Data centre interconnect drives coherent
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NeoPhotonics announced at OFC a high-speed modulator and intradyne coherent receiver (ICR) that support an 800-gigabit wavelength
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It also announced limited availability of its nano integrable tunable laser assembly (nano-ITLA) and demonstrated its pico-ITLA, an even more compact silicon photonics-based laser assembly
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The company also showcased a CFP2-DCO pluggable
NeoPhotonics unveiled several coherent optical transmission technologies at the OFC conference and exhibition held in San Diego last month.
“There are two [industry] thrusts going on right now: 400ZR and data centre interconnect pizza boxes going to even higher gigabits per wavelength,” says Ferris Lipscomb, vice president of marketing at NeoPhotonics.
Ferris Lipscomb
The 400ZR is an interoperable 400-gigabit coherent interface developed by the Optical Internetworking Forum (OIF).
Optical module makers are developing 400ZR solutions that fit within the client-side QSFP-DD and OSFP pluggable form factors, first samples of which are expected by year-end.
800-gigabit lambdas
Ciena and Infinera announced in the run-up to OFC their latest coherent systems - the WaveLogic 5 and ICE6, respectively - that will support 800-gigabit wavelengths. NeoPhotonics announced a micro intradyne coherent receiver (micro-ICR) and modulator components that are capable of supporting such 800-gigabit line-rate transmissions.
NeoPhotonics says its micro-ICR and coherent driver modulator are class 50 devices that support symbol rates of 85 to 90 gigabaud required for such a state-of-the-art line rate.
The OIF classification defines categories for devices based on their analogue bandwidth performance. “With class 20, the 3dB bandwidth of the receiver and the modulator is 20GHz,” says Lipscomb. “With tricks of the trade, you can make the symbol rate much higher than the 3dB bandwidth such that class 20 supports 32 gigabaud.” Thirty-two gigabaud is used for 100-gigabit and 200-gigabit coherent transmissions.
Class 50 refers to the highest component performance category where devices have an analogue bandwidth of 50GHz. This equates to a baud rate close to 100 gigabaud, fast enough to achieve data transmission rates exceeding a terabit. “But you have to allow for the overhead the forward-error correction takes, such that the usable data rate is less than the total,” says Lipscomb (see table).
Source: Gazettabyte, NeoPhotonics
Silicon photonics-based COSA
NeoPhotonics also announced a 64-gigabaud silicon photonics-based coherent optical subassembly (COSA). The COSA combines the receiver and modulator in a single package that is small enough to fit within a QSFP-DD or OSFP pluggable for applications such as 400ZR.
Last year, the company announced a similar COSA implemented in indium phosphide. In general, it is easier to do higher speed devices in indium phosphide, says Lipscomb, but while the performance in silicon photonics is not quite as good, it can be made good enough.
“It [silicon photonics] is now stretching certainly into the Class 40 [that supports 600-gigabit wavelengths] and there are indications, in certain circumstances, that you might be able to do it in the Class 50.”
Lipscomb says NeoPhotonics views silicon photonics as one more material that complements its indium phosphide, planar lightwave circuit and gallium arsenide technologies. “Our whole approach is that we use the material platform that is best for a certain application,” says Lipscomb.
In general, coherent products for telecom applications take time to ramp in volumes. “With the advent of data centre interconnect, the volume growth is much greater than it ever has been in the past,” says Lipscomb.
NeoPhotonics’ interested in silicon photonics is due to the manufacturing benefits it brings that help to scale volumes to meet the hyperscalers’ requirements. “Whereas indium phosphide has very good performance, the infrastructure is still limited and you can’t duplicate it overnight,” says Lipscomb. “That is what silicon photonics does, it gives you scale.”
NeoPhotonics also announced the limited availability of its nano integrable tunable laser assembly (nano-ITLA). “This is a version of our external cavity ITLA that has the narrowest line width in the industry,” says Lipscomb.
The nano-ITLA can be used as the source for Class 50, 800-gigabit systems and current Class 40 600 gigabit-per-wavelength systems. It is also small enough to fit within the QDFP-DD and OSFP client-side modules for 400ZR designs. “It is a new compact laser that can be used with all those speeds,” says Lipscomb.
NeoPhotonics also showed a silicon-photonics based pico-ITLA that is even smaller than the nano-ITLA.“The [nano-ITLA’s] optical cavity is now made using silicon photonics so that makes it a silicon photonics laser,” says Lipscomb.
Instead of having to assemble piece parts using silicon photonics, it can be made as one piece. “It means you can integrate that into the same chip you put your modulator and receiver on,” says Lipscomb. “So you can now put all three in a single COSA, what is called the IC-TROSA.” The IC-TROSA refers to an integrated coherent transmit-receive optical subassembly, defined by the OIF, that fits within the QSFP-DD and OSFP.
Despite the data centre interconnect market with its larger volumes and much faster product uptakes, indium phosphide will still be used in many places that require higher optical performance. “But for bulk high-volume applications, there are lots of advantages to silicon photonics,” says Lipscomb.
400ZR and 400ZR+
A key theme at this year’s OFC was the 80km 400ZR. Also of industry interest is the 400ZR+, not an OIF specification but an interface that extends the coherent range to metro distances.
Lipscomb says that the initial market for the 400ZR+ will be smaller than the 400ZR, while the ZR+’s optical performance will depend on how much power is left after the optics is squeezed into a QSFP-DD or OSFP module.
“The next generation of DSP will be required to have a power consumption low enough to do more than ZR distances,” he says. “The further you go, the more work the DSP has to do to eliminate the fibre impairments and therefore the more power it will consume.”
Will not the ZR+ curtail the market opportunity for the 400-gigabit CFP2-DCO that is also aimed at the metro?
“It’s a matter of timing,” says Lipscomb. “The advantage of the 400-gigabit CFP2-DCO is that you can almost do it now, whereas the ZR+ won’t be in volume till the end of 2020 or early 2021.”
Meanwhile, NeoPhotonics demonstrated at the show a CFP2-DCO capable of 100-gigabit and 200-gigabit transmissions.
NeoPhotonics has not detailed the merchant DSP it is using for its CFP2-DCO except to say that it working with ‘multiple ones’. This suggests it is using the merchant coherent DSPs from NEL and Inphi.
NeoPhotonics ups the baud rate for line and client optics
- Neophotonics’ 64 gigabaud optical components are now being designed into optical transmission systems. The components enable up to 600 gigabits per wavelength and 1.2 terabits using a dual-wavelength transponder.
- The company’s high-end transponder that uses Ciena’s WaveLogic Ai coherent digital signal processor (DSP) is now shipping.
- NeoPhotonic is also showcasing its 53 gigabaud components for client-side pluggable optics capable of 100-gigabit wavelengths at the current European Conference on Optical Communication (ECOC) show being held in Rome.
NeoPhotonics says its family of 64 gigabaud (Gbaud) optical components are being incorporated within next-generation optical transmission platforms.
Ferris LipscombThe 64Gbaud components include a micro intradyne coherent receiver (micro-ICR), a micro integrable tunable laser assembly (micro-ITLA) and a coherent driver modulator (CDM).
The micro-ICR and micro-ITLA are the Optical Internetworking Forum’s (OIF) specification, while the CDM is currently being specified.
“Three major customers have selected to use all three [64Gbaud components] and several others are using a subset of those,” says Ferris Lipscomb, vice president of marketing at NeoPhotonics.
NeoPhotonics also unveiled and demonstrated two smaller 64Gbaud component designs at the OFC show held in March. The devices - a coherent optical sub-assembly (COSA) and a nano-ITLA - are aimed at 400-gigabit coherent pluggable modules as well as compact line-card designs.
“These [two compact components] continue to be developed as well,” says Lipscomb.
Baud rate and modulation
The current 100-gigabit coherent transmission uses polarisation-multiplexing, quadrature phase-shift keying (PM-QPSK) modulation operating at 32 gigabaud. The 100 gigabits-per-second (Gbps) data rate is achieved using four bits per symbol and a symbol rate of 32Gbaud.
Optical designers use two approaches to increase the wavelength’s data rate beyond 100Gbps. One approach is to increase the modulation scheme beyond QPSK using 16-ary quadrature amplitude modulation (16-QAM) or 64-QAM, the other is to increase the baud rate.
“The baud rate is the on-off rate as opposed to the bit rate. That is because you are packing more bits in there than the on-off supports,” says Lipscomb. “But if you double the on-off rate, you double the number of bits.”
Doubling the baud rate from 32Gbaud to 64Gbaud achieves just while using 64-QAM trebles the data sent per symbol compared to 100-gigabit PM-QSPK. Combining the two - 64Gbaud and 64-QAM - creates the 600 gigabits per wavelength.
A higher baud rate also has a reach advantage, says Lipscomb, with its lower noise. “For longer distances, increasing the baud rate is better.”
But doubling the baud rate requires more capable DSPs to interpret things at twice the rate. “And such DSPs now exist, operating at 64Gbaud and 64-QAM,” he says.
Three major customers have selected to use all three [64Gbaud components] and several others are using a subset of those
Coherent components
NeoPhotonics’ 64Gbaud optical components are suitable for line cards, fixed-packaged transponders, 1-rack-unit modular platforms used for data centre interconnect and the CFP2 pluggable form factor.
For data centre interconnect using 600-gigabits-per-wavelength transmissions, the distance achieved is up to 100km. For longer distances, the 64Gbaud components achieve metro-regional reaches at 400Gbps, and 2,000km for long-haul at 200Gbps.
But to fit within the most demanding pluggable form factors such as the OSFP and QSFP-DD, smaller componentry is required. This is what the coherent optical sub-assembly (COSA) and nano-ITLA are designed to address. The COSA combines the coherent modular driver and the ICR in a single gold-box package that is no larger than the individual 64Gbaud micro-ICR and CDM packages.
Source: Gazettabyte
“There is a lot of interest in 400-gigabit applications for a CFP2, and in that form factor you can use the separate components,” says Lipscomb. “But for data centre interconnect, you want to increase the density as much as possible so going to the smaller OSPF or QSFP-DD requires another generation of [component] shrinking.”
NeoPhotonics says there are two main approaches. One, and what NeoPhotonics has done with the nano-ITLA and COSA, is to separate the laser from the remaining circuitry such that two components are needed overall. A benefit of a separate laser is also lower noise. “But the ultimate approach would be to put all three in one gold box,” says Lipscomb.
For data centre interconnect, you want to increase the density as much as possible so going to the smaller OSPF or QSFP-DD requires another generation of [component] shrinking
Both approaches are accommodated as part of the OIF’s Integrated Coherent Transmitter-Receiver Optical Sub-Assembly (IC-TROSA) project.
Another challenge to achieving coherent designs such as the emerging 400ZR standard using the OSFP or QSFP-DD is accommodating the DSP with the optics while meeting the modules’ demanding power constraints. This requires a 7nm CMOS DSP and first samples are expected by year-end with limited production occurring towards the end of 2019. Volume production of coherent OSFP and QSFP-DD modules are expected in 2020 or even 2021, says Lipscomb.
100G client-side wavelengths
NeoPhotonics also used the OFC show last March to detail its 53Gbaud components for client-side pluggables that are 100-gigabit single-wavelength and four-wavelength 400-gigabit designs. Samples of these have now been delivered to customers and are part of demonstrations at ECOC this week.
The components include an electro-absorption modulated laser (EML) and driver for the transmitter, and photodetectors and trans-impedance amplifiers for the receiver path. The 53Gbaud EML can operate uncooled, is non-hermetic and is aimed for use with OSFP and QSFP-DD modules.
To achieve a 100-gigabit wavelength, 4-level pulse-amplitude modulation (PAM-4) is used and that requires an advanced DSP. Such PAM-4 DSPs will only be available early next year, says NeoPhotonics.
The first 400-gigabit modules using 100-gigabit wavelengths will gain momentum by the end of 2019 with volume production in 2020, says Lipscomb.
The various 8-wavelength implementations such as the IEEE-defined 2km 400GBASE-FR8 and 10km 400GBASE-LR8 are used when data centre operators must have 400-gigabit client interfaces.
The adoption of 100-gigabit single-wavelength implementations of 400 gigabits, in contrast, will be adopted when it becomes cheaper on a cost-per-bit basis, says Lipscomb: “It [100-gigabit single-wavelength-based modules] will be a general replacement rather than a breaking of bottlenecks.”
NeoPhotonics is also making available its DFB laser technology for silicon-photonics-based modules such as the 2km 400G-FR4, as well as the 100-gigabit single-wavelength DR1 and the parallel-fibre 400-gigabit DR4 standards.
WaveLogic AI transponder
NeoPhotonics has revealed it is shipping its first module using Ciena’s WaveLogic Ai coherent DSP. “We are shipping in modest volumes right now,” says Lipscomb.
The company is one of three module makers, the others being Lumentum and Oclaro, that signed an agreement with Ciena to use of its flagship WaveLogic Ai DSP for their coherent module designs.
Lipscomb describes the market for the module as a niche given its high-end optical performance, what he describes as a fully capable, multi-haul transponder. “It has lots of features and a lot of expense too,” he says. “It is applied to specific cases where long distance is needed; it can go 12,000km if you need it to.”
The agreement with Ciena also includes the option to use future Ciena DSPs. “Nothing is announced yet and so we will have to see how that all plays out.”
OIF moves to raise coherent transmission baud rate
"We want the two projects to look at those trade-offs and look at how we could build the particular components that could support higher individual channel rates,” says Karl Gass of Qorvo and the OIF physical and link layer working group vice chair, optical.
Karl Gass
The OIF members, which include operators, internet content providers, equipment makers, and optical component and chip players, want components that work over a wide bandwidth, says Gass. This will allow the modulator and receiver to be optimised for the new higher baud rate.
“Perhaps I tune it [the modulator] for 40 Gbaud and it works very linearly there, but because of the trade-off I make, it doesn’t work very well anywhere else,” says Gass. “But I’m willing to make the trade-off to get to that speed.” Gass uses 40 Gbaud as an example only, stressing that much work is required before the OIF members choose the next baud rate.
"We want the two projects to look at those trade-offs and look at how we could build the particular components that could support higher individual channel rates”
The modulator and receiver optimisations will also be chosen independent of technology since lithium niobate, indium phosphide and silicon photonics are all used for coherent modulation.
The OIF has not detailed timescales but Gass says projects usually take 18 months to two years.
Meanwhile, the OIF has completed two projects, the specification outputs of which are referred to as implementation agreements (IAs).
One is for integrated dual polarisation micro-intradyne coherent receivers (micro-ICR) for the CFP2. At OFC 2015, several companies detailed first designs for coherent line side optics using the CFP2 module.
The second completed IA is the 4x5-inch second-generation 100 Gig long-haul DWDM transmission module.