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I was reading a book about Optical Wireless Communication.

In the chapter where the transmitters are explained, there is a figure showing two types of bandwidth for LEDs.

This confused me a bit, why there are:

  • Electrical bandwidth
  • Optical bandwidth

What is the difference and the relationship between them? Moreover which one is relevant when it comes to the modulation (modulation bandwidth)?

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Light is part of the electro-magnetic spectrum: -

enter image description here

To the left is AC power and to the right are gamma rays and some cosmic rays. Somewhere in the middle is visible light. This is called the electro-magnetic spectrum and the optical spectrum for an optical device refers to the bandwidth (or range of colours) that it produces or is most sensitive to.

For an LED, how quickly you can "modulate" the light level with an electrical signal is related to the electrical bandwidth of the device. How fast a photodiode responds to modulations of light is also related to the electrical spectrum of the device.

Here is an expanded slice of the radio part of the spectrum: -

enter image description here

Somewhere approximately in this region is where the electical bandwidth limits for a given device will lie.

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  • \$\begingroup\$ so the optical bandwidth is relevant only if we care about the colors "emitted" by the device? \$\endgroup\$ – cross May 13 '16 at 15:39
  • \$\begingroup\$ Yes, that is correct. \$\endgroup\$ – Andy aka May 13 '16 at 16:14

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