The emerging market opportunity for pluggable coherent modules is causing companies to change their strategies.
Ciena is developing and plans to sell its own coherent modules. And now Acacia Communications, the coherent technology specialist, says it is considering changing its near-term coherent digital signal processor (DSP) roadmap to focus on coherent pluggables for data centre interconnect and metro applications.
DSP roadmap
Acacia’s coherent DSP roadmap in recent years has alternated between an ASIC for low-power, shorter-reach applications followed by a DSP to address more demanding, long-haul applications.
In 2014, Acacia announced its Sky 100-gigabit DSP for metro applications that was followed in 2015 by its Denali dual-core DSP that powers its 400-gigabit AC-400 5x7-inch module. Then, in 2016, Acacia unveiled its low-power Meru, used within its pluggable CFP2-DCO modules. The high-end 1.2-terabit dual-core Pico DSP used for Acacia’s board-mounted AC1200 coherent module was unveiled in 2017.
“The 400ZR is our next focus,” says Tom Williams, senior director of marketing at Acacia.
The 400ZR standard, promoted by the large internet content providers, is being developed to link switches in separate data centres up to 80km apart. Acacia’s subsequent coherent DSP that follows the 400ZR may also target pluggable applications such as 400-gigabit CFP2-DCO modules that will span metro and metro-regional distances.
“There is a trend to pluggable, not just the 400ZR but the CFP2-DCO [400-gigabit] for metro,” says Williams. “We are still evaluating whether that causes a shift in our overall cadence and DSP development.”
AC1200 trials
Meanwhile, Acacia has announced the results of two transatlantic trials involving its AC1200 module whose production is now ramping.
In the first trial, Acacia, working with ADVA, transmitted a 300-gigabit signal over a 6,800km submarine cable. The 300-gigabit wavelength occupied a 70GHz channel and used ADVA’s Teraflex technology, part of ADVA’s FSP 3000 CloudConnect platform. Teraflex is a one-rack-unit (1RU) stackable chassis that supports three hot-pluggable 1.2-terabit sleds, each sled incorporating an Acacia AC1200 module.
In a separate trial, the AC1200 was used to send a 400-gigabit signal over 6,600km using the Marea submarine cable. Marea is a joint project between Microsoft, Facebook and Telxius that links the US and Spain. The cable is designed for performance and uses an open line system, says Williams: “It is not tailored to a particular company’s [transport] solution”.
There is a trend to pluggable, not just the 400ZR but the CFP2-DCO [400-gigabit] for metro
The AC1200 module - 40 percent smaller than the 5x7-inch AC400 module - uses Acacia’s patented Fractional QAM (quadrature amplitude modulation) technology. The technology uses probabilistic constellation shaping that allows for non-integer constellations. “Instead of 3 or 4 bits-per-symbol, you can have 3.56 bits-per-symbol,” says Williams.
Acacia’s Fractional QAM also uses an adaptive baud rate. For the trial, the 400-gigabit wavelength was sent using the maximum baud rate of just under 70 gigabaud. Using the baud rate to the full allows a lower constellation to be used for the 400-gigabit wavelength thereby achieving the best optical signal-to-noise ratio (OSNR) and hence reach.
In a second demonstration using the Marea cable, Acacia demonstrated a smaller-width channel in order to maximise the overall capacity sent down the fibre. Here, a lower baud rate/ higher constellation combination was used to achieve a spectral efficiency of 6.41 bits-per-second-per-Hertz (b/s/Hz). “If you built out all the channels [on the fibre], you achieve of the order of 27 terabits,” says Williams.
Pluggable coherent
The 400ZR will be implemented using the same OSFP and QSFP-DD pluggable modules used for 400-gigabit client-side interfaces. This is why an advanced 7nm CMOS process is needed to implement the 400ZR DSP so that its power consumption will be sufficiently low to meet the modules’ power envelopes when integrated with Acacia’s silicon-photonics optics.
There is also industry talk of a ZR+, a pluggable module with a reach exceeding80km. “At ECOC, there was more talk about the ZR+,” says Williams. “We will see if it becomes standardised or just additional proprietary performance.”
Another development is the 400-gigabit CFP2-DCO. At present, the CFP2-DCO delivers up to 200-gigabitwavelengths but the standard, as defined by the Optical Internetworking Forum (OIF), also supports 400 gigabits.
Williams says that there a greater urgency to develop the 400ZR than the 400-gigabit CFP2-DCO. “People would like to ramp the ZR pretty close to the timing of the 400-gigabit client-side interfaces,” says Williams. And that is likely to be from mid-2019.
In contrast, the 400-gigabit CFP2-DCO pluggable while wanted by carriers for metro applications, is not locked to any other infrastructure build-out, says Williams.