10 Gbit/s dispersion compensation techniques abound

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While the bulk of today’s networks still operate at 2.5 Gbit/s transmission speeds, more and more carriers are moving to 10 Gbit/s, with an eye towards 40 and even 100 Gbit/s. However, chromatic dispersion, characterised as a spreading or broadening of a signal due to the propagation of different wavelengths at different speeds, represents a formidable hurdle to this higher-speed transmission. That said, several companies have developed and announced new dispersion compensation techniques in recent months, using optical or electronic technologies-or some combination of both.

The primary method of dispersion compensation in use today involves the deployment of spools of dispersion-compensating fibre (DCF), but such fibre is both expensive and inefficient, necessitating alternative compensation methods. “You have to have a certain amount of fibre very close to your equipment to compensate for the effects happening in the reel distance,” explains Oswin Schreiber, senior product marketing scientist at AMCC (www.amcc.com). “And your power budget looks horrible because you are putting additional fibre in without covering any distance.”06q4lwe Europe 18

As such, several companies have made recent announcements in the dispersion compensation space, including TeraXion (www.teraxion.com) and Civcom (www.civcom.com), both of which used the recent European Conference on Optical Communication (ECOC) to tout the benefits of optical dispersion techniques.

At the conference, Civcom unveiled its Free-Path Manageable Dispersion Compensation Module (M-DCM), designed as a DCF replacement in metro and regional DWDM networks. According to Yair Itzhar, vice president of international sales and marketing, the benefit of the M-DCM is three-fold: it’s tunable, it’s remotely manageable, and it’s multichannel. Based on the company’s existing etalon-based Tunable Optical Dispersion Compensator (TODC), the M-DCM compensates for chromatic dispersion values ranging from -1,700 ps/nm to +1,700 ps/nm.

TeraXion, meanwhile, demonstrated its fibre Bragg grating (FBG)-based ClearSpectrum Tunable Dispersion Compensator (TDC), which provides flexible chromatic dispersion management and colorless tuning at each receiver, say company representatives. It also can be used in reconfigurable optical add/drop multiplexed (ROADM) networks to perform simultaneous dispersion trimming of multiplexed DWDM channels. TeraXion says it has shipped more than 600 TDC modules to date.

On the other hand, silicon vendors such as AMCC, Broadcom (www.broadcom.com), and Scintera (www.scintera.com) argue that electronic dispersion compensation (EDC) is not only less expensive than optical dispersion techniques, it also enables the use of less expensive laser sources. Employed on the receive side of transponders/transceivers, EDC eliminates dispersion via an optical-to-electrical conversion.

Schreiber compares selecting the appropriate EDC technology with buying a new car; there are high-end options and low-end options. On the high end are technologies like Maximum Likelihood Sequence Estimation (MLSE), the preferred digital equalization method of 10 Gbit/s transponder vendor CoreOptics (www.coreoptics.com). On the low end is AMCC’s EDC technology, which can be integrated with the clock data recovery (CDR) function in an XFP transceiver, for example, to transmit up to 120 km. “It’s a tradeoff between power and performance,” says Schreiber, who explains that the MLSE version provides greater compensation but is more power-hungry than the low-end EDC.

The EDC chip vendors currently are developing the next generation of the technology, incorporating EDC into cost-optimised SFP+ form factors for short reach (<10 km) connections, for example. For tunable, higher-power applications, EDC also is being incorporated into the Extended XFP form factor or XFP-E, reports Schreiber.

If there is one subject upon which several of the proponents from both the EDC and optical camps seem to agree, it appears to be the emergence of duobinary technology, an alternative to the traditional non-return to zero (NRZ) transmission format.

“If carriers want to upgrade to 10 Gbit/s, they don’t want to touch the hardware that is buried in the ground, and they don’t want to put additional amplifiers in,” explains Schreiber. “That is why people are also working on schemes on the transmit side, like optical spectrum shaping and duobinary coding, to precondition the signal to go longer distances in the existing network.”

If you combine optical duobinary technology on the transmit side with EDC on the receive side, you could develop modules that transmit from 160 to 200 km, Schreiber explains. “That’s using EDC plus special shaping mechanisms on the optical side to launch the signal,” he says.

In fact, AMCC demonstrated its EDC technology in conjunction with AZNA Corp.’s (www.aznacorp.com) optical spectrum shaping-an alternative to optical duobinary-and achieved distances of up to 250 km in the test lab. AZNA has developed an optical spectrum shaping technology, implemented on its 10 Gbit/s Chirped Managed Laser (CML) that preconditions the launch to make it less dispersive as it traverses the fibre.

The optical folks also are interested in optical duobinary coding and alternative modulation schemes, which they are integrating into their existing optical dispersion compensation devices. To complement its TODC, for example, Civcom has developed a duobinary technology. Itzhar says it became increasingly important for Civcom to offer both technologies because “most Tier 1 and Tier 2 carriers I’m in contact with ask about duobinary.”

The company has developed a duobinary transponder capable of transmitting up to 200 km on its own, but when Civcom combined duobinary schemes with its existing TODC in a standard 300-pin transponder, it achieved distances of up to 350 km. Most metro networks are between 200 km and 350 km, Itzhar reports, so the combined duobinary and TODC technology (known as the Duobinary-TODC transponder) satisfies most metro network needs.

Fellow 10 Gbit/s optical transponder and subsystem vendor Kodeos Communications (www.kodeos.com) also has combined optical and electronic approaches, acquiring MLSE-based EDC vendor Intersymbol Communications in March to complement its existing optical duobinary technology. While products that combine the two technologies aren’t expected until next year, the company has introduced its 10 Gbit/s Marathon transponder line, which it claims supports longer distances and higher dispersion tolerance up to ±3,200 ps/nm using optical duobinary modulation.

Neil Salisbury, vice president of marketing at Kailight Photonics (www.kailight.com), believes there are several reasons why 40 Gbit/s networking has not been more readily adopted. The first is the high cost of the 40 Gbit/s transponders themselves and the second is the need to compensate for chromatic dispersion-for which recent advances in optical duobinary technology should help, he says. But the third problem, polarisation-mode dispersion (PMD), has not yet been solved, and this is where Kailight hopes to differentiate itself from its competitors.

PMD is caused by different polarisations within the signal traveling through the fibre at different speeds and arriving at different times. The net effect is a blurring or smearing of the received signal.

Kailight has developed an analog-based PMD mitigation technology, which it is integrating into its existing Tunable All-optical Signal Regenerator (TASR). “It is kind of a spin-out technology from our TASR line,” says Salisbury. While the company’s PMD mitigation technology is patent-pending, Salisbury will say that it creates an averaging effect on PMD in the network. “If your network is jumping all over the place very quickly, you are still getting the benefit [of the PMD mitigation], which you wouldn’t get if it were being done electrically,” he says. “Electrical is not fast enough.”

For his part, Salisbury is bullish on the use of analog or optical technology to compensate for dispersion. He cites a recent customer request for more than 100 ps of differential group delay (DGD) improvement at 10 Gbit/s and more than 30 ps of improvement at 40 Gbit/s. “Analog is the only way to get there,” he says, noting that Kailight’s 10 Gbit/s TASR with built-in PMD mitigation achieves those specifications. “I believe we are quoting better improvement than anyone who would be doing it electronically,” he adds.

Meghan Fuller is senior editor at Lightwave

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