Variable Optical Attenuator are used in fiber optic transmission systems to reduce the optical power received by photo-detectors. The light intensity of laser diodes and LED's Should be controlled to be within their dynamical range. Otherwise, excess power level can result in undesired radiation pattern, modal structure and central wavelength. Suchlssue is even more critical for high-speed singlemode system where the laser is very sensitive to back reflection.

Controlling the light intensity emitted by the transmitter requires extra electronic circuitry. With the use of adaptor type attenuator, system tuning can now be accomplished at the destination office, immediately in front of the receiver, instead of in the field from the top of a bucket truck. This is much more convenient and can significantly speed up system installation. In addition, this will also reduce downtime in case of equipment failure.