What does an optical attenuator do?

An optical attenuator, or fibre optic attenuator, is a device used to reduce the power level of an optical signal, either in free space or in an optical fibre. The basic types of optical attenuators are fixed, step-wise variable, and continuously variable.
Optical attenuators are commonly used in fibre optic communications, either to test power level margins by temporarily adding a calibrated amount of signal loss, or installed permanently to properly match transmitter and receiver levels. Sharp bends stress optic fibres and can cause losses. If a received signal is too strong a temporary fix is to wrap the cable around a pencil until the desired level of attenuation is achieved.