
Harnessing light for wireless communications - dwaxe
https://code.facebook.com/posts/484014178459953/harnessing-light-for-wireless-communications/?utm_source=codedot_rss_feed&utm_medium=rss&utm_campaign=RSS+Feed
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rosalinekarr
Maybe with this technology, we can finally build the 80's neon cyberpunk
future we've all been waiting for.

~~~
liveshops_
I wanted to upvote this twice.

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zw123456
One of the major challenges for FSO is that it requires direct line of sight
(LoS). One reason that electromagnetic radiation in the RF range is more
desirable for communications is because it can be non-Los (think about your
cell phone, you do not have to see the tower for it to work, you can be
indoors for example).

The second issue is the free space pathloss (see
[https://en.wikipedia.org/wiki/Free-
space_path_loss](https://en.wikipedia.org/wiki/Free-space_path_loss) ) you can
see by the formula that the loss goes up exponentially by not only the
distance but also the frequency, which is why the wireless carriers are so
interested in the 600Mhz spectrum, you need to build fewer towers using 600Mhz
than say 3.5Ghz.

The sudden interest in mm-wave (e.g. 28Ghz) for 5G is because the spectrum is
much wider, that is the channels can be very wide allowing more data to be
encoded, the trade off is that you start losing the NLoS capability, or it
becomes much more difficult.

With light, all those problems are orders of magnitude more difficult to
overcome. Also, there are regulations around the level of emissions of light
for safety reasons, if a laser is turned up too high it can harm a persons
eyes if they happen to look at it, so although the spectrum itself is not
regulated, the amount of power you can emit is. That makes it very difficult
to implement an FSO system that can safely transmit more than one or two
kilometers, but if you get fog, rain or snow, that distance quickly becomes
100's of meters at best.

They are correct that the best way to overcome this issue is by developing
sensors with much better sensitivity, by making them larger you can
theoretically collect more photons, like a telescope. Since it is unlikely you
can overcome the safety issue of increasing the transmission power level, it
is smart to focus on the receive end and improve the sensitivity. But that
does not help the LoS problem.

But a very interesting development nevertheless.

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tlb
Loss goes up quadratically with distance, not exponentially.

The graphs showing the quadratic dependence of loss vs. frequency assume a
half-wave antenna, meaning the antenna size decreases inversely to wavelength,
so most of the effect is from having a smaller antenna.

In any case, path loss doesn't usually limit network capacity. Densely
deployed systems are limited by interference, and the interference experiences
the same path loss as the signal.

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zw123456
Yes, by the square, this is HN I cannot speak so loosely:) For Multiple access
RF systems yes, but got point to point systems, that is line of sight, noise
is not an issue, free space loss, also impairments from fog, trees and other
things are dominant.

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mdturnerphys
After reading this post, I was left with the question of why bother with the
luminescence--why not just collect the transmitted light with the fibers and
transmit it directly to the detector. The linked journal article makes it
clear that the transmitted light is not collected by it entering the ends of
the fibers. Instead, the transmitted light "scatters" off of the dye (changing
wavelengths in the process), regardless of where and at what angle it hits the
fiber, and some of it is scattered in the proper angular range to be
transmitted along the fiber.

~~~
mstresh
Efficiently directing light into a fiber optic cable is a difficult problem,
in general. The luminescence is their proposed solution to this problem and
one of the key results in their journal article.

You have to shine light end-on into a fiber--you can't couple light into the
fiber by shining on it at the side. Even for large, visible-light
scintillating fiber used in this paper, you only have a few degrees of
possible incidence angles that direct the light into the fiber. See for
example, (one of the first results from Google), the figure on page 4 of this
datasheet [0].

[0] [https://www.ethz.ch/content/dam/ethz/special-
interest/phys/p...](https://www.ethz.ch/content/dam/ethz/special-
interest/phys/particle-physics/precisionphysicsatlowenergy-
dam/TeachingContent/ASL/bicronfiber.pdf)

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antonios
Reminded me of the Ronja project:
[http://ronja.twibright.com/about.php](http://ronja.twibright.com/about.php)

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vturner
Isn't it amazing how we in the "developed" nations always presume that the
"developing" nations need our technology? Colonial Europe assumed the savage
nations needed railroads and hospitals, and now us enlightened 21st century
liberals assume the developing world needs our internet, our network of
connectivity.

But do they? Are they not already connected, and are we truly connected?
Consider our societies. We are epidemically depressed, living alone, and
becoming increasingly friendless. The internet has undoubtedly created
business opportunity (I'm work remotely as a researcher); but has it
"connected" us as we were not already connected? Instead, has it in fact
disconnected us in its facade of relationships?

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seanalltogether
> (I'm work remotely as a researcher)

Sounds like you're projecting your own existential crisis onto others. Maybe
you should try quitting your job and finding something local and more hands
on.

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vturner
No projection of my own. I have a significant person in my life and friends.
I'm commenting on the well-documented reality of some western cultures. I few
searches brought these stories up:

Depression:
[http://www.nimh.nih.gov/health/statistics/prevalence/major-d...](http://www.nimh.nih.gov/health/statistics/prevalence/major-
depression-among-adults.shtml)

Living Alone: [https://www.washingtonpost.com/politics/more-americans-
livin...](https://www.washingtonpost.com/politics/more-americans-living-alone-
census-says/2014/09/28/67e1d02e-473a-11e4-b72e-d60a9229cc10_story.html)

Friends in UK: [http://www.telegraph.co.uk/men/active/mens-
health/11996473/2...](http://www.telegraph.co.uk/men/active/mens-
health/11996473/2.5-million-men-have-no-close-friends.html)

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femto
My immediate question was isn't their bundle of optical fibres performing
essentially the same function as a lens? Why not use a lens?

Answering my own question, having just read the abstract (in case anyone else
had the same reaction). The claim is that they get the light collecting
powers, combined with a wide field of view. A lens collects the light, but has
a narrow field of view, which requires steering/alignment.

Edit: Another possibility is a Luneburg lens, a type of lens that can focus
plane waves from many directions to a small area?
[https://en.wikipedia.org/wiki/Luneburg_lens](https://en.wikipedia.org/wiki/Luneburg_lens)

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xigency
> In addition, this portion of the spectrum is not regulated

Apart from not pointing a laser at a flying aircraft. I'm guessing this would
be affected by weather.

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metaobject
Clouds and optical turbulence are both problems that have to be mitigated by
FSO systems

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beloch
I wonder what their detector's dark count rate and activation threshold is. If
both are low enough, this is precisely the sort of detector people doing free
space quantum crypto need.

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hathym
Are they trying to re-invent Li-Fi ? [https://en.wikipedia.org/wiki/Li-
Fi](https://en.wikipedia.org/wiki/Li-Fi)

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pyvpx
the article is easily digestible and you'll find that no, they aren't. not
even close.

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xigency
Hm, they seem similar because both Li-Fi and free-space optical communication
(FSO) use visible or near-visible spectra to send and receive information. I
guess they might be seen differently as Li-Fi is potentially designed for
indoor use? I don't see why they are substantially different, however, or at
least incomparable.

What the post discusses: [https://en.wikipedia.org/wiki/Free-
space_optical_communicati...](https://en.wikipedia.org/wiki/Free-
space_optical_communication)

~~~
metaobject
One significant difference is that FSO operates over a much longer distance
and has to contend with atmospheric interference (clouds, turbulence, etc).
Also, the transmitter/receiver hardware used is much different.

