Heavy Reading’s take on optical module trends
The industry knows what the next-generation 400-gigabit client-side interfaces will look like but uncertainty remains regarding what form factors to use. So says Simon Stanley who has just authored a report entitled: From 25/100G to 400/600G: A Competitive analysis of Optical Modules and Components.
Implementing the desired 400-gigabit module designs is also technically challenging, presenting 200-gigabit modules with a market opportunity should any slip occur at 400 gigabits.
Simon StanleyStanley, analyst-at-large at Heavy Reading and principal consultant at Earlswood Marketing, points to several notable developments that have taken place in the last year. For 400 gigabits, the first CFP8 modules are now available. There are also numerous suppliers of 100-gigabit QSFP28 modules for the CWDM4 and PSM4 multi-source agreements (MSAs). He also highlights the latest 100-gigabit SFP-DD MSA, and how coherent technology for line-side transmission continues to mature.
Routes to 400 gigabit
The first 400-gigabit modules using the CFP8 form factor support the 2km-reach 400Gbase-FR8 and the 10km 400Gbase-LR8; standards defined by the IEEE 802.3bs 400 Gigabit Ethernet Task Force. The 400-gigabit FR8 and LR8 employ eight 50Gbps wavelengths (in each direction) over a single-mode fibre.
There is significant investment going into the QSFP-DD and OSFP modules
But while the CFP8 is the first main form factor to deliver 400-gigabit interfaces, it is not the form factor of choice for the data centre operators. Rather, interest is centred on two emerging modules: the QSFP-DD that supports double the electrical signal lanes and double the signal rates of the QSFP28, and the octal small form factor pluggable (OSFP) MSA.
“There is significant investment going into the QSFP-DD and OSFP modules,” says Stanley. The OSFP is a fresh design, has a larger power envelope - of the order of 15W compared to the 12W of the QSFP-DD - and has a roadmap that supports 800-gigabit data rates. In contrast, the QSFP-DD is backwards compatible with the QSFP and that has significant advantages.
“Developers of semiconductors and modules are hedging their bets which means they have got to develop for the QSFP-DD, so that is where the bulk of the development work is going,” says Stanley. “But you can put the same electronics and optics in an OSFP.”
Given there is no clear winner, both will likely be deployed for a while. “Will QSFP-DD win out in terms of high-volumes?” says Stanley. “Historically, that says that is what is going to happen.”
The technical challenges facing component and module makers are achieving 100-gigabit-per-wavelength for 400 gigabits and fitting them in a power- and volume-constrained optical module.
The IEEE 400 Gigabit Ethernet Task Force has also defined the 400GBase-DR4 which has an optical interface comprising four single-mode fibres, each carrying 100 gigabits, with a reach up to 500m.
“The big jump for 100 gigabits was getting 25-gigabit components cost-effectively,” says Stanley. “The big challenge for 400 gigabits is getting 100-gigabit-per-wavelength components cost effectively.” This requires optical components that will work at 50 gigabaud coupled with 4-level pulse-amplitude modulation (PAM-4) that encodes two bits per symbol.
That is what gives 200-gigabit modules an opportunity. Instead of 4x50 gigabaud and PAM-4 for 400 gigabits, a 200-gigabit module can use existing 25-gigabit optics and PAM-4. “You get the benefit of 25-gigabit components and a bit of a cost overhead for PAM-4,” says Stanley. “How big that opportunity is depends on how quickly people execute on 400-gigabit modules.”
The first 200-gigabit modules using the QSFP56 form factor are starting to sample now, he says.
100-Gigabit
A key industry challenge at 100 gigabit is meeting demand and this is likely to tax the module suppliers for the rest of this year and next. Manufacturing volumes are increasing, in part because the optical module leaders are installing more capacity and because of the entrance of many, smaller vendors into the marketplace.
End users buying a switch only populate part of the ports due to the up-front costs. More modules are then added as traffic grows. Now, internet content providers turn on entire data centres filled with equipment that is fully populated with modules. “The hyper-scale guys have completely changed the model,” says Stanley.
The 100-gigabit module market has been coming for several years and has finally reached relatively high volumes. Stanley attributes this not just to the volumes needed by the large-scale data centre operators but also the fact that 100-gigabit modules have reached the right price point. Another indicator of the competitive price of 100-gigabit is the speed at which 40-gigabit technology is starting to be phased out.
Developments such as silicon photonics and smart assembly techniques are helping to reduce the cost of 100-gigabit modules, says Stanley, and this will be helped further with the advent of the new SFP-DD MSA.
SFP-DD
The double-density SFP (SFP-DD) MSA was announced in July. It is the next step after the SFP28, similar to the QSFP-DD being an advance on the QSFP28. And just as the 100-gigabit QSFP28 can be used in breakout mode to interface to four 25-gigabit SFP28s, the 400-gigabit QSFP-DD promises to perform a similar breakout role interfacing to SFP-DD modules.
Stanley sees the SFP-DD as a significant development. “Another way to reduce cost apart from silicon photonics and smart assembly is to cut down the number of lasers,” he says. The number of lasers used for 100 gigabits can be halved from four using 28 gigabaud signalling and PAM-4). Existing examples of two-wavelength/ PAM-4 styled 100-gigabit designs are Inphi’s ColorZ module and Luxtera’s CWDM2.
The industry’s embrace of PAM-4 is another notable development of the last year. The debate about the merits of using 56-gigabit symbol rate and non-return-to-zero signalling versus PAM-4 with its need for forward-error correction and extra latency has largely disappeared, he says.
The first 400-gigabit QSFP-DD and OSFP client-side modules are expected in a year’s time with volumes starting at the end of 2018 and into 2019
Coming of age
Stanley describes the coherent technology used for line-side transmissions as coming of age. Systems vendors have put much store in owning the technology to enable differentiation but that is now changing. To the well-known merchant coherent digital signal processing (DSP) players, NTT Electronics (NEL) and Inphi, can now be added Ciena which has made its WaveLogic Ai coherent DSP available to three optical module partners, Lumentum, NeoPhotonics and Oclaro.
CFP2-DCO module designs, where the DSP is integrated within the CFP2 module, are starting to appear. These support 100-gigabit and 200-gigabit line rates for metro and data centre interconnect applications. Meanwhile, the DSP suppliers are working on coherent chips supporting 400 gigabits.
Stanley says the CFP8 and OSFP modules are the candidates for future pluggable coherent module designs.
Meanwhile, the first 400-gigabit QSFP-DD and OSFP client-side modules are expected in a year’s time with volumes starting at the end of 2018 and into 2019.
As for 800-gigabit modules, that is unlikely before 2022.
“At OFC in March, a big data centre player said it wanted 800 Gigabit Ethernet modules by 2020, but it is always a question of when you want it and when you are going to get it,” says Stanley.
Acacia looks to co-package its coherent PIC and DSP-ASIC
- Acacia Communications is working to co-package its coherent DSP and its silicon photonics transceiver chip.
- The company is also developing a digital coherent optics module that will support 400 gigabit.
Acacia Communications is working to co-package its coherent DSP and its silicon photonics transceiver chip. The line-side optical transceiver company is working on a digital coherent optics module that will support 400 gigabits.
Acacia announced last November that it was sampling the industry’s first CFP2 Digital Coherent Optics (CFP2-DCO) that supports 100- and 200-gigabit line rates. The CFP2-DCO integrates the DSP and its silicon photonics chip within a CFP2 module, which is half the size of a CFP module, with each chip packaged separately.
The CFP2-DCO adds to the company’s CFP2-ACO design that was announced a year ago. In the CFP2-ACO, the CFP2 module contains just the optics with the DSP-ASIC chip on the same line card connected to the module via a special high-speed interface connector.
Now, Acacia is working to co-package the two chips, which will not only improve the performance of its CFP2-DCO but also enable new, higher-performance optical modules such as a 400-gigabit DCO. The Optical Internetworking Forum announced a new implementation agreement last December for an interoperable 400-gigabit ZR (80km) coherent interface.
Both [the DSP and silicon photonics chip] are based on CMOS processes. The next step for Acacia is to bring them into a single package.
Portfolio upgrades
Acacia has also upgraded its existing portfolio of coherent transceivers. The company has integrated the enhanced silicon photonics coherent transceiver in its AC100-CFP and its AC-400 5x7-inch modules.
The silicon-photonics transceiver achieves a more efficient coupling of light in and out of the chip and uses an improved modulator driver design that reduces the overall power consumption. The design also supports flexible grid, enabling channel sizes of 37.5GHz in addition to fixed-grid 50GHz channels.
The resulting AC100-CFP module has a greater reach of 2,500km and a lower power consumption than the first generation design announced in 2014. The enhanced PIC has also been integrated within the AC-400. The AC-400, announced in 2015, integrates two silicon photonics chips to support line rates of 200, 300 and 400 gigabits.
CFP2-DCO
Acacia is using the coherent transceiver photonic integrated circuit (PIC), first used in its CFP2-ACO, alongside a new coherent DSP to integrate the optics and DSP within the compact CFP2.
“The third-generation PIC is a mini PIC; in a gold box that is about the size of a dime, which is a third of the size of our original PIC,” says Benny Mikkelsen, founder and CTO of Acacia.
One design challenge with its latest DSP was retaining the reach of the original DSP used in the AC100-CFP while lowering its power consumption. Having an inherently low-power coherent DSP design in the first place is one important factor. Mikkelsen says this is achieved based on several factors such as the DSP algorithms chosen and how they are implemented in hardware, the clock frequencies used within the chip, how the internal busses are implemented, and the choice of bits-per-symbol used for the processing.
The resulting DSP’s power consumption can be further reduced by using an advanced CMOS process. Acacia uses a 16nm CMOS process for its latest DSP.
Other challenges to enable a CFP2-DCO module include reducing the power consumption of the optics and reducing the packaging size. “The modulator driver is the piece part that consumes the most power on the optics side,” says Mikkelsen.
Acacia's CFP2-DCO supports polarisation multiplexing, quadrature phase-shift keying (PM-QPSK) for 100 gigabits, and two modulation schemes: polarisation multiplexing, 8-ary quadrature amplitude multiplexing (PM-8QAM) and 16-ary QAM - for 200-gigabit line rates. In contrast, its -ACO supports just PM-QPSK and PM-16QAM.
At 100 gigabits, the DSP consumes about half the power of the Sky DSP used in the original AC100. Using PM-8QAM for 200 gigabits means the new DSP and optics support a higher baud rate - some 45 gigabaud compared to the traditional 32-35 gigabaud used for 100 and 200-gigabit transmission. However, while this increases the power consumption, the benefit of 8QAM is a 200-gigabit reach beyond 1,000km.
Mikkelsen stresses that a key reason the company can achieve a CFP2-DCO design is having both technologies in-house: “You can co-optimise the DSP and the silicon photonics”.
We think, at least in the near term, that the OSPF module seems to be a good form factor to work on
ACO versus DCO
Since Acacia now offers both the CFP2-ACO and CFP2-DCO modules, it is less concerned about how the relative demand for the two modules develops. “We don’t care too much which one is going to have the majority of the market,” says Mikkelsen. That said, Acacia believes that the CFP2-DCO market will become the larger of the two.
When the CFP2-ACO was first considered several years ago, the systems vendors and optical module makers shared a common interest. Systems vendors wanted to use their custom coherent DSP-ASICs while the -ACO module allowed component makers that didn't have the resources to develop their own DSP to address the market with their optics. It was also necessary to separate the DSP and the optics if the smaller CFP2 form factor was to be used.
But bringing CFP2-ACOs to volume production has proved more difficult than first envisaged. The CFP2-DCO is far easier to use, says Mikkelsen. The module can be plugged straight into equipment whereas the CFP2-ACO must be calibrated by a skilled optical engineer when a wavelength is first turned up.
Future work
Acacia is now looking at new module form factors and new packaging technologies. “Both [the DSP and silicon photonics chip] are based on CMOS processes,” says Mikkelsen. “The next step for Acacia is to bring them into a single package.”
In addition to the smaller size, a single package promises a slightly lower power consumption as well as manufacturing cost advantages. “We also expect to see higher performance once the DSP and optics are sitting next to each other which we believe will improve signal integrity between the two,” says Mikkelsen.
Acacia is not waiting for any industry challenges to be overcome for a single-package design to be achieved. The company points out that its silicon photonics chip is not temperature sensitive, aiding its co-packaging with the DSP.
Acacia is working on a 400-gigabit DCO design and is looking at several potential module types. The company is a member of the OSFP module MSA as well as the Consortium of On-Board Optics (COBO) which has started a coherent working group. “We think, at least in the near term, that the OSPF module seems to be a good form factor to work on,” says Mikkelsen.