New Process Builds Electronic Function into Optical Fibre

Optical fibre helped bring us the Internet, and silicon/germanium devices brought us microelectronics. Now, a joint team from the University of Southampton and Penn State University has developed a new way to combine these technologies. The team has made semiconductor devices, including a transistor, inside microstructured optical fibres. The resulting ability to generate and manipulate signals inside optical fibres could have applications in fields as diverse as medicine, computing, and remote sensing devices.

Optical fibre has proved to be the ideal medium for transmitting signals based on light, while crystalline semiconductors are the best way to manipulate electrons. One of the greatest current technological challenges is exchanging information between optics and electronics rapidly and efficiently. This new technique may provide the tools to cross the divide. The results of this research will be published in the 17 March edition of the journal Science. 

"This advance is the basis for a technology that could build a large range of devices inside an optical fiber," said John Badding, associate professor of chemistry at Penn State University. While the optical fibre transmits data, a semiconductor device allows active manipulation of the light, including generating and detecting, amplifying signals, and controlling wavelengths. "If the signal never leaves the fiber, then it is faster, cheaper and more efficient," said Badding.

"This fusion of two separate technologies opens the possibility of true optoelectronic devices that do not require conversion between optical and electronic signals," said Pier Sazio, senior research fellow in the Optoelectronics Research Centre at the University of Southampton (UK). "If you think of the fibre as a water main, this structure places the pumping station inside the pipe. The glass fibre provides the transmission and the semiconductor provides the function."

Beyond telecommunications, optical fibres are used in a wide range of technologies that employ light. "For example, in endoscopic surgery, by building a laser inside the fibre you might be able to deliver a wavelength that could not otherwise be used," said Badding.

The key breakthrough was the ability to form crystalline semiconductors that nearly fill the entire inside diameter, or pore, of very narrow glass capillaries. These capillaries are optical fibres--long, clear tubes that can carry light signals in many wavelengths simultaneously. When the tube is filled with a crystalline semiconductor, such as germanium, the semiconductor forms a wire inside the optical fibre. The combination of optical and electrical capabilities provides the platform for development of new optoelectronic devices.

The crystals were formed using chemical vapor deposition (CVD) to deposit germanium and other semiconductors inside the long, narrow pores of the hollow optical fibre. In the CVD process, a germanium compound is vaporized and then forced through the pores of the fibre at pressures as high as 1000 times atmospheric pressure and temperatures up to 500°C. A chemical reaction within the fibre allows germanium to coat the interior walls of the hollow fiber and then form crystals that grow inward. "The process works so perfectly that you can get a germanium tube that has an opening in the center of only 25 nanometers through the length of the fibre," said Sazio. "This is only a tiny fraction of the diameter of the fibre pore, so it is essentially a wire." This is the first demonstration of building crystalline structures, which are best for semiconductor devices, inside the pores of the capillaries.

The team has built a simple in-fibre transistor, and they point to the success of the Erbium Doped Fibre Amplifier, which was invented at Southampton in the late 1980s, to illustrate the transformational possibilities of this technology. By incorporating the chemical element erbium into the fibre, the Erbium Amplifier allows efficient transmission of data signals in transoceanic optical fibres. "Without that kind of device, it would be necessary to periodically convert the light to an electronic signal, amplify the signal, and convert it back to light, which is expensive and inefficient" said Sazio. "Since its inception, the Erbium Amplifier has made the internet possible in its current form."

Beyond the simple devices that this research has demonstrated, the research team sees the potential for the embedded semiconductors to carry optoelectronic applications to the next level. "At present you still have electrical switching at both ends of the optical fibre," says Badding. "If we can get to the point where the signal never leaves the fibre, it will be faster and more efficient. If we can actually generate signals inside a fibre, a whole range of optoelectronic applications become possible."

• University of Southampton Media Relations
• Penn State University Press Release
• Paper published in Science