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Optical processors from Ciena deliver flexible modulation

8 April 2011 
The latest 100 Gigabit ASICs from Ciena support programmable modulation schemes that trade transmission speed for optical reach. The WaveLogic coherent optical processors are already shipping inside Ciena’s ActivFlex 6500 packet-optical platform and will be added to the ActivSpan 4200 advanced services platform in the coming months.
The latest generation of WaveLogic comprises the T2 transmitter and R2 receiver ASICs. The T2 device implements dual-polarisation bipolar phased-shift keying (DP-BPSK) at 40Gbps for ultra-long-haul transmission, or dual-polarisation quadrature phase-shift keying (DP-QPSK) at 40 and 100Gbps. The R2 chip comprises four analogue-to-digital converters that sample the received 100Gbps signal channels and a digital signal processor (DSP) to compensate for the various dispersions introduced by the fibre including chromatic dispersion, polarisation mode dispersion and polarisation dependent loss.
“These [WaveLogic] devices build on Moore’s Law,” says Mark Gibbon, optical transport product line leader, EMEA, at Ciena. “Much of the innovation in optical transmission is starting to come through on the digital signal processing side.” 
Ciena has not detailed what CMOS process node technology it is using for the WaveLogic devices. But it has said that future WaveLogic devices will support 400Gbps and 1Tbps speeds; Ciena has already demonstrated 400Gbps optical transmission using 16-QAM, which
adds amplitude modulation to phase and polarisation modulation.
Coherent detection improves optical transmission in several ways, says Gibbon. It enables more cost-effective 100Gbps light paths; it simplifies deployments due to its ability to tackle a variety of fibre dispersion impairments; and enables more agile optical networking since the technology is flexible enough to adapt to route changes when light paths are switched.
Ciena plans to introduce sophisticated, soft-decision forward error correction (FEC) in future versions of WaveLogic to enhance optical performance. Instead of making absolute decisions on a state of the received symbol, soft-decision FEC attaches a probability to the symbol’s possible state. This results in a more powerful coding scheme and enhanced optical transmission performance. However, Gibbon stresses that traditional, hard-decision FEC continues to improve, as the latest WaveLogic optical processor demonstrates.
By Roy Rubenstein