HP 86140A Series User Manual page 272

Reference
Cleaning Connections for Accurate Measurements
Figure 6-4. A dirty endface from poor cleaning
Repeated connections made without removing loose particles, or using
improper cleaning tools can lead to physical damage of the glass fiber endface,
as shown in Figure 6-4. When severe, the damage on one connector end can
be transferred to another good connector that comes in contact with it.
Figure 6-5. A damaged fiber end from using an improper cleaning procedure
The cure for these problems is disciplined connector care. as described in the
following list and in "Cleaning Connections for Accurate Measurements" on
page 6-10.
6-14