Sponges grow sturdy optical
Technology Research News
What do deep-sea sponges have to do with
human communications? Possibly a lot.
Researchers from Lucent Technologies' Bell Laboratories, OFS,
and Tel Aviv University in Israel have found that the optical properties
of the skeleton of the deep-sea sponge Euplectella -- also known as the
glass sponge -- are similar to conventional fiber-optic cables. At the
same time, the skeleton, made up of three layers of material, is tougher
than optical fibers.
Methods borrowed from the sponge could eventually be used to build
or grow tougher fiber-optic lines that require less protection, and control
light more finely than today's lines, according to Vikram Sundar, a member
of technical staff at Bell Labs.
The sponge caught the researchers' attention because it stands
out in the murky ocean depths where it resides. "Euplectella is brighter
in appearance than the surrounding environments within which they are
typically found," said Sundar. "We were interested in understanding if
they have any unique optical properties that lead them to stand out,"
The researchers were surprised to find that the refractive properties
of sponge skeleton, or spicule material, is very similar to conventional
telecommunications fiber, said Sundar. "Their diameters are comparable,
and they [both] have a high refractive index core that is surrounded by
a low refractive index shell," he said.
A material's refractive index is a measure of how light bends
as it passes through. The illusion that a drinking straw bends at the
water line is due to the different refractive indexes of light and water.
The combination of high and low refractive index materials allows
spicule fibers to trap light -- similar to the way telecommunications
fibers confine light in order to transmit it.
The sea sponge fibers are multimode when surrounded by air or
seawater, and single mode when embedded in a material. Multimode fiber
can carry multiple lightwaves over shorter distances, and single mode
fiber can carry a single lightwave over longer distances. The free-standing
spicules are multimode because the refractive index contrast is greater
between the spicule shell and air than between its core and shell. This
allows light to fill the whole fiber rather than just the core.
The natural fiber material has three properties of interest to
First, the three layers were created using nature's bottom-up
approach rather than the usual top-down approach used to manufacture conventional
optical fibers, said Sundar. "Small structural units -- silica spheres
that are between 50 and 200 nanometers in diameter -- are assembled together
to yield the final 100 micron fiber," he said. A nanometer is one thousand
of a micron and one millionth of a millimeter, or the span of ten hydrogen
atoms. One hundred microns is about the size of a thick human hair.
In contrast, commercial fibers are slowly stretched, or pulled
into shape. Borrowing the bottom-up approach from nature would mean greater
design flexibility, said Sundar. A bottom-up approach would make it easier
to create hybrid materials that contain well-defined layers of material,
Second, the sponge material was assembled without the need for
the high temperatures used in commercial fiber processes. "The low temperature
synthesis... means that these fibers can be doped with materials that
are not possible in conventional fibers," said Sundar. Adding sodium,
for instance, can raise the refractive index of the material, he said.
And third, the three-layered spicule fiber is tougher than commercial
fiber materials, said Sundar. "In a conventional fiber any crack that
is initiated at the surface propagates through the bulk of the fiber and
results in giant mechanical failure," he said. Things are different in
spicule fibers, however. The middle layer is a crack-arresting organic
material that makes the spicule fiber resistant to fractures, he said.
Using this type of material in commercial applications would mean
fibers would not need to be protected by a polymer jacket, said Sundar.
Spicules are strong enough that they function as structural elements,
The researchers' next steps are to analyze spicules from other
species of sponges to find how the characteristics of the spicules change
depending on how deep the sponges reside. The idea is to see "if there's
a correlation between the spicule's optical properties and the availability
of ambient light," said Sundar.
The discovery provides another way of thinking about constructing
conventional fibers, Sundar said. "The varying of the refractive index...
is finer than is possible in optical fiber, and could be an advantage
for future applications," he said.
Commercial fiber-optic materials made from or inspired by sponge
spicules are a long way off because a lot of other work needs to be done
to make such materials practical, said Sundar. One key is figuring out
how to draw the material into the very long fibers -- on the order of
kilometers -- needed for communications applications, he said.
The glass sponge research could yield practical results within
ten years, said Sundar.
Sundar's research colleagues were Andrew D. Yablon from Lucent
spin-off OFS, John L. Grazul and Joanna Aizenberg from Bell Laboratories,
and Micha Ilan from Tel Aviv University. The work appeared in the August
21, 2003 issue of Nature. The research was funded by Lucent Technologies.
TRN Categories: Optical Computing, Optoelectronics and Photonics
Story Type: News
Related Elements: Technical paper, "Fibre-Optical Features
of a Glass Sponge," Nature, August 21, 2003.
September 10/17, 2003
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