
'Optical fibre' made out of thin air - WestCoastJustin
http://www.cbc.ca/news/technology/optical-fibre-made-out-of-thin-air-1.2715321
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spott
So, I do a little bit of related research in this area:

This is typically called a "filament", a "plasma channel", and sometimes
described as a waveguide. It requires high powers (~10^13 W/cm^2, usually
femtoseconds in duration) depending on gas. It essentially works because the
index of refraction has two nonlinear terms:

    
    
        n(ω) = n_0 + n_2 I - n_plasma
    

the n_2 I makes it so that higher intensities (which occur closer to the
center of the filament for a typical gaussian laser beam), have a higher index
of refraction, causing the field to focus, increasing the electric field
intensity at the center. However, when the field gets high enough, it ionizes
the material, causing a plasma to form, which has a negative contribution,
defocusing the filament. These two contributions balance out and cause a
"waveguide" where the light essentially travels along a narrow channel,
allowing the focusing pulse to travel longer without as much spatial
dispersion.

You can't send another light beam along these channels, because after the beam
has left, all that is there is a plasma channel, which defocuses and absorbs
light instead.

This has been around for a while.

What is NEW here is that they modified their beam profile to have 4 or 8 or
even an annular ring of these plasma channels. Accoustic waves propagate
toward the center of this filament arrangement causing a change in the density
(which changes the index of refraction, the index of refraction increases
linearly with density) that makes the channels act like a waveguide for
another light source. This source of the density change has a duration of
microseconds. When the acoustic waves have dispersed, a thermal gradient has a
similar effect, and this has a duration of milliseconds.

They used this method to channel light from a spark source while mostly
maintaining the spectra of the spark.

Now, what this allows, or doesn't allow:

LIDAR is a possible use, but whatever you point this source at is going to
have to be able to take 10^13 W/cm^2... and not a lot of materials can take
that without breaking down. While there is good focusing and guiding of the
light, I think single photons are probably going to have a high probability of
being absorbed (the mode of the photon will overlap into the plasma channels,
which will likely absorb it), The nice thing is that signals could likely
travel through one of these channels (though there likely will have to be some
dispersion compensation at the receiving end).

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filearts
I feel like I'm missing something important, but if laser photons naturally
scatter over distances then wouldn't that issue apply to the laser being used
to create the compressed air channel?

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icegreentea
The paper goes into more detail.

Basically, if you shoot a pulsed laser of sufficiently high power, you can
generate a phenomenon where the laser's interaction with air will basically
cause it to self focus - resulting in a filament of plasma that traces out a
length several times longer than the normal distance of the laser to double in
cross section area.

By using this effect to generate waveguides, the paper specifically singles
out utility in two situations (laser induced breakdown spectroscopy and
LIDAR), focusing heavily on the first. In laser induced breakdown
spectroscopy, you shoot a laser at a remote target, basically blowing it up,
and reading the spectrum it gives off as it blows up. It is sensing this
spectrum that the paper demonstrates the technique on. By providing the return
signal with a waveguide, it is in theory possible to get a much better signal,
at further distances.

As for using this technique to help transmit other lasers? I dunno if it makes
sense or not. I kind of expect (speculating out of my ass here), at the crazy
interactions that let you create the self-focusing/propagation effect also
makes it difficult to encode a signal into the laser, so it still might make
sense to use this technique to guide a lower power, normally modulated laser
signal... maybe.

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bprater
LIDAR -- that's related to driverless vehicles. I wonder if this might have
implications in that space?

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icegreentea
Little or none. LIDAR for road driving applications is hardly limited by range
or resolution at this point. The problems are cost (which this does nothing to
help with - it requires MORE lasers), and dealing with rain/snow/dust, which
this also does nothing about.

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alex_duf
Anyone has any idea of how long such a structure will stay in thin air ?
Because to me it seems like a proof of concept but it wouldn't be viable : you
need to keep the "fiber" open long enough to transmit data.

Since it's thin air, the soundwave created at the beginning will just continue
and the effect would dissipate.

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mbreese
The article mentioned microseconds... so not long, but possibly long enough,
if you could keep the pulses going and not cancel each other out, but it
sounds like this technique still has a long way to go. And carrying data may
not be the ultimate end-point of this particular technique.

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beloch
This could have applications in free space quantum encryption links. Signals
sent in quantum communications must be at the level of single photons to
ensure security. Traditionally, high repetition rates are used to overcome
loss. i.e. If you have a 1 in a million chance of getting a single photon
through, then trying a million times a second lets you get one photon through
per second, on average. Increasing the transmission of a long free-space link
could increase the maximum distance at which the link will be usable. This may
be especially useful for satellite-to-ground links in less than perfect
atmospheric conditions. Very cool research!

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femto
It would have limited application in satellite-to-ground links, as the photons
would soon be outside the atmosphere. It is a neat idea though.

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beloch
Loss happens in atmosphere, not vacuum. If it increases the overall
transmission the link, what does it matter if the waveguide doesn't go the
full distance?

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femto
I agree that it will help, to the extent that it counters diffraction of the
beam whilst is in the atmosphere, hence it having "limited" application rather
than no application.

Diffraction still happens in a vacuum. The beam spreads, so photons are lost
by not falling on the detector.

The waveguide effect would counteract scattering to some degree. Even then,
wouldn't scattering emit photons in random directions, meaning that the
majority of scattered photons would be moving at too great an angle to the
high/low density boundary to be refracted back into the beam?

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physonaught
I would be interested in looking at applications with applied electric/optical
fields.

Could you shape the plasma after it was created, shaping the waveguide?

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sophacles
Say they manage to sustain these channels over distance, and figure out the
pulse timing to be usable for a relatively sustained link...

What happens on a snowy day? (or other weather types with lots of
interference... hazy/dusty days, etc).

What effect would a stiff wind have on this?

What effect would it have on a person walking through the plasma creation
beam?

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Istof
The connection will fail if your destination moves out of sight?

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phreeza
I have wondered if such a thing would be possible in the interstellar medium
for a sufficiently powerful laser.

