The new technology, which also makes use of other exotic materials, such as yttrium-iron-garnet, is aimed at components for dense wavelength-division multiplexing (DWDM) in the rapidly growing metropolitan-area optical network market. Within a few months the company expects to have a full, automated wafer fab up and running, and will use it to offer a cost-competitive optical-application integrated-circuit service.

"Optical-switching technology is at the point where electronics was in the 1960s — based exclusively on components," said Fadi Daou, Telephotonics' vice president of product management. "If you look at any current optical system, what you see is a spaghetti bowl of components connected with optical fibers."

After doing market research on the optical-components industry, Daou believed that, based on some recent technological advances, the market was ripe for the same kind of integration strategy that has fueled the growth of electronics. He was particularly impressed with the work of Louay Eldada on thermo-optic polymers, and Daou convinced Eldada to join an enterprise that would evolve a new integrated photonics. Eldada, who had done extensive work on optical polymers in both academic and industrial labs, is now the chief technical officer and a co-founder of Telephotonics (Wilmington, Mass.).

"The basic strategy is to use silicon wafers and the processes that have evolved in the electronics industry as a basis," Daou said. "To that we add a special optical polymer that is used to establish optical interconnects on the wafer. Researchers have been trying to do that for some time, but I was attracted to Louay's work on thermo-optic polymers because I felt it would work."

Tunable components

The company is not ready to disclose the specific polymer being used as the "platform" for its integrated optical technology, but the basic operation is a change in refractive index in response to temperature changes.

"We first 'pixelate' the substrate with electronic contacts and then pattern polymer waveguides onto the chip with a low-temperature process," explained Dave Freihofer, vice president of marketing. "This first step produces the capability of building tunable filters and optical multiplexers on a silicon chip. In addition, the integrated systems can perform optical-signal monitoring and modulation. The electronic subsystems are able to activate a change in refractive index by locally changing the temperature of the polymer."

That capability can be applied in novel ways to create tunable optical components, the company maintains. One important example is a tunable filter than can scan for wavelengths over the entire DWDM range. The component is simply a diffraction grating induced within the waveguide by varying the refractive index with temperature. The temperature variation is controlled by the electronics, a scheme that would make it possible to implement wavelength-based service provisioning remotely.

"Right now, service providers have to roll a truck out to a site and change components, since all current waveguides only operate at a fixed wavelength," Daou said. "So this technology can also offer cost savings at higher levels in metro networks."

Telephotonics had to solve some hard problems in order to make this scheme workable. First of all, optical propagation in a waveguide is more complex than electron conduction in copper or aluminum. The light tends to scatter when the direction of the waveguide changes. That problem was solved by a dual-material approach to create a high-refractive-index cladding that reflects the photons back into the core. Just as in glass optical fibers, the polymer channels are able to bend around corners with negligible loss, Daou explained.

The other problem was the effect that variations in ambient temperature can have on the performance of the thermally activated components. "We were able to license some [intellectual property] from Columbia University that controls those effects," Daou said.

With the basic implementation problems of the combined polymer and silicon electronics solved, the company plans to introduce more functionality with magneto-optic effects.

The material that will be the basis for magneto-optics is yttrium-iron-garnet (YIG), an inorganic crystal that is physically quite different from the polymer. "The availability of this material is very limited, so we have a couple of physicists working full-time on producing pure crystalline YIG in-house," said Daou.

YIG will allow Telephotonics to integrate optical isolators — essentially an optical diode — and circulators, which are used to add or drop wavelength channels from a DWDM fiber. The magneto-optic effect in general will produce asymmetric operations with light by allowing the control of polarization with electronics.

Not only will the company be able to integrate these polarization-control components on-chip, but the problem of optical alignment between the magneto-optic components and the optical waveguides will be automatically solved by the integration process.

One significant function that the magneto-optics will add to the integrated optical circuits is dispersion control. Currently this function is most critical in long-haul optical networks, for which large subsystems are built to control optical dispersion over long distances.

However, the same problem is starting to appear in metropolitan-area networks as system bit rates skyrocket. "We now have a means of controlling dispersion at high bit rates right on the chip," Daou said.

"Apart from some unusual materials that we pattern onto wafers at room temperature, the fab line operates just like a typical chip-manufacturing line," Daou said.

Currently it takes about four hours to process a wafer, after which it is diced and the die tested. Telephotonics is offering samples now.