In a paper delivered today at OFC/NFOEC 2007 in Anaheim, CA, Alcatel-Lucent describes how it has developed a tunable optical waveguide equalizing filter that is fabricated entirely in a CMOS manufacturing line, the same manufacturing technology that produces electronic integrated circuits. Alcatel-Lucent says its breakthrough technology “eliminates the package walls separating the photonic circuit from the electronic circuit,” thereby opening the door to new optical networking architectures.
While the electronics industry is a multibillion-dollar per year industry, the photonics industry remains essentially a boutique operation. Components are built on separately optimized material platforms, including lithium niobate (LiNbO3), indium phosphide (InP), and indium gallium arsenide (InGaAs). As a result, the photonics market is fractured; each material sees a portion of the market, and none of the materials enjoy the economies of scale inherent in the electronics world.
“What we can envisage,” says Alice White, vice president of enabling technologies at Alcatel-Lucent Bell Labs, “is a world in which these two worlds—electronics and photonics—come together seamlessly on a single platform.” Consider, for example, her recently purchased hybrid car. “This car chooses when to use the gas engine and when to use the electronic motor based on which is best, efficiency-wise and performance-wise,” she explains. “Sometimes it uses both together, but it does this seamlessly. We could imagine, down the road, a situation in which the electronics and photonics are on the same chip. And thinking about trying to do that gives us some real architecture and design advantages,” White notes.
Of course, White and her team have gone beyond just thinking about it. They say they have developed a CMOS-compatible tunable optical equalizer that leverages inherent advantages from both the electronic and photonic worlds.
The research is funded by the Defense Advanced Research Projects Agency’s (DARPA’s) EPIC Program, or Electronic and Photonic Integrated Circuits Program. Alcatel-Lucent Bell Labs’ partners on the project include BAE Systems, which provides the CMOS foundry; MIT; Cornell University; and Applied Wave Research, a computer-aided design (CAD) vendor.
As dictated by DARPA’s EPIC Program, all deliverables have to be made in a commercial foundry. And while she admits that this was a formidable challenge, White also notes, “The fact that we can do it makes this all the more interesting. It really leapfrogs the tech transfer issue.”
Silicon-based tunable optical equalizer
Bell Labs says its zero/pole filters enable network operators to clean signals within a transmission channel on the silicon chip—either directly or by modifying the signal in anticipation of later distortion—as well as balance the power of different transmission channels. The key to the demonstration is a new control configuration that uses a single voltage to adjust the signal equalization and an innovative new architecture to realize complex responses in a low-order filter.
The base structure of the filter is a Mach-Zehnder interferometer. The filter itself is symmetric; light comes into the filter and is split among two ring resonators on the upper arm of the filter and two ring resonators on the lower arm. “What the rings do is create a nonlinear response in frequency,” explains Doug Gill, MTS, Integrated Photonics Research, Alcatel-Lucent Bell Labs. “The resonance makes the phase change in a very nonlinear way. We control how sharply that phase changes and where the phase change is located in frequency space. We call it ‘The Dance of the Resonances.’ We align them and tune them to get the response that we want,” he says.
At the output of the filter, the light from the two arms interferes constructively and destructively to get the desired type of magnitude response. “But,” says Gill, “you can also get the type of phase response that you want, and that’s why this filter can compensate both for intensity distortion across the profile of the channel and phase distortion across the profile of the channel.”
Sanjay Patel, technical manager of Integrated Photonics Research at Ball Labs says the team purposely selected a filter design for its first implementation of the technology in part because optical loss has historically been an issue. Silicon, by contrast, is a high index contrast material, which has enabled Bell Labs to make very tight bends and create an optical circuit that is much smaller than, say, a planar lightwave circuit (PLC)-based device. “It is about a factor of twenty smaller,” Patel reports.
Moreover, he asserts, using a combination of silicon and optics gives you “the advantage of doing the optical compensation ranges in compact form factors with electronic assist. So you kind of get the best of all worlds, and you can say, ‘I know where optics provides an advantage, and I know where electronics provides an advantage.’ I can seamlessly move between the two and complement the necessary strengths that both of these bring to the area of dispersion compensation, just as an example,” he adds.
Particularly in the case of dispersion compensation, there are advantages to doing that in the optical domain, notes Gill. When you go into the electrical domain, you lose the phase information of the signal. “So here is an example of where there is an inherent advantage of doing that particular process in the optical domain,” Gill says. “And then you can have the support of the electronics around that process to add electronic dispersion compensation on top to whatever degree might be necessary.”
While generally available technologies are not expected for 3 to 5 years, synergistically combining such optical filters with on-chip electronic circuits can provide a commercially viable path to providing reconfigurable, low-cost, low-power-consumption devices that could fit into small-form-factor pluggable modules, say Bell Labs representatives. These new devices also are ideal for dual-use applications in systems that route data over both electronic and optical networks, depending on the most appropriate delivery and/or transport format.
Eliminating walls
At the end of the day, says Bell Labs, this announcement is about tearing down the barriers between electronics and photonics that exist both in the physical world and in the mindset of network engineers. “What we’re trying to do here is stop that [mentality of] ‘Do it electronically’ or ‘Do it photonically,’” says White. “Instead, let’s just say, ‘In this particular functionality, what makes the most sense?’ And then not have a package wall separating the photonic circuit from the electronic circuit.”
Every time you have a package wall, adds Gill, you increase the cost and size of the device, and you introduce constraints on design. “When you have a package wall, you have to have some common standard mating characteristic between the various packages. Eliminating those package walls opens up new dimensions in component design.”
“We’re really thinking about these things in terms of not a fight between electronics and optics but a collaboration,” muses Patel. “There are synergies.”