Q&A: silicon photonics in the cloud

Mellanox’s Arlon Martin reveals why SiP is an appealing option for hyperscale datacentre networks and considers how optical solutions could evolve beyond 400G networks. Interview by James Tyrrell.

Q1 - What makes silicon photonics so appealing?

Cloud datacentres are looking for technology sweet spots that can enable them to scale their networks to next-generation speeds at lower cost, with lower power consumption, and with faster manufacturing ramps than previous generations. Silicon photonics addresses this sweet spot. Silicon photonics chips are already widely used in today’s 100Gb/s networks with four 25Gb/s lanes, supporting datacentres with reaches of 500 metres to 2 km. For the next generation of 400Gb/s networks, Mellanox has already demonstrated the capability to transmit at speeds of 100Gb/s per lane -- four times faster than today’s products -- using the same size chip. This will lower networking cost and improve density. In addition, the energy efficiency improves by roughly a factor of two, meaning that datacentres run cooler and cost less to operate. Because silicon photonics chips eliminate the assembly complexity of traditional optical transceivers with hundreds of lenses, filters, isolators and other optical components, the manufacturing is far simpler. While traditional optical assembly processes can take several years to scale to high volume, silicon photonics optical engines are fabricated in standard foundries, so the volume ramp-up can be extremely fast. For hyperscale datacentre networks of 500m to 2km in reach, silicon photonics really hits the sweet spot.

Q2 - What are the key roles for optical chips in Mellanox's portfolio of solutions? 

Mellanox is well-known as a leading supplier of end-to-end Ethernet and InfiniBand intelligent interconnect solutions and services for servers, storage, and hyperconverged infrastructure. Recently, at Supercomputing 17, Mellanox announced what we believe is the world’s most scalable switch platform based on HDR 200G InfiniBand Quantum switch technology. With up to 1600-ports in a single platform, Quantum Switch Systems enable the highest performance while reducing datacentre network expenses by 4X. The new Quantum switch will use the same QSFP pluggable form factor as with previous generations, but double the speed to 200Gb/s. Silicon photonics transceivers easily fit in this compact package, and support the higher bandwidth and the longer reaches required by web 2.0 and cloud data centres. Not all 200Gb/s networks perform the same. Customers want the fastest switching times, the lowest latency and the lowest Bit Error Rate (BER). Our Silicon photonics-based optical transceivers ensure the highest throughput with the lowest BER for any InfiniBand or cloud data centre reach. Our transceivers have a BER of 1E-15 which, at 200Gb/s transmission, is one bit error every hour or two. Many competing solutions have a BER of 1E-12, which is 1000x worse, with an error occurring every five seconds. Silicon photonics transceivers just perform better, in my opinion. 

Q3 - Looking ahead at even greater utility of CPU and storage elements in the cloud - what are the pinch points and how do you see optical solutions evolving?

For the past 20 years, datacentre networks have been built around front-panel pluggable transceivers. The transceiver speeds have scaled with the switching speeds of the switches: 1 Gb/s transceivers for 1Gb/s ports; 10Gb/s transceivers for 10Gb/s ports; etc. Having ports on the front panel, allows many options for technologies in their networks. For example –

  • Direct Attach Copper (DACs) could be used to connect servers to Top-Of-Rack switches
  • Active Optical Cables might be used for storage and appliances up to 100m away
  • VCSEL optical transceivers for reaches of several hundred meters on multi-mode fibre
  • Silicon photonics optical transceivers for reaches of up to 2km or more on single mode fibre

But, sometime after the 400G generation of networks, it’s likely that the “Front panel pluggable” model breaks and that the optics will migrate inside the switch itself, either on the board with the switching chip or co-packaged in a Multi-Chip Module (MCM) with the switching chips. Mellanox is an active participant in the Consortium for On Board Optics (COBO), an industry effort to standardize packaging for on board optics. For COBO and MCM applications, the advantages of silicon photonics integration become large. SiP chips can work at higher temperatures than traditional laser technologies. Smaller sizes mean that SiP solutions can be closer to the switching chip. Functional blocks, like the modulator driver, can be incorporated into the switching ASIC itself. Finally, the entire switch assembly process favours the electronics-style assembly of silicon photonics.


Arlon Martin is senior director of marketing at Mellanox Technologies.

More information