Why Is Binary Modulation Preferred in Visible Light Communication?

[RAHIL008]

I read that for VLC, they use toggling the LEDs ON/OFF to represent binaries. Why cannot we modulate the visible light like radio waves.

 

[Drakkith]

The frequency is far too high for electronics to respond, so we aren't able to modulate a signal in the same way we can with a radio wave.

 

[RAHIL008]

I guess that is because "electronics" are made to handle lower frequencies of radio waves. Why cannot we make compatible electronics.?
the question is, can we modulate light? Handling is different issue.

 

[Drakkith]

Modulating and handling go hand in hand. To modulate a radio signal, the signal is generally created by the electronics and sent through the wires, whose electrons are able to respond to the frequency. Visible light oscillates too quickly for the electrons to respond, and since all materials depend on electrons to carry the signal, if they can't oscillate back and forth with the signal then the signal can't propagate. There aren't any materials that can handle light because there are no particles other than electrons that can carry an electric signal. So we can't make new electronics because the materials simply don't exist.

 

[RAHIL008]

Hmm...got it...thanks :-)

 

[NascentOxygen]

I read that for VLC, they use toggling the LEDs ON/OFF to represent binaries. Why cannot we modulate the visible light like radio waves.

You can modulate the intensity of the light beam---by driving the LEDs with current that has a component proportional to the amplitude of the audio signal. But digital signal transmission can be made more immune to interfering sources, including sunlight.

 

[berkeman]

As already said, amplitude modulation is often used in optical communication systems. However, other modulation techniques have been used as well. Here is a good paper from Stanford reviewing modulation schemes used in long-haul optical fiber links:

www-ee.stanford.edu/~jmk/pubs/mod.and.det.tech.COTA.6-06.pdf

:)

 

[Drakkith]

Thanks for the link, Berkeman, I wasn't aware we were using those modulation methods in fiber optics. Am I correct in assuming they are still using either the light from the original laser or from another source to mix with/interfere with the signal and perform modulation/demodulation?

 

[tech99]

Thanks for the link, Berkeman, I wasn't aware we were using those modulation methods in fiber optics. Am I correct in assuming they are still using either the light from the original laser or from another source to mix with/interfere with the signal and perform modulation/demodulation?

The LED or semiconductor laser can be pulsed on and off with its power supply.

 

[sophiecentaur]

Analogue transmission by Amplitude Modulating light tends to be hampered by non linearity of emitters and sensors. Frequency Modulation is difficult, too. One form of modulation by an analogue signal is to use pulse width modulation. The level of the light signal is binary (switched on of off) - so linearity doesn't matter and the timing of beginning and ends of the pulses is easy to make proportional to the analogue signal you want to carry.