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Acacia announces a 1.2 terabit coherent module

Acacia Communications has given first details of its AC1200 coherent optical module, capable of transmitting up to 1.2 terabits of data. The custom coherent transceiver is being aimed at applications ranging from linking data centres to long-haul and even sub-sea transmissions and was announced at the recent ECOC show held in Gothenburg.

Channel capacity and link margin can be maximised by using the fractional QAM scheme. Source: Acacia.

The company is facing increasing market competition. Ciena has teamed up with Lumentum, NeoPhotonics, and Oclaro, sharing its high-end coherent DSP expertise with the three optical module makers. Meanwhile, Inphi has started sampling its 16nm CMOS M200, a 100- and 200-gigabit coherent DSP suitable for CFP2-ACO, CFP-DCO, and CFP2-DCO module designs.

The AC1200 is Acacia’s response, extending its high-end module offering beyond a terabit to compete with the in-house system vendors and preserve its performance lead against the optical module makers.


Enhanced coherent techniques

The AC1200 has an architecture similar to the company’s AC400 5x7-inch 400-gigabit module announced in 2015. Like the earlier module, the AC1200 features a dual-core coherent DSP and two silicon photonics transceiver chips. But the AC1200 uses a much more sophisticated DSP - the 16nm CMOS Pico device announced earlier this year - capable of supporting such techniques as variable baud rate, advanced modulation and coding schemes so that the bits per symbol can be fine-tuned, and enhanced soft-decision forward error correction (SD-FEC). The AC400 uses the 1.3 billion transistor Denali dual-core DSP while the Pico DSP has more than 2.5 billion transistors.

The result is a two-wavelength module design, each wavelength supporting from 100-600 gigabits in 50-gigabit increments.

Acacia is able to triple the module’s capacity to 1.2 terabits by incorporating a variable baud rate up to at least 69 gigabaud (Gbaud). This doubles the capacity per wavelength compared to the AC400 module. The company also uses more modulation formats including 64-ary quadrature amplitude modulation (64-QAM), boosting capacity a further 1.5x compared to the AC400’s 16-QAM.

Acacia has not detailed the module’s dimensions but says it is a custom design some 40 percent smaller in area than a 5x7-inch module. Nor will it disclose the connector type and electrical interface used to enable the 1.2-terabit throughput. However, the AC1200 will likely support 50 gigabit-per-second (Gbps) 4-level pulse-amplitude modulation (PAM-4) electrical signals as it will interface to 400-gigabit client-side modules such as the QSFP-DD.

The AC1200’s tunable baud rate range is around 35Gbaud to 69Gbaud. “The clock design and the optics could truly be continuous and it [the baud rate] pairs with a matrix of modulation formats to define a certain resolution,” says Tom Williams, senior director of marketing at Acacia Communications. Whereas several of the system vendors’ current in-house coherent DSPs use two baud rates such as 33 and 45Gbaud, or 35 and 56Gbaud, Acacia says it uses many more rates than just two or three.

The result is that at the extremes, the module can deliver from 100 gigabits (a single wavelength at some 34Gbaud and quadrature phase-shift keying - QPSK) to 1.2 terabits (using two wavelengths, each 64-QAM at around 69Gbaud).

The module also employs what Acacia refers to as very fine resolution QAM constellations. The scheme enables the number of bits per symbol to be set to any value and not be limited to integer bits. Acacia is not saying how it is implementing this but says the end result is similar to probabilistic shaping. “Instead of 2 or 3 bits-per-symbol, you can be at 2.5 or 2.7 bits-per-symbol,” says Williams. The performance benefits include maximising the link margin and the capacity transmitted over a given link. (See diagram, top.) 

The SD-FEC has also been strengthened to achieve a higher coding gain while still being a relatively low-power implementation.

Using a higher baud rate allows a lower order modulation scheme to be used. This can more than double the reach. Source: Acacia

The company says it is restricted in detailing the AC1200’s exact performance. “Because we are a merchant supplier selling into system vendors that do the link implementations, we have to be careful about the reach expectations we set,” says Williams. But the combination of fractional QAM, a tunable baud rate, and improved FEC means a longer reach for a given capacity. And the capacity can be tuned in 50-gigabit increments. 


Platforms and status

ADVA Optical Networking is one vendor that has said it is using Acacia’s 1.2-terabit design for its Teraflex product, the latest addition to its CloudConnect family of data centre interconnect products.

Is ADVA Optical Networking using the AC1200? “Our TeraFlex data centre interconnect product uses a coherent engine specifically developed to meet the performance expectations that our customers demand,” says ADVA's spokesperson.

Teraflex is a one-rack-unit (1RU) stackable chassis that supports three hot-pluggable 1.2-terabit ‘sleds’. Each sled’s front panel supports various client-side interface module options: 12 x 100-gigabit QSFP28s, 3 x 400-gigabit QSFP-DDs and lower speed 10-gigabit and 40-gigabit modules using ADVA Optical Networking’s MicroMux technology.

Samples of the AC1200 module will be available in the first half of 2018, says Acacia. General availability will likely follow a quarter or two later. 

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