I've been doing a bit of work on optical open-air transmission, and have been thinking, any hays that varies the intensity of the light or whatever else would affect the amplitude.

Would it be possible to send and receive a frequency modulated light transmission? And if so how hard would it be?

  • $\begingroup$ Transmit PWM, receive AM. Error correction FEC. $\endgroup$ – Optionparty Aug 21 '17 at 14:50

Technically speaking, frequency modulation of light would involve shifting the color since, if you look at the EM spectrum, you basically see a rainbow in the visible part of it from red (lowest frequency) to violet (highest frequency). If you wanted to, you could take the original signal, run it through an analog-to-digital converter, and do the calculations to modulate the different parts of an RGB LED. The receiving side of that wouldn't be trivial, though, but it could be done.

Most light modulation that I've seen has instead been amplitude modulation. The details for the circuit would depend on what you're using for a light source, but it's basically a matter of making the light bright or dim based on the input signal. The receiving side for this could be a simple phototransistor circuit.

One possibility that would combine both would work better for digital signals, and that would be to approach it like the way that RTTY works for radio waves. Use two colors of light, the more distinguishable the better, one for your "high" and one for your "low" frequency. You then shift between the two to transmit, and use 2 phototransistor circuits behind 2 color filters matching what you're sending to receive. You would probably need to use amplitude modulation as well, if only to filter out stray light, just like IR remote controls do.

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    $\begingroup$ It is definitifely NOT possible to do frequency modulation of light by using a RGB-LED as it is not possible to do FM modulation of radio waves by using three CM transmitters of different but fixed frequency. All you can get by this method is that it would appear to the human eye the same as real FM light would look like. $\endgroup$ – Curd Oct 12 '14 at 13:54
  • $\begingroup$ You're still thinking of amplitude modulation, which isn't what he was looking for. If you've ever changed the color of an RGB LED, you've changed the frequency of its output. Think about it. $\endgroup$ – Brian WB4ES Oct 13 '14 at 14:36
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    $\begingroup$ Your are wrong because "color" != "frequency". Color is the effect of a whole sprectrum of frequencies. If you vary the intensities of three (red, green, blue) LEDs you will still always get only those three frequencies (e.g. 620nm, 550nm, 450nm if expressed as wavelength). You will never ever get light with 580nm although if you mix 620nm with 550nm it will appear to the human eye as the same color as e.g. a single frequency of 580nm. $\endgroup$ – Curd Oct 13 '14 at 15:22
  • $\begingroup$ Now you're talking about perception, but that doesn't negate the fact that frequency/wavelength is directly related to color in visible light. If you use a scheme similar to RTTY, as I described in my 3rd paragraph, there's no need to mix anything. $\endgroup$ – Brian WB4ES Oct 14 '14 at 16:30
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    $\begingroup$ Like Curd says, an RGB-LED signal does not work analogous to FM radio. An RGB led would be more like three AM signals modulated in concert. It's just that the human eye cannot distinguish between red and green light combined to form yellow or the single frequency of light that is perceived as yellow. But a spectroscope can certainly see the difference. $\endgroup$ – JanKanis Aug 21 '17 at 7:56

There exist tunable lasers, where the output frequency can be modulated. And there exists such a thing as an acousto-optic modulator though I'm not sure if those are used to actually modulate the frequency of light.

Or a more D.I.Y. method could be: shine a narrow beam of continuous spectrum light (e.g. incandescent light) through a prism. Then capture a small part of the diffracted beam, e.g. by having it shine into an optical fiber, or using a small slit. When you rotate the prism a few degrees, the frequency of the captured light varies accordingly.

And accordingly to demodulate the signal: have an array of photoelectric cells and a prism in front of it. Which cell is hit by the light depends on the wavelength. Though you will then need more electronics to combine the outputs of all the cells into a single signal.

  • $\begingroup$ excellent answer! I happened to stumble across AOMs a while back when a customer of mine told me about the trouble they had controlling theirs. I instantly fell in love with the working principle of these devices :) But: AOMs don't actually change the frequency; but I could imagine them being used together with a dispersive device (eg. a prism) to "select" the colored light that is coupled into an optical device. $\endgroup$ – Marcus Müller Aug 25 '17 at 20:18
  • $\begingroup$ I think AOMs can also function as dispersive devices (though I don't have any experience with them), so all you need is a wide spectrum light source. $\endgroup$ – JanKanis Sep 24 '20 at 12:33

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