PERIMETER SECURITY SYSTEMS WITH FIBER OPTIC SENSORS..

PERIMETER SECURITY SYSTEMS WITH FIBER-OPTIC SENSORS..

VVEDENSKY Boris Sergeevich, Candidate of Physical and Mathematical Sciences

PERIMETER SECURITY SYSTEMS WITH FIBER-OPTIC SENSORS

General principles of operation of security systems with fiber-optic sensors

Fiber optic cables, which are usually used for transmitting information, can also be used as sensors for perimeter security systems. Deformation of the optical fiber changes its optical parameters and, as a consequence, the characteristics of the radiation passing through the fiber. Due to the specificity of the physical principles used, fiber optic systems are characterized by very low susceptibility to electromagnetic interference, which allows them to be used in unfavorable electrophysical conditions.

Optical fiber is generally a coaxial light guide. Light propagates along the central part (core) of the cable. The core of the fiber is adjacent to a transparent cladding, which has a lower refractive index than the core. Light propagating at an angle to the axis of the fiber is reflected from the interface between the core and the cladding and is concentrated in the central part of the fiber. The outer opaque coating serves to mechanically protect the cable.

Miniature semiconductor lasers or LEDs are usually used as radiation sources. At the cable output, the radiation is recorded by a photodetector, which converts the optical signal into an electrical signal. When the fiber is deformed, the internal reflection conditions change, which results in changes in the phase and spatial characteristics of the beam at the cable output. These changes are recorded by the photodetector and processed by the signal analyzer.

Optical fibers are divided into multimode and single-mode. The core diameter of multimode fibers is usually 50…100 microns. A large number of wave types (modes) with different geometric parameters simultaneously propagate through such a fiber. These rays experience multiple reflections from the boundary between the core and the cladding, which leads to noticeable signal attenuation.

The core diameter of single-mode optical fibers is no more than 10 microns. Only one type of wave (mode) can propagate in such a fiber, and light attenuation here is significantly less than in multimode optical fibers.

Fiber-optic security systems use various methods of recording intrusion signals:

Method of recording intermode interference

A semiconductor laser usually generates several dozen modes (spectral lines) close in frequency with a certain energy distribution over the radiation spectrum. If a multimode fiber-optic cable is subjected to mechanical stress, then the radiation spectrum recorded by the receiver at its output changes, which allows cable deformations to be detected.

Speckle structure registration method

At the output of a multimode optical fiber, a so-called “speckle structure” is observed, which is an irregular system of light and dark spots. When the fiber is deformed or vibrated, the speckle structure of the radiation undergoes changes. Spatially sensitive photodetectors are used here to detect cable deformations.

Interference method

This method uses the principle of two-beam interferometry. The laser beam is split into two and directed into two identical single-mode optical fibers, one of which is the detecting one and the other is the reference one. At the receiving end, both beams form an interference pattern. Mechanical impacts on the sensitive cable lead to changes in the interference pattern, which are recorded by the photodetector.

In recent years, a large number of security systems with fiber optic sensors have been developed, which are used to protect various types of perimeters. Some of the modern systems are briefly described below.

Systems for the protection of elastic fencing

Australian company Future Fibre Technologies (FFT) uses two main detection technologies using fiber optic sensors.

The first technology, called M/V, detects cable movement and vibration (Movement & Vibration – M/V). The sensor cable (Fig. 1) is connected to the start and end modules. The M/V analyzer is connected to the start module via a passive optical cable. Radiation from a semiconductor laser is fed to the sensing element and the system records the signal reflected from the end module.

Fig. 1. Block diagram of FFT M/V technology for detecting movement and vibrations of fiber optic cable

When a multimode optical fiber moves or vibrates, the energy distribution between the individual modes changes. These changes are recorded by an optical photodetector and processed by an analyzer. The M/V system uses multimode optical fibers with a core diameter of 62.5 µm. The light source is a semiconductor laser with a power of 1…2 mW, operating at a wavelength of 1.31 µm. The M/V technology allows for the recording of vibrations in a frequency range from several hertz to 300…600 hertz. The system based on multimode fiber allows for the organization of security zones up to 6 km long and is used mainly on elastic (deformable) fences.

The second technology from FFT is based on the principle of detecting microstrains in optical fiber and is abbreviated as MSL (from MicroStrain Locator). In Fig. 2The structural diagram of the system is shown. The extended sensor includes three separate fibers of a multi-core optical cable. The two upper fibers act as sensitive elements: radiation from a semiconductor laser operating in continuous mode is fed to them. The third (output) fiber is used to transmit signals to the system analyzer. The radiation source is located in the analyzer block, from which the laser radiation is fed through the input passive cable to the initial module. In this module, the radiation is split into two beams, which are fed to two fibers. The radiation is transmitted through both fibers to the final module, in which interference of both beams occurs. In fact, this system is an interferometer. If both arms of this interferometer are in an unperturbed state, then the interference pattern at its output, i.e. at the final module, remains unchanged. In this case, the signal transmitted from the final module through the output optical fiber to the analyzer does not have a variable component. When the cable is deformed or vibrates, the optical path difference in the sensitive fibers (i.e. the interferometer arms) changes, and the end module records the variable component of the signal, transmitting it to the analyzer. The MSL system uses commercially available single-mode optical fibers with a core diameter of 9 μm.

Fig. 2. Structural diagram of the MSL technology from FFT
for detecting microdeformations in fiber-optic cable

The peculiarity of the MSL system is that single-mode cores of a standard multi-core fiber-optic cable intended for transmitting signals can be used as sensitive elements. In Fig. 3The structure of such a cable is shown, where two single-mode cores are the arms of a sensitive interferometer. The cores should be located on diametrically opposite edges of the cable so that the sensor's sensitivity to bending is maximum.

Fig. 3. Schematic diagram of a multi-core fiber-optic cable from FFT

The MSL technology uses semiconductor lasers with an output power of 12…50 mW, operating at a wavelength of 1.31 or 1.55 µm, as light sources. High radiation power and low losses in the sensor allow increasing the length of a separate zone to 60 km. According to the developers, the MSL technology is approximately three orders of magnitude more sensitive than the M/V technology. The MSL system registers vibrations in the frequency range of approximately 300 Hz to 2 kHz, which corresponds to the characteristic frequencies that occur in typical metal fences when someone tries to overcome them.

Obviously, a zone length of several tens of kilometers is inconvenient for practical use. Without information about a specific intrusion location, the alarm signal will be almost useless. Therefore, it is very interesting that the modified MSL technology has made it possible to implement the function of determining the intrusion location with a fairly high accuracy. For this, three active sensor fibers are used, structurally combined in a multi-core optical cable. The two upper fibers (Fig. 4) are used to detect intrusion using the interferometric method, and a probing signal is fed into the third fiber, determining the distance from the beginning of the cable to the point where microdeformations occur. The start and end modules are used here to process signals from all three fibers. The data on the applied method of intrusion localization is a company secret, but it can be assumed that a modified optical time domain reflectometry (OTDR) technology used to diagnose damage to communication fiber-optic cables is used here.

Fig. 4. FFT fiber-optic security system diagram with intrusion detection function

The technologies developed by FFT are implemented in several versions of security systems with extended sensors. Secure Fence systemin the M/V version is designed for elastic mesh fences. The length of a separate zone is up to 2 km. The sensor cable is attached directly to the mesh using plastic ties. The initial and final modules (photo 1) are placed underground in standard telecommunication wells located at the edges of the zone.

Photo 1. Initial module of the Secure Fence system in an underground well

FFT Company Releases Secure Fence Taut Wire Perimeter Security System, which is a combination of a fiber-optic sensor and a wire-tension barrier. The sensor cable is mounted on 3.2 m high support posts. The barbed wire beams are mechanically connected to the fiber sensors on the support posts, which register changes in the wire tension. The maximum length of a single zone is 4 km. The system is resistant to winds of up to 100 km/h; the automatic correction system adjusts the sensor parameters when the temperature changes in the range from -40° to +75° C. The Secure Fence Taut Wire system detects attempts to climb over the fence, move the wire beams apart, or cut them. The manufacturer notes the very high detection capacity of the system at a very moderate cost of its maintenance.

For all FFT systems, the analyzer is structurally an industrial computer installed at the security post. The sensor signal processing algorithm allows filtering out environmental interference (wind, rain, traffic noise, birds, etc.). The analyzers are equipped with relay outputs for controlling additional equipment.

The English company Remsdaq produces several systems of the Sabre series with fiber-optic sensors. SabreFonic Systemis designed to protect perimeters of mesh or lattice metal fences. The cable attached to the mesh generates low-frequency signals when there is an attempt to overcome the fence or cut it. The sensor element is a pair of optical fibers of the LS2H cable type with a protective sheath reinforced with Kevlar. The core diameter is 62.5 μm, the sheath diameter is 125 μm, the outer diameter of the cable is 4.8 mm. The maximum cable length between the transmitter and receiver is 1000 m; standard optical connectors of the SMA type are used for connection to the analyzer. The radiation source is a semiconductor laser with a wavelength of 0.78 μm. Changes in the speckle structure during cable deformation are detected by a position-sensitive photodetector.

For the SabreFonic system, the company has developed a new analyzer, the Sabre II Processor, equipped with a powerful digital signal processing system. The analyzer has a built-in user interface in the form of three 7-segment LED indicators, as well as menu selection and mode setting buttons. The analyzer also has communication ports for remote configuration and diagnostics. The analyzer supply voltage is 10…14 V, the current consumption is 300 mA, the operating temperature range is from -10° to +70° C.

In the SabreTape system from Remsdaq, the fiber-optic sensor is attached to a cutting tape mounted on a fence or canopy (photo 2). A galvanized steel tape 0.5 mm thick and 20 mm wide is stretched so that an attempt to climb over the fence causes mechanical deformations registered by the sensor. The system is designed to detect only very energetic actions of the intruder, but practically does not give false alarms. The sensor is a multimode fiber with a core/shell diameter of 50/125 μm; the radiation source is a laser with a wavelength of 0.85 μm. Optical losses in the fiber do not exceed 3 dB/km. The system was developed according to the specifications of the UK Ministry of Defense; It is designed for operation in adverse atmospheric conditions (sea fog, acid vapors, industrial emissions, sand) and a temperature range from -30° to +70° C.

Photo 2. SabreTape signal barrier by Remsdaq

F-5000 Fibernet perimeter system sensor by the Israeli company TRANS Security Systems and Technology (TSS) is a network made of single-core multimode optical fiber, protected by a plastic sheath reinforced with Kevlar. The sheath provides protection for the fiber from UV radiation, moisture, salt water, etc. The network consists of cells with a side of 16 cm; at each intersection, the fibers are ultrasonic fused and protected by a plastic overlay. Pulsed radiation from a LED operating in the near IR range (wavelength 0.85 or 1.3 μm) is distributed throughout the network. A PIN photodiode serves as the radiation receiver. The emitter and receiver are connected to the network using standard ST-type optical connectors. Signal processing is performed by an F-5000 series processor, designed for 2 or 4 zones up to 100 m long each. The F-5000-1 stand-alone processor has relay contacts at the output, and the F-5000-2 processor is connected to the TEE-400 computer control system. The processors are housed in sealed cases measuring 500x400x200 mm; they are powered by a 48 V DC source; the system operating temperature range is from -30° to +70° C.

Depending on the selection of the response threshold, the F-5000 system generates an alarm signal when a fiber is strained or broken in any of the network cells. Fiber-optic network 1 (Fig. 5) is installed stand-alone or attached next to an existing fence 2. It is divided into two parts: the lower part, 2-3 m high, is attached to the fence, and the upper part of the net is made in the form of a canopy attached to elastic fiberglass posts 3, installed at an angle every 2 meters. The lower part of the network is stretched between horizontal metal tubes 4, reinforced along the upper and lower edges of the main fence, respectively. The lower part of the alarm barrier forms a separate security zone, which is configured to operate only in the event of a break in the network cells, which eliminates triggering from random factors (animals, people passing by, transport, etc.) when using the system in densely populated areas.

The upper part of the F-5000 alarm barrier forms a separate security zone. Along the upper end of the canopy, a fiber-optic cable 5 in a durable braid is mounted, allowing this cable to be used as a tension element. The cable is optically connected to the sensitive network using converters 6. The design of the canopy is flexible enough, and the processor of this security zone is configured to register climbing over the canopy. Note that if individual cells are broken, there is no need to change the entire network. The sensor is restored using cable sections and special optical jumpers.

Fig. 5. Signal barrier of the F-5000 security system by TSS:
1 – fiber optic cable network; 2 – existing fence; 3 – fiberglass posts; 4 – tension metal tubes; 5 – fiber optic cable in reinforced braiding; 6 – optical converters

The F-5000 system can also be built into walls (protection of buildings and premises) or mounted underground at a depth of up to 50 cm (anti-undermining barriers). For facilities with a very high degree of protection, the company produces a modified F-6000 system, which forms a 4-meter-high signal barrier.

The American company Fiber SenSys Inc. produces several fiber optic perimeter systems in the Fiber Defender (FD) series. Model FD-205is designed for both fence and wall protection and for underground installation. The maximum length of one protection zone is 2000 m. The system uses digital processing of sensor signals; the system processor automatically adjusts the system parameters depending on the noise generated by the wind. An anemometer connected to it, which records the wind speed, can be used to adjust the processor. The FD-205 series processors are mounted on the fence; up to 127 processors can be connected to a single system using a single communication fiber cable.

FD-208 Systemis designed for facilities with unfavorable conditions for the operation of electronic equipment (electromagnetic interference, aggressive environments, etc.). All processors are installed in a rack at a security post, which can be removed from the perimeter at a distance of up to 10 km.

A modification of the system, called FD-220, uses both standard fiber cable sensors (SC3 type) and sensors in a protective sheath (SC4). The maximum length of one security zone is 2000 m. Optical cables are connected to the processor using standard ST type connectors. According to the manufacturers, the service life of a cable sensor is at least 20 years. The FD-220 system processor can be used both as a stand-alone security device and as part of a network system with serial polling via the RS-232C interface. The processor is powered by a source with a voltage of 10 … 24 V, the power consumption is 2.1 W. The operating temperature range of the SDI series systems is from -30 ° to + 55 ° C.

Israeli company Magal released a fiber-optic signal barrier FiberMESH 2005, installed on mesh fences. The fiber-optic sensor is covered with a protective polyester sheath; the outer diameter of the cable is 3.5 mm. A pulsed semiconductor infrared laser is used as a radiation source. The two-dimensional sensor grid has cells measuring 15 x 15 cm; fiber cables are welded at each grid node. The lower edge of the grid is attached to a horizontal pipe, which prevents attempts to lift the grid. The upper part of the grid, about 1 m high, is made in the form of a canopy above the main fence. The upper element of the signal canopy is a wire-tension sensor, to which a sensitive fiber-optic grid is attached. If an attempt is made to cut or deform the grid, an alarm signal is given by the fiber-optic sensor. When attempting to climb over the fence, a wire-tension (electromechanical) sensor is activated, which has an adjustable response threshold (tension from 15 to 40 kg).

The FiberMESH 2005 signal fence is supplied in sections 10 m long and 2.0, 2.5 or 3.0 m high. Adjacent sections are mechanically connected and joined via an optical channel. The standard length of one zone is 100 m.

The electronic signal processing unit is located in the center of the zone; two 50-meter halves of the fiber-optic barrier and the tension sensor converter are connected to it. The electronic unit of the perimeter system is connected to the control post via the RS-422 interface. The system is powered by a 10-30 V source; consuming 4 mA in standby mode and 45 mA in alarm mode. According to the developers, the system is resistant to winds of up to 70 km/h and remains operational in rain and snow; the operating temperature range is from -30° to +72° C.

Canadian company Senstar-Stellar produces the fiber-optic security system IntelliFIBER, designed to protect mesh perimeter fences. The sensor cable contains two fiber-optic cores in a protective sheath; the sensor is attached directly to the fence. The optical cores are connected using standard ST-type optical connectors to the output of the semiconductor laser and the input of the photodetector on the board of the electronic unit (analyzer). The electronic unit registers changes in the optical parameters of the cable caused by deformations of the fence during intrusion attempts. Interestingly, the design solution of the new system is based on the use of the Intelli-FLEX electronic unit from a well-known security system of the same company, which uses a vibration-sensitive coaxial cable as a sensor. When using an optical cable, the electronic unit is supplemented with an optical module, which contains a laser emitter, a photodetector and a meter of the received radiation power with an LED indicator. The optical module itself consumes 1.2 W; When installed, the functions of all signal cables connected to a standard Intelli-FLEX series processor are preserved.

The Intelli-FLEX processor is configured to detect two main types of intrusion — climbing over the fence or destroying it. For each channel in the processor, sensitivity thresholds, minimum intrusion duration and event counter time window are set. Using an autonomous programmer, processor sensitivity parameters and weather compensation modes are set. The maximum length of one security zone with a fiber-optic sensor is 2 km; the operating temperature range of the system is from -40 ° to +70 ° C. Like all fiber-optic devices, this system is characterized by immunity to electromagnetic and radio frequency interference. To protect the electronic unit from lightning discharges, gas dischargers are used on all relay outputs, as well as on the terminals of the power and alarm cables.

Systems for the protection of rigid fences and walls

The sensitivity of a fiber-optic sensor is usually insufficient for direct recording of vibrations of rigid metal fences, so manufacturers of security systems develop special barriers with integrated fiber-optic sensors.

For the protection of rigid metal fences, the Israeli company Magal has developed the INNO-FENCE system. A distinctive feature of the system is that the fiber sensor is built into the upper horizontal channel of the fence panel, through which the vertical fence posts pass. The sensor is completely hidden by a cover; it responds to deformations of the horizontal channel of the fence that occur when trying to overcome it. The multimode fiber cable has a core with a 100/140 μm structure, specific absorption is 7 dB/km. The system uses a semiconductor laser with a wavelength of 850 nm and an effective frequency band of at least 200 MHz. The electronic unit of the receiver/transmitter analyzes the cable signals and generates an alarm when a certain threshold of mechanical impact on the fence is exceeded. According to the developers, the system is characterized by a low level of false alarms and practically does not require technical maintenance. However, the disadvantage of the system is its relatively low sensitivity. For the system to be triggered, it is necessary to apply a force of more than 40 kg to the fence or deform the fence bars, creating a gap of at least 220 mm between them. Therefore, the system will register only “forceful” intrusions accompanied by significant mechanical impacts. The two-zone electronic unit of the INNO-FENCE system – FOST (Fiber Optic Sensor Transponder) – is powered by a 12 V source, consuming only 4 mA in standby mode. It has relay outputs and an RS-422 interface for data transmission. The operating temperature range of the system is from -20° to +71° C.

The Australian company FFT has developed a series of special barriers of the Secure Fence Palisade type.with sensors integrated into them. The optical sensor cable is hidden in the upper box-shaped channel through which the vertical fence posts pass. These posts have a certain degree of freedom and can move slightly, rotate or vibrate under mechanical action, generating a signal in the sensor cable attached to the posts. The system detects various types of intrusion — climbing over, sawing or pushing the posts apart.

The same principle is used to build the semi-rigid Secure Fence Palisade alarm barrier, which is installed on top of walls or fences. The metal elements of such an alarm barrier are mechanically fastened to a fiber sensor laid along the end of the wall. When an intruder leans on the barrier, the latter is deformed and the system generates an alarm signal.

To protect rigid walls and barriers, Remsdaq produces the Optimesh system, where fiber-optic cables form a network with square cells, which is mounted inside walls or partitions. The system is triggered only when a cable breaks, so it only registers hard “force” impacts (for example, a wall break). However, with such a detection criterion, the probability of false alarms is quite low.

Underground systems with fiber-optic cables

The SabreLine system from Remsdaq (Fig. 6) is designed to protect approaches to objects or for restricted areas. The optical cable is located along the border of the protected perimeter and is masked with a protective coating. The cable is placed between two elastic mats and laid in the form of parallel loops with a step of 20 cm under the ground surface at a depth of 5 cm. The sensor detects pressure changes caused by a walking or crawling person. The cable has a core with a diameter of 100/140 μm; the outer diameter of the cable is 2.4 mm. The emitter and analyzer are similar in their characteristics to the SabreFonic system. The electronic unit is installed underground in a special well closed with a metal cover. An underground signal cable connects the analyzer to the control panel. Remsdaq claims that with proper preparation of the perimeter trench, the underground security system works effectively in deserts, on grassy and gravel soils, and under asphalt roads.

Fig. 6. Structural diagram of the underground fiber-optic system SabreLine by Remsdaq

System TSS's F-7000-FOBS is also designed for underground installation and records the soil pressure created by an intruder (photo 3). To do this, the optical cable is placed at a depth of 5 — 10 cm under the ground surface, bending it in the form of a loop, covering a strip 1 — 2 meters wide. To ensure high and uniform sensitivity, the cable is laid on a light metal grid and covered with the same grid on top. Such a system can be used in almost all types of soil — sand, gravel, clay soils, etc.