The optical loss of a fiber optic cable plant is the most common
measurement installers make. Also called insertion loss, it is
measured by using a test source, a power meter, reference test
cables and connector-mating adapters. The insertion loss test
mimics the actual usage of the cable plant with a transmitter
and receiver, so it is the best indication of the condition of
the cable plant. Another loss test, using an optical time-domain
reflectometer (OTDR), is generally not allowed for acceptance
testing because it uses a different measurement technique that
may not correlate with actual system link loss. We’ll discuss
OTDR testing in future articles. The test source for loss
measurements should be compatible to the fiber type and
wavelengths of systems that may use the cable plant. Most test
sources for multimode fiber use LED sources at 850 and 1,300 nm
wavelengths, although many systems today only require testing at
850 nm and use a 850 nm vertical cavity surface emitting laser
(VCSEL), a low-cost laser