Tidy format & related comments here

               http://electronics.stackexchange.com/questions/24214/using-a=
-led-to-transmit-data/24225#24225

___________________

Untidy gist:


>From Mark Rages Ronja reference

[Through air optical coms page](http://www.modulatedlight.org/)

It says:

 - This page deals with long-distance atmospheric (through-the-air)
optical ("lightbeam") communications of various types using coherent
and noncoherent light sources, methods of mitigating atmospheric
effects on such communications, as well as various technologies
involved in transmitting and receiving such communications.  The
majority of the content on these pages is produced by self-funded
hobbyists that have taken on the challenge of furthering the state of
the art in this somewhat arcane field.)

[More on same](http://www.modulatedlight.org/optical_comms/optical_index.ht=
ml)
same people

[Ronja](http://ronja.twibright.com/)

They say:

 - Ronja is a free technology project for reliable optical data links
with a current range of 1.4km and a communication speed of 10Mbps full
duplex.

 Applications of this wireless networking device include backbone of
free, public, and community networks, individual and corporate
Internet connectivity, and also home and building security. High
reliability and availability linking is possible in combination with
WiFi devices. The Twibright Ronja datalink can network neighbouring
houses with cross-street ethernet access, solve the last mile problem
for ISP=92s, or provide a link layer for fast neighbourhood mesh
networks.

_______________________

Well worth a look:

[This August 2009 "letter" shows how fast you can push a slow white LED !!!=
]

              (http://northumbria.openrepository.com/northumbria/bitstream/=
10145/119149/2/Le%2520Minh%2520et%2520al%2520-%2520100-Mbs%2520NRZ%2520visi=
ble%2520light%2520communications%2520using%2520a%2520postequalized%2520whit=
e%2520LED%2520-%2520article.pdf).

 They use a white LED with phosphor response in the few MHz range,
filter out the slow yellow component and  equalise, to get 50 MHz
modulation bandwidth, which allows on/off NRZ at 100 Mb/s.

 They note that the achieved 50 Mb/s is 25x the unequalised unfiltered
bandwidth.

I'd wonder, why not use a blue LED without phosphor?


________________

[This abstract]

            (http://ieeexplore.ieee.org/Xplore/login.jsp?url=3Dhttp%3A%2F%2=
Fieeexplore.ieee.org%2Fiel5%2F3%2F23011%2F01070555.pdf%3Farnumber%3D1070555=
&authDecision=3D-203)

notes that InGaAsP LEDS are good for 300 Mbps at full power if you
talk to them nicely (reverse bias off pulsing to sweep charge out
faster) and 200 Mbps if you drive with non reverse bias.

 They say:

 - The application of reverse bias pulses at on-off transitions
increased the maximum bit rate of full power operation of long
wavelength InGaAsP LED's from 200 to 300 Mbits/s by reduction of the
stored charge fall time.

 Although designed primarily for nonreturn to zero (NRZ) DS-4
experiments, the circuitry operates from 50 to 300 Mbits/s for the
return to zero (RZ) or NRZ format with fixed or pseudo-random word
patterns.

______________

Here is
[a useful but compact answer to "How fast is an LED"]

           (http://www.eng.yale.edu/posters150/pdf/woodall2.pdf) and
it is worth noting that they say 'about 2 GHz modulation bandwidth or
about 1 Gb/s' compared to the  300 Mb/s above. Note that for
engineering purposes 300 ~~~=3D 1000 :-)


__________________


  [1]: http://i.stack.imgur.com/ZPCTC.jpg

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