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When you make a phone call or browse the internet, chances are a lot of the communication happens over fiber-optic links transmitting billions of bits every second.
A recent experiment shows it may be possible to "twist" light waves, cram in more information than ever before, and send the signal over a practical distance. In this case, the physicists used twisted laser light to send the message "Hello World" between two islands.
Light waves are used in communications all the time. Radio is a form of light, as are lasers that are common in fiber optics. To get information in and out, one can use the amplitude of the wave (as in AM radio), the wave's frequency (used in FM radio) and even the phase and polarization (used in fiber optics along with the first two).
The fact that one can use just four features, or so-called degrees of freedom, to encode information into a single light wave limits how much can be communicated via a photon. An international team from the University of Vienna wanted to see if they could encode information into another feature, the angular momentum, of a light wave, and send it far enough to be useful—in this case, about 88 miles (142 kilometers) between two observatories in the Canary Islands.
(Score: 0) by Anonymous Coward on Tuesday December 06 2016, @08:58PM
Nope. Photon orbital angular momentum a.k.a "twisted light" is a form of spatial modulation, and is a subset of MIMO. There is no added degree of freedom in the photons themselves. The added degree of freedom is in the spatial pattern over which the light is transmitted and received. This is a near-field effect that completely disappears in the far field.
The fiber equivalent would be different "modes", which is something that fiber producers try to get rid of, which is why you see "single mode" fiber for sale.
(Score: 2) by Zz9zZ on Tuesday December 06 2016, @10:48PM
I only had a vague sense of disagreement, I didn't see why the orbital angular momentum couldn't go through a fiber optic cable though it was likely to have a lot of issues. A few mins of searching found: https://www.extremetech.com/computing/159986-twisted-laser-vortexes-carry-1-6-terabits-per-second-over-fiber-optic-network [extremetech.com]
Seems it requires special cables that can keep the signals separated, and I'm very curious why doping the cable allows the signal to remain coherent.
~Tilting at windmills~
(Score: 0) by Anonymous Coward on Tuesday December 06 2016, @11:27PM
Still not sure if Shannon is still happy or this is something more interesting.
Consider a PDM-QPSK modulator driving an optical channel.
(Modulation inputs are I&Q each for up and sideways polarizations)
Normally for each baud you pick a constant value for these modulation inputs.
But you could vary them during the baud.
With the correct modulation input (really high bandwidth,etc?), you should be able to send a rotating light beam down the channel.
One might even be able to superimpose multiple rotations during the baud and sort them out at the rx end.
Obviously, the bandwidths involved would make this an optical gadget, but maybe thinking about an unbuildable simpler system is good for understanding the scheme.
Questions are:
1) Is this the same on the channel as what the article is talking about.
2) If so, then why doesn't the really wide bandwidth modulation make a wide bandwidth signal which spreads to adjacent colors on the channel?
3) If so, don't these separate modulations cut into the SNR of the simple pick-a-value-per-baud modulations?