Optical attenuators used in fiber optic communications systems may use a variety of principles for their functioning. Those using the gap-loss principle are sensitive to the modal distribution ahead of the attenuator, and should be used at or near the transmitting end, or they may introduce less loss than intended. Optical attenuators using absorptive or reflective techniques avoid this problem.
The basic types of optical attenuators are fixed, step-wise variable, and continuously variable.
Source: from Federal Standard 1037C
See also: telecommunication