Magazine article Security Management

Detecting Intrusion with Fiber Optics

Magazine article Security Management

Detecting Intrusion with Fiber Optics

Article excerpt

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. …

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