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.
- 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.
- Scattering - Atoms in glass cause a certain amount of scattering of light and scattered light will not emerge at the output.
- Leakage - Light will leak out of fiber, especially if the are a lot of bends in the fiber.