Sponges grow sturdy optical fiber

By Kimberly Patch, 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," he said.

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 the researchers.

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, he said.

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, he added.

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.

Timeline:   Unknown
Funding:   Corporate
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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