A method and system of intrusion detection system for a multimode fiber
optic cable. A light signal is launched into the cable fiber to establish
a narrow spectral width, under-filled non-uniform mode field power
distribution in the cable. A small portion of the higher order signal
modes arriving at the remote end of the cable is sampled by use of a
coupler and monitored for transient changes in the mode field power
distribution. The power distribution changes with physical disturbance of
the cable. When those changes are detected as being characteristic of
fiber intrusion, the system activates an alarm. This method can sense and
alarm any attempt to access the optical fibers in a fiber optic
communication cable. In preferred embodiments, the active signal of a
multimode optical fiber is monitored for both signal degradation and
transient power disturbance patterns that could indicate fiber damage or
physical intrusion. A translator can be provided in an existing optical
fiber system in which the data signals are translated in wavelength
and/or launch conditions to optimize the monitoring signals in an
otherwise non-optimized system.