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If light is an electromagnetic wave, why is it not affected by electromagnetic interference? I've heard it's because fiber optic do not use electrical voltages. Can someone go deeper into the subject?

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Free-air laser communications can be affected by EMI if that EMI matches the of the detector (i.e. is some other matching light). This is just another way of saying stray light will interfere with your communications if it can get into your detector.

But in fiber optics, the internal laser is isolated from external light sources. High frequency EMI (i.e. stray or ambient light) from outside hitting a bare optical fiber can't travel down the fiber in a stable manner because it hits it at the wrong angle, and if the optical fiber is sheathed in an opaque material, external light isn't able to make contact with the fiber at all.

Lower frequencies EMI , like radio waves, can penetrate the sheath and get into the optical fiber. The typical way EMI produces noise is to induce movement of charges in a conductor, but the fiber is not a conductor so no charge movement can be induced. And even if it was a conductor where charge movement could be induced, it would not matter since the optical fiber is carrying its signal with light, not charge movement. Therefore, the optical fiber is immune.

On the flip side: if light is an electromagnetic wave, why does it not cause EMI in electrical signals?

Traditional electric signals that travel down wires are based off the movement of charges. In this case, EMI is from electromagnetic waves inducing unwanted charge movement in the wire. Radio waves can pass through insulation to do this and penetrate deep down into the conductor to do this.

But light is much higher frequency so gets absorbed more easily and can't penetrate the insulation. And even for bare wire, even if it does induce something, the frequencies induced are so high that the impedance due to parasitic inductance is so high the signal can't get anywhere. So light doesn't really interfere with electrical signals the same way radio waves can.

For stray electromagnetic radiation to produce interference in an optical fiber in an analogous manner to electrical signals, you would need to light to be able to penetrate any opaque sheath the optical fiber has, and then induce the fiber to fluoresce and emit photons, some of which would be at the right angle to travel down the fiber.

That is my understanding of it, anyways.

EDIT: Atoms of a material have something called a plasma frequency and if too low, electrons around the atom cannot oscillate fast enough to respond the the EM frequencies (like light) trying to induce them to emit light.

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