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Scientists Double Optical Fiber Transmission Capacity
ms writes: "Yesterday golem.de reported that the Optical Communication and High-Frequency Engineering Group at the University of Paderborn (Germany) claims to have made a technology practical which doubles the transmission capacity of optical fibers to 80 GBit/s. In their so-called "polarization division multiplex data transmission system" they don't only send one but two mutually orthogonal light waves through the fiber. They say the only big problem was the dispersal of the light waves which limits the data rate. Additional they had to fight against the phenomena that the polarization direction of the light waves changes while it goes through the fiber. Now, after two years of research, they invented an "automatic optical compensator of polarization mode dispersion" which fights both the limitations. With this gadget they were able to send data at a rate of twice 40 GBit/s (that's 85,899,345,920 Bps) over a test-line of 212 km. And "only the available equipment limited distance and data rate". As we all know, optical fibers build the (cronically overloaded) backbone of our beloved Net. (BTW: That's Net., not .Net!)" Here's the babelfish translation, too.
"mutually orthogonal" means (for a set of two or more elements) that each pair of elements is orthogonal--AFAIK, it's a synonym for "pairwise orthogonal". "orthogonal," of course, has lots of synonyms, including "linear independence," "at right angles," "having zero dot-product," "statistically uncorrelated," etc.
So, the three spacial dimensions, the set {phase of the moon, day of the week, time of day}, etc. are all "mutually orthogonal." When talking about a set of only two elements, the "mutually" is superfluous, but not redundant.
-- MarkusQ
The beams in this article are orthogonal in the sense that channel #1 has it's E-field pointed prependicular to channel #2's E-field so they won't interfere with each other (so they're `orthogonal' in the usual compu-geek sense of the term, too.)
The german team seems to have solved two big engineering problems with sending two channels of information this way. One is to send a mean-polarized signal so that you can compare the two channels against it (kind of a carrier signal for polarization) to see which channel is which.
The other I confess to not understanding. Apparently there are sync problems -- signals carried one polarization may travel faster than the other polarization. I can only guess that this is a problem caused by inhomogenaities in fibre. Whatever its caused by, they've managed to measure it and compensate for it.
As for your other question, they definately can and do use frequency as a way of encoding information. Just like with radio signals, you can use the brightness of the light (amplitude modulation, or AM) or its color (frequency modulation, FM). In practice, FM is less problematic; the amplitude of a signal is easily confused by noise, whereas frequency is much less so.
Hmm, and same Timothy posted this article [slashdot.org] on June 25th about a lot of fiberoptic cables that have been put into the ground but haven't been put to work. :)
You gotta love the consistency of Slashdot posts
Dark fiber is fiber with no optical equipment connected to it. Fiber is not the expensive part of optical networking. Air-conditioned environment-controlled closet space filled with millions of dollars of self-healing optical equipment is the expensive part. A lot of metro optical carriers use the benchmark of $100,000 per month per 7 foot rack in operating costs. The denser the equipment, the cheaper the equipment, the more of that dark fiber the carriers can light to form the backbone of the Internet.
So, in short, Slashdot was right and you were totally wrong. Or Insightful. Your choice.