How Are Fiber Optic Cables Used For (Physical) Intrusion Detection?

Many of us have fiber optic cable that delivers Internet access to our homes or offices. But optical fiber cable can be used for more than high-speed data transfer. I’m not talking about artwork or illumination, but rather an important security application: physical intrusion detection.

Optical fiber types

In order to understand how this works, you need to know a little bit about fiber optic cable. There are two types of fiber:  single-mode and multimode. Without getting deep into the weeds and mathematics of the transmission of light in the two types of fibers, there are some important differences:

  • Single-mode fiber can be used in applications where higher speed or longer distance is required.
  • Single-mode fiber has a very small diameter, compared to multimode.
  • Single-mode fiber is more expensive per meter and the equipment is more expensive than that for multimode.

Single mode and multimode fiber can each be used for intrusion detection, but the mechanisms are significantly different. Vendors who sell intrusion detection schemes based on fiber keep their exact schemes proprietary, but the concepts are similar. One factor the vendors keep confidential is how to determine whether the intrusion was by, say, a pedestrian or a vehicle.

Beam profile of a single-mode (left) and a multi-mode (right) fiber

How it works

In the case of multimode fiber, the concept is straightforward: a laser is used to send light through the fiber (as is done when the fiber is used to transmit data). The light at the receiving end forms a particular speckle pattern when the fiber is undisturbed. When the fiber is under pressure or significantly bent, the pattern of the light at the receiving end changes. This allows a device at the receiving end to analyze the distortion and notify a monitoring device accordingly.

In the case of single-mode fiber, things get a bit more complicated. The nature of light traveling in a single path or mode through the fiber, there is no pattern at the receiving end that can be disturbed. Instead, one wavelength or color of light is sent through the fiber in one direction and another wavelength of light is sent in the other direction. Pressure on the cable changes the way the cable transmits light that can be detected at the end of the cable.

(Another technique involving light pulses has been used to detect earthquakes, but that is beyond the scope of security uses.)

When might it be used?

Using fiber optic cable for intrusion detection is not inexpensive, so when might it be appropriate. Significant applications are in areas where a clear view of a perimeter or internal area cannot be easily viewed directly.

Consider a facility where one or more portions of a perimeter border an area of dense vegetation such as a forest. In that case, the physical plant may want to erect a fence on that perimeter. A cable could be integrated with the fence or place inside or outside the fence in order to detect vehicles or individuals at the fence, or trying to cross it.

Inside a facility, a system might be deployed in an area where there was little activity at a particular time of day. Specifically, a system might be used in a server room not generally occupied at night.

Rightly, organizations are often reluctant to discuss their specific security measures. That makes knowing how many and what types of fiber optic intrusion detection systems are in use. But a web search for vendors indicates that the size of the market is not trivial. Who knows, you or I may traverse such a system daily. If you do – knowingly or unknowingly – at least you have an idea of how it works.

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