An attenuator is a device that reduces the intensity of the light signal passing through it.
Here are some single fiber attenuators. There also variable attenuator arrays made with MEMS technology, one is shown here.
One of the most popular applications is on short fiber optic links where the light level is so high that it is outside of the dynamic range of the light detector (receiver). An attenuator is placed in the circuit to reduce the light level such that it is in the receiver’s dynamic range.
Another application of attenuators is in WDM systems where the power in each channel is tailored so that the power delivered to the first in-line EDFA (Erbium Doped Fiber Amplifier) is flat in its spectrum.