What is the function of a variable optical attenuator

Optical attenuators can take a number of different forms and are typically classified as fixed or variable attenuators. What's more, they can be classified as LC, SC, ST, FC, MU, E2000 etc. according to the different types of connectors. Fixed optical attenuators used in fiber optic systems may use a variety of principles for their functioning. Preferred attenuators use either doped fibers, or mis-aligned splices, or total power since both of thes. [pdf]