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Detecting intrusion with fiber optics.

Innovative developments in fiber-optic technology make it ideal for intrusion detection above and below ground, as well as under water.

After years of testing, a new family of sensors based on advanced fiber-optic material and optimized signal processing is available to meet the requirements of today's physical security managers.

Optical fiber is a fine, strong strand of glass or other optical medium. It is often called a "wave guide" because the optical fiber guides light waves from a light source at one end to a detector at the other end of the fiber optic.

A glass fiber optic is drawn from a melt comprising two different types of glass. The difference is in the refractive index, or the glass's light-bending ability.

Light of visible, infrared, or other wavelength, when introduced into the core of the fiber optic, is retained by a process of total internal reflection until it exits onto a receiving device.

In operation, light is pulsed through the fiber optic in a manner similar to an electric signal through a wire. Fiber optics have certain distinct advantages over electrically conductive materials, and these natural features are beneficial when applied to physical security. They include the following:

* Optical fiber is immune to electrical interference from power supplies, generators, power cables, lights, lightening, radio frequencies, storms, or static in the air. This means fewer false alarms for fiber-optic security systems.

* Optical fiber is naturally safe in operation, making it ideal for explosives and other hazardous applications.

* Optical fiber is secure; it cannot be electrically bridged like an electric conductor.

* The drive circuits, light sources, and detectors required to operate fiber optics are low-power, long-life, and operationally reliable components that contribute to highly reliable systems.

A typical commercial light-emitting diode (LED) produces approximately 30 decibels (db) of light energy. (The power of light passing through a fiber optic is usually measured in decibels.) Glass fiber-optic material absorbs approximately 3 db of light per kilometer of cable, so it is possible to "sense" using optical fiber over great distances.

Fiber optics can be used in a number of ways to combine their operational reliability and immunity to interference with a high probability of intruder detection. The two principal forms of using fiber optics as a security sensor are continuity sensing and change sensing.

In continuity sensing, the fiber optic must be broken to set off an alarm. This is probably the most reliable form of sensing technology when presented in the correct manner. Change sensing means the fiber optic's light transmission properties are used to detect events along the fiber optic, which in the case of security are intrusion events.

Speckle-pattern sensing, a form of change sensing, adopts the measurable changes of light passing through a fiber optic when the fiber optic experiences disturbance.

Speckle-pattern sensing is achieved by using monochromatic light (light of a single wavelength, like the light emitted by a compact disk player laser diode). The light waves interfere in a known optical process that is readily detectable by a suitable receiver component.

The speckle-pattern sensor, now that optimized signal processing exists, offers exciting prospects for economical and effective general physical security applications.

By applying fiber-optic sensing in a variety of physical forms, a wide range of intrusion detection scenarios can be addressed by a single fundamental fiber-optic technology. Several continuity and speckle-pattern sensors are described here.

Continuity sensor one - barbed tape. This form of fiber-optic sensor is well-known in the United Kingdom and parts of Europe, Australia, the Middle East, and in a few recent systems introduced in the United States by Stellar Systems of Santa Clara, CA.

A composite strand sensor combines a fiber-optic continuity sensor within a barbed steel tape. An installed system appears similar to a taut wire product. However, it does not require mechanical activation or complex processing.

The preventive maintenance and repair of these systems is straightforward, and the cost is low - typically it amounts to 1.5 percent of the system cost per year.

The fiber optic-alarmed barbed tape system can be a free-standing fence or an existing fence, and it attaches to walls and buildings. It is currently used in prisons, military bases, and power stations, where it offers the highest possible level of operational reliability and security and few false alarms. It has a proven high probability of intruder detection.

Continuity sensor two - security web. In this type of system, a fiber-optic cable woven and formed into a net is deployed in walls as a covert sensor.

These systems, where the net is cast within glass-reinforced concrete, are deployed in prison walls, mobile high-security containers, and vehicles, and within light structures to detect attempts to cut or break through the structure.

Since the net is formed from a continuity sensor, it has to be broken to set off an alarm, so it is not activated by anything other than significant physical damage. The net is immune to interference from lights, power points, computers, and radios, and it is undetectable.

The net is woven like a chain-link fence, and buttons are welded at the nodal points. Fiber-optic cable that is woven to form the net becomes the communications cable leading to the low-power control electronics, which can be housed a considerable distance from the zone or zones being protected.

Continuity sensor three-underwater barrier/sensor. One of the few remaining areas where technology can offer real high-security protection is under water. A system built on a rugged version of the fiber-optic net is now being used in production installations.

The Aquamesh system encapsulates the basic fiber-optic continuity sensor, using a proven composite strand electronics and communications system.

The underwater alarmed barrier can take the form of a light-weight barrier sensor that has to be cut to pass through. Alternatively, the barrier can be formed to incorporate strength members that can resist the impact (not explosion) of a torpedo or effectively delay an intruder for a required response period.

The highly reliable fiber-optic system can be enhanced by adding a second fiber optic within the single structure. This addition enables parallel redundancy in the control system and offers high system "mean time between failure" predictions. This is particularly useful under water, where false alarms are unacceptable when the rules of engagement may be enunciated by the intruder detection system.

Continuity sensor four - simple loop and lace. Basic modules of the continuity sensor are used frequently. Fiber-optic cable is laced through ceiling and roof voids, used to secure large areas of glazing, and set into vault walls to detect break-ins. A loop of optical fiber can be used to prevent the undetected turning of valves in oil and water terminals, for example.

The application of a simple fiber-optic microswitch can remove the need for wiring to a door contact. This is particularly helpful in explosives areas.

Speckle-pattern sensor one - buried pressure sensor. As described, the speckle-pattern sensor family uses the harnessed power of light to detect disturbance in an optical fiber in a range of physical environments and forms.

The buried application deploys a continuous fiber-optic cable across an open area of ground, and the system detects the pressure of an intruder through the slight bending of the fiber optic and the output change in the light signal that is produced.

To operate correctly, the pressure sensor must be installed in a manner that enables it to feel the pressure of an intruder, so ground conditions must be correctly prepared for installation.

Where the ground is extremely hard because of baking or freezing, it is necessary to add a resilient matting material and possibly improve the ground conditions around the sensor to retain some slight spring. Excellent results have been reported during several years of testing in soft sand and peatbog environments.

As with all sensors, inappropriate locations exist, such as under concrete or moving sand or snow.

The buried version of the speckle-pattern sensor is typically buried approximately 50 mm below the ground and is quick and easy to install.

The pressure sensor can be installed under road surface materials like blacktop and in high radio frequency areas where it performs well. It is immune to the interference and stimulation that a proximity to water can cause in other systems.

Speckle-pattern sensor two - acoustic/vibration sensor. One of the most exciting developments in recent years is the application of a distributed fiberoptic cable to detect the vibrations and noises caused by cutting or climbing a fence and by breaking through building fabrics of various types.

Using the speckle-pattern detection method, Sabrefonic, one such cable, is naturally immune to the false alarm phenomena that causes problems in conductive materials. In addition, the sensor is sensitive to the specific events caused by intrusion.

However, as with other vibration sensors it is important to note that while a cable sensor enunciates cuts or break-ins with a high degree of probability, it detects and differentiates climbing forms of attack and environmental conditions with a lower level or probability.

The lower probability of climb detection is brought about because a cautious, light, agile climber in soft shoes is less likely to produce the noises and vibrations that the sensor is set to detect.

A higher security site would combine an anticlimb device like an alarmed outrigger with an acoustic cable sensor to detect cutting of the fence. Both of these forms of physical security are available from fiber-optic sensors.

The fiber-optic cable sensor is immune to lights, power cables, and other emissions, so it can be fitted in or on walls of explosives stores, computer rooms, or industrial properties.

A variety of physical security problems can be solved by the adoption of a single core technology applied in the two basic sensing forms described here.

The telecommunications industry has achieved a great deal in simplifying installation, repair, and maintenance practices for fiber-optic cables so security systems installers can adopt a proven approach.

The applications for this technology are limited only by the imagination. Fiber optics enable a creativity on the part of the installer, systems house, and consulting engineer. For years fiber optics were the answer looking for the question. Now the applications are apparent and the technology is being applied in many environments.

Barry Griffiths is general manager of security systems for Pilkington Optronics in Denbigh, Clwyd, United Kingdom. He is a member of ASIS.
COPYRIGHT 1992 American Society for Industrial Security
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Author:Griffiths, Barry
Publication:Security Management
Date:Jul 1, 1992
Words:1718
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