400-gigabit coherent modules finally hit their stride

NeoPhotonics discusses 400-gigabit coherent modules, the move to 130-gigabaud symbol-rate optical components and a company tunable laser milestone.

NeoPhotonics’ 400ZR pluggable optical modules are now available and will ship in volume from the autumn.

“The QSFP-DD and OSFP 400ZR [optical modules] have passed qualification tests and we are engaged in numerous customer qualifications around the world,” says Ferris Lipscomb, vice president of marketing at NeoPhotonics.

Ferris Lipscomb

400ZR modules implement the OIF’s 400-gigabit standard to connect directly equipment in data centres up to 120km apart without needing separate dedicated dense wavelength-division multiplexing (DWDM) optical transport equipment. The first 400ZR modules will be used by data centre operators.

But coherent pluggables support longer-reach modes. These may be interoperable if implementing the OpenZR+ multi-source agreement (MSA) or when delivering custom optical performance that are referred to as ZR+ modules.

NeoPhotonics has reported that its 400-gigabit coherent QSFP-DD when operated as a ZR+ module can achieve an 800km reach.

At the OFC 2021 virtual conference and exhibition, NeoPhotonics discussed its multi-rate CFP2-DCO module that has a reach of 1,500km at 400 gigabits. Here, 64 gigabaud (GBd) and 16-ary quadrature amplitude modulation (16-QAM) are used along with probabilistic constellation shaping and a proprietary forward error correction scheme.

Probabilistic shaping manipulates the data before transmission so that the inner four constellation points more frequently carry data than the outer 12. Using this signal processing approach improves the overall distance between the constellation points used.

“Taken together, these two [the ZR+ and the CFP2-DCO] show that pluggable modules will have much broader applications than just data centre interconnect over 80-120km,” says Lipscomb.

NeoPhotonics’ CFP2-DCO module uses indium phosphide-based components while its QSFP-DD ZR/ZR+ module employs a silicon photonics-based coherent optical sub-assembly (COSA).

When Western telcos will adopt the CFP2-DCO pluggable is to be determined, says Lipscomb, but the Chinese operators are already using it. In China, the CFP2-DCO module employs quadrature phase-shift keying (QPSK) modulation instead of 16-QAM to enable distances beyond 1,000km at 200-gigabit wavelengths.

Two-hundred gigabits implemented using 64GBd and QPSK rather than a 32GBd symbol rate and 16-QAM enables more space between the points on the constellation diagram so that there is more resiliency to degradation before the received points blend.

At 400 gigabits, the performance of these modules is approaching that of line cards, says Lipscomb, yet they are much cheaper, have a lower power consumption and are more convenient to use.

NeoPhotonics has also shown 36 of its QSFP-DD coherent modules in an Arista Networks’ switch. “This shows it has low-enough power management capability to fully load an Arista switch,” says Lipscomb.

Adding coherent optics in the form of pluggables to a switch reduces the overall cost to a quarter that of existing architectures where a separate optical networking platform is required as well as extra optical transceivers linking the two.

Class 80 components

Tim Jenks, chairman and CEO of NeoPhotonics, has detailed how the company will extend the bandwidth of its indium phosphide coherent integration platform to operate at a symbol rate of 130GBd.

NeoPhotonics is already shipping in limited quantities its Class 60 devices. These are defined as having a three decibel (dB) bandwidth at 60GHz such that they operate at a symbol rate of 96GBd. Such devices enable coherent wavelengths up to 800 gigabit.

In contrast, Class 80 optical components - the driver modulator and the integrated coherent receiver - have a 3dB bandwidth at 80GHz, suitable for 130GBd. Such devices will enable 1 terabit, 1.2-terabit and even greater speed wavelengths. (See Table.)

NeoPhotonics, Gazettabyte

The hare and the tortoise

Class 80 coherent designs will require new optics as well as a next-generation 5nm coherent digital signal processor (DSP).

Such coherent systems will extend the reach and reduce cost but will not improve spectral density. “As you go to higher baud rates, you have to expand the channel width,” says Lipscomb. “So you get fewer channels in the same band.” This is also why cost comes down as fewer modules will be needed overall.

While there is a clear path for the optics to reach 130GBd, says Lipscomb, developing the faster analogue-to-digital and digital-to-analogue converters, the 5nm DSP, and the connectors, is more challenging.

Borrowing from the Aesop fable, Lipscomb equates the optics to the hare and the DSP to the tortoise.

It is not that the optics is so much ahead but that the DSP depends on the availability of the next CMOS process node and that is highly predictable.

“The DSP is the tortoise,” says Lipscomb. “It is going to get there at a certain rate, you can count on it.” The optics might sprint ahead but then it may take longer to scale up production. Overall though, the optics tracks the DSP’s availability.

Lipscomb expects 128GBd systems to be trialled next year with deployments starting in 2023. The individual baud rate used by a system can vary between 120-130GBd but in general 128GBd, as a multiple of 32 and 64GBd rates, is used as a reference symbol rate for Class 80 devices.

Pluggables versus line cards

Lipscomb says the high-end, longest-reach optical networking applications will be served using line cards based on 130GBd coherent systems.

People will pay for the high-end optical performance of such line card-based coherent optics because they will need it, so the segment will continue to be profitable, says Lipscomb.

But the bulk of the [coherent] work will increasingly be served using pluggables, first 400 gigabits at 64GBd and then at 800 gigabits operating at 96GBd.

“They won’t be the most efficient,” he says. “But they will be the cheapest because of the pluggability and will have lower power.”

Coherent sensing and tunable lasers

NeoPhotonics is also applying its coherent know-how for other non-telecom applications such as driverless cars (LiDAR), industrial sensing and medical sensing.

Industrial sensing refers to precise measurement of components while the medical sensing application - optical coherent tomography - allows probing a few millimetres deep under the skin.

“We provide principally lasers but also some other components that are used in coherent-sensing applications,” says Lipscomb.

NeoPhotonics recently shipped its 2 millionth narrow linewidth laser since 2011.

NeoPhotonics acquired Santur and its narrow linewidth laser in 2011 followed by Emcore in 2014.  Initially what was used was the integrable tunable laser assembly (ITLA), followed by the more compact micro-ITLA and then the nano-iTLA. Lipscomb says that the size of the original ITLA is comparable to a QSPD-DD coherent module.

“In that timeframe [since 2011], some 3 million coherent ports have been shipped,” says Lipscomb. “Some parts use one laser and some use two; you can do the maths but we have a very significant presence in the majority of the coherent ports that have shipped.”