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New Fiber Development
Maaaac writes "Just read this on GMSV: 'British researchers are developing a new kind of optical fiber that could surpass the known data transmission limitations of fiber. Augmented with a pattern of microscopic air holes that runs their entire length, these aptly-named holey fibers have a variety of surprising optical properties, not the least of which is single mode operation at all wavelengths and the ability to withstand the transmission of huge amounts of energy or data. To produce the fibers, researchers aligned an array of thin glass tubes, melted them together, and then stretched them to make a single fiber several kilometers long and about 125 microns across. While it's previously been suggested that such fibers would be predominantly used to transmit power -- or even matter -- their data transmission capabilities could be instrumental to the development of optical computers.' Now if only they would run this to my curb..."
In an optical fiber, light rays traveling through the core can bounce off of the outer boundary between the cladding (lower index of refraction) and the core via total internal reflection. However, interference only allows rays at certain angles with respect to the fiber propagate. Each of these valid ray directions represents a mode.
Single mode operation means only the axial mode, where the ray travels straight down the core, is valid. The reason single mode operation is desired is because the higher modes do penetrate into the lower-index cladding where the speed of light is higher when they reflect off of it, which causes the higher-index modes to propagate faster than lower modes. Basically, if you fire a very sharp pulse of light of all modes into an optical fiber, the modes will all reach the other end of the fiber at different times. Since your sharp impulse has been spread over time, there is a limit to how many different pulses can be resolved over a certain period of time. Single mode operation means that there are no higher modes and hence less spread and higher bandwidth. (There are other causes of spread, but not much can be done about most of them).