foolZONE Special Reports for April 1, 2006

by Lee H Goldberg, Senior Fiber-Lighting Editor, analogZONE

April 1st 2006, St-Roumald, Québec...Until now technologists have viewed the web-work of optical fiber that now links the world's computers as an infinitely wide pipeline whose capacity is only limited by the speed at which the lasers and photo detectors can send and receive pulses of laser light. But this assumption has recently been turned upside down with the discovery that the ever-increasing data rates being used in optical networks may be the source of fiber fatigue that is slowly degrading their capacity. Until a cure can be found, the International Telecommunications Union (ITU) has issued a strongly-worded communiqué advising carriers adhere to a strict set of speed limits to protect the world's fiber infrastructure.

The phenomenon, known as photonic erosion, was first described by researchers at the Québec-based Académie de St Germain's Institute des Optiques Gigantesque (IOG). It occurs when high-energy quantum packets of light interact with the glass fiber's lattice-like molecular structure. The exact mechanism of photonic erosion is still not completely understood, but researchers speculate that the modulation processes normally used to transmit data create photons with peculiar transitional energies that resonate with the silicon dioxide (SiO2) bonds in the glass. Although such interactions are statistically rare with unmodulated light, recent experiments showed that these incidents of photonic entrapment increases in proportion to the data rate being sent along the fiber. "The structural disruptions we see are tiny, but over time they degrade the fiber, and its ability to carry light over long distances," said Frederic Truffaut, Directeur de IOG's Centre de Recherche de Photonics.

Initial studies show that this effect is negligible below 5 Gbit/s but that the degradation seems to increase logarithmically with higher data rates. Even at the 10 Gbit/s rates commonly used today, Truffaut estimates that it will take 15 - 20 years before a 1 dB loss in signal will be evident in the average 20 km run of fiber, but warns that doubling data rates will cut the fiber's useful lifetime in half. "And if carriers move to speeds of 100 Gbit/s or more, we'll be using up the fiber plant faster than we are currently laying it," concluded Truffaut.

IOG and several other optics research centers have already begun work on transmission methods that don't cause damage to the optical fiber. Scientists are enjoying some early success with a method called bit balancing which subjects the fiber to equal bit pressure in both directions. Initial experiments indicate that maintaining equal numbers of ONEs and ZEROs in each direction over a period of 10 ns or less seems to avoid, or greatly diminish, the erosive effects of unbalanced photonic activity. Scientists are still unclear as to whether this is due to a cancellation of resonant energies or if there is some sort of other mechanism at work. Other research groups are experimenting with high-powered X-ray lasers to see if they can bounce the fiber's lattice structures back to their original states and thereby anneal the large lengths of fiber already suffering from photonic erosion.

Until a solution to mitigate the effects of photonic erosion is found, the ITU has proposed a voluntary 10 Gbit/s speed limit, intended to preserve the planet's fiber infrastructure. Under the plan carriers running over public networks will be asked to limit their speeds to 5-Gbit/s for most applications, and pay a fiber-usage premium for links running at 10 Gbit/s. The ITU has loosened its guidelines for LANs and enterprise-class networks to allow speeds of 20 Gbit/s to 40 Gbit/s since their fiber runs are shorter and usually more accessible for replacement.

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