Orbital-angular momentum (OAM) modes are a class of transverse spatial light modes where phase distribution spirals through the beam – leading it to be called ‘twisted light’.
According to the a paper published in the New Journal of Physics, the requirement for accurate alignment at the receiver to coherently detect phase had persuaded scientists that OAM modes were too fragile to be used over distance through turbulent atmospheres.
This is unfortunate because, while there are limited ways to phase modulation lasers, there are endless ways to modulate OAM modes, said the Vienna Institute for Quantum Optics and Quantum Information, where the research was conducted, allowing a vast number of data symbols to be encoded as modes.
The team’s innovative step was to avoid phase coherent detection by choosing modes that result in an amplitude distribution across the laser beam – the laser makes a visible pattern when it strikes a screen. In this case, modes with patterns like the petals around a flower were chosen – see diagram.
Light from the sending laser (20mW 532nm) was reflected off a spatial light modulator (essentially a matrix of LCD pixels) to phase-modulate OAM modes on to the beam. This was then projected across the city through a telescope.
At the far end, the beam simply hit a screen, viewed by a camera.
The patterns corresponding to the different modes were easily be distinguishable by eye, and were decoded via the camera using a trained artificial neural network.
Turbulence, it was discovered, moves the beam rapidly around the screen with a deflection of several cm, making practical coherent measurement of phase across the beam impossible, but does not badly affect phase within the beam, so the structure within the beam due to modulation remains.
Data rate was four modes (=symbols) per second and the 16 transmitted modes were received with 1.7% error rate.
The researchers equate the turbulence experienced over 3km of city to 6km vertically through the atmosphere, suggesting OAM could be used to communicate with satellites.
They also propose that a quantum version of the technique could pass cryptography keys from place to place.
There have been reports on this research linking it with 2.5Tbit/s data transmission. This is a figure issued by the Institute of Physics, which has also published the paper, to indicate data rates achieved using OAM modulation in labs.
from News http://ift.tt/14X2B4L
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