The attenuation of an optical fiber measures the amount of light lost between input and output. Total attenuation is the sum of all losses. It is dominated by imperfect light coupling into the fiber and absorption and scattering within the fiber. Sometimes other effects
can cause important losses, such as light leakage from fibers that suffer severe micro bending. Attenuation limits how far a signal can travel through a fiber before it becomes too weak to detect.
Absorption and scattering are both cumulative, with their effects increasing with fiber length. In contrast, coupling losses occur only at the ends of the fiber. The longer the fiber, the more important are absorption and scattering losses, and the less important coupling losses. Conversely, attenuation and scattering may be much smaller than end losses for short fibers.

To briefly review these losses, when you deliver an input power, P0 , to a fiber, a fraction of that light, ΔP, is lost. Thus only the power P0 — ΔP gets into the fiber. This light then suffers absorption and scattering loss in the bulk of the fiber. These losses depend on length. If the light lost to absorption per unit length is a and the light lost to scattering per unit length is S, the fraction of light that remains is (1 — α — S). Outside the research laboratory, the quantity that matters is the attenuation per unit length, which is the sum of the absorption and scattering (α + S).
Recall that to calculate the power remaining after a distance D, you raise the fraction of light remaining after attenuation to the power D. This gives a formula for power at a distance D

That formula is more useful for looking at the process of light loss than for calculations.
It reminds us that absorption and scattering combine to make attenuation. It also reminds us that attenuation acts only on light that gets into the fiber, because some light is lost on entry.