This paper explores using fiber optic cabling and sensors to achieve cost-effective, long-distance intrusion monitoring. Also covered are the advantages of these non-electric, spark-free fiber optic sensors, which enable their use in chemical plants, underground installations and other environments where explosive gases may be present.
The telecommunications industry has long known the advantages of using optical fiber to send information over great distances. Now the security industry has the opportunity to use the same technology to achieve long distance intrusion monitoring using the same technology and components as that used in a telecommunications network.
It must be recognized that using fiber optics in security applications is not a new idea. Security systems exist that use specialized, highly sensitive optical fiber involving doped fiber cladding and interferomic sensing equipment.
These highly sensitive security systems are not the subject of this paper. These systems are excellent for monitoring small areas, but they are not usually deployed to monitor distances greater than 1000 feet, such as the perimeter around an airport or large factory complex. These systems are too expensive for such large-scale deployment, and they are so sensitive they can be subject to frequent nuisance alarms caused by environmental factors when monitoring large areas.
Instead, this paper focuses a new technique of applying common, low-cost optical fiber to monitor large-scale facilities.
A Cost-Effective Approach
This new approach takes advantage of optical fiber’s sensitivity to optical losses resulting from “macrobending,” i.e. bending fiber to a radius of curvature that is tight enough to produce measurable light loss. Because this approach uses standard communication optical fiber, the tools, installation and maintenance of this type of security system is no different than that required for a standard fiber optic telecommunications link.
Actually, this technology has been used in communication closets for years to signal an alert if a fiber optic cable in a large network becomes broken, severely bent, or is otherwise damaged.
For example, if a backhoe operator were to accidentally break a buried fiber optic telecommunications cable, repair personnel are alerted. They then use a device called an Optical Time Domain Reflectometer (OTDR) to identify the type of problem and pinpoint exactly where it occurred along many miles of cable.
An OTDR is required to average thousands of reflections at intervals along the fiber to identify this break point, typically requiring 10 seconds or more to complete the process.
This inherent ability of fiber optic technology to pinpoint the location of a bent or broken cable makes this same technology ideal for pinpointing the location of an intruder. For example, if an intruder breaks or bends an optical fiber that has been installed around the perimeter of a facility, the location of their intrusion attempt can be pinpointed with an OTDR that is built into the system.
One minor problem in using this approach in a security application is that for an OTDR to detect a measurable amount of optical loss, the optical fiber must either be broken or bent at a relatively sharp angle. In most cases, an intruder would likely bend the cable only slightly to gain access to a protected facility; the light loss produced by this slight bend would not be enough to be detected by the OTDR.
Fortunately, the solution is straightforward. It involves installing simple spring-loaded triggering devices along the cable route that can sense a slight disturbance to the perimeter cable, and then magnify that disturbance by creating a much more pronounced bend in the cable. This tighter bend produces enough light loss to be detected by the OTDR.