Deep-sea sponges have a thing or two to teach us about making fiber-optic cables. The skeleton of the deep-sea sponge Euplectella—also called the glass sponge—possesses optical properties that are very much like those of commercial fiber-optic lines, a discovery made by researchers from Lucent Technologies' Bell Laboratories, OFS, and Tel Aviv University in Israel. What's more, the skeleton—composed of three layers of material—is tougher than optical fibers.

The glass sponge could inspire methods for building or growing tougher fiber-optic cables that need less protection and direct light more finely than current lines, says Vikram Sundar, a member of the technical staff at Bell Labs.

Researchers were drawn to the sponge because it's conspicuous in the dark depths of the ocean where it lives. "Euplectella is brighter in appearance than the surrounding environments within which they are typically found," says Sundar. "We were interested in understanding if they have any unique optical properties that lead them to stand out."

To the researchers' surprise, they discovered that the refractive properties of sponge skeleton—or spicule material—and those of standard telecommunications fibers are very much alike, says Sundar. "Their diameters are comparable, and they (both) have a high refractive index core that is surrounded by a low refractive index shell," he notes. Refractive index measures how much light bends as it travels through a material. By combining high and low refractive index materials, spicule fibers can catch light much in the same way that telecommunications fibers trap light in order to transmit it.

The natural fiber material possesses three properties that have captured the interest of the researchers. First, its three layers are formed with nature's bottom-up approach, instead of the typical top-down method used to build conventional optical fibers, says Sundar. Switching to the bottom-up approach from nature would boost design flexibility, he explains, facilitating the manufacture of hybrid materials composed of clearly defined layers.

Second, the sponge material is created without the use of high temperatures, which are necessary in current commercial fiber processes. "The low temperature synthesis…means that these fibers can be doped with materials that are not possible in conventional fibers," says Sundar. For example, incorporating sodium can increase the refractive index of the material, he notes.

Third, the three-layered spicule fiber is tougher than conventional fiber materials, says Sundar. "In a conventional fiber any crack that is initiated at the surface propagates through the bulk of the fiber and results in a giant mechanical failure," he says. In contrast, spicule fibers can resist fractures because of their hardy middle layer, a crack-defying organic material. Utilizing this type of material in commercial applications would eliminate the need for protective polymer jackets, says Sundar. In fact, spicules are sturdy enough that they act as structural elements, he adds.

The researchers will next examine spicules from other sponge species to see how characteristics vary according to the depth of the sponges' natural environment. Their goal is to ascertain "if there's a correlation between the spicule's optical properties and the availability of ambient light," explains Sundar.

The discovery gives researchers a new perspective on the assembly of conventional fibers, says Sundar. "The varying of the refractive index…is finer than is possible in optical fiber, and could be an advantage for future applications," he notes.

But don't expect commercial fiber-optic materials produced from or inspired by sponge spicules to become available anytime soon. They're still far from practical, says Sundar. One challenge will be finding a way to stretch the material into the kilometers-long fibers needed for commercial purposes, he notes. As a result, the glass sponge research could take up to 10 years to produce practical results.

The research—which was funded by Lucent Technologies—was published in the August 21, 2003 issue of Nature.

Source: Sponges Grow Sturdy Optical Fiber
Kimberly Patch
Technology Research News, Sept. 10-17, 2003
www.trnmag.com/Stories/2003/091003/Sponges_grow_sturdy_optical_fiber_091003.html