Acacia targets access networks with coherent QSFP-DD

• Acacia Communications has announced a 100-gigabit coherent QSFP-DD pluggable module.
• The module is the first of several for aggregation in the access network.

The second article addressing what next for coherent

Part 2: 100-gigabit coherent QSFP-DD

Acacia Communications has revisited 100-gigabit coherent but this time for access rather than metro networks.

Acacia’s metro 100-gigabit coherent pluggable product, a CFP, was launched in 2014. The pluggable has a reach from 80km to 1,200km and consumes 24-26W.

Tom Williams

The latest coherent module is the first QSFP-DD to support a speed lower than the 400-gigabit 400ZR and ZR+ applications that have spurred the coherent pluggable market. 

The launching of a 100-gigabit coherent QSFP-DD reflects a growing need to aggregate 10 Gigabit Ethernet (GbE) links at the network edge as 5G and fibre are deployed.

“The 10GbE links in all the different types of access networks highlight a need for a cost-effective way to do this aggregation,” says Tom Williams, vice president of marketing at Acacia.

Why coherent?

The deployment of 5G, business services, 10-gigabit passive optical networking (PON) and distributed access architecture (DAA) are driving greater traffic at the network edge.

Direct-detection optics is the main approach used for aggregation but Acacia argues coherent is now a contender.

Until now, Acacia has only been able to offer coherent metro products for access. The company believes a 100-gigabit coherent module is timely given the network edge traffic growth coupled with the QSFP-DD form factor being suited for the latest aggregation and switch platforms. Such platforms are not the high-capacity switches used in data centres yet port density still matters.

“We think we can trigger a tipping point and drive coherent adoption for these applications,” says Williams.

Using coherent brings robustness long associated with optical transport networks. “You just plug both ends in and it works,” he says.

In access, the quality of fibre in the network varies. With coherent, there is no need for an engineer to do detailed characterisations of the link thereby benefiting operational costs.

Adopting coherent technology for access also provides a way to scale. “You may only need 100 gigabits today but there is a clear path to 200 and 400 gigabit and the use of DWDM [dense wavelength-division multiplexing],” says Williams.

100-gigabit QDFP-DD

Acacia's 100-gigabit QSFP-DD uses a temperature-controlled fixed laser and has a reach of 120km. The 120km span may rarely be needed in practice - up to 80km will meet most applications - but the extra margin will accommodate any vagaries in links.

The module uses Acacia’s 7nm CMOS low-power Greylock coherent digital signal processor (DSP). The Greylock is Acacia’s third-generation low power DSP chip that is used for its 400ZR and ZR+ modules.

The 100-gigabit QSFP-DD shares the same packaging as the 400ZR and ZR+ modules. The DSP, silicon-photonics photonic integrated circuit (PIC), modulator driver and trans-impedance amplifier (TIA) are all assembled into one package using chip-stacking techniques, what Acacia calls an opto-electronic multi-chip module (OEMCM).

“Everything other than the laser is in a single package,” says Williams. “The more we make optics look like electronics and the fewer interconnect points we have, the higher the reliability will be.”

The packaging approach brings size and optical performance benefits. The optics and DSP must be tightly coupled to ensure signal integrity as the symbol rates go up for 400-gigabit and soon 800-gigabit data rates. But this is less of an issue at 100-gigabit given the symbol rate is 32-gigabaud only. 

Opportunities

The 100-gigabit QSFP-DD is now sampling and undergoing qualification. Acacia has yet to announce its general availability.

The company is planning other coherent modules for access including a tunable laser-based QSFP-DD as well as designs that meet various environmental requirements. 

“We view coherent as moving into the access market and that will require solutions that address the entire market,” says Williams. That said, Acacia admits uncertainty remains as to how widely coherent will be adopted.

“The market has to play out and there are other competitive solutions,” says Williams. “We believe coherent will be the right solution but how that plays out near- to mid-term is uncertain.”