3.Installing the sensing fiber
The SL50x detects intruders by sensing small disturbances caused by attempts to gain access
to cables inside a conduit. The SL50x detects these disturbances by using one of the fibers
within the conduit as a vibration/motion sensor. The sensor can be any multimode fibers,
including currently installed dark fibers and all OM fiber types, that are not used for transmitting
data.
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 light that is transmitted through the fiber. The SL50x uses precision
lasers and detectors, along with sophisticated digital signal processing, to measure these
changes in the laser radiation; analyzing them 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 application note on fiber optics titled: AN-SM-007 Fiber
Optics.
Using existing (dark) fiber or non-Fiber SenSys fiber
If a given cable/conduit in the system has enough unused (dark) fibers, one of them may be
used as the sensing fiber.
Sometimes unused (dark) fiber may not be available, or for various reasons, it may be
undesirable to use existing fibers. In these instances, new fiber can be pulled through the
existing conduit and used as the sensing fiber.
Whenever dark fiber or a non-Fiber SenSys fiber is used, performance will vary compared to the
Fiber SenSys sensor cables. To verify satisfactory performance the fiber must be tested.
Depending on site requirements some dark fibers may not provide sufficient sensitivity due to
the way the fiber is mechanically built into the cable/conduit.
Connectors
The SL50x is a time-domain-multiplexed system that can monitor multiple fully independent
zones (sensing fibers) using a single APU. The SL504 can monitor up to four zones and the
SL508 can monitor up to eight zones. To maintain 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 fiber-optic connectors (APC). Physical Contact (PC) and Ultra Physical Contact (UPC)
connectors should not be used. For more information about fiber-optic connectors, refer to the
application note on fiber optics titled: AN-SM-007 Fiber Optics.
