I seem to recall hearing about devices that
could detect a tap in fiber. They worked by
sending light at frequencies slightly different
than the primary channel. If the fiber were
bent to tap the light, the other frequencies
would attenuate a larger amount than the
primary channel thus allowing a monitor
to "see" the tap.
The tool used to detect taps on fiber as well as on cables is a Time Domain
Reflectometer known lovingly as a cable radar. Both send out a signal and look
for signal loss over the length. When you first set up a path you run an TDR to
identify connections which since the line is new have been planned and
authorized. You can periodically run the TDR to compare the known with the
unknown to identify illegal connections or breaks.
Deloitte & Touche LLP