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By mirroring beam in line, noise is cancelled

It is an elegant and simple solution, but one that is relatively new to the world of fiber optic signaling -- beam mirroring.  The approach, already frequently used in the noise-cancelling headphones, is being touted as a promising new route to improving fiber optic routing in a new study by researchers at telecommunications company Alcatel-Lucent SA's (EPA:ALU) Bell Labs unit.

The idea involves making "twins" -- dual beams of light in a fiber that mirror each other.  Each peak in one beam is a trough in its twin.  Together they bounce along the line, much as a single beam would.

One major limiting factor in fiber optics is noise.  In order to send signals farther, beams are transmitted at higher power.  But the higher the power, the more beams tend to interact with the material in the fiber's walls, adding noise.  Beams have a limiting maximum distance they can travel and maintain fidelity -- after that they need to be received and rerouted, hopping along the next link.

By adopting the "twin" approach, light can travel four times farther that a single beam could.  In their study, the Bell Labs stream piped data at 400 Gigabits per second (Gbps) -- four times faster than the best commercially available speeds, down 12,800 kilometers (7953 miles) of fiber.  That's a longer line than the longest transoceanic link.

While note the first study to suggest the phase conjugate approach, Bell Labs claims its work offers the most straightforward implementation and is proven to travel farther without rerouting.  That means the need to reroute or boost signals during long transoceanic links may no longer be needed.

Fiber optic cable
[Image Source: Guardian UK]

Lead author Xiang Liu of Bell Laboratories in New Jersey comments in an interview with BBC News:

Sometimes you may send data from London to New York, sometimes you may send it from London to Paris. The links are changing and you cannot keep sending people to the middle of the link.

At the receiver, if you superimpose the two waves, then all the distortions will magically cancel each other out, so you obtain the original signal back.  This concept, looking back, is quite easy to understand, but surprisingly, nobody did this before.

Nowadays everybody is consuming more and more bandwidth - demanding more and more communication.  We need to solve some of the fundamental problems to sustain the capacity growth.

The approach may allow faster data transfer speeds too.  As it reduces signal noise, it allows for less repetition of information in a given beam.

The study on the work was published in the prestigious peer-reviewed journal Nature Photonics.

Alcatel-Lucent in currently in the midst of constructing a 100 Gbps undersea fiber link between Malaysia, India, and the Middle East.

Sources: Nature Photonics, BBC News

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Another use for cling film.
By drycrust3 on 5/28/2013 1:04:38 PM , Rating: 2
The idea involves making "twins" -- dual beams of light in a fiber that mirror each other. Each peak in one beam is a trough in its twin.

You may not need two beams of light to produce this effect, you may be able to produce it with just one beam of light. Since the light beam is monochromatic, then by passing half the beam of light through a carefully constructed piece of transparent material, e.g. a piece of cling film, the light going through the plastic membrane (which has a refractive index greater than 1) you create a phase difference between the two halves of the beam. The choice of refractive index and thickness of the transparent medium would need to be peculiar to the frequency of light used so as to get a pure 180 degrees phase difference.
There is, however, a problem with this approach, in that you would need to make sure you have high quality anti-relflective coating on the transparent medium to keep the power outputs of the two halves about the same, but even so you'll still get some light reflected back towards the light source, which probably isn't significant for this purpose, but in other optical devices it could be significant.
I should point out that one reason no one had thought of this before is the fact that you're sending a signal and its inverse down the same line, and normally this would produce a canceling effect, meaning the sum of the two signals is zero power.
I'm guessing, if this report is true, that it does work is because the photons are working independently of each other, so a 0 degree photon and a 180 degree photon can be right next to each other and completely ignorant of each others existence. You could get problems where there are impedance mismatches though, e.g. where there a joins in the cable or at the receiving equipment.
The distance between repeaters used on trans-oceanic cables is, I believe, about 300 km (200 miles). There is an advantage in having repeaters when you have a break in the cable because by identifying which repeaters are still working from each end you at least have a rough idea where to start looking (i.e. is there a fishing boat in between these two repeaters). It may not be so easy to do this if there aren't any repeaters.

RE: Another use for cling film.
By boeush on 5/28/2013 7:27:56 PM , Rating: 2
I imagine one could send two mutually phase-inverted signals down the same fiber, simply by using orthogonal polarization (e.g. signal A is polarized along the X axis, while signal B that is the inverted version of A is polarized along the Y axis in the plane of the fiber's transverse cross-section.) Any defects in the fiber that affect signal polarization, should 'rotate' the polarization of both A and B by equal amount, so at the receiver you can still recover both signals, e.g. via some kind of a birefringent crystal.

This would assume that the distributions of photon polarization don't degrade or spread out too much over the fiber's span, and that the fiber itself doesn't suffer from too much birefringence (which might play havoc with the phases of the signals, although can probably be dynamically compensated out at the receiver if the transmitter periodically sends a standard-shape waveform down the fiber...)

RE: Another use for cling film.
By PaFromFL on 5/28/2013 7:35:45 PM , Rating: 2
... or maybe circular polarization to even things out.

By drycrust3 on 5/29/2013 12:42:46 AM , Rating: 2
e.g. signal A is polarized along the X axis, while signal B that is the inverted version of A is polarized along the Y axis in the plane of the fiber's transverse cross-section

In theory your idea should work. As I understand it, inter-galactic light is polarised like this, but I don't think there is the phase synchronisation as well.

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