Tuesday, September 18, 2012

Attenuation in Fiber Communications Systems

I'm teaching a fiber optics communications course this semester and - like just about every communications course - we started out talking about attenuation.

Attenuation is just a fancy word for loss. In any communications system you've got a certain amount of signal strength going in and a certain amount of signal strength coming out. If there is no amplification in a system there is always going to be loss and the output signal will always be weaker than the input signal.

In fiber systems attenuation is caused by three things:

  1. Absorption - Glass, whether it is fiber or the windows in your house, will always absorb a small amount of light going through it. The amount depends on the wavelength of light and what the glass is made of.
  2. Scattering - Atoms in glass cause a certain amount of scattering of light and scattered light will not emerge at the output.
  3. Leakage - Light will leak out of fiber, especially if the are a lot of bends in the fiber.
Fiber manufacturers typically provide specifications for all three of these, along with total attenuation per kilometer.

One of the primary goals in any communications system is to keep the attenuation to a minimum. Even so, there will always be a loss in signal intensity when comparing output power to input power. Calculating attenuation in a system is pretty simple. Attenuation is cumulative so basically you just add up the signal loss for each component in the system. Here's an example:

Question: A 50 km fiber run has been spec'd at 99% transmission per km. What percentage of light will emerge at the output?

The fiber run is transmitting 99% per km so after the first km 99% of the input signal will be available, after the second km, 99% of what's left after the first km will be available, etc. So we can say:
60.5% of the original input signal strength will emerge at the output.

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