectivity deployments,” she says.

As for demand, Bjorlin expects equal demand for the two module types in the early phases: “Longer term, we will probably see more demand for the duplex solution”. 

LightCounting says that 100-gigabit PSM4 modules took an early lead in the rollout of 100 Gigabit Ethernet, with VCSEL-based modules not far behind. 

“Some are shipping CWDM4/CLR4 and we expect that market to ramp,” says Murray. “Microsoft and Amazon Web Services seem to like PSM4 modules while others want to stick with modules that can use duplex fibre. 

Source: Intel

Data centre switching

“One of the most compelling reasons to drive silicon photonics in the future is that it is an integratable platform,” says Bjorlin.

Switch silicon from the likes of Broadcom support 3.2 terabits of capacity but this will increase to 6.4 terabits by next year and 12.8 terabits using 4-level pulse amplitude modulation (PAM-4) signalling by 2018 (see chart). And by 2020, 25.6-terabit capacity switch chips are expected.

The demand for 100 gigabit is for pluggable modules that fit into the front panels of data center switches. But the market is evolving to 400-gigabit embedded optics that sit on the line card, she says, to enable these emerging higher-capacity switches. Intel is a member of the Consortium of On-Board Optics (COBO) initiative that is being led by Microsoft.

“When you get to 25.6-terabit switches, you start to have a real problem getting the electrical signals in and out of the switch chip,” says Bjorlin. This is where silicon photonics can play a role in the future by co-packaging the optics alongside the switch silicon.

“There will be a need for an integrated solution that affords the best power consumption, the best bandwidth-density that we can get and effectively position silicon photonics for optical I/O [input/output],” says Bjorlin. “Ultimately, that co-packaging is inevitable.”