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. 

Tom WilliamsThe 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). 

“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.”