Frequency modulation using visible light

Question

Currently I am doing research into visible light communication from an Embedded Systems background (MSc), but I am struggling to relate the concepts of modulation of radio to visible light. I already apply modulation to a working VLC system, but the underlying system still puzzles me.

My goal of this post is to create an understanding of the underlying fundamental concepts, so if there are flaws in my assumptions below please point me to it.

Radio

In RF communication, two modulation techniques are used: amplitude modulation and frequency modulation. All other modulation techniques are variations on these types.

These modulation techniques change the electromagnetic field in terms of coherent photons with a specific frequency and amplitude (source).

Visible light communication

In VLC, amplitude modulation is the most common technique (changing the intensity), but frequency modulation is also used. As frequency relates to color, some research uses RGB LEDs to do this in combination with color filtered photodiodes.

(1) However, and this is where the problem is, some research applies frequency and phase modulation using a single white LED. They use a photodiode as a receiver which is sensitive to intensity but not to color variations, so how is frequency modulation then possible?

(2) It seems that VLC has a third kind of main modulation technique in comparison with RF, is this correct?

(3) If so, what makes light so different from RF then, as it also is electromagnetic radiation? And can this third type be applied to RF?

Answer 1

Frequency modulation in VLC is not done by changing the frequency of the light itself. The light intensity is pulsed at a high frequency to create a carrier wave. This carrier wave is then frequency modulated to transmit the information, typically using frequency shift keying.

That's why it works with white light, where the frequency of the light isn't well defined.

Answer 2

Physics first: light and radio waves are the same thing, just at vastly different frequencies. Radio works between $1\text{ Hz}$ and approx. $(10^{11}-10^{12})\text{ Hz}$ (at the moment). The frequency of visible light is around $10^{15}\text{ Hz}$. The range between these frequencies is usually called infrared radiation and is of little interest for communication.

Communication with visible light does usually NOT modulate the carrier wave at $10^{15}\text{ Hz}$ directly. Instead we modulate an RF signal onto that light (as an amplitude modulated signal) and then we modulate the phase or frequency of that RF signal. The reason for that is because we don't have easy ways to mix light (it can be done, but requires complicated optical setups). This leaves you with pretty much the same choices of modulation technique as with any other IF scheme. For data communication, of course, one can modulate the codes directly, because the semiconductor lasers are fast enough for that (LEDs are not).