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Posted: Mar 27, 2006
New process builds electronic function into optical fiber
(Nanowerk News) Optical fiber helped bring us the Internet, and silicon/germanium devices brought us microelectronics.
Now, a joint team from Penn State University and
of Southampton has developed a new way to combine these technologies.
The team has made semiconductor devices, including a transistor, inside
microstructured optical fibers. The resulting ability to generate and manipulate
signals inside optical fibers could have applications in fields as diverse
as medicine, computing, and remote sensing devices.
Optical fiber 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, titled "Microstructured Optical Fibers as High-Pressure Microfluidic Reactors" were 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
professor of chemistry at Penn State University. While the optical fiber 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
"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 fiber as a water main, this structure places the pumping station inside
the pipe. The glass fiber provides the transmission and the semiconductor provides
This photo shows a glass fiber with a bundle of semiconductor wires emanating from it. Each wire is just 2 microns in diameter--20 times smaller than a human hair. The glass fiber is glowing from blue laser light. (Source: Neil Baril, Penn State)
Beyond telecommunications, optical fibers are used in a wide range of technologies
that employ light. "For example, in endoscopic surgery, by building a
laser inside the fiber 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 fibers--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 fiber. 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 fiber. In the CVD process, a germanium compound is vaporized and then
forced through the pores of the fiber at pressures as high as 1000 times atmospheric
pressure and temperatures up to 500°C. A chemical reaction within the fiber
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 fiber," said Sazio. "This is only a tiny
fraction of the diameter of the fiber 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.
Scanning electron micrograph (SEM) of the cross section of a partially silicon filled holey fiber (Source: Neil Baril, Penn State)
The team has built a simple in-fiber transistor, and they point to the success
of the Erbium Doped Fiber 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 fiber, the Erbium Amplifier
allows efficient transmission of data signals in transoceanic optical fibers. "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 fiber," says Badding. "If we
can get to the point where the signal never leaves the fiber, it will be faster
and more efficient. If we can actually generate signals inside a fiber, a whole
range of optoelectronic applications become possible."