EXFO IQS-3150 Variable Attenuator for IQS-500/600 - Manual (Page 71)

Variable Attenuator 65
7 Measuring Multimode
Insertion Loss
Often, when using a multimode source (mostly LED sources), part of the
optical energy is transmitted into the fiber cladding. These cladding modes
attenuate rapidly, but will affect power meter readings if the source is
connected using only a short jumper, as would be the case when taking a
reference before measuring insertion loss.
Due to the internal optics of the Variable Attenuator (and the majority of
attenuators), most of the cladding modes are filtered out. This means that,
with the attenuator connected, the power meter will display a loss greater
than the attenuation setting of the attenuator (cladding modes have been
partially stripped).
Cladding modes are not used in communication systems and, therefore,
should not have been part of the source power measurement in the first
place.
short distance
long distance