Ciena's multi-format 400G coherent QSFP-DD pluggable
Ciena showcased a working 400-gigabit Universal coherent pluggable module at the ECOC 2022 conference and exhibition in Basel, Switzerland.
Ciena is using its WaveLogic 5 Nano coherent digital signal processor (DSP) for the Universal QSFP-DD coherent pluggable module.
“We call it universal because it supports many transmission modes – interoperable and high performance; the most in the industry,” says Helen Xenos, senior director of portfolio marketing at Ciena.
The pluggable has custom extended-performance modes and supports three industry formats: the 400ZR interoperable standard, the 400ZR+ multi-source agreement (MSA), and the OpenROADM MSA. (See tables below).
IP over DWDM
Communications service providers (CSPs) want to add pluggable coherent modules to their IP routers, removing the need for a separate transponder card or box linking the router to the optical line system.
The advent of coherent QSFP-DD pluggables has meant the same form factor can be used for client-side and line-side optics, ensuring efficient use of the router ports.
The CSPs want the coherent QSFP-DD module to have sufficient optical performance to meet their demanding networking requirements. For example, the module’s output signal can pass through the filtering stages of reconfigurable optical add-drop multiplexers (ROADMs) along the optical link.
Optical amplification and filtering
Ciena’s coherent QSFP-DD adds a fibre-based optical amplifier and a tunable optical filter to the coherent photonics and electronic ICs.
The optical amplification enables the high-performance mode and the launching of a 4dBm output signal. In contrast, 400ZR and 400ZR+ have a launch power of -10dBm.
“This is the industry’s highest [QSDP-DD] transmit power,” says Xenos.
The tunable optical filter improves the optical performance of the coherent receiver.
In an optical line system with colourless ROADMs, the Erbium-doped fibre amplifiers (EDFAs) generate out-of-band transmission noise – amplified spontaneous emission (ASE). The noise sources superimpose and become significant, impairing wavelength and system performance dramatically.
The tunable optical filter eliminates this effect and simplifies deployment over any photonics line system. In addition, Ciena says the pluggables can now work alongside high-baud rate transponders in existing ROADM applications.
The QSFP-DD’s tunable optical filter means its optical performance closely matches that of the CFP2-DCO, aiding the two classes of pluggables working together.
Modes of operation
400ZR defines the module’s baseline coherent performance. The OIF developed the 400ZR standard so hyperscalers can link their equipment in two separate data centres up to 120km apart.
The 400ZR specification delivers just enough optical performance to meet the optical link budget. The OIF produced a low-cost, interoperable, pluggable coherent specification.
400ZR supports a single baud rate – 60 gigabaud (GBd), and modulation scheme – dual-polarisation 16-QAM, and carries Ethernet frames.
Google, Meta, Microsoft and Alibaba were all involved in the OIF development, with the 400ZR Implementation Agreement published in early 2020.
400ZR supports two-channel widths: 75GHz and 100GHz, while the forward error correction scheme used is CFEC.
The 400ZR+ MSA enhances the performance by supporting other data rates – 100, 200 and 300 gigabits-per-second (Gbps) – as well as 400Gbps. In addition, it uses several modulation schemes and the enhanced O-FEC error correction scheme that extends reach.
Ciena’s module also meets the OpenROADM MSA, supporting Ethernet and OTN and an enhanced reach at 400Gbps.
Ciena’s Universal module’s extended performance modes up the symbol rate to 65 and 70 gigabaud (GBd) and uses probabilistic constellation shaping (PCS).
PCS maps the bitstream onto the constellation to maximise the data recovery at the coherent receiver, thereby improving overall optical performance. The scheme also allows the fine-tuning of the data rate sent.
At ECOC, Ciena showed the module implementing the high-performance mode at 70GBd and PCS.
ECOC innovation award
The ECOC Exhibition Market Focus Advisory Committee awarded the most innovative product award to Ciena’s WaveLogic 5 Nano 400G Universal QSFP-DD.
Effect Photonics buys the coherent DSP team of Viasat
Effect Photonics has completed the acquisition of Viasat’s staff specialising in coherent digital signal processing and forward error correction (FEC) technologies and the associated intellectual property.
The company also announced a deal with Jabil Photonics – a business unit of manufacturing services firm Jabil – to co-develop coherent optical modules that the two companies will sell.
The deals enable Effect Photonics to combine Viasat’s coherent IP with its indium phosphide laser and photonic integrated circuit (PIC) expertise to build coherent optical designs and bring them to market.
Strategy
Harald Graber, chief commercial officer at Effect Photonics, says the company chose to target the coherent market after an internal strategic review about how best to use its PIC technology.
The company’s goal is to make coherent technology as affordable as possible to address existing and emerging markets.
“We have a kind of semiconductor play,” says Graber. By which he means high-volume manufacturing to make the technology accessible.
“When you go to low cost, you cannot depend 100 per cent on buying the coherent digital signal processor (DSP) from the merchant market,” he says. “So the idea was relatively early-born that somehow we had to address this topic.”
This led to talks with Viasat and the acquisition of its team and technology.
Markets
“We also saw, as with some of our competitors, that making modules for satellite or free-space optics has a natural harmony for the roadmaps,” says Graber.
Effect Photonics and Jabil Photonics will bring to market an advanced, low-power coherent module design based on the QSFP-DD form factor.
Graber says 400ZR+ coherent modules fall short in their output power which is noticeable for networks with multiple reconfigurable optical add/drop multiplexing (ROADM) stages.
“So you need a little more [output power], and our technology allows us to do more,” he says.
By owning a coherent DSP and PIC, the company can integrate closely the two to optimise the coherent engine’s optical performance.
“You have a lot of room for improvement, which you cannot do when you buy a merchant DSP, especially when we talk about a 1.6 terabit design and above,” says Graber. “Our optical machine is already fully integrated, including the laser. It’s just now this last piece part to alleviate the current industry barriers.”
Effect Photonics’ focus is the communications sector. “We are putting everything in place to serve the hyperscalers,” says Graber.
The company is also looking at satellite communications and free-space optics.
Effect Photonics is working with Aircision, a company developing a free-space optics system that can send 10 gigabit-per-second (Gbps) over a 5km link for mobile backhaul and broadband applications.
Having all the parts for coherent designs will enable the company to address other markets like quantum key distribution (QKD) and lidar.
“The main problem with QKD is you cannot use amplification,” says Graber. “You need to have something fully integrated, with a nice output power to achieve the links.”
Graber says that for QKD, the company will only have to tweak its chip.
“We just have to make sure that the internal noise is in the right levels and these kinds of things,” says Graber. “So there’s a lot of opportunities; it puts us in a nice position.”
Company
Effect Photonics is headquartered in The Netherlands and has offices in four countries.
Last year, the company raised $43M in Series-C funding. The company raised a further $20 million with the Viasat deal.
The company has 250 staff, split between engineering and a large manufacturing facility.
OFC highlights a burgeoning coherent pluggable market
A trend evident at the OFC show earlier this month was the growing variety of coherent pluggable modules on display.
Whereas a coherent module maker would offer a product based on a coherent digital signal processor (DSP) and a basic design and then add a few minor tweaks, now the variety of modules offered reflects the growing needs of the network operators.
Acacia, part of Cisco, announced two coherent pluggable to coincide with OFC. The Bright 400ZR+ QSFP-DD pluggable form factor is based on Acacia’s existing 400ZR+ offering. It has a higher transmit power of up to 5dBm and includes a tunable filter to improve the optical signal-to-noise ratio (OSNR) performance.
Acacia’s second coherent module is the fixed wavelength 400-gigabit 400G ER1 module designed for point-to-point applications.
“I can understand it being a little bit confusing,” says Tom Williams, vice president of marketing at Acacia. “We have maybe five or six configurations of modules based on the same underlying DSP and optical technology.”
Bright 400ZR+
The Bright 400ZR+ pluggable addresses a range of network architectures using the high-density QSFP-DD form factor, says Williams.
“Before you had to use the [larger] CFP2-DCO module, now we are bringing some of the functionality into the -DD,” he says. “The Bright 400ZR+ doesn’t replace the CFP2-DCO but it does move us closer to that.” As such, the module also supports OTN framing.
The Bright 400ZR+ has a higher launch power than the optical specification of the OpenZR+ standard but supports the same protocol so it can operate with OpenZR+ compliant pluggables.
The module uses internal optical amplification to achieve the 5dB launch power. The higher launch power is designed for various architectures and ROADM configurations.
“It is not that it allows a certain greater reach so much as the module can address a wider range of applications,” says Williams. “When you talk about colourless, directionless or colourless-directionless-contentionless (CDC-) reconfigurable optical add-drop multiplexing (ROADM) architectures, these are the types of applications this opens up.”
The integrated tunable filter tackles noise. In colourless ROADM-based networks, because the optical multiplexing occurs without filtering, the broadband out-of-band noise can raise the overall noise floor. This then decreases the overall OSNR. Amplification also increases the noise floor.
The tunable filter is used to knock down the overall noise floor, thereby improving the transmit OSNR.
The output power of the Bright 400ZR+ is configurable. The 5dBm launch power is used for ROADMs with array-waveguide gratings while for colourless multiplexing the tunable filter is used, reducing the output power to just above 1dBm.
“You are seeing an anchoring of interoperability that operators can use and then you are seeing people build on top of that with enhancements that add value and expand the use cases,” says Williams.
400 gigabits over 40km
As part of the OIF industry organisation’s work that defined the 400ZR specification, a 40km point-to-point unamplified link was also included. Acacia’s 400G ER1 is such an implementation with the ‘ER’ referring to extended reach, which IEEE defines as 40km.
“At every data rate there has always been an application for these ER reaches in access and enterprise,” says Williams. “The link is just a fibre, it’s like the 10km LR specification, but this goes over 40km.”
The ER1 has been designed to reduce cost and uses a fixed laser. ”We are not doing OSNR testing, it is based on a power-limited 40km link,” says Williams.
The OIF standard uses concatenated forward-error correction (CFEC) while Acacia employs its openFEC (oFEC) that enhances the reach somewhat.
Shipment updates
Acacia also reported a significant ramp in the shipment of its pluggables that use its Greylock coherent DSP.
It has shipped over 50,000 such pluggables, 20,000 alone shipped in Cisco’s last (second) fiscal quarter. “This is being driven by the expected early adopters of 400ZR, as well as a range of other applications,” says Williams.
Acacia says it has also shipped over 100,000 Pico DSP ports. Each AC1200 multi-haul module has two such ports.
The AC1200 sends up to 1.2 terabits over two wavelengths using Acacia’s 7nm CMOS Pico DSP. The multi-haul module is being used in over 100 networks while three of the four largest hyperscalers use the technology.
Acacia also demonstrated at OFC its latest multi-haul module announced last year, a 1.2 terabits single-wavelength design that uses its latest 5nm CMOS Jannu DSP and which operates at a symbol rate of up to 140 gigabaud.
Acacia says samples of its latest multi-haul module that uses its own Coherent Interconnect Module 8 (CIM 8) form factor will be available this year while general availability will be in 2023.
Post-deadline
Williams also presented a post-deadline paper at OFC.
The work outlined was the demonstration of the optical transmission of 400 Gigabit Ethernet flows over a 927km link. The trial comprised transmission through several networks and showed the interoperability of 400-gigabit QSFP-DD and CFP2 modules.
The work involved Orange Labs, Lumentum, Neophotonics, EXFO and Acacia.
Huawei sets transmission record with new modulator
Coherent discourse: Part 1
A paper from Huawei and Sun Yat-Sen University in the January issue of the Optica journal describes a thin-film lithium niobate modulator. The modulator enabled a world-record coherent optical transmission, sending nearly 2 terabits of data over a single wavelength.
Much of the industry’s focus in recent years has been to fit coherent optical technology within a pluggable module.
Such pluggables allow 400-gigabit coherent interfaces to be added to IP routers and switches, serving the needs of the data centre operators and telecom operators.
But research labs of the leading optical transport vendors continue to advance high-end coherent systems beyond 800-gigabit-per-wavelength transmissions.
Optical transport systems from Ciena, Infinera and Huawei can send 800-gigabit wavelengths using a symbol rate of 96-100 gigabaud (GBd).
Acacia Communications, part of Cisco, detailed late last year the first 1.2-terabit single-wavelength coherent pluggable transceiver that will operate at 140GBd, twice the symbol rate of 400-gigabit modules such as 400ZR.
Now Huawei has demonstrated in the lab a thin-film lithium niobate modulator that supports a symbol rate of 220GBd and beyond.
Maxim Kuschnerov, director of the optical and quantum communications laboratory at Huawei, says the modulator has a 110GHz 3dB bandwidth but that it can be operated at higher frequencies, suggesting a symbol rate as high as 240GBd.
Thin-film lithium niobate modulator
Huawei says research is taking place into new materials besides the established materials of indium phosphide and silicon photonics. “It is a very exciting topic lately,” says Kuschnerov.
He views the demonstrated thin-film lithium niobate optical modulator as disruptive: “It can cover up several deficiencies of today’s modulators.”
Besides the substantial increase in bandwidth – the objective of any new coherent technology – the modulator has performance metrics that benefit the coherent system such as a low driving voltage and low insertion loss.
A driving voltage of a modulator is a key performance parameter. For the modulator, it is sub-1V.
The signal driving the modulator comes from a digital-to-analogue (D/A) converter, part of the coherent digital signal processor (DSP). The D/A output is fed into a modulator driver. “That [driver] requires power, footprint, and increases the complexity of integrating the [modem’s] modules tighter,” says Kuschnerov.
The modulator’s sub-1V drive voltage is sufficiently small that the DSP’s CMOS-based D/A can drive it directly, removing the modulator driver circuit that also has bandwidth performance limitations. The modulator thus reduces the transmitter’s overall cost.
The low-loss modulator also improves the overall optical link budget. And for certain applications, it could even make the difference as to whether optical amplification is needed.
“The modulator checks the box of very high bandwidth,” says Kuschnerov. “And it helps by not having to add a semiconductor optical amplifier for some applications, nor needing a driver amplifier.”
One issue with the thin-film modulator is its relative size. While not large – it has a length of 23.5mm – it is larger than indium phosphide and silicon photonics modulators.
1.96-terabit wavelength
Huawei’s lab set-up used a transmit coherent DSP with D/As operating at 130 Giga-samples-per-second (GS/s) to drive the modulator. The modulation used was a 400-quadrature amplitude modulation (400-QAM) constellation coupled with probabilistic constellation shaping.
A 10 per cent forward error correction scheme was used such that, overall, 1.96-terabits per second of data was sent using a single wavelength.
The D/A converter was implemented in silicon germanium using high-end lab equipment to generate the signal at 130GS/s.
“This experiment shows how much we still need to go,” says Kuschnerov. “What we have done at 130GBd shows there is a clear limitation with the D/A [compared to the 220GBd modulator].”
Baud-rate benefits
Increasing the baud rate of systems is not the only approach but is the favoured implementation choice.
What customers want is more capacity and reducing the cost per bit for the same power consumption. Increasing the baud rate decreases the cost and power consumption of the optical transceiver.
By doubling the baud rate, an optical transceiver delivers twice the capacity for a given modulation scheme. The cost per bit of the transceiver decreases as does the power consumed per bit. Instead of two transceivers and two sets of components, one transceiver and one set are used instead.
But doubling the baud rate doesn’t improve the optical system’s spectral efficiency since doubling the baud rate doubles the channel width. That said, algorithmic enhancements are added to each new generation of coherent modem but technically, the spectral efficiency practically no longer improves.
Huawei acknowledges that while the modulator promises many benefits, all the coherent modem’s components – the coherent ASIC, the D/A and analogue-to-digital (D/A) converters, the optics, and the analogue circuitry – must equally scale. This represents a significant challenge.
Kuschnerov says optical research is finding disruptive answers but scaling performance, especially on the electrical side, remains a critical issue. “How do you increase the D/A sampling rates to match these kinds of modulator technologies?” he says. “It is not straightforward.”
The same is true for the other electrical components: the driver technologies and the trans-impedance amplifier circuits at the receiver.
Another issue is combining the electrical and optical components into a working system. Doubling the signalling of today’s optical systems is a huge radio frequency design and packaging challenge.
But the industry consensus is that with newer CMOS processes and development in components and materials, doubling the symbol rate again to 240GB will be possible.
But companies don’t know – at least they are not saying – what the upper symbol rate limit will be. The consensus is that increasing the baud rate will end. Then, other approaches will be pursued.
Kuschnerov notes that if a 1.6-terabit transceiver could be implemented using a single wavelength or with eight 200Gbps ones with the same spectral performance, cost, footprint and power consumption, end users wouldn’t care which of the two were used.
However, does optics enable such greater parallelism?
Kuschnerov says that while decades of investment has gone into silicon photonics, it is still not there yet.
“It doesn’t have the cost-effectiveness at 16, 32 or 64 lanes because the yield goes down significantly,” he says. “We as an industry can’t do it yet.”
He is confident that, soon enough, the industry will figure out how to scale the optics: “With each generation, we are getting better at it.”
Coherent engineers will then have more design options to meet the system objectives.
And just like with microprocessors, it will no longer be upping the clock frequency but rather adopting parallel processing i.e. multiple cores. Except, in this case, it will be parallel coherent optics.
Nokia adds 400G coherent modules across its platforms
Nokia is now shipping its 400-gigabit coherent multi-haul CFP2-DCO. The module exceeds the optical performance of 400ZR and ZR+ coherent pluggables.
Nokia’s CFP2-DCO product follows its acquisition of silicon photonics specialist, Elenion Technologies, in 2020.
Nokia has combined Elenion’s coherent optical modulator and receiver with its low-power 64-gigabaud (GBd) PSE-Vc coherent digital signal processor (DSP).
Nokia is also adding coherent pluggables across its platform portfolio.
“Not just optical transport and transponder platforms but also our IP routing portfolio as well,” says Serge Melle, director of product marketing, IP-optical networking at Nokia.
“This [amplifier and filter] allows for much better optical performance,”
“This [amplifier and filter] allows for much better optical performance,”
Melle is an optical networking industry veteran. He joined Nokia two years ago after a 15-year career at Infinera. Melle started at Pirelli in 1995 when it was developing a 4×2.5-gigabit wavelength-division multiplexing (WDM) system. In between Pirelli and Infinera, Melle was at Nortel Networks during the optical boom.
400ZR, ZR+ and the multi-haul CFP2-DCO
The CFP2-DCO’s optical performance exceeds that of the QSFP-DD and OSFP form factors implementing 400ZR and ZR+ but is inferior to line-card coherent transponders used for the most demanding optical transport applications.
The 400ZR coherent OIF standard transmits a 400-gigabit wavelength up to 120km linking equipment across data centres. Being a standard, 400ZR modules are interoperable.
The ZR+ adds additional transmission speeds – 100, 200 and 300-gigabits – and has a greater reach than ZR. ZR+ is not a standard but there is the OpenZR+ multi-source agreement (MSA).
Implementing 400ZR and ZR+ coherent modules in a QSFP-DD or OSFP module means they can be inserted in client-side optics’ ports on switches and routers.
The OIF did not specify a form factor as part of the 400ZR standard, says Melle, with the industry choosing the QSFP-DD and OSFP. But with the modules’ limited power dissipation, certain modes of the coherent DSP are turned off, curtailing the feature set and the reach compared to a CFP2-DCO module.
The modules also have physical size restrictions.
“You don’t have enough thermal budget to put an optical amplifier inside the QSFP-DD package,” says Melle. “So you are left with whatever power the DWDM laser outputs through the modulator.” This is -7dBm to -10dBm for 400ZR and ZR+ optics, he says.
The CFP2-DCO is larger such that the DSP modes of encryption, OTN client encapsulation, LLDP snooping (used to gather data about attached equipment), and remote network monitoring (RMON) can be enabled.
The CFP2-DCO can also house an optical amplifier and tunable filter. The filter reduces the out-of-band optical signal-to-noise ratio (OSNR) thereby increasing the module’s sensitivity. “This [amplifier and filter] allows for much better optical performance,” says Melle. A 400-gigabit multi-haul module has a 0dBm optical output power, typically.
The different transceiver types are shown in the table.
Nokia’s paper at the recent OFC virtual conference and exhibition detailed how its 400-gigabit multi-haul CFP2-DCO achieved a reach of 1,200km.
The paper details the transmission of 52, 400-gigabit signals, each occupying a 75GHz channel, for a total capacity of 20.8 terabits-per-second (Tbps).
Melle stresses that the demonstration was more a lab set-up than a live network where a signal goes through multiple reconfigurable optical add-drop multiplexers (ROADMs) and where amplifier stages may not be equally spaced.
That said, the CFP2-DCO’s reach in such networks is 750km, says Nokia.
IP-optical integration
Having coherent pluggables enables 400 Gigabit Ethernet (400GbE) payloads to be sent between routers over a wide area network, says Nokia.
“Given this convergence in form factor, with the QSFP-DD and ZR/ ZR+, you can now do IP-optical integration, putting coherent optics on the router without sacrificing port density or having locked-in ports,” says Melle.
Nokia is upgrading its IP and optical portfolio with coherent pluggables.
“In the routers, ZR/ ZR+, and in transponders not only the high-performance coherent optics – the [Nokia] PSE-Vs [DSP] – but also the CFP2-DCO multi-haul,” says Melle. “The 400-gigabit multi-haul is also going to be supported in our routers.”
Accordingly, Nokia has developed two sets of input-output (I/O) router cards: one supporting QSFP-DDs suited for metro-access applications, and the second using CFP2-DCO ports for metro and regional networks.
The choice of cards adds flexibility for network operators; they no longer need to have fixed CFP2-DCO slots on their router faceplates, whether they are used or not. But being physically larger, there are fewer CFP2-DCO ports than QSFP-DD ports on the I/O cards.
While the QSFP-DD MSA initially defined the module with a maximum power dissipation of 14.5W, a coherent QSFP-DD module consumes 18-20W. Dissipating the heat generated by the modules is a challenge.
Nokia’s airflow cooling is simplified by placing a module on both sides of the line card rather than stacking two CFP2-DCOs, one on top of the other.
Nokia is adding its CFP2-DCO to its 1830 optical transport portfolio. These include its PSI-M compact modular systems, the PSS transponder systems and also its PSS-x OTN switching systems.
The 400ZR/ZR+ module will be introduced with all its routing platforms this summer – the 7250 IXR, 7750 SR, 7750 SR-s, and the 7950 XRS, whereas the CFP2-DCO will be added to its 7750 and 7950 series later this year.
Nokia will source the 400ZR/ZR+ from third parties as well as from its optical networks division.
Its routers use QSFP-DD form-factor for all 400GbE ports and this is consistent for most router vendors in the industry. “Thus, our use and supply of 400ZR/ZR+ pluggable DCOs will focus on the QSFP-DD form-factor,” says Melle. However, the company says it can offer the OSFP form-factor depending on demand.
Network planning study
Nokia published a paper at OFC on the ideal coherent solution for different applications.
For metro aggregation rings with 4-5 nodes and several ROADM pass-throughs, using ZR+ modules is sufficient. Moreover, using the ZR+ avoids any loss in router port density.
For metro-regional core applications, the ZR+’s optical performance is mostly insufficient. Here, the full 400-gigabit rate can not be used but rather 300 gigabit-per-second (Gbps) or even 200Gbps to meet the reach requirements.
Using a 400-gigabit multi-haul pluggable on a router might not match the density of the QSFP-DD but it enables a full 400-gigabit line rate.
For long-haul, the CFP2-DCO’s performance is “reasonable”, says Nokia, and this is where high-performance transponders are used.
What the OFC paper argues is that there is no one-size-fits-all solution, says Melle.
800-Gigabit coherent pluggables
Traditionally, the IEEE has defined short-reach client-side optics while the OIF defines coherent standards.
“If we want this IP-optical convergence continuing in the next generation of optics, those two worlds are going to have to collaborate more closely,” says Melle.
That’s because when a form-factor MSA will be defined, it will need to accommodate the short-reach requirements and the coherent optics. If this doesn’t happen, says Melle, there is a risk of a new split occuring around the IP and optical worlds.
The next generation of coherent pluggables will also be challenging.
All the vendors got together in 2019 and said that 400ZR was just around the corner yet the modules are only appearing now, says Melle.
The next jump in pluggable coherent optics will use a symbol rate of 90-130GBd.
“That is very much the cutting-edge so it brings back the optics as a critical enabling technology, and not just optics but the packaging,” concludes Melle.
100-gigabaud optics usher in the era of terabit transmissions
Telecom operators are in a continual battle to improve the economics of their optical transport networks to keep pace with the relentless growth of IP traffic.
One approach is to increase the symbol rate used for optical transmission. By operating at a higher baud rate, more data can be carried on an optical wavelength.
Ferris Lipscomb
Alternatively, a higher baud rate allows a simpler modulation scheme to be used, sending the same amount of data over greater distances. That is because the fewer constellation points of the simpler modulation scheme help data recovery at the receiver.
NeoPhotonics has detailed two optical components - a coherent driver-modulator and an intradyne coherent receiver (micro-ICR) - that operate at over 100 gigabaud (GBd). The symbol rate suits 800-gigabit systems and can enable one-terabit transmissions.
NeoPhotonics’ coherent devices were announced to coincide with the ECOC 2020 show.
Class 60 components
The OIF has a classification scheme for coherent optical components based on their analogue bandwidth performance.
A Class 20 receiver, for example, has a 3-decibel (dB) bandwidth of 20GHz. NeoPhotonics announced at the OFC 2019 show Class 50 devices with a 50GHz 3dB bandwidth. The Class 50 modulator and receiver devices are now deployed in 800-gigabit coherent systems.
NeoPhotonics stresses the classes are not the only possible operating points. “It is possible to use baud rates in between these standard numbers,” says Ferris Lipscomb, vice president of marketing at NeoPhotonics. “These classes are shorthand for a range of possible baud rates.”
“To get to 96 gigabaud, you have to be a little bit above 50GHz, typically a 55GHz 3dB bandwidth,” says Lipscomb. “With Class 60, you can go to 100 gigabaud and approach a terabit.”
It is unclear whether one-terabit coherent transponders will be widely used. Instead, Class 60 devices will likely be the mainstay for transmissions up to 800 gigabits, he says.
Source: NeoPhotonics, Gazettabyte
Design improvements
Several aspects of the components are enhanced to achieve Class-60 performance.
At the receiver, the photodetector’s bandwidth needs to be enhanced, as does that of the trans-impedance amplifier (TIA) used to boost the received signals before digitisation. In turn, the modulator driver must also be able to operate at a higher symbol rate.
“This is mainly analogue circuit design,” says Lipscomb. “You have to have a detector that will respond at those speeds so that means it can’t be a very big area; you can’t have much capacitance in the device.”
Similarly, the silicon germanium drivers and TIAs, to work at those speeds, must also keep the capacitance down given that the 3dB bandwidth is inversely proportional to the capacitance.
Systems vendors Ciena, Infinera, and Huawei all have platforms supporting 800-gigabit wavelengths while Nokia‘s latest PSE-Vs coherent digital signal processor (DSP) supports up to 600 gigabit-per-wavelength.
Next-generation symbol rate
The next jump in symbol rate will be in the 120+ gigabaud range, enabling 1.2-terabit transmissions.
“As you push the baud rate higher, you have to increase the channel spacing,” says Lipscomb. “Channels can’t be arbitrary if you want to have any backward compatibility.”
A 50GHz channel is used for 100- and 200-gigabit transmissions at 32GBd. Doubling the symbol rate to 64GBd requires a 75GHz channel while a 100GBd Class 60 design occupies a 100GHZ channel. For 128GBd, a 150GHz channel will be needed. “For 1.2 terabit, this spacing matches well with 75GHz channels,” says Lipscomb.
It remains unclear when 128GBd systems will be trialled but Lipscomb expects it will be 2022, with deployments in 2023.
Upping the baud rate enhances the reach and reduces channel count but it does not improve spectral efficiency. “You don’t start getting more data down a fibre,” says Lipscomb.
To boost transport capacity, a fibre’s C-band can be extended to span 6THz, dubbed the C++ band, adding up to 50 per cent more capacity. The L-band can also be used and that too can be extended. But two sets of optics and optical amplification are required when the C and L bands are used.
400ZR and OpenZR+
Lipscomb says the first 400ZR coherent pluggable deployments that link data centres up to 120km apart will start next year. The OIF 400ZR coherent standard is implemented using QSFP-DD or OSFP client-side pluggable modules.
“There is also an effort to standardise around OpenZR+ that has a little bit more robust definition and that may be 2022 before it is deployed,” says Lipscomb.
NeoPhotonics is a contributor member to the OpenZR+ industry initiative that extends optical performance beyond 400ZR’s 120km.
800-gigabit coherent pluggable
The OIF has just announced it is developing the next-generation of ZR optics, an 800-gigabit coherent line interface supporting links up to 120km. The 800-gigabit specification will also support unamplified fixed-wavelength links 2-10km apart.
“This [800ZR standard] will use between Class 50 and Class 60 optics and a 5nm CMOS digital signal processor,” says Lipscomb.
NeoPhotonics’ Class 60 coherent modulator and receiver components are indium phosphide-based. For the future 800-gigabit coherent pluggable, a silicon photonics coherent optical subassembly (COSA) integrating the modulator with the receiver is required.
NeoPhotonics has published work showing its silicon photonics operating at around 90GBd required for 800-gigabit coherent pluggables.
“This is a couple of years out, requiring another generation of DSP and another generation of optics,” says Lipscomb.
Telecoms embraces 400ZR optics for IP-over-DWDM
Verizon Media has trialled 400-gigabit coherent pluggable optics to improve the delivery of video content to subscribers.
Verizon Media added a 400ZR QSFP-DD module from Inphi to a switch already using 100-gigabit optics.
Adding dense wavelength-division multiplexing (DWDM) optics to a switch enables it to send IP traffic (IP-over-DWDM) directly without needing a separate DWDM data centre interconnect box and additional client-side optics to link the two platforms (see diagram).
“Verizon Media, showing leadership outside the hyperscalers, is moving to IP-over-DWDM,” says Tomas Maj, senior director, marketing, optical interconnect at Inphi. “It shows the maturity of the ecosystem and the confidence of more and more operators in IP-over-DWDM and 400ZR.”
Content distribution network
Inphi cites three applications driving traffic growth between data centres: cloud network virtualisation, content distribution and edge analytics, and data mirroring and backup.
The primary users of these applications are the hyperscalers – it is the hyperscalers that spurred the creation of the OIF’s 120km 400ZR standard – but these applications increasingly apply to the telcos.
Verizon Media uses its content delivery network to share and back-up video between its data centres dubbed super PoPs (points-of-presence). Video is also sent to smaller outlying sites, closer to subscribers, where the most popular content is hosted.
ColorZ II
Verizon Media’s network uses Inphi’s existing 100-gigabit ColorZ QSFP28 pluggable optics.
The ColorZ is a direct-detect module that uses 4-level pulse amplitude modulation (PAM-4) to convert 4×25-gigabit electrical signals to two 50-gigabit PAM-4 optical wavelengths that fit within a 100GHz channel.
The ColorZ module, of which Inphi has now shipped over 100,000 units, has an 80km reach.
Inphi’s second-generation ColorZ II uses the OIF’s 400ZR coherent standard. Both generations employ an silicon photonics chip to implement the optics.
“As you go up in PAM-4 speed, you are taking hits in optical signal-to-noise ratio and receiver sensitivity and the design becomes costly,” says Maj. “At some point, you look at coherent and you have better yield and optical performance.”
For Verizon Media’s trial, the ColorZ II 400ZR QSFP-DD was added to switches from Arista Networks. Using ColorZ II optics in the same 100GHz channels quadruples fibre capacity from 4 to 16 terabits while halving the transmission cost-per-bit.
Nitin Batta, principal infrastructure architect at Verizon Media, said in a press release that the ColorZ II was chosen to enable it to “rapidly, easily and cost-effectively add terabits of capacity in response to customer demand.”
The 400ZR standard ensures interoperability and gives customers confidence by having several module companies to choose from, says Maj. Adopting the module also provides important diagnostic information regarding a link’s performance.
All the elements for a 400-gigabit ecosystem are coming together, says Inphi.
Four-hundred-gigabit client-side optical modules are leading the way and now 400-gigabit coherent pluggables are at the testing and validation stage before volume deployment.
The ColorZ II will be generally available at the year’s end.
Is traffic aggregation the next role for coherent?
Ciena and Infinera have each demonstrated the transmission of 800-gigabit wavelengths over near-1,000km distances, continuing coherent's marked progress. But what next for coherent now that high-end optical transmission is approaching the theoretical limit? Can coherent compete over shorter spans and will it find new uses?
Part 1: XR Optics
“I’m going to be a bit of a historian here,” says Dave Welch, when asked about the future of coherent.
Interest in coherent started with the idea of using electronics rather than optics to tackle dispersion in fibre. Using electronics for dispersion compensation made optical link engineering simpler.
Dave Welch
Coherent then evolved as a way to improve spectral efficiency and reduce the cost of sending traffic, measured in gigabit-per-dollar.
“By moving up the QAM (quadrature amplitude modulation) scale, you got both these benefits,” says Welch, the chief innovation officer at Infinera.
Improving the economics of traffic transmission still drives coherent. Coherent transmission offers predictable performance over a range of distances while non-coherent optics links have limited spans.
But coherent comes at a cost. The receiver needs a local oscillator - a laser source - and a coherent digital signal processor (DSP).
Infinera believes coherent is now entering a phase that will add value to networking. “This is less about coherent and more about the processor that sits within that DSP,” says Welch.
Aggregation
Infinera is developing technology - dubbed XR Optics - that uses coherent for traffic aggregate in the optical domain.
The 'XR’ label is a play on 400ZR, the 400-gigabit pluggable optics coherent standard. XR will enable point-to-point spans like ZR optics but also point-to-multipoint links.
Infinera, working with network operators, has been assessing XR optics’ role in the network. The studies highlight how traffic aggregation dictates networking costs.
“If you aggregate traffic in the optical realm and avoid going through a digital conversion to aggregate information, your network costs plummet,” says Welch.
Are there network developments that are ripe for such optical aggregation?
“The expansion of bandwidth demand at the network edge,” says Rob Shore, Infinera’s senior vice president of marketing. “It is growing, and it is growing unpredictably.”
XR Optics
XR optics uses coherent technology and Nyquist sub-carriers. Instead of a laser generating a single carrier, pulse-shaping at the optical transmitter is used to create multiple carriers, dubbed Nyquist sub-carriers.
The sub-carriers carry the same information as a single carrier but each one has a lower symbol rate. The lower symbol rate improves tolerance to non-linear fibre effects and enables the use of lower-speed electronics. This benefits long-distance transmissions.
But sub-carriers also enable traffic aggregation. Traffic is fanned out over the Nyquist sub-carriers. This enables modules with different capacities to communicate, using the sub-carrier as a basic data rate. For example, a 25-gigabit single sub-carrier XR module and a 100-gigabit XR module based on four sub-carriers can talk to a 400-gigabit module that supports 16.
It means that optics is no longer limited to a fixed point-to-point link but can support point-to-multipoint links where capacities can be changed adaptively.
“You are not using coherent to improve performance but to increase flexibility and allow dynamic reconfigurability,” says Shore.
Rob Shore
XR optics makes an intermediate-stage aggregation switch redundant since the higher-capacity XR coherent module aggregates the traffic from the lower-capacity edge modules.
The result is a 70 per cent reduction in networking costs: the transceiver count is halved and platforms can be removed from the network.
XR Optics starts to make economic sense at 10-gigabit data rates, says Shore. “It depends on the rest of the architecture and how much of it you can drive out,” he says. “For 25-gigabit data rates, it becomes a virtual no-brainer.”
There may be the coherent ‘tax’ associated with XR Optics but it removes so much networking cost that it proves itself much earlier than a 400ZR module, says Shore.
Applications
First uses of XR Optics will include 5G and distributed access architecture (DAA) whereby cable operators bring fibre closer to the network edge.
XR Optics will likely be adopted in two phases. The first is traditional point-to-point links, just as with 400ZR pluggables.
“For mobile backhaul, what is fascinating is that XR Optics dramatically reduces the expense of your router upgrade cost,” says Welch. “With the ZR model I have to upgrade every router on that ring; in XR I only have to upgrade the routers needing more bandwidth.”
Phase two will be for point-to-multipoint aggregation networks: 5G, followed by cable operators as they expand their fibre footprint.
Aggregation also takes place in the data centre, has coherent a role there?
“The intra-data centre application [of XR Optics] is intriguing in how much you can change in that environment but it is far from proven,” says Welch.
Coherent for point-to-point links will not be used inside the data centre as it doesn’t add value but configurable point-to-multiple links do have merit.
“It is less about coherent and more about the management of how content is sent to various locations in a point-to-multiple or multipoint-to-multipoint way,” says Welch. “That is where the game can be had.”
Uptake
Infinera is working with leading mobile operators regarding using XR Optics for optical aggregation. Infinera is talking to their network architects and technologists at this stage, says Shore.
Given how bandwidth at the network edge is set to expand, operators are keen to explore approaches that promise cost savings. “The people that build mobile networks or cable have told us they need help,” says Shore.
Infinera is developing the coherent DSPs for XR Optics and has teamed with optical module makers Lumentum and II-VI. Other unnamed partners have also joined Infinera to bring the technology to market.
The company will detail its pluggable module strategy including XR Optics and ZR+ later this year.
Acacia unveils its 400G coherent module portfolio
Acacia Communications has unveiled a full portfolio of 400-gigabit coherent optics and has provided test samples to customers, one being Arista Networks.
Delivering a complete set of modules offers a comprehensive approach to address the next phase of coherent optics, the company says.
The 400-gigabit coherent designs detailed by Acacia are implemented using the QSFP-DD, OSFP and CFP2 pluggable form factors.
Collectively, the pluggables support three performance categories: the 400ZR standard, OpenZR+ that is backed by several companies, and the coherent optics specification used for the Open ROADM multi-source agreement (MSA).
OIF-defined 400ZR standard designed for hyperscalers
“These are challenging specifications,” says Tom Williams, vice president of marketing at Acacia. “Even the 400ZR, where the objective has been to simplify the requirements.”
400ZR and OpenZR+
The OIF-defined 400ZR standard is designed for hyperscalers to enable the connection of switches or routers in data centres up to 120km apart.
The 400ZR standard takes in a 400 Gigabit Ethernet (GbE) client signal and outputs a 400-gigabit coherent signal for optical transmission.
“Hyperscaler customers want a limited subset of performance [with the ZR] because they don’t want to introduce operational complexity,” says Williams.
Acacia is implementing the 400ZR standard with two module offerings: the QSFP-DD and the OSFP.
Acacia is also a founding member of OpenZR+, the industry initiative that supports both 400ZR and extended optical performance modes. The other OpenZR+ members are NEL, Fujitsu Optical Components, Lumentum, Juniper Networks and Cisco Systems which is in the process of acquiring Acacia.
OpenZR+ supports 100GbE and its multiples (200GbE and 300GbE) input signals, not just 400GbE as used for ZR. To transmit the 200- 300- and 400GbE client signals, OpenZR+ uses quadrature phase-shift keying (QPSK), 8-ary quadrature amplitude modulation (8-QAM), and 16-QAM, respectively.
OpenZR+ also employs an enhanced forward-error correction (oFEC) used for the Open ROADM specification and delivers improved dispersion compensation performance.
“OpenZR+ is not just about going further but also being able to offer more functionality than 400ZR,” says Williams.
Acacia is implementing OpenZR+ using the QSFP-DD and OSFP form factors.
Open ROADM
The Open ROADM specification is the most demanding of the three modes and is targeted for use by the telecom operators. Here, a CFP2-DCO module is used due to its greater power envelope. And while the Open ROADM optics is aimed at telcos, the CFP2-DCO also supports OpenZR+ and 400ZR modes.
“The telcos are not as focussed on [face plate] density,” says Williams. “The CFP2-DCO has a higher output and is not limited to just Ethernet but also multiplexed client signals and OTN.”
Since line cards already use CFP2-DCO modules, the Open ROADM module enables a system upgrade. Existing line cards using the 200-gigabit CFP2-DCO may not support 400GbE client signals but with the Open ROADM CFP2’s higher symbol rate, it offers enhanced reach performance.
This is because the Open ROADM CFP2-DCO uses a 64 gigabaud (GBd) symbol rate enabling a 200-gigabit signal to be transmitted using QPSK modulation. In contrast, 32GBd is used for the existing 200-gigabit CFP2-DCOs requiring 16-QAM. Using QPSK rather than 16-QAM enables better signal recovery.
There is also an interoperability advantage to the new CFP2-DCO in that its 200-gigabit mode is compliant with the CableLabs specification.
All three designs – 400ZR, OpenZR+ and Open ROADM – use Acacia’s latest 7nm CMOS Greylock low-power coherent digital signal processor (DSP).
This is the company’s third-generation low-power DSP following on from its Sky and Meru DSPs. The Meru DSP is used in existing 32GBd 100/ 200-gigabit CFP2-DCOs.
3D stacking
Acacia has spent the last year and a half focusing on packaging, using techniques from the semiconductor industry to ensure the pluggable form factors can be made in volume.
The higher baud rate used for the 400-gigabit coherent modules means that the electronic ICs and the optics need to be closely coupled. “Moving up the baud rate means that the interconnection between the [modulator] driver [chip] and the modulator can become a limiting factor,” says Williams.
Acacia is not detailing the 3D design except to say that the Greylock DSP, its silicon-photonics photonic integrated circuit (PIC), and the modulator driver and trans-impedance amplifier (TIA) are all assembled into one package using chip-stacking techniques. The chip is then mounted onto a printed circuit board much like a BGA chip, resulting in a more scalable process, says Acacia.
“We have taken the DSP and optics and turned that into an electronic component,” says Williams. “Ultimately, we believe it will lead to improvements in reliability using this volume-repeatable process.”
Acacia says its modules will undergo qualification during most of this year after which production will ramp.
No one module design will be prioritised, says Williams: “There are a lot of benefits of doing all three, leveraging a lot of common elements.”
Nokia buys Elenion for its expertise and partnerships
Nokia will become the latest systems vendor to bolster its silicon photonics expertise with the acquisition of Elenion Technologies.
The deal for Elenion, a privately-held company, is expected to be completed this quarter, subject to regulatory approval. No fee has been disclosed.
“If you look at the vertically-integrated [systems] vendors, they captured the lion’s share of the optical coherent marketplace,” says Kyle Hollasch, director of optical networking product marketing at Nokia. “But the coherent marketplace is shifting to pluggables and it is shifting to more integration; we can’t afford to be left behind.”
Elenion Technologies
Elenion started in mid-2014, with a focus of using silicon as a platform for photonics. “We consider ourselves more of a semiconductor company than an optics company,” says Larry Schwerin, CEO of Elenion.
Elenion makes photonic engines and chipsets and is not an optical module company. “We then use the embedded ecosystem to offer up solutions,” says Schwerin. “That is how we approach the marketplace.”
The company has developed a process design kit (PDK) for photonics and has built a library of circuits that it uses for its designs and custom solutions for customers.
A PDK is a semiconductor industry concept that allows circuit designers to develop complex integrated circuits without worrying about the underlying transistor physics. Adhering to the PDK ensures the circuit design is manufacturable at a chip fabrication plant (fab).
But developing a PDK for optics is tricky. How the PDK is designed and developed must be carefully thought through, as has the manufacturing process, says Elenion.
“We got started on a process and developed a library,” says Larry Schwerin, CEO of Elenion. “And we modelled ourselves on the hyperscale innovation cycle, priding ourselves that we could get down to less than three years for new products to come out.”
The “embedded ecosystem” Elenion refers to involves close relationships with companies such as Jabil to benefit from semiconductor assembly test and packaging techniques. Other partnerships include Molex and webscale player, Alibaba.
Elenion initially focussed on coherent optics, providing its CSTAR coherent device that supports 100- and 200-gigabit transmissions to Jabil for a CFP2-DCO pluggable module. Other customers also use the design, mostly for CFP2-DCO modules.
The company has now developed a third-generation coherent design, dubbed CSTAR ZR, for 400ZR optics. The optical engine can operate up to 600 gigabits-per-second (Gbps), says Elenion.
Elenion’s work with the cloud arm of Alibaba covers 400-gigabit DR4 client-side optics as well as an 800-gigabit design.
Alibaba Cloud has said the joint technology development with Elenion and Hisense Broadband covers all the production stages: the design, packaging and testing of the silicon photonics chip followed by the design, packaging, assembly and testing of the resulting optical module.
Bringing optics in-house
With the acquisition of Elenion, Nokia becomes the latest systems vendors to buy a silicon photonics specialist.
Cisco Systems acquired Lightwire in 2012 that enabled it to launch the CPAK, a 100-gigabit optical module, a year ahead of its rivals. Cisco went on another silicon photonics shopping spree more recently with the acquisition of Luxtera in 2019, and it is the process of acquiring leading merchant coherent player, Acacia Communications.
In 2013 Huawei bought the Belgium silicon photonics start-up, Caliopa, while Mellanox Technologies acquired silicon photonics firm, Kotura, although subsequently, it disbanded its silicon photonics arm.
Ciena bought the silicon-photonics arm of Teraxion in 2016 and, in the same year, Juniper bought silicon photonics start-up, Aurrion Technologies.
Markets
Nokia highlights several markets – 5G, cloud and data centres – where optics is undergoing rapid change and where the system vendor’s designs will benefit from Elenion’s expertise.
“5G is a pretty obvious one; a significant portion of our optical business over the last two years has been mobile front-haul,” says Nokia’s Hollasch. “And that is only going to become more significant with 5G.”
Front-haul is optics-dependent and requires new pluggable form factors supporting lower data rates such as 25Gbps and 100Gbps. “This is the new frontier for coherent,” says Hollasch.
Nokia is not looking to be an optical module provider, at least for now. “That one we are treading cautiously,” says Hollasch. “We, ourselves, are quite a massive customer [of optics] which gives us some built-in scale straight away but our go-to-market [strategy] is still to be determined.”
Not being a module provider, adds Schwerin, means that Nokia doesn’t have to come out with modules to capitalise on what Elenion has been doing.
Nokia says both silicon photonics and indium phosphide will play a role for its coherent optical designs. Nokia also has its own coherent digital signal processors (DSPs).
“There is an increasingly widening application space for silicon photonics,” says Hollasch. “Initially, silicon photonics was looked at for the data centre and then strictly for metro [networks]; I don’t think that is the case anymore.”
Why sell?
Schwerin says the company was pragmatic when it came to being sold. Elenion wasn’t looking to be acquired and the idea of a deal came from Nokia. But once the dialogue started, the deal took shape.
“The industry is in a tumultuous state and from a standpoint of scenario planning, there are multiple dynamics afoot,” says Schwerin.
As the company has grown and started working with larger players including webscales, their requirements have become more demanding.
“As you get more into bigs, they require big,” says Schwerin. “They want supply assurance, and network indemnification clauses come into play.” The need to innovate is also constant and that means continual investment.
“When you weigh it all up, this deal makes sense,” he says.
Schwerin laughs when asked what he plans to do next: “I know what my wife wants me to do.
“I will be going with this organisation for a short while at least,” he says. “You have to make sure things go well in the absorption process involving big companies and little companies.”