Optical attenuators serve two primary purposes. The first purpose is to reduce the optical power transmitted in a system to allow for transmitter to detector balance. There is some loss of optical power over the length of a fiber optic cable. Additional optical power losses occur in each connector interface. An optical signal that travels a substantial distance between a transmitter and a detector or that travels through a number of connector assemblies will lose substantial optical power before the signal reaches a detector. Due to this loss of optical power, optical attenuators may be undesirable. On the other hand an optical signal that travels a relatively short distance through a relatively small number of connectors may have optical power at the detector that exceeds the detectors optimum optical power range. In situations in which signals with very high optical power reach a detector, it may be necessary to reduce the optical power with an optical attenuator to obtain optimum detector performance. The second purpose for employing optical attenuators is to test system performance. To test optical signal transmission systems the optical power must be reduced by a measurable quantity. Such tests can be used to test the system limits. To determine if an optical system will work when the optical power is reduced by 50% or some other selected amount, the optical power has to be reduced by 50% or another selected amount. Such a reduction is preferably made without changing the transmitter or the detector. To test the system performance as if under adverse conditions, it is necessary to degrade the system by a known quantity and then test for function. This will insure that the system will function under all conditions.
Optical attenuators that are currently used in operating optical systems either change the gap between two fiber optic cable ends or change the alignment between two fiber optic cable ends. The amount of attenuation obtained by these procedures depends on a number of different factors. These factors include fiber optic cable end shape and surface finish. Currently the ends of fiber optic cables in high performance systems are ground and polished. No two ground and polished fiber optic cable ends are exactly the same even when made in a laboratory. The variations in fiber optic cable end shapes make control of the gap between two cable ends difficult. It has not been possible to provide large air gaps in connectors that normally have abutting termini. Fiber optic cable connection end misalignment is difficult to control and measure. The misalignment angle, cable center line alignment, the gap between two cable ends, cable end shape and cable end surface condition all make a difference. Control of all these factors in the field is impossible.