Acacia unveils 400 Gigabit coherent transceiver

• The AC-400 5x7 inch MSA transceiver is a dual-carrier design
• Modulation formats supported include PM-QPSK, PM-8-QAM and PM-16-QAM
• Acacia’s DSP-ASIC is a 1.3 billion transistor dual-core chip 

Acacia Communications has unveiled the industry's first flexible rate transceiver in a 5x7-inch MSA form factor that is capable of up to 400 Gigabit transmission rates. The company made the announcement at the OFC show held in Los Angeles. 

Dubbed the AC-400, the transceiver supports 200, 300 and 400 Gigabit rates and includes two silicon photonics chips, each implementing single-carrier optical transmission, and a coherent DSP-ASIC. Acacia designs its own silicon photonics and DSP-ASIC ICs.

"The ASIC continues to drive performance while the optics continues to drive cost leadership," says Raj Shanmugaraj, Acacia's president and CEO.

The AC-400 uses several modulation formats that offer various capacity-reach options. The dual-carrier transceiver supports 200 Gig using polarisation multiplexing, quadrature phase-shift keying (PM-QPSK) and 400 Gig using 16-quadrature amplitude modulation (PM-16-QAM). The 16-QAM option is used primarily for data centre interconnect for distances up to a few hundred kilometers, says Benny Mikkelsen, co-founder and CTO of Acacia: "16-QAM provides the lowest cost-per-bit but goes shorter distances than QPSK."  

Acacia has also implemented a third, intermediate format - PM-8-QAM - that improves reach compared to 16-QAM but encodes three bits per symbol (a total of 300 Gig) instead of 16-QAM's four bits (400 Gig). "8-QAM is a great compromise between 16-QAM and QPSK," says Mikkelsen. "It supports regional and even long-haul distances but with 50 percent higher capacity than QPSK." Acacia says one of its customer will use PM-8-QAM for a 10,000 km submarine cable application.

Source: Gazettabyte 

Other AC-400 transceiver features include OTN framing and forward error correction. The OTN framing can carry 100 Gigabit Ethernet and OTU4 signals as well as the newer OTUc1 format that allows client signals to be synchronised such that a 400 Gigabit flow from a router port can be carried, for example. The FEC options include a 15 percent overhead code for metro and a 25 percent overhead code for submarine applications. 

The 28 nm CMOS DSP-ASIC features two cores to process the dual-carrier signals. According to Acacia, its customers claim the DSP-ASIC has a power consumption less than half that of its competitors. The ASIC used for Acacia’s AC-100 CFP pluggable transceiver announced a year ago consumes 12-15W and is the basis of its latest DSP design, suggesting an overall power consumption of 25 to 30+ Watts. Acacia has not provided power consumption figures and points out that since the device implements multiple modes, the power consumption varies.

The AC-400 uses two silicon photonics chips, one for each carrier. The design, Acacia's second generation photonic integrated circuit (PIC), has a reduced insertion loss such that it can now achieve submarine transmission reaches. "Its performance is on a par with lithium niobate [modulators]," says Mikkelsen.

It has been surprising to us, and probably even more surprising to our customers, how well silicon photonics is performing

The PIC’s basic optical building blocks - the modulators and the photo-detectors - have not been changed from the first-generation design. What has been improved is how light enters and exits the PIC, thereby reducing the coupling loss. The latest PIC has the same pin-out and fits in the same gold box as the first-generation design. "It has been surprising to us, and probably even more surprising to our customers, how well silicon photonics is performing," says Mikkelsen.

Acacia has not tried to integrate the two wavelength circuits on one PIC. "At this point we don't see a lot of cost savings doing that," says Mikkelsen. "Will we do that at some point in future? I don't know." Since there needs to be an ASIC associated with each channel, there is little benefit in having a highly integrated PIC followed by several discrete DSP-ASICs, one per channel. 

The start-up now offers several optical module products. Its original 5x7 inch AC-100 MSA for long-haul applications is used by over 10 customers, while it has two 5x7 inch modules for submarine operating at 40 Gig and 100 Gig are used by two of the largest submarine network operators. Its more recent AC-100 CFP has been adopted by over 15 customers. These include most of the tier 1 carriers, says Acacia, and some content service providers. The AC-100 CFP has also been demonstrated working with Fujitsu Optical Components's CFP that uses NTT Electronics's DSP-ASIC. Acacia expects to ship 15,000 AC-100 coherent CFPs this year.

Each of the company's module products uses a custom DSP-ASIC such that Acacia has designed five coherent modems in as many years. "This is how we believe we out-compete the competition," says Shanmugaraj.  

Meanwhile, Acacia’s coherent AC-400 MSA module is now sampling and will be generally available in the second quarter.