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.
Background
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.
Example # 1 - Perimeter Security
In this first example, standard fiber optic cable (with a U.V. resistant jacket) is attached to the existing fencing around a perimeter of a large facility, such as a military base. The fiber is held in place by a number of triggering devices installed on the fencing at intervals around the perimeter.
A laser injects infrared light into one end of the fiber optic cable and the light is continually monitored with the system's built-in OTDR to determine if there has been any change in light output. If the fiber optic cable is broken, or one of the trigger devices is activated by a slight disturbance to the cable, the system triggers an alarm.
The triggering devices installed along the perimeter can include mechanical adjustments to control their level of sensitivity. System sensitivity can also be adjusted by varying the wavelength of the injected infrared light.
Example # 2 - Security for Underground Facilities
In a similar example, the fiber optic cable is attached beneath a number of manhole covers, to protect against unauthorized access to underground utilities.
An opto-magnetic triggering device is installed beneath each manhole cover. When one of these devices sense that a manhole cover has been removed, it triggers an internal mechanism that creates a tight bend in the fiber. As in the first example, the OTDR at the head end of the system senses the resulting loss of light, triggers an alert, and pinpoints the exact location of the event.
Problems Solved
This new approach provides solutions to some of the classic challenges faced by security system designers, such as:
How to monitor very large distances in a cost-effective manner?
Fiber carries signals over a much greater distance than copper wire without requiring re-amplification of the signal. With fiber, no electrical source is required to power the sensors. This makes fiber optics a cost-effective security solution for monitoring very large perimeters, such as those around military bases, large factory complexes, and so forth.
How to reduce the greater incidence of nuisance alarms that result when monitoring large areas?
The perimeter monitoring approach discussed here involves using opto-mechanical sensors that include mechanisms that allow for tension adjustments of the fiber optic sensing cable. The sensors can be adjusted to resist wind, snow and other environmental factors to reduce or even eliminate nuisance alarms. Varying the wavelength of the light injected into the cable can also adjust the sensitivity of the system.
How to guard against someone defeating the security system?
A unique attribute of a fiber optic security system is that, unlike copper wire, optical fiber cannot be cut, spliced into, or jumped without being detected.
How to monitor environments subject to explosive gases? Fiber optics use infrared light, not electricity, to transmit signals. Unlike copper wire, optical fiber cannot arc or produce a spark that could trigger an explosion. This makes fiber optic sensors ideal for monitoring chemical plants, underground installations, storage tanks and many other areas where combustible gases may be present.
How to monitor environments where there are conductive or corrosive liquids or gases?
Optical fiber is made of glass, which is inert. Unlike metallic wire, optical fiber will not corrode when subject to chemicals, and it cannot short out, even when exposed to water.
How to design a system that can survive electrical storms?
A security system that covers a wide area is more subject to damage or destruction caused by lightning storms. Optical fiber, which is glass, does not conduct electricity or lightning. If one sensor is destroyed by a lightning strike, the lightning cannot travel over the optical cable to destroy other sensors and components within the system.
Installation Considerations
As one might conclude, security systems that use light (photons) instead of electricity (electrons) require a different skill set as far as security installers are concerned. The handling of fiber optics is not necessarily more difficult, but it does require some new knowledge and tools.
Fiber optic security systems as described above would also require a user-friendly software program to integrate the system's OTDR with a display monitor to show tripped sensors along the cable run. Carefully labeled visual maps, showing the attenuation locations, would enable the security guard to quickly locate an intrusion attempt and provide a quick response.
As mentioned earlier, there are different types of fiber optic security systems. Installers must be familiar with the particular type of fiber optic system specified by the end user as well as the installation specifications required by the manufacturer of the system. Only factory trained and authorized installers should attempt installation of fiber optic security systems.
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