Despite the effectiveness of OTDR technology, the existence of dead zones in fiber optic links are a potential source of fiber monitoring uncertainty. A dead zone in an OTDR measurement can occur when there is a high reflectance event in the fiber line. This can be caused by air gaps, splices or connectors that produce a sufficient level of reflectance to temporarily saturate the OTDR detector.
During this “black-out” period while the detector is recovering from the saturation, the OTDR will not be capable of accurately discerning other nearby events in the fiber line. This becomes important when a fault condition arises close enough to an existing splice or connection such that a new issue is obscured by a pre-existing reflectance source.
Another common component of fiber optic networks that can lead to challenges as well as opportunities is the prevalence of dark fiber. Despite the ominous-sounding moniker, this term simply refers to the presence of unused or “unlit” fiber in the network. This descriptor is also sometimes used to describe fiber optic cables that are leased from the original carrier to another party.
This dark fiber still requires testing and monitoring to ensure integrity, particularly if the fiber is designated for future expansion opportunities. The presence of unused, terminated fibers can sometimes prove advantageous for monitoring. Fault conditions will usually affect all fibers in a cable, so monitoring of selected dark fibers is an effective way to continually verify the cable integrity without disrupting the active fibers.