spins mirrored fibers
Technology Research News
The different colors reflected by a film
of oil floating on a puddle of water depend on the thickness of the oil.
The thinner the oil layer, the shorter the wavelength of light it reflects.
An oil film reflects more colors at its edge because its thickness varies
more there than in the middle.
Researchers at the Massachusetts Institute of Technology have taken advantage
of this principle to make long plastic fibers containing layers of metal
that reflect light of specific wavelengths, giving the fibers mirrored
The fibers could be used as a type of bar-code or as optical filters.
They also bring fiber-optic technology a step closer to being able to
increase the number of wavelengths that can be transmitted at once through
hollow fibers, which could speed communications.
The researchers made the optical fibers using a common manufacturing process
-- heating a thick tube of the layered materials, then drawing it into
a long fiber with layers that are much thinner, but retain the same order
and relative proportions.
Although the process has been used to make fibers of different strengths
and resistance to moisture, "very little work has been done controlling
optical properties of fibers," said Yoel Fink, an assistant professor
of materials science and engineering at MIT.
The key to producing the mirrored strands was using two materials with
opposite optical properties, but similar thermal properties. A reflective
material requires layers that refract, or bend light differently, but
keeping the fiber layers the same proportions as the starting tube requires
that the layers soften the same amount under the influence of heat. "We
found two materials that behave very differently optically, yet... behave
identically under heating [so they can] be codrawn at the same temperature,"
One of the materials has a high index of refraction, while the other has
a low index of refraction, and the layers work together to block and thus
reflect light of a certain wavelength or mix of wavelengths. "If you have
the layered thicknesses commensurate with a wavelength or a certain fraction
of a wavelength... certain wavelengths are not permitted to transmit and
they can only be reflected," said Fink. At every layer of the materials,
part of the light comes back and part is transmitted through; given enough
layers few or none of the given wavelengths can get through, he said.
The researchers drew the fiber from a one-inch tube containing 20 to 40
alternating layers of the materials. The individual layers were 20 to
30 microns thick, or about half the thickness of a hair. Drawing the fiber
is essentially "shrinking this tube," said Fink.
Drawing the fiber shrinks the layers to 100 nanometers thick, which is
about one tenth the width of a bacterium, and 200 to 300 times thinner
than the original layers. "In the process... we end up with hundreds of
meters or even a few kilometers of fibers that have multilayers [that]
basically are mirrors," said Fink.
Depending on the thicknesses, the mirrors can reflect all or certain portions
of certain wavelengths of light, said Fink. A single fiber could reflect,
for instance, a precise ratio of three parts green light, which has wavelengths
550 nanometers long, one and one-half parts red light at 650 nanometers,
and one part 450-nanometer blue light.
A fiber with a very specific optical signature could be woven into a garment,
and could act like a bar-code, said Fink. "You could basically encode
information into any specific fiber and be able to read it off your clothes,"
The fibers could also be used as optical filters that would be very low
cost and also flexible, said Fink. Optical filters, which block specific
wavelengths of light, are a common component of telecommunications equipment.
"You're also able to build a tunability into these fibers," said Fink.
Stretching the fiber can cause the layers to elongate, thinning the layers
and making them reflect light of shorter wavelengths, he said.
The researchers are now working on using the mirrors as part of a fiber
laser to produce cloth that emits light, said Fink. "These would perform
very well as laser mirrors, but you'd have to get a light source into
there as well," he said. "Our next step will be to try to get light-emitting
Longer-term, hollow fibers could be made with the mirrors on the inside,
which could potentially guide many more channels of information than today's
fiber-optic communications lines, Fink said. "We would use that mirror
that's capable of reflecting light very efficiently at certain wavelengths...
to line the inner core of a hollow fiber. You would be able to [transmit
light] at wavelengths where silica is not very good, such as visible or
infrared," he said.
In addition, because transmitting light through air is more efficient
than transmitting light through silica, this type of fiber could potentially
transmit many more channels, or colors of information at once, increasing
the bandwidth of a single fiber line. Because of the way light travels
through silica, different wavelengths of light carrying different streams
of information must be distinct so they will not interact. "You have to
space the colors very far apart [in silica.] You could put many more colors
into the same fiber," if the light is traveling through air, said Fink.
Fink's research colleagues were Shandon D. Hart, Gary R. Maskaly, Burak
Temelkuran, Peter H. Prideaux and John D. Joannopoulos. They published
the research in the April 19, 2002 issue of the journal Science. The research
was funded by The Defense Advanced Research Projects Agency (DARPA) and
the National Science Foundation (NSF).
Timeline: 18 months
TRN Categories: Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "Multilayered Mirror
Fibers," Science, April 19, 2002.
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