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I've heard that Edison (maybe) invented a way to talk over a light beam. Is this true and how do you modulate the light beam?

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It's Marconi, it's called radio. – Ron Maimon Mar 14 '12 at 22:50
@RonMaimon Some elaboration would not hurt. – Stephen Mc Ateer Mar 14 '12 at 23:20

Heliographs permit communication over large distances with visible light, albeit with a rather low modulation frequency. It seems fairly clear that if you simply increase the switching rate you can create enough bandwidth to carry voice. As Ron has already pointed out, radio, however there are many extant technologies that use light around the visible region for communication, such as the through-space infrared links briefly popular on computers and PDAs, as well as fibre optics which are ubiquitous in long-range communication and can also be seen in the household in the form of toslink connections for high definition audio. In this case, the light is constrained within the fibre by total internal reflection.

High frequency modulation of light is simple with modern LEDs and laser diodes.

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The answer is "yes", particularly in that radio communication is by a sort of light. But if by "light" you mean something close to visible, then it is also true. Terabeam Corporation sold a communication system that used a laser beam. Their most common unit was the Elliptica which was provided 100Mbps (fast ethernet) two-way communication. This is called "free space optics" as opposed to "fiber optics" where the light travels down a fiber optic. The light was created using a laser diode and was modulated by changing the voltage / current supplied to the diode.

There was also a 1Gbps unit that didn't go to market. My recollection is that it was too fast for laser diode type modulation and so used an AOM or "acoustic-optic modulator". This is a way of using sound waves to change the path of a laser beam. For this application, the "sound" waves are at radio frequencies, i.e. 1.25GHz for a 1Gbps data transmission rate using the usual 8B/10B encoding.

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I designed the programmable logic for this system and am listed as an inventor for one of the resulting patents: – Carl Brannen Mar 15 '12 at 4:07
Two students designed a system like this in a lab course I taught. It only worked one way over a distance of two meters and used cheap components, but it worked by modulating the voltage to a laser diode. – ptomato Mar 15 '12 at 7:39

In a general sense, radio waves are light at a different frequency, so @RonMaimon 's comment is correct, radio does allow you to talk using modulated light. Reading up on how radio works would be a good place to start.

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