Acacia's single-wavelength terabit coherent module

• Acacia has developed a 140-gigabaud, 1.2-terabit coherent module

• The module, using 16-ary quadrature amplitude modulation (16-QAM), can deliver an 800-gigabit wavelength over 90 per cent of the links of a North American operator.  

Acacia Communications, now part of Cisco, has announced the first 1.2-terabit single-wavelength coherent pluggable transceiver.

Tom Williams

And the first vendor, ZTE, has already showcased a prototype using Acacia’s single-carrier 1.2 terabit-per-second (Tbps) design.

The coherent module operates at a symbol rate of up to 140 gigabaud (GBd) using silicon photonics technology. Until now, indium phosphide has always been the material at the forefront of each symbol rate hike.

The module uses Acacia’s latest Jannu coherent digital signal processor (DSP), implemented in 5nm CMOS. The coherent transceiver also uses a custom form-factor pluggable dubbed the Coherent Interconnect Module 8 (CIM-8).

Trends

Acacia refers to its 1.2-terabit coherent pluggable as a multi-haul design, a break from its product categorisation as either embedded or pluggable.

“We are introducing a pluggable module that supports what has traditionally been the embedded market,” says Tom Williams, senior director of marketing at Acacia. “It supports high-capacity edge applications all the way out to long-haul and submarine.”

Pluggables are the fastest-growing segment of the coherent market. Whereas the mix of custom embedded designs to pluggable interoperable is 2:1, that is forecast to change with coherent pluggables accounting for two-thirds of the total ports.

Acacia highlights the growth of coherent pluggables with two examples.

Data centre operator Microsoft used Inphi’s (now Marvell’s) ColorZ direct-detect 100-gigabit modules for data centre interconnect for up to 80km whereas now the industry is moving to the 400ZR coherent MSA.

In turn, while proprietary embedded coherent solutions would be used for reconfigurable optical add-drop multiplexers (ROADMs), now, interoperable pluggable coherent modules are being adopted with the OpenROADM MSA.

“There is still a significant need in the market for full-performance multi-haul solutions but we think their development needs to be informed and influenced by pluggables,” says Williams.

1.2-terabit capacity

As coherent technology matures, the optical transmission performance is approaching the theoretical limit as defined by Claude Shannon.

“There is still opportunity for improvement,” says Williams. “We still have performance enhancements with each generation but it is becoming more incremental.”

Williams highlights how its latest design offers a 20–25 per cent spectral efficiency improvement compared to Acacia’s AC1200 that uses two wavelengths to deliver up to 1.2Tbps.

“As we increase baud rate, that alone does not give any improvement in spectral efficiency,” says Williams. It is the algorithmic enhancements that still boost performance.

Acacia is adopting an enhanced probabilistic constellation shaping (PCS) algorithm as well as an improved forward-error correction scheme. “There are also some benefits of a single carrier as opposed to using multiple carriers,” says Williams.

Source: Acacia 

Design

The latest design is a natural extension of the AC1200 which can send 400 gigabits over ultra-long-haul distances, 800 gigabits using two wavelengths over most spans, and three 400-gigabit payloads over shorter, network-edge reaches. Now, this can all be done using a single wavelength.

A 150GHz channel is used when transmitting the module’s full rate of 1.2Tbps. And with the module’s adaptive baud rate feature, the rate can be reduced to fit a wavelength in a 75GHz-wide channel. Existing 800-gigabit transmissions use 112.5GHz channel widths and the multi-rate module also supports this spacing.

Williams says 16-QAM is the favoured signalling scheme used for transmission. This is what has been chosen for the 400ZR standard at 64GBd. Doubling the symbol rate means 800 gigabits can be sent using 16-QAM.

Acacia also highlights that future generation coherent designs, what it calls class 4 (see diagram above), will double the symbol rate again to some 240GBd. But the company is not saying whether the technology enabling such rates will be silicon photonics.

The company has long spoken of the benefits of using a silicon packaging approach for its coherent modules in terms of size, power and automated manufacturing. But as the symbol rate doubles, packaging plays a key role to help tackle challenging radio frequency (RF) design issues.

Acacia stacks the driver and trans-impedance amplifier (TIA) circuitry directly on top of its photonic integrated circuit (PIC) while its coherent DSP is also packaged as part of the design. “This gives us much better signal integrity than if we have the optics and DSP packaged separately,” says Williams.

The key to the design is getting the silicon photonics - the optical modulator, in particular - operating at 140GBd. “If you can, the packaging advantages of silicon are significant,” says Williams.

Acacia points out that with the migration of traffic from 100GbE to 400GbE it makes sense to offer a single-wavelength multi-rate design. And 400GbE will remain the mainstay traffic for a while. But once the transition to 800 gigabit occurs, the idea of supporting two coherent wavelengths - a future dual-wavelength “AC2400” - may make sense.

CIM-8

Acacia is using its own form factor and not a multi-source agreement (MSA) because the 1.2-terabit technology exceeds all existing client-side data rates.

In turn, the power consumption of the 1.2-terabit coherent module requires a custom form factor while launching an MSA based on the CIM-8 would have tipped off the competition, says Williams.

That said, Acacia has made no secret that its next high-end design following on from its 64GBd AC1200 would double the symbol rate and that the company would skip the 96GBd rate used by vendors such as Ciena, Huawei and Infinera already offering 800-gigabit wavelength systems.

For Acacia’s multi-rate design that needs to address submarine applications, the goal is to maximise transmission performance. In contrast, for a ZR+ coherent design that fits in a QSFP-DD, the limited power budget of the module constrains the design’s performance.

With 5nm Jannu DSP, Acacia realised it could not fit the design in the QSFP-DD or OSFP. But it could produce a pluggable multi-haul design with its CIM-8 that is slightly larger than the CFP2 form factor. And pluggables are advantageous when 4-8 can be fitted in a one-rack-unit (1RU) platform.

Acacia says its 140GBd module using 16-QAM will deliver an 800-gigabit wavelength over 90 per cent of the links of a North American operator. For the remaining, longest-distance links (the 10 per cent), it will revert to 400 gigabits.

In contrast, existing 800-gigabit systems operating at 96GBd cover up to 20 per cent of the links before having to revert to the slower speed, says Acacia.

Applications

Hyperscaler data centre operators are the main drivers for 1.2Tbps interconnects. The interface would typically be used in the metro to link smaller data centres to a larger aggregation data centre.

“The 1.2-terabit interface is just trying to maximise cost per bit; pushing more bits over the same set of optics,” says Williams.

The communications service providers’ requirements, meanwhile, are focussed on 400 gigabits and at some point will migrate to 800 gigabits, says Williams.

Several system vendors are expected to announce products using the new module in the coming months.