
Fiber Defender FD508TM User Manual 8
3.The Sensing Fiber
The FD508 detects intruders by sensing small disturbances caused by vibrations induced within
a fiber optic sensor attached to the perimeter. The optical sensor is a thin strand of multimode
optical fiber inside a specially designed 3 mm fiber optic cable. The fiber optic cable should be
installed in such a way that, when intruders attempt to cross the perimeter, they create slight
vibrations that disturb the sensing fiber. These disturbances are then detected by the FD508
APU, which generates the appropriate alarm(s).
Fiber Optic Sensing
When an optical fiber is exposed to vibration, the vibrations cause small asymmetric
changes in the fiber’s density. In turn, these changes in density cause measurable
changes in certain characteristics of laser radiation transmitted through the fiber. The
FD508 uses precision lasers and detectors, along with sophisticated digital signal
processing, to measure these changes in the laser radiation. The processor analyzes the
incoming signals in order to determine whether they are caused by intruders, or harmless
nuisances, such as vibrating equipment. To learn more about fiber optics and their use as
sensors, refer to the fiber optics application note titled: AN-SM-007 Fiber Optics.
Types of Installations
There are many different ways to use the FD508 system. The most common installation is
on chain link fence. For fence-mounted applications, the fiber-optic cable is installed inside
a flexible conduit, which is then secured to the fence using stainless steel wire ties. Other
applications involve installing the optical cable inside the channels of decorative metal
fence, or running the flexible conduit (with optical cable inside) along the tops of concrete
walls. For detailed information about the possible fenced perimeter installations, and site
design techniques, see the application note on site design and assessment titled: AN-SM-
036 FD500 Series –Site Design and Assessment.
Connectors
The FD508 is a time-domain-multiplexed system that can monitor up to eight fully
independent zones (sensing fibers) using a single APU. To maintain a high signal-to-noise
ratio, it is important that all optical connections in the system be made with either fusion
splices or angled physical-contact (APC) fiber optic connectors. PC and UPC connectors
should not be used. For more information about fiber optic connectors, refer to the fiber
optics application note: AN-SM-007 Fiber Optics.