Hollow Fiber: No Pipe Dream
At the OFC postdeadline paper session tomorrow, U.K. startup BlazePhotonics Ltd. will present results on a hollow-core fiber with an optical loss that's ten times better than what's been achieved before. What's more, BlazePhotonics says its results suggests that hollow core fiber could meet the target of having lower loss than standard transmission fiber.
"We're getting close to breakeven," says VP of research and development Dr. Henrick Sabert. "Now we can begin to understand the physics that take you there."
Being better than standard transmission fiber is no mean feat. After decades of development, the loss of optical fiber has settled at a figure of around 0.2 dB per kilometer (dB/km). The very best commercially available fiber is from Sumitomo Electric Industries Ltd., with a loss of just 0.151 dB/km.
Hollow-core Optical Fiber has the potential for virtually zero losses because light travels predominantly inside hollow spaces inside the fiber, rather than in glass. "Our results show that more than 99 percent of the light travels in the spaces," Sabert contends. BlazePhotonics' fiber has a honeycomb-like cross section, although other types have been proposed by other vendors (see Holey Fibers! for some images).
But achieving these low losses has turned out to be hard work, causing some folk to question if it is even achievable (see OmniGuide: Hollow Promises?).
Naturally, BlazePhotonics believes that ultra-low-loss hollow fiber is a viable proposition and says its latest results point the way. The new results that it will report tomorrow are losses of 1.7 dB/km, which is a significant improvement over the previous best of 13 dB/km, which was set by Corning Inc. (NYSE: GLW) in 2002.
Earlier fibers had so much loss that it was impossible to tell what the limiting factors were, according to Sabert. In the latest fibers, loss is caused predominantly by scattering of light at the surfaces between the spaces and the glass, and this mechanism appears to be fundamental -- it will set the limit on the lowest loss that can be achieved.
"For hollow core fiber to be commercially interesting, it would need to be a factor of two or four times better than standard fiber," says Sabert. He reckons a loss of 0.1 dB/km would be good enough. Lower-loss fiber would allow signals to travel farther between amplifier points, reducing or even eliminating the cost of using Erbium Doped-Fiber Amplifiers (EDFAs). Even small improvements in loss could result in significant cost savings.
Lower loss would be especially useful in two applications, he says: unrepeatered submarine links and quantum communications. "Folk in submarine battle to gain a fraction of a decibel." In quantum communications, which encode data using the physical properties of single photons, the signal cannot be amplified, because this would destroy the information it is carrying (see Quantum Crypto Gets a Chance).
BlazePhotonics also came to the conclusion that the optimum operating wavelength (the one with lowest loss) of a hollow core fiber is likely to be around 1,900 to 2,000 nanometers, rather than the usual 1,550 nm found in DWDM systems today. That's because the absorption by glass, which usually increases rapidly at wavelengths above 1,600 nm, is drastically reduced (most of the light travels in the spaces not the glass, remember). Since the surface scattering falls as wavelength increases, the new optimum point is at a higher wavelength.
This kind of argument isn't likely to make folk rip out their existing fiber and replace it. But in new-build scenarios, it could lead to some novel solutions.
— Pauline Rigby, Senior Editor, Light Reading