Whether you know it or not you are probably using fibre optics every day, they are the very thin glass fibres which carry most of the information around the internet and telephone systems. The information travels in the form of incredibly short pulses of infra red laser light, this is picked because the glass is very transparent in this colour, and can travel tens of kilometers and still be detected, it can then be amplified and continue travelling.
The problem is that the further the pulses travel the more they tend to spread out due to imperfections in the glass's properties, this means that you have to make them longer, so you can get fewer though per second and so you can't transmit as much data, or you need to regenerate the signal every few hundred kilometres in a very expensive process.
But Xiang Lui and collegues from Bell labs in New Jersey, have come up with a system that might help. A pulse of light will disperse and spread out along an optical fibre, but there is a pattern of light that the imperfections of the fibre will cause to have exactly the opposite effect, which is called it's phase conjugate. The group sent these two signals down the fibre optic one with a horizontal polarisations and one with a vertical one, and then recombined them at the end and reduced the distortions by a factor of nearly 30.
This has allowed them to send 400Gbits per second down an optical fibre which is 12800km long which is very impressive, and could reduce the price and increase the capacity and reliability of long data cables
I'm an engineer who oversees a lot of bespoke fibre cabling and I found the article hard to understand! Essentially I'm thinking of it as analogous to the common-mode rejection used in copper balanced transmission lines. If anyone can shed any light on "phase-conjugated twin waves" I'd appreciate it.
Ahh, "The group sent these two signals down the fibre optic one with a horizontal polarisations and one with a vertical one, and then recombined them at the end and reduced the distortions by a factor of nearly 30." That should be a analogue to using a beam splitter, getting opposite spins from it. So you would need a different transmission system than what is used today I guess, even though the fibre will be the same.
Without actually buying the article, the print is a bit too small to read...
Channel Packing - OFDM http://en.wikipedia.org/wiki/OFDM and the like are very standard for high speed RF and copper-transmission standards but I assumed that when they hit fibre they'd start in multi-mode applications before they worked their way to single-mode. FunkyWorm, Mon, 10th Jun 2013
I had a quick look at the paper today: They are using the cross-polarisation method to obtain two simultaneous measurements of the fiber distortion using two different pulse shapes. The error correction performance drops if the measurements are too far apart in time
There are quite a few modulation schemes that can be used. The more complex ones enable more bits per second to be carried withing a particular bandwidth. In fibre, keeping the bandwidth narrow is useful as it reduces the effects of dispersion. In RF it is useful mainly useful so that more of the available spectrum is utilised. In both cases, the more bits/second you send the shorter is the range however. However, if the aim is to just get as much data tranferred as possible (and the distance between repeaters is not a factor) then elaborate modulation schemes are worthwhile. It makes a lot of sense to send two signals differentially because any "common mode" distortions can be cancelled effectively improving the Signal to Noise+Distortion ratio. It has been a long time since I worked on fibre transmission so it is interesting to see the latest developments. graham.d, Tue, 11th Jun 2013
evan_au - couple of notes; although multi-mode is cheaper to manufacturer than single-mode cable (it has a 50 micron core as oppose to 9 microns) they have different application. Typically multi mode is used for SAN traffic and the like. Additionally being able to launch many modes down a cable reduced the cost of host-bus adaptors; it's how you can have a fibre card in your PC for only a few hundred quid. The SFPs are cheaper as well.
We should not ignore the possibility of further advances in optical fiber materials. For example Fluoride glasses offer the possibility of lower attenuation than conventional silica fibers.
I first read about using phase-conjugate mirrors (PCM) to amplify and clean up optical fiber light pulses some years ago (a decade? 15 years?). PCMs are pretty cool. They make use of optically-nonlinear materials (ONM) that alter their optical density in response to light. Imagine a set-up for creating a hologram but instead of photographic film, you have a block of ONM. There's an image light signal coming in which interferes with a reference beam also going into the ONM. The trick is that a second reference beam, exactly conjugate to the first one (identical but coming in from the opposite direction), is sent into the ONM. This beam interacts with the pattern of varying optical densities created by the interference of the first reference beam and the image light signal and produces light that goes back along the paths that the original signal did. If a particular ray of light enters the ONM along some path, a duplicate beam comes back out retracing that exact path.