Portland, Ore. -- Converting from optical to electrical signals, then back to optical, is the bane of modern networks, often requiring a $10,000 optical-to-electronic converter just to perform some simple signal processing, then another $10,000 electronic-to-optical converter to put the signal back on the fiber-optic cable. Now researchers have invented a method that merges electronics with optics by inserting semiconductor devices inside a hollow optical fiber, potentially integrating the electronic signal-processing functions into the cable carrying the signal.

"The idea is to fuse optics with electronics by building semiconductors right into the structure of the fibers, enabling the properties of those semiconductors to be used inside optical fibers," said professor John Badding at Pennsylvania State University (University Park). "Eventually we want to use integrated semiconductors to make light in the fiber, detect light in the fiber, modulate light in the fiber, reshape pulses--whatever you needed out-board electronics to perform before." Badding performed the research with Penn State colleagues Venkatraman Gopalan and Vincent Crespi, as well as with Pier Sazio, a senior research fellow in the Optoelectronics Research Center at the University of Southampton, England.

The technique adapts chemical vapor deposition, at 1,000-times atmospheric pressure and at temperatures up to 500°C, to coat the inside of the hollow cores of fiber-optic cables with semiconductors at a rate of tens of nanometers per minute. At the end of the process, the hollow cores--which may measure 100 nm to 5 microns in diameter--close down to as small as 10 nm.

So far the researchers have successfully fabricated silicon germanium heterojunctions inside a fiber, demonstrated that the fiber still behaves as a waveguide, then implemented a field-effect transistor (FET) that could modulate the signal passing though the core.

"We have demonstrated our chemical vapor deposition technique by building a depletion-mode FET inside a fiber, with the [metallic] gate wrapped around the outside," said Badding. "First we deposited a semiconducting wire inside the fiber so that we could pass current through that wire and modulate it by changing the gate bias. Then we showed that the semiconductor core of these fibers can guide 1.5-micron light."

Usable systems taking this approach are likely to be years away from commercialization. However, the fruit of that labor could realize the elusive goal of optical networks that do not require expensive optical-to-electrical-to-optical converters. "The basic idea is simple: to use the properties of semiconductors to generate, modulate and detect light in an optical-fiber geometry," said Badding. "We are just beginning to explore all the possibilities, but the potential to integrate optical fibers with semiconductors is revolutionary."

To turn their current proof-of-concept demonstrations into a manufacturable technology, the research team is experimenting with lithographic techniques to pattern the semiconducting core of a fiber-optic cable. Since circuits must fit into micron-size cores that are meters long, the aspect ratio of these patterns will be narrow and long. Yet the researchers say that no technological barriers need be surmounted; it will just take a lot of hard engineering effort to realize their goal.

By 2007, the researchers hope to demo a device that integrates a common electronic signal-processing function into a fiber-optic cable.