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CWDM8 MSA targets compact lower power optics for 400G

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The CWDM8 multi-source agreement (MSA) Group has been formed to define optical specifications for 400G singlemode optics using 50G per wavelength and non-return-to-zero (NRZ) optical modulation.

The consortium aims to address what they see as a near-term need for lower power and more cost-effective options for distances of 2km and 10km within data centre and campus networks. This will support the deployment of 12.8Tb/s Ethernet switches and other advanced networking equipment with 50G electrical I/O.

The IEEE’s 400GBASE-LR8 module specification has a 10km reach using a cooled laser diode, but the optics won’t fit into next-generation form factors like QSFP-DD and OSFP, according to Robert Blum, director of strategic marketing and business development for Intel’s silicon photonics division.

Current LR8 modules come in the larger CFP8 format which has a 20W power envelope (see, for example, Oclaro demonstrates 400G CFP8 module at OFC 2017).

“The goal is to have an MSA where you can integrate into a small form factor and stay well within the power budget of 12W while going 2km and 10km,” Blum explained.

As the name suggests, the new interfaces defined by the CWDM8 MSA will employ eight wavelengths on the CWDM grid, which allows uncooled lasers to be used. They will pair this with NRZ signalling rather than PAM-4.

Though multilevel optical signalling enables a lower baud rate optics to be used, the signal-to-noise ratio takes a hit because the amplitude difference between the levels is one third of what it is in NRZ.

“It makes a lot of sense to go with NRZ if you have the high-bandwidth components available,” Blum asserted. “We will have more than 6dB better link budget than using PAM-4, and will also have a much faster time to market.”

The parts to support 50G PAM-4 based optics, such as compact digital signal processors and low-power 50Gb/s SerDes chips, have yet to be released and won’t be available in volume before the end of 2018, according to Blum.

In contrast, the MSA members hope to start shipping modules compliant with its specifications in the same time frame. Intel has already developed a prototype transmitter with eight lasers on one chip, Blum said.

MSA participants expect to address industry needs by developing technologies to create a competitive supply chain, while providing products that are optically compatible and interoperable. On the electrical side, the new interfaces defined by the MSA will maintain compatibility with existing standards, such as the CDAUI-8 chip-to-module electrical interface.

Founding members of the CWDM8 MSA include module makers Intel, Macom, Mellanox, Neophotonics, and Rockley Photonics. The consortium also includes switch silicon manufacturers Barefoot Networks and Innovium, as well as Accton Technology, Credo Semiconductor and Hisense.

The initiative has also received the backing of equipment manufacturers Nokia, Huawei, Quanta Cloud Technology and ZTE, who provided supporting quotes in the announcement.

“There is currently no adequate solution in the market to meet our needs for cost effective and uncooled 400G optical interfaces for 2km and 10km reaches. We welcome the formation of the CWDM8 MSA, which offers proven 50G NRZ technology to enable compact and low power dissipation form factors for 400G,” said Yu Li, vice president of data centre switching, Huawei.

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