Alcatel-Lucent demos dual-carrier Terabit transmission
"Without [photonic] integration you are doubling up your expensive opto-electronic components which doesn't scale"
Peter Winzer, Alcatel-Lucent's Bell Labs
Part 1: Terabit optical transmission
Alcatel-Lucent's research arm, Bell Labs, has used high-speed electronics to enable one Terabit long-haul optical transmission using two carriers only.
Several system vendors have demonstrated one Terabit transmission including Alcatel-Lucent but the company is claiming an industry first in using two multiplexed carriers only. In 2009, Alcatel-Lucent's first Terabit optical transmission used 24 sub-carriers.
"There is a tradeoff between the speed of electronics and the number of optical modulators and detectors you need," says Peter Winzer, director of optical transmission systems and networks research at Bell Labs. "In general it will be much cheaper doing it with fewer carriers at higher electronics speeds than doing it at a lower speed with many more carriers."
What has been done
In the lab-based demonstration, Bell Labs sent five, 1 Terabit-per-second (Tbps) signals over an equivalent distance of 3,200km. Each signal uses dual-polarisation 16-QAM (quadrature amplitude modulation) to achieve a 1.28Tbps signal. Thus each carrier holds 640Gbps: some 500Gbps data and the rest forward error correction (FEC) bits.
In current 100Gbps systems, dual-polarisation, quadrature phase-shift keying (DP-QPSK) modulation is used. Going from QPSK to 16-QAM doubles the bit rate. Bell Labs has also increased the symbol rate from some 30Gbaud to 80Gbaud using state-of-the-art high-speed electronics developed at Alcatel Thales III-V Lab.
"To achieve these rates, you need special high-speed components - multiplexers - and also high-speed multi-level devices," says Winzer. These are indium phosphide components, not CMOS and hence will not be deployed in commercial products for several years yet. "These things are realistic [in CMOS], just not for immediate product implementation," says Winzer.
Each carrier occupies 100GHz of channel bandwidth equating to 200GHz overall, or a 5.2b/s/Hz spectral efficiency. Current state-of-the-art 100Gbps systems use 50GHz channels, achieving 2b/s/Hz.
The 3,200km reach using 16-QAM technology is achieved in the lab, using good fibre and without any commercial product margins, says Winzer. Adding commercial product margins would reduce the optical link budget by 2-3dB and hence the overall reach.
Winzer says the one Terabit demonstration uses all the technologies employed in Alcatel-Lucent's photonic service engine (PSE) ASIC although the algorithms and soft-decision FEC used are more advanced, as expected in an R&D trial.
Before such one Terabit systems become commercial, progress in photonic integration will be needed as well as advances in CMOS process technology.
"Progress in photonic integration is needed to get opto-electronic costs down as it [one Terabit] is still going to need two-to-four sub-carriers," he says. A balance between parallelism and speed needs to be struck, and parallelism is best achieved using integration. "Without integration you are doubling up your expensive opto-electronic components which doesn't scale," says WInzer.
AppliedMicro samples 100Gbps CMOS multiplexer
AppliedMicro has announced the first CMOS merchant multiplexer chip for 100Gbps coherent optical transmission. The S28032 device supports dual polarisation, quadrature phase-shift keying (DP-QPSK) and has a power consumption of 4W, half that of current multiplexer chip designs implemented in BiCMOS.
The S28032 100 Gig multiplexer IC. Source: AppliedMicro
"CMOS has a very low gain-bandwidth product, typically 100GHz," says Tim Warland, product marketing manager, connectivity solutions at AppliedMicro. “Running at 32GHz, we have been able to achieve a very high bandwidth with CMOS."
Significance
The availability of a CMOS merchant device will be welcome news for optical transport suppliers and 100Gbps coherent module makers. CMOS has better economics than BiCMOS due to the larger silicon wafers used and the chip yields achieved. The reduced power consumption also promotes the move to smaller-sized optical modules than the current 5x7-inch multi-source agreement (MSA).
"By reducing the power and the size, we can get to a 4x6-inch next-generation module,” says Warland. “And perhaps if we go for a shorter [optical transmission] reach - 400-600km - we could get into a CFP; then you can get four modules on a card.”
"Coherent ultimately is the solution people want to go to [in the metro] but optical duo-binary will do just fine for now"
Tim Warland, AppliedMicro
Chip details
The S28032 has a CAUI interface: 10x12Gbps input lanes that are multiplexed into four lanes at 28Gbps to 32Gbps. The particular data rate depends on the forward error correction (FEC) scheme used. The four lanes are DQPSK-precoded before being fed to the polarisation multiplexer to create the DP-QPSK waveforms.
The device also supports the SFI-S interface - 21 input channels, each at 6Gbps. This is significant as it enables the S28032 to be interfaced to NTT Electronics' (NEL) DSP-ASIC coherent receiver chip that has been adopted by 100Gbps module makers Oclaro and Opnext (now merged) as well as system vendors including Fujitsu Optical Systems and NEC.
The mux IC within a 100Gbps coherent 5x7-inch optical module. Source: AppliedMicro
The AppliedMicro multiplexer IC, which is on the transmit path, interfaces with NEL's DSP-ASIC that is on the receiver path, because the FEC needs to be a closed loop to achieve the best efficiency, says Warland. "If you know what you are transmitting and receiving, you can improve the gain and modify the coherent receiver sampling points if you know what the transmit path looks like," he says.
The DSP-ASIC creates the transmission payloads and uses the S28032 to multiplex those into 28Gbps or greater speed signals.
The SFI-S interface is also suited to interface to FPGAs, for those system vendors that have their own custom FPGA-based FEC designs.
"Packet optical transport systems is more a potential growth engine as the OTN network evolves to become a real network like SONET used to be"
Francesco Caggioni. AppliedMicro
The multiplexer chip's particular lane rate is set by the strength of the FEC code used and its associated overhead. Using OTU4 frames with its 7% overhead FEC, the resulting data rate is 27.95Gbps. With a stronger 15% hard-decision FEC, each of the 4 channel's data rate is 30Gbps while it is 31.79Gbps with soft-decision FEC.
"It [the chip] has got sufficient headroom to accommodate everything that is available today and that we are considering in the OIF [Optical Internetworking Forum],” says Warland. The multiplexer is expected to be suitable for coherent designs that achieve a reach of up to 2,000-2,500km but the sweet spot is likely to be for metro networks with a reach of up to 1,000km, he says.
But while the CMOS device can achieve 32Gbps, it has its limitations. "For ultra long haul, we can't support a FEC rate higher than 20%," says Warland. "For that, a 25% to 30% FEC is needed."
AppliedMicro is sampling the device to lead customers and will start production in 1Q 2013.
What next
The S28032 joins AppliedMicro's existing S28010 IC suited for the 10km 100 Gigabit Ethernet 100GBASE-LR4 standard, and for optical duo-binary 100Gbps direct detection that has a reach of 200-1,000km.
"Our next step is to try and get a receiver to match this chip," says Warland. But it will be different to NEL's coherent receiver: "NEL's is long haul." Instead, AppliedMicro is eyeing the metro market where a smaller, less power-hungry chip is needed.
"Coherent ultimately is the solution people want to go to [in the metro] but optical duo-binary will do just fine for now," says Warland.
Two million 10Gbps OTN ports
AppliedMicro has also announced that it has shipped 2M 10Gbps OTN silicon ports. This comes 18 months after it announced that it had shipped its first million.
"OTN is showing similar growth to the 10 Gigabit Ethernet market but with a four-year lag," says Francesco Caggioni, strategic marketing director, connectivity solutions at AppliedMicro.
The company sees OTN growth in the IP edge router market and for transponder and muxponder designs, while packet optical transport systems (P-OTS) is an emerging market.
"Packet optical transport systems is more a potential growth engine as the OTN network evolves to become a real network like SONET used to be," says Caggioni. "We are seeing development but not a lot of deployment."
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