EXFO iOLM - Manual (Page 48)

Setting User Preferences
42 iOLM
Configuring your iOLM
Configuring your iOLM
The iOLM tab displays the settings used for taking measurements. The
iOLM performs the acquisition on the port and wavelengths selected in the
iOLM tab; the values are preserved for the next acquisition.
The loopback mode allows you to test two fibers at once, which saves time
and manipulations. If you have activated the loopback option for your
application, it is ready to use when needed.
When working in loopback mode, there are three rules to follow so the
loopback can give appropriate results:

Both fibers under test and connected to the loop must have the same
length.

The launch and receive lengths must be specified correctly.

The length of the loop must be specified correctly.
The loopback measurement is especially designed for testing cables
formed of a pair of identical fibers of an equivalent length. The measuring
time is optimized because two fibers are checked in a single
measurement.
Note: Files created using the loopback mode cannot be used to perform a second
loopback operation again.