Fiber-optic cables are an Internet and telephone communications backbone, connecting buildings, cities, and nations. Below the Atlantic Ocean, for example, lie thousands of kilometers of fiber that can carry about one million simultaneous phone calls.
But these lines require external electronic devices to generate, amplify, receive, and manipulate the data. And the process of moving the data between optical fibers and these electronic devices consumes enormous amounts of power, requires costly technology, and reduces overall bandwidth.
Now researchers at the University of South Hampton in the U.K. and Penn State University have found a way to put the semiconductor materials for those electronic devices directly into the fiber, and have fabricated a working transistor within a fiber. They achieved this by turning the capillaries of ordinary optical fiber into chemical reaction chambers – they deposited high-quality semiconductor materials of crystalline silicon and germanium into fiber by tweaking a conventional chip-making process.
[For an image of the fiber with semiconductor material in it, click here.]
“We’re fusing two very important bits of technology,” says Pier Sazio, research fellow at the University of Southampton, and lead author on the March 17 Science paper about their work. “The leap we’ve made here is being able to introduce technically important semiconductor material into fiber.” While the work is early-stage research, it represents a clear advance. His group is the first to use a conventional semiconductor deposition technique to produce electronics – made of high-quality crystalline semiconductors found in computer chips – inside the capillary of a fiber.
Specifically, the researchers were able to deposit germanium, a common semiconductor material, into fiber capillaries 70 centimeters long and only 25 nanometers in diameter. By etching away some fiber and affixing electrical contacts, the researchers were also able to fabricate a basic transistor inside a fiber capillary 11 millimeters long and five micrometers in diameter. This first inter-fiber transistor was crude, and merely a proof of principle, says John Badding, professor of chemistry at Penn State University and an author on the paper.
Further research is needed to go from transistors to more complicated devices – such as lasers for producing light, detectors for receiving it, and modulator that separate wavelengths of light and disperse them through appropriate channels. But Badding expects that further fine-tuning of deposition conditions, such as temperature gradients and chemical purity, will make this possible.
While the integration of electronic devices into telecommunication fiber is still many years away, the researchers say that such an electro-optic fiber could save time, energy, and money, because the signal would not have to leave the fiber and be processed in an external electric device. “Right now, we’re really good at sending torrential amounts of information under the ocean, converting it to electrical signals, and routing it,” Badding says. “That’s expensive and requires megawatts of electricity. To route that information inside a fiber would be revolutionary.”
The team’s electro-optic fiber was developed by taking advantage of the geometry of conventional optic lines, which are made of hollow, capillary-like glass tubes bundled together. One of the challenges of depositing semiconductors into the tubes – which can range in shape from circular to hexagonal, with diameter from nanometers to micrometers – is to uniformly apply the material down the length of the tube without clumps or defects, Badding says. Sending semiconductor compounds into these tubes is “the equivalent of filling a garden hose over a mile long,” he says.
To overcome this challenge, the group modified the well-known deposition technique called chemical vapor deposition (CVD). In this process, compounds of silicon, germanium, or other semiconductors are vaporized and typically sprayed onto flat substrates. Badding says his team used CVD, but forced the vaporized material through the long, thin capillaries at pressures as high as 1,000 times atmospheric pressure and many times more than conventional CVD. While the material filled the tubes, the researchers heated the fiber so that the material assembled into a crystallized semiconductor.
Another group that’s also working to integrate electronics into fiber is led by Mehmet Bayindir at MIT. This group adds semiconductor materials, metals, and polymers to raw glass before the fiber is stretched into its eventual length (see Smart Fibers). The advantage is that “our technique uses traditional fiber drawing technology” and is completed in one step, says Ayman Abouraddy, a postdoc researcher at MIT who works with Bayindir. In contrast, using CVD to build electronics in fiber is a two-step process, in which the fiber is drawn and then the devices are deposited.
It would be challenging, Abouraddy suspects, to form a semiconductor the entire length of a meters-long fiber by using CVD. However, the technique could have applications where short fiber is needed, such as for a splice to replace an external switch or other electronic device, he says.
Abouraddy also notes that the crystalline semiconductors made by the Southampton and Penn State researchers produce faster detectors and light modulators than the slower, so-called amorphous semiconductors used by the MIT group. While slower detectors are good enough for medical applications, such as detecting concentrations of a chemical in the body, crystalline semiconductors are required in telecommunications networks, where information needs to be processed quickly.
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