Infinera has taped out its 7nm CMOS FlexCoherent 6 digital signal processor (DSP) and expects samples of the coherent chip in the next fortnight.
“We are ready to start fabricating prototype modules,” says Robert Shore, senior vice president of marketing at Infinera.
The DSP, along with Infinera’s photonic integrated circuit (PIC) that was demonstrated at the OFC show held in March, will be integrated into a module that supports two wavelengths, each capable of supporting up to 800 gigabits of data.
The company expects to have a prototype of the 1.6-terabit Infinite Coherent Engine 6 (ICE6) module by year-end, while the first ICE6-based products are scheduled for the second half of 2020.
Core, edge and cognitive networking
Infinera outlined its networking vision earlier this year after the acquisition of Coriant.
Dubbed Infinite Network, the vision covers the core and network edge as well as cognitive networking whereby the network makes intelligent decisions and adapts using status data collected from the network (see diagram).
For the core, the focus remains to advance the capacity and reach of optical transmissions. “There is no question that long-haul and subsea networks need this kind of functionality,” says Shore.
The latest coherent systems coming to market have much greater flexibility and programmability. “There are a lot more knobs to play with,” says Shore.
The changeable parameters include symbol rate, modulation scheme as well as probabilistic constellation shaping that is used to fine-tune the data rate to better exploit the capacity available on a given route.
At the network edge, the adoption of cloud technologies is causing change. Such cloud deployments not only include distributed data centres in the metro but also multi-access edge computing located in building basements and at cell towers.
Such deployments mean applications will become more distributed to massively dynamic traffic patterns, says Shore. In turn, the traditional networking model whereby thousands of endpoints are aggregated in the metro network before being passed onto the core will no longer hold.
“There is a need to classify that traffic close to the edge, handle the traffic on a flow-by-flow basis, and connect the end-user to the application without backhauling it,” says Shore.
Classifying traffic implies IP and that is an area Infinera is addressing with its disaggregated router strategy. But there is also a coherent opportunity here: the growth in edge-network traffic is outpacing the ability of direct-detect optics. “This increases the opportunity for edge-optimised coherent-based solutions,” says Shore.
Infinera has enhanced its PIC and coherent DSP technology for its ICE6 coherent module.
The current ICE4 uses two PICs: one for the transmitter and one for the receiver. The PICs support up to 1.2 terabits, six channels each capable of transmitting up to 200-gigabit signals using 16-ary quadrature amplitude modulation (16-QAM). The ICE6 uses one PIC that supports two optical channels for a total capacity of 1.6 terabits.
Meanwhile, the FlexCoherent 6 implements coherent toolkit features such as sub-carriers, soft-decision forward-error correction (SD-FEC), and probabilistic constellation shaping.
Earlier this year, Infinera outlined two planned module options, one addressing the core, the ICE6c, and a second for the edge, dubbed the ICE6e.
The ICE6e was also a two-channel module but tailored for the metro, with lower optical performance and lower power consumption. However, the merit of such a metro module has become unclear in the era of the 400ZR interface, says Shore.
Infinera continues to monitor the 400ZR pluggable module opportunity but admits it has less significance for the company. “The 400ZR is really for short distance, point-to-point links,” he says.
The 400ZR+ that extends 400-gigabit coherent performance to metro distances has still to be specified. Like the ZR, it will be implemented using a pluggable module such as the QSFP-DD or the OSFP. However, while the 400ZR+ is not a direct competitor to the envisaged ICE6e, it would overlap.
For now, the ICE6e has been demoted with Infinera focusing on the flagship ICE6 module for the core while investigating other coherent options for the metro.
Infinera has already mentioned a pluggable module design which would take a 400-gigabit input and convert it to lower streams, for example, operating at 100 gigabits or 25 gigabits, and implemented using a ‘virtualised laser’.
Unlike the 400ZR which supports a 400-gigabit link only, such a design suits metro-access, taking multiple streams from the access network.
“This is a different approach to metro-optimised optics and might be an alternative to ZR, ZR+ as well as something like the ICE6e,” says Shore. For now, the company is not ready to add further detail.
One key feature of the latest FlexCoherent 6 DSP is its support for Nyquist sub-carriers.
Infinera first announced its adoption of sub-carriers in 2016. Here, multiple Nyquist sub-carriers are spread across a channel instead of modulating the data onto a single carrier.
The benefit is that high data rates are possible while the baud rate used for each individual sub-channel is much lower, and a lower baud rate is more tolerant to non-linear channel impairments during optical transmission.
Sub-carriers improve spectral efficiency as the channels have sharper edges that enable tighter packing.
The ICE4 DSP was Infinera’s first design to use sub-carriers, adopting four or six per channel, for example. The FlexCoherent 6 is more sophisticated using eight sub-channels, each independently controlled and operating at 11 gigabaud (GBd) for an overall baud rate of 88GBd.
“We apply probabilistic constellation shaping to each sub-carrier,” says Shore. “This enables dynamic bandwidth allocation where we can increase or decrease the amount of bandwidth on each sub-carrier.”
This allows more data to be sent on the inner sub-carriers and less data on the two outer sub-carrier signals that are typically harder to recover.
Shore says that the sweet spot for sub-carriers is 8-11GBd and that the number of sub-carriers used can be changed to remain within the sweet-spot region (see diagram above). Infinera says the overall aggregate baud rate can go up to 94GBd.
Another feature of the DSP is its use of SD-FEC gain sharing. By sharing the FEC codes, processing resources can be shifted to one of the two optical channels that needs it the most. The result is that some of the strength of the stronger signal can be traded to bolster the weaker one, extending its reach or potentially allowing a higher modulation scheme to be used.
Infinera also uses a single oscillator for both of the module’s optical channels. This avoids having to use extra channel spacing to cater to the independent laser drift when two oscillators are used, one for each laser. Using a single oscillator thus benefits spectral efficiency.
Infinera expects to announce more details related to its metro plans at the upcoming ECOC show in Dublin in September.
“It [the announcement] will play very nicely with metro-style white-box routers,” says Shore.
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