Luxtera's 100 Gigabit silicon photonics chip
Luxtera has detailed a 4x28 Gigabit optical transceiver chip. The silicon photonics company is aiming the device at embedded applications such as system backplanes and high-performance computing (HPC). The chip is also being used by Molex for 100 Gigabit active optical cables. Molex bought Luxtera's active optical cable business in January 2011.
“Do I want to invest in a copper backplane for a single generation or do I switch over now to optics and have a future-proof three-generation chassis?”
Marek Tlalka, Luxtera
What has been done
To make the optical transceiver, a distributed-feedback (DFB) laser operating at 1490nm is coupled to the silicon photonics CMOS-based chip. One laser only is required to serve the four individually modulated 28Gbps transmit channels, giving the chip a 112Gbps maximum data rate. There are also four receive channels, each using a germanium-based photo-detector that is grown on-chip.
The DFB is the same laser that Luxtera uses for its 4x10Gbps and 4x14Gbps designs. What has been changed is the Mach-Zehnder waveguide-based modulators that must now operate at 28Gbps, and the electronics amplifiers at the receivers. “The chip [at 5mmx6mm] is pretty much the same size as our 4x10 and 4x14 Gig designs,” says Marek Tlalka, director of marketing at Luxtera.
Source: Luxtera
Luxtera is announcing the 100 Gigabit chip which it is sampling to customers. Molex, for example, will package the chip and the laser to make its active optical cable products. Luxtera will package the transceiver chip and laser in a housing as an OptoPHY, a packaged product it already provides at lower speeds. The company will sell the 100Gbps OptoPHY for embedded applications such as system backplanes and HPC.
Applications
The 100GbE transceiver chip is targeted at next-generation backplane applications as well as active optical cables. And it is enterprise vendors that make switches, routers and blade servers that are considering adopting optical backplanes for their next-generation platforms, says Luxtera.
According to Tlalka, system vendors are moving their backplanes from 15Gbps to 28Gbps: “It is pretty obvious that building an electrical backplane at this data rate will be extremely challenging.”
When vendors design a new chassis, they want it to support three generations of line cards. Even if a system vendor develops a 28Gbps copper-based backplane, it will need to go optical when the backplane data rate increases to 40-50Gbps in 2-3 years’ time and 100Gbps when that speed transition occurs. “Do I want to invest in a copper backplane for a single generation or do I switch over now to optics and have a future-proof three-generation chassis?” says Tlalka.
Exascale computers, 1000x more powerful than existing supercomputers planned for the second half of the decade, is another application area. Here there is a need for 25-28Gbps links between chips, says Tlalka.
System platforms and HPC are ideal candidates for the packaged transceiver chip but longer term Luxtera is eyeing the move of optics inside chips such as ASICs. Such system-on-chip optical integration could include Ethernet switch ICs (See example switch ICs from Broadcom and Intel (Fulcrum)) and network interface cards. Another example highlighted by Tlalka is CPU-memory interfaces.
However such applications are at least five years away and there are significant hurdles to be overcome. These include resolving the business model of such designs as well as the technical challenges of coupling the ASIC to the optics and the associated mechanical design.
Standards
Luxtera's 100Gbps transceiver chip supports a variety of standards.
Operating at 25Gbps per channel, the chip supports 100GbE and Enhanced Data Rate (EDR) Infiniband. The ability to go to 28Gbps per channel means that the transceiver can also support the OTN (optical transport network) standard as well as proprietary backplane protocols that add overhead to the basic 25Gbps data rate.
In addition the chip supports the OIF's short reach and very short reach interfaces that define the interface between an ASIC and the optical module.
The chip is also suited for some of the IEEE Next Generation 100Gbps Optical Ethernet Study Group standards now in development. These interfaces will cover a reach of 30m to 2km.
400GbE and HDR Infiniband
Luxtera says that it is working on different channel ’flavours' of 100G. It is also following developments such as Infiniband Hexadecimal Data Rate (HDR) and 400GbE.
HDR will use 40Gbps channels while there is still an industry debate as to whether 400GbE will be implemented using ten channels, each at 40Gbps, or as a 16x25Gbps design.
The InfiniBand roadmap gets redrawn
“We can already demonstrate in silicon a 30Gbps transmitter."
Marek Tlalka, Luxtera
“Our June 2008 roadmap originally projected 4x EDR at less than 80Gbps data rate for 2011,” says Skip Jones, director of technology at QLogic and co-chair of the IBTA’s marketing working group. “The IBTA has increased the data speeds for 2011 due to demand for higher throughput.” A 26Gbps channel rate - or 104Gbps for 4x EDR - is to accommodate the overhead associated with 64/66bit encoding.
The IBTA has also added an interim speed, dubbed Fourteen Data Rate (FDR), operating at 14Gbps per channel or 56Gbps for 4x FDR. This, says the IBTA, is to address midrange enterprise applications in the data centre. “Many server OEMs’ backplanes can support speeds up to 56Gbps,” says Jones. “For those OEMs doing a server refresh using existing backplanes, 56Gbps will be the solution they’ll be looking to implement.”
The IBTA dismisses claims by some industry voices that the re-jigged roadmap is to stop InfiniBand falling behind 100 Gigabit Ethernet (GbE) while FDR is to advance InfiniBand while laser vendors grapple with the challenge of developing 26Gbps vertical-cavity surface-emitting lasers (VCSELs) for EDR.
Jones points out that 4x Quad Data Rate (QDR) InfiniBand (4x10Gbps) now accounts for between 60 and 70 percent of newly deployed InfiniBand systems, and that 100Gbps EDR will appear in 2011/ 2012. “The IBTA has a good track record of releasing products on time; as such, 100Gbps InfiniBand will come out much faster than 100 Gigabit Ethernet.” FDR, meanwhile, will benefit from 14Gbps VCSELs for Fibre Channel that will be available next year. Jones admits that developing a 26Gbps VCSEL poses a challenge but that “InfiniBand markets are mostly electrical interconnects”.
“The 4x25G short reach is not going to rise and dominate for quite awhile."
Scott Schube, LightCounting
“VCSELs are going to have a tough time at 26Gbps per lane, though they'll get there,” says Scott Schube, senior analyst and strategist at optical transceiver market research firm, LightCounting. “There's definitely a push to go to 26Gbps per lane to reduce pin counts, and the chip guys look like they will be ready before the VCSELs.”
One company looking to benefit from the emerging market for EDR is Luxtera. The silicon photonics specialist says its modulator has already been demonstrated at 30Gbps. This is fast enough to accommodate EDR, 100 Gigabit Ethernet (a 4-channel design) and the emerging 28Gbps Fibre Channel standard.
“We can already demonstrate in silicon a 30Gbps transmitter using the same laser as in our existing products and modulated in our silicon waveguides,” says Marek Tlalka, vice president of marketing at Luxtera. “That allows us to cover 14Gbps, 26Gbps EDR, parallel Ethernet as well as 28Gbps for serial Fibre Channel.”
Luxtera will need to redesign the transistor circuitry to drive the modulator beyond the current 15Gbps before the design can be brought to market. It will also use an existing silicon modulator design though the company says some optimisation work will be required.
There are two main product offerings from Luxtera: QSFP-based active optical cables and OptoPHY, one and four-channel optical engines. Luxtera’s OptoPHY product is currently being qualified and is not yet in volume production.
For multi-channel designs, Luxtera uses a continuous-wave 1490nm distributed feedback (DFB) laser fed to the modulated channels. Addressing 28Gbps Fibre Channel, an SFP+ form factor will be used. Luxtera may offer a transceiver product or partner with a module maker with Luxtera providing the optical engine. “It’s an open question,” says Tlalka.
“The IBTA has a good track record of releasing products on time; as such, 100Gbps InfiniBand will come out much faster than 100 Gigabit Ethernet.”
Skip Jones, IBTA
The company has said that the single-channel and four-channel 10Gbps OptoPHY engine consumes 450mW and 800mW respectively. Going to 26Gbps will increase the power consumption but only by several tens of percent, it says.
The first product from Luxtera will be a pluggable cable followed by a companion OptoPHY. The pluggable active optical cable from Luxtera will support 100GbE and EDR Infiniband. “I’d still place my bets on InfiniBand deploying first followed by 100GbE,” says Tlalka.
But Schube warns that Luxtera faces a fundamental challenge “Leading-edge designs based on proprietary technology to solve commodity problems - more bandwidth for out-of-the-box connections - are never going to get widely adopted, though Luxtera can fill a niche for awhile," he says.
There is also much work to be done before 100Gbps interfaces will be deployed. “The 4x25G short reach is not going to rise and dominate for quite awhile, no matter what the component availability is,” says Schube. That is because switch ASICs, backplanes, connectors and line cards will all first need to be redesigned.
Meanwhile the IBTA has also announced two future placeholder data rates on its InfiniBand roadmap: High Data Rate (HDR) due in 2014 and the Next Data Rate (NDR) sometime after. “We will refrain from identifying the exact lane speed until we are closer to that timeframe to avoid confusion and the possibility - and probability - of changing future lane speeds,” says Jones.
And Luxtera says its modulator can go faster still. “I think we can easily go 40 and 50Gbps,” says Tlalka. “After 50Gbps we’ll have to look at new magic.”