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.
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.
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.
ADVA’s 800-gigabit CoreChannel causes a stir
ADVA’s latest addition to its FSP 3000 TeraFlex platform provides 800-gigabit optical transmission. But the announcement has caused a kerfuffle among its optical transport rivals.
ADVA’s TeraFlex platform supports various coherent optical transport sleds, a sled being a pluggable modular unit that customises a platform’s functionality.
The coherent sleds use Cisco’s (formerly Acacia Communication’s) AC1200 optical engine. Cisco completed the acquisition of Acacia in March.
The AC1200 comprises a 16nm CMOS Pico coherent digital signal processor (DSP) that supports two wavelengths, each up to 600-gigabit, and two photonic integrated circuits (PICs), for a maximum capacity of 1.2 terabits.
The latest sled from ADVA, dubbed CoreChannel, supports an 800-gigabit stream in a single channel.
ADVA states in its press release that the CoreChannel uses “140 gigabaud (GBd) sub-carrier technology” to deliver 800-gigabit over distances exceeding 1,600km.
This, the company says, improves reach by over 50 per cent compared with state-of-the-art 95GBd symbol rate coherent technologies.
It is these claims that have its rivals reacting.
“Despite their claims – they are not using actual digital sub-carriers,” says one executive from a rival optical transport firm, adding that what ADVA is doing is banding two independent 70GBd 400-gigabit wavelengths together and trying to treat that as a single 800-gigabit signal.
“This isn’t necessarily a bad solution for some applications – each network operator can decide that for themselves,” says the executive. However, he stresses that the CoreChannel is not an 800-gigabit single-channel solution and uses 4th generation 16nm CMOS DSP technology rather than the latest 5th generation, 7nm CMOS DSP technology.
A second executive, from another optical transport vendor providing 800-gigabit single-wavelength solutions, adds that ADVA’s claim of 140GBd is too ‘creative’ for a two-lambda solution.
“It’s not a real 800 gigabit. Not that this must be bad, but one should call things as they are,” the spokesperson said. “What matters to the operators is the cost, power consumption, reach and density of a modem; the number of lambdas is more of an internal feature.”
CoreChannel
ADVA confirms it is indeed using Cisco’s Pico coherent DSP to drive two wavelengths, each at 400 gigabits-per-second (Gbps).
“You can say the CoreChannel is a less challenging requirement because we are not driving it [the Pico DSP] to the maximum modulation or constellation complexity,” says Stephan Rettenberger, senior vice president, marketing and investor relations at ADVA. “It is the lower end of what the AC1200 can do.”
Until now the two wavelengths have been combined externally, and have not been integrated from a software or a command-and-control approach.
“The CoreChannel sled is just another addition to the TeraFlex toolbox,” says Rettenberger. “It has one physical line interface that drives an 800Gbps stream using two wavelengths, each one around 70GBd, that are logically and physically combined.”
The resulting two-wavelength 800-gigabit stream sits within a 150GHz channel. However, the channel width can be reduced to 125GHz and even 112.5GHz for greater spectral efficiency.
ADVA says the motivation for the design is the customers’ requirement for lower-cost transport and the ability to easily transport 400 Gigabit Ethernet (GbE) client signals.
“With this 800-gigabit line speed, you can go something like 2,000km, that is 50-100 per cent more than what 95GBd single-wavelengths solutions will do,“ says Rettenberger. “And you can also drive it at 400 gigabits and you can do something like 6,000km.”
The reaches quoted are based on a recent field trial involving ADVA.
ADVA uses a single DSP, similar to the latest 800-gigabit systems from Ciena, Huawei and Infinera. Alongside the DSP are two non-hermetically-sealed PICs whereas the 95GBd indium-phosphide solutions use a single hermetically sealed gold box.
ADVA’s solution also requires two lasers whereas the 800-gigabit single-wavelength solutions use one laser.
“Yes, we have two lasers versus one but that is not killing the cost,” says Rettenberger. “And it is also not killing the power consumption because the PIC is so much more power efficient.”
Rettenberger stresses that ADVA is not saying its offering is necessarily a better solution. “But it is a very interesting way to drive 800 gigabits further than these 95 gigabaud solutions,” says Rettenberger. “It has the same cost, space, power efficiency, just greater reach.”
ADVA also agrees that it is not using electrical sub-carriers such as Infinera uses but it is using optical sub-carrier technology.
These two wavelengths are combined logically and also from a physical port interface point of view to fit within a 150GHz window.
The 95GBd, in contrast, is an interim symbol rate step and the resulting 112.5GHz channel width doesn’t easily fit with legacy 25GHz and 50GHz band increments, says ADVA, while the 150GHz band the CoreChannel sled uses is the same channel width that will be used once single-wavelength 140GBd technology becomes available.
Acacia has also long talked about the merit of doubling the baud rate suggesting Cisco’s successor to the AC1200 will have a 140GBd symbol rate. Such a design is expected in the next year or two.
“We feel this [CoreChannel] implementation is already future-proofed,” says Rettenberger.
ADVA says it undertook this development in collaboration with Acacia.
Acacia announced a dual-wavelength single-channel AC1200 solution in 2019. Then, the company unveiled its AC1200-SC2 that delivers 1.2 terabits over an optical channel.
The SC2 (single chip, single channel) is an upgrade of Acacia’s AC1200 module in that it sends 1.2 terabits using two sub-carriers that fit in a 150GHz-wide channel.
Customer considerations
Choosing an optical solution comes down to five factors, each having its weight depending on the network application, says the first executive.
These are capacity-per-wavelength, cost-per-bit, capacity-per- optical-engine or -module, spectral efficiency and hence capacity-per-fibre, and power-per-bit.
“Each is measured for a given distance/ network application,” says the executive. “And the reason the weight changes for different applications is that the importance of each factor is different at different points in the network. For example, the importance of spectral efficiency changes depending on how expensive it is to light up a link (fibre and line system costs).”
For long-haul and submarine, spectral efficiency is the most important factor, while for metro it is typically cost-per-bit. Meanwhile, for data centre interconnect applications, it’s a mix between cost-per-bit and power-per-bit. Capacity-per-wave and capacity-per-optical-engine are valuable because they can reduce the number of wavelengths and modules that need to be deployed, reducing operating expenses and accelerating service activation.
“The reason that 5th generation [7nm CMOS technology] is superior to fourth generation [16nm] DSP technology is that it provides superior performance in every single one of those key criteria,” says the executive. “This fact minimised any potential benefits that could be achieved by banding together two wavelengths using 4th generation technology when compared to a single wavelength using 5th generation technology.”
“It sounds like others feel we have misled the market; that was not the intent,” says Rettenberger.
ADVA does not make its own coherent DSP so it doesn’t care if the chip is implemented using a 16nm, 7nm or a 5nm CMOS process.
“We are trying to build a good solution for transmitting 400GbE signals and, for us, the Pico chip is a wonderful piece of technology that we have now implemented in four different [sled] variants of TeraFlex.”
Acacia heralds the era of terabit-plus optical channels
Acacia Communications has unveiled the AC1200-SC2 that delivers 1.2 terabits over a single optical channel.
The SC2 (single chip, single channel) is an upgrade of Acacia’s high-end AC1200 module. The AC1200 too is a 1.2-terabit module but uses two optical channels, each transmitting a 600-gigabit wavelength. The SC2 sends 1.2 terabits using two sub-carriers that fit within a single 150GHz-wide channel.
Each line is a data rate. Shown is the scope of how the baud rate and the modulation scheme can be varied and its impact on channel width, reach and data rate. Source: ADVA.
“In the SC2, we take care of everything so the user configures a single channel that is easier to manage in their network,” says Tom Williams, vice president of marketing at Acacia.
1.2-terabit channel
Acacia unveiled the AC1200 at the ECOC show in 2017. With its introduction, Acacia gained an advantage over its system-vendor rivals in bringing a 1.2-terabit coherent module to market using 600-gigabit wavelengths. The module supports up to 64-ary quadrature amplitude modulation (64-QAM) and a symbol rate of 69 gigabaud (GBd).
Systems vendors such as Ciena, with its WaveLogic 5, and Infinera, with its Infinite Coherent Engine 6 (ICE6), responded with their next-generation coherent designs that use symbol rates approaching 100GBd and support an 800-gigabit wavelength.
Sell-side research analysts interpreted the coherent developments as Acacia having a window of opportunity to exploit the AC1200 until the systems vendors’ coherent designs come to market in the coming year. The analysts also noted how 400 Gigabit Ethernet client signals better fit in an 800-gigabit wavelength compared to a 600-gigabit wavelength.
Then, in July, Acacia’s status as a merchant coherent technology supplier changed with the announcement that Cisco Systems is to acquire the company for $2.6 billion. Now, Acacia has detailed the SC2 as its acquisition awaits completion.
AC1200-SC2
The SC2 uses the same form factor and electrical connector as the AC1200 module, simplifying the upgrading of system designs using the AC1200. However, the SC2 module uses a single fibre pair for its optical output whereas the AC1200 uses two pairs, one for each channel.
The SC2 module shares the same Pico coherent digital signal processor (DSP) and baud rates as the AC1200. The Pico DSP uses fractional quadrature amplitude modulation (QAM) and an adjustable baud rate.
Fractional QAM allows the tuning of the transmitted data rate by using a mix of adjacent modulation formats. For example, 8-QAM and 16-QAM are alternated, and the percentage of time each is used determining the resulting data rate. In turn, the baud rate can be increased to widen the signal’s spectrum, if the optical channel permits, such that using a lower modulation scheme may become possible, improving the reach (see diagram above).
The AC1200 uses 50GHz- and 75GHz-wide channels while the SC2 uses 50-150GHz channels. For 600-gigabit and 1.2-terabit transmissions, the widest channels are used: 75GHz for the AC1200, and 150GHz for the SC2. “But as you go down in data rate, you can address the transmission in multiple ways,” says Williams. “You can run a higher modulation scheme in a narrow channel or, with a wider channel, run a lower modulation scheme to go further.”
The result optical performance means that the SC2 can be used for multiple applications: from short-span data centre interconnect where the full 1.2-terabit capacity is sent using 64-QAM, to metro-regional and long-haul distances using 800-gigabit and 16-QAM, all the way to ultra-long-haul terrestrial and subsea links with 400-gigabitand quadrature phase-shift keying (QPSK) modulation.
The AC1200 and the SC2 have comparable optical performance in terms of spectral efficiency and reach. This is unsurprising given how both modules use the same Pico DSP, baud rates and modulation schemes.
The AC1200 uses two 75GHz channels, each carrying 600 gigabits, to send 1.2 terabits, while the SC2 uses two sub-carriers in a 150GHz channel. However, the SC2 has a slight advantage since no guard band is needed between the two channels as is required with the AC1200 (unless the AC1200 is sending a two-channel ‘superchannel’ whereby no dead zone is needed between the channels).
Acacia is not detailing how it generates the optical sub-carriers besides saying the change stems from the interface between the Pico DSP and its silicon photonics-based photonic integrated circuit (PIC). The company will also not say if the SC2 uses a new PIC design.
Operational benefits
The fact that the SC2 and AC1200 deliver the same reach and capacity may explain why Acacia downplays the argument that the company has again leapfrogged its rivals with the advent of a module that sends 1.2 terabits over a single channel.
Instead, Acacia stresses the system and operational benefits resulting from doubling the data transmitted per channel.
“The SC2 module allows the entire capacity to be managed as a single channel,” says Williams. “The original [AC1200] module is well-suited to brownfield networks operating with 50GHz or 75GHz spacing, while the SC2 offers advantages in greenfield network architectures that can use channel plans up to 150GHz.”
Using a higher-capacity channel requires fewer optical components and reconfigurable optical add/ drop multiplexer (ROADM) ports thereby reducing networking costs, says Williams.
Using 150GHz-wide channels also aligns with an emerging consensus among network operators regarding wavelength roadmaps. “Network operators want to operate on some standardised grid based on regular multiples [50GHz, 75GHz] because it avoids fragmentation,” says Williams.
Availability
Acacia is already providing the SC2 module to certain customers that are undertaking validation testing. The firm is ready to ramp production based on particular customer demand.
Acacia will also be demonstrating its latest module at this week’s ECOC show being held in Dublin.
Acacia eyes pluggables as it demos its AC1200 module
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.
Source: Gazettabyte
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.
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“There is a trend to pluggable, not just the 400ZR but the CFP2-DCO [400-gigabit] for metro”
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”.
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.
Acacia announces a 1.2 terabit coherent module
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.