EXFO iOLM - Manual (Page 13)

Introducing the intelligent Optical Link Mapper
intelligent Optical Link Mapper 7
Launch, Receive, and Loop Fiber
Launch, Receive, and Loop Fiber
Unlike a traditional OTDR, the iOLM requires only a short launch fiber
(>50 m) to benefit from all the advantages of this referencing method,
regardless of the link length and loss. A launch fiber longer than 200 m is
not recommended when testing PON links. Because the output port of an
OTDR may degrade its loss and ORL performances after multiple
connections, the use of a launch cable is always recommended.
The first element of the tested link is tagged with the letter (A) into the link
view. A launch cable allows you to properly characterize the first connector
of the fiber link under test (A) and exclude the OTDR connector's wear
from the link evaluation. A reasonable amount of degradation of the OTDR
connector is acceptable when using an APC interface; the ORL remains
low due to the angle polish, preventing poor near-end resolution. By using
a launch fiber, the OTDR connector loss is excluded from the
measurement. The iOLM evaluates the OTDR connector loss each time a
measurement is performed to inform you about the condition of the
connector. It is important to understand that excessive loss at this
connector will eventually degrade the measurement capabilities of the
instrument. In addition, using a launch cable will help protect your OTDR
connector by limiting the number of connections performed directly on
that connector. It is easier to repair or replace a launch cable than to
replace the OTDR connector.
The last element of the tested link is tagged with the letter (B) into the link
view. A receive fiber cable can be used at the end of the link opposite the
test module in order to characterize the last connector of the link (B) and
increase the precision of the total insertion loss result by comparing
differential level of two known fibers (to avoid errors due to different
backscatter coefficients of the fiber used in the link). If no receive cable is
used, the iOLM application will be able to measure the position and ORL of
this connector in unmatched condition, but not its loss. No pass/fail status
will be displayed for that connector. The required length of the receive
fiber will depend on the loss of the link under test. A higher loss requires a
longer pulse to reach the receive fiber level. Unlike the launch fiber, the