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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.
First, twice 40Gbps is not 85,899,345,920 bps, it is actually 80,000,000,000 +/- 1,000,000,000. We don't measure Gbps in powers of 2. Secondly, the internet backbone is not overloaded, but is running at about 20% capacity according to the people who operate it.
Hmm, and same Timothy posted this article 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
"A door is what a dog is perpetually on the wrong side of" - Ogden Nash
In geometry, orthogonal just means perpendicular. But, according to searchStorage: "In computer terminology, something - such as a programming language or a data object - is orthogonal if it can be used without consideration as to how its use will affect something else. " So, the light waves are mutually orthogonal (they are data objects in this case), but I'm not exactly sure how to apply the definition to exactly what the scientists are doing with fiber optic cables.
F-bacher
James Tiberius Kirk: "Spock, the women on your planet are logical. No other planet in the galaxy can make that claim."
"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
A more appropriate comparison is to 56kbps modems we have today. 300 to 56000 in 14 years. That roughly a doubling of speed every 2 years. Slower than computer CPU development, but then we are limited by the telco equipment, aren't we.
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.
This is kind of an intresting experiemnt, but this is not news. The "otherwise possibe" part makes it sound like no one has done PMD compensation before, this is false. Here is why:
1. PMD compensators are being built by many research groups. You still can't call up an order one (AFAIK), but soon.
2. PMD (mean DGD, differential group delay). DGD changes with time and wavelength.
3. PMD on buried fiber varies slowly. It is easy to compensate.
4. Nortel, Alcatel and others with be releasing 40 GB/s (per WAVELENGTH) systems next year. They are suppose to run 100's of km, between regens and at many wavelengths (160?).
Here is a link with almost all peer reviewed papers on PMD:
Here one can see many references to PMD compensation and even some at bit rates of 160 GB/s. With PMD compensation the line speed isn't that important, it is the accuracy and speed of your compensation.
This is not a break through.
I see some others posting explanations about physics behind this, but it seems a bit unsatisfactory for some. Here's my best shot at it:
There are two orthogonal polarization modes that propagate down fiber, meaning the there's a sort of up-down oscillation of the electric field (one mode), and a left-right oscillation (other mode). If fiber were perfect, you could send a signal along each polarization, and they wouldn't bother (interfere with) one another, but it's not. If you send polarized light down a fiber, it will not keep the same polarization (unless you use polarization-maintaining fiber, but that's a pain, and you can only send one polarization down).
So people generally send down (relavitively) unpolarized light. They modulate this one signal as fast as they can (getting about 40Gb/s), and then deal with dispersion as best they can.
Dispersion results from the spread in frequencies (colours) of your signal (each colour travels a different speed in the fiber) and also from the fact that a fiber has polarization mode dispersion (the part of the signal along one polarization axis travels at a different speed than the other part, called PMD from here on in). Both of these effects cause a pulse that you send down the fiber to be distorted (part of the pulse travels at a different speed than the other part). Chromatic dispersion (the first kind) has been dealt with (fibers have a wavelength at which the loss is lowest and a wavelength at which the chromatic dispersion is lowest, and it's been worked such that these two things are at basically the same wavelength), but PMD is a big limitation to pushing the capabilities of fiber. This was stated on the front page post:
They say the only big problem was the dispersal of the light waves which limits the data rate.
I think that should read "dispersion", not "dispersal".
So, what these guys have done is made a PMD compensator. Somehow it automatically makes sure that a given polarization of light stays in that polarization as it travels down the fiber. If one can preserve the polarization of both modes (which is different than polarization maintaining fiber, which takes ONE polarization of light and keeps it polarized), and then send a signal along each polarization axis, then one doesn't need to deal with PMD, because within a given signal, all the pulses are travelling at the same rate.
Then, if you don't have to deal with PMD, then there's very little to slow you down in pushing data through the fiber, basically just how fast you can modulate your laser (I think you could drive a LiNbO3 Mach-Zhender modulator up to about 80Gb/s or so, whereas I think in the article they were driving it at 40Gb/s). That's why they say the data rate was only limited by available equipment. I'm not sure how the PMD compensator works, I'll have to read the actual article more closely. I hope this helps!
"Now gluttony and exploitation serves eight!" - TV's Frank
DWDM (dense wavelength division multiplexing) referrs to multiplexing multiple optical signals on a fiber by having them exist at different wavelengths of light. This is very similar to how the cable TV line carries 100 or so channels of TV signal by having them at different frequencies.
The D (for dense) means that there are many such channels, often 40+. This article referrs to having two 40Gb/s channels at the same wavelength, but with opposite polarizations so they don't interfere with each other much. This same signal could be used as a base for a DWDM system to effectively double the current maximum speed of like 10Tb/s (40Gb/s * 250 channels).
main(O){10<putchar((O--,102-((O&4)*16| (31&60>>5*(O&3)))))&&main(2+ O);}
LN2 is cool!
This phenomenon is called polarization-mode dispersion and we just covered it in my fiber-optic communications class. It occurs because of birefrigence, which is the phenomenon where different polarizations see different refractive indices. Since refractive index is the speed of light in vacuum divided by speed of light in a medium, this means signals with different polarizations will travel different speeds. Even worse, since fiber birefringence is probably stress-induced and varies over the length of the fiber, it is difficult to tell what the polarization axes of the fiber are so that you can minimize this effect.
Polarization-mode dispersion is a problem even when you're not multiplexing by polarization because it results in the ordinary and extraordinary polarization of a light pulse separating and possibly colliding with other pulses, thereby limiting the bandwidth of the fiber. On the other hand, if you use the ordinary polarization as one channel and the extraordinary polarization as a separate channel, both channels will propagate with zero polarization-mode dispersion and double the effective bandwidth of the fiber. They will propagate at different speeds, but that really isn't an issue as long as the light pulses that represent your 0's and 1's aren't spreading.
The trick is determining the ordinary and extraordinary axes of the fiber, which is the breakthrough that this group made. It sounds like they use a reference channel to determine the ordinary and extraordinary polarization axes of the fiber and also to measure the change in polarization introduced by the fiber so that they can demultiplex the two polarization channels. This is a very simple and elegant way to negate polarization mode dispersion and to enable polarization-division multiplexing.
"It take 9 months to bear a child, no matter how many women you assign to the job."
maybe they meant they were using two, orthoganal (via the polarization) signals of the same wavelength, so that you can densely divide and multiplex that however you like, and you can double the resulting bandwidth via polarizing and recycling that wavelength.
16 wavelengths = 160 Gbps
16 wavelengths, twice each (one's all sideways from thuther) = 320 Gbps.
Though I'm just talking out of my ass.
For some reason, Babel Fish translates "unterbrechungsfrei" as "noly-break". A much better translation would be "free of interruption".
No, 2 is the max. Circular polarisation is just horizontal AND vertical at the same time, with the same amplitude and 90 out of phase. So you will not be able to discriminate the channels if you add in extra horizonal and vertically polarised light as well.
And yes, fibre can handle circular modes, or any other polarisation state for that matter.
Last geeky point. Orthogonal can be rephrased as '2 polarisation states occupying antipodean points on the Poincaré Sphere'