Advanced networks need advances in components

Mike Scott is vice president for technology and product development in high-performance optical-component solutions at Nortel Networks (Brampton, Ont., Canada). He has a doctorate in materials from University of Cambridge, England, and 20 years experience in the optical communications industry.

WDM Solutions: Nortel Networks has been a leader in developing and deploying WDM systems, especially for 10-Gbit/s transmission. What fundamental component technologies will be necessary for the next-generation of long-haul DWDM systems?

Scott: There are two ways of getting more capacity on a fiber—higher line rates or more channels. For higher line rates, electronics are critical and will be a significant differentiator. To get to 40 Gbit/s you'll need gallium arsenide electronics such as HBTs (heterojunction bipolar transistors) and HEMTs (high-electron-mobility transistors). At even faster speeds you are likely to see indium phosphide-based electronics making an entrance. I do not believe that silicon germanium will be used at these speeds, but it is increasingly being used for 10-Gbit/s applications—an area where a few years ago gallium arsenide dominated.

You're also going to need high-speed receivers and modulators. Lithium niobate and gallium arsenide are the most promising modulators.

In terms of adding more WDM channels, there will always be a tradeoff between channel spacing and speed. I don't see a major discontinuity in passive components in going from 10 to 40 Gbit/s, though the specifications on the filters will need to be tighter.

Management of fiber nonlinearities becomes a much bigger challenge at higher line rates, so there will be a need for components to correct for both chromatic and polarization-mode dispersion.

WDM Solutions: What about the use of technologies such as Raman amplification and solitons?

Scott: Raman amplification is very useful but will be used as a supplement to the EDFA [erbium-doped fiber amplifier] to obtain greater reach and to maintain signal-to-noise ratios at higher rates.

My personal feeling on solitons is that they are very difficult to manage and will probably not prove out. The use of RZ pulses can be considered a pseudo-soliton approach.

WDM Solutions: On a different subject, how will the acquisition of the JDS Uniphase pump-diode facility in Zurich, Switzerland, be integrated into Nortel?

Scott: The Zurich facility will become part of Nortel's high-performance optical-component solutions business and provide the laser chips for our pump-laser modules. We will sell the chips to other laser-module makers on the open market and use them internally for our own products. In acquiring the facility, we not only acquired a manufacturing site that makes the most reliable pump-laser chips in the world, but we got a very talented pool of people as well.

The most important issues are cost and space. You've got to have low manufacturing costs, which will drive automation in an industry that still employs an awful lot of people to hand-assemble products. Size is another key issue. A metric I like to quote is gigabits per second per cubic meter. This will drive higher levels of integration.

WDM Solutions: What about wavelength tunability?

Scott: Tunability is a hot topic right now. The immediate benefit to service providers is for sparing, where a tunable laser can greatly reduce the need for backup lasers. For system manufacturers, [tunable lasers] significantly reduce the inventory they need to carry. Tunable lasers will also provide the ability to do wavelength switching and wavelength conversion, and so allow providers to offer wavelength-on-demand services.

WDM Solutions: What do you see in the future for integration in component design?

Scott: We have to be very careful here. Monolithic integration at the component level is often touted as solving many problems, and it's often compared to the integration seen in the electronics world. But optics and electronics are very different. IC integration means integrating many thousands of identical functions on a common substrate. Photonics are different: one needs to integrate a number of very different functions, each of which is often best achieved in a very different material system. There are some examples where this makes sense; the integrated laser and modulator is a good example, and I believe that the interconnect problems at high speeds will lead us to revisit monolithically integrated receivers. We are also likely to see integration of some passive functions. I am a great fan of hybrid integration, particularly onto silicon. This is the obvious route to achieving the economies of manufacturing and space that will be needed.

WDM Solutions: When will we see the widespread deployment of all-optical switches?

Scott: As you know, Nortel bought Xros for its MEMS-switching capabilities. Optical switching will become a core enabler of future agile optical networks, but I am not in a position to tell you when we might release products.

WDM Solutions: How much of your manufacturing process is automated and how important is that to you?

Scott: I can't give you exact numbers, but I can tell you that automation is absolutely vital to this industry, and we're automating considerable portions of our components and module manufacturing lines.

WDM Solutions: What potential next-generation technologies are you watching closely?

Scott: Well, there's lots of interesting work going on right now and I am obviously not going to tell you what we see as the big differentiators. There's been a lot of hype surrounding photonic bandgap technologies, and I am sure they will eventually offer novel functionality once someone works out how to produce them in volume. I also think that polymeric materials have a future so long as the stability issues can be solved.

WDM Solutions: So what do you see as your biggest challenge—what keeps you awake at night?

Scott: Nothing keeps me awake at night! But I can tell you that we intend to be the world leader in components. Our goal is to continue to provide our customers and their customers with timely, cost-effective, innovative solutions. That is a real challenge that we intend to win.