A fiber Bragg grating sensing intelligent perimeter fence alarm system

Traditional perimeter security or fence alarm systems: such as active infrared counter-radiation, microwave counter-radiation, leakage cables, vibration cables, electronic fences, power grids, etc., have made certain contributions to security technology prevention, but they are limited by some objective technical conditions and other factors and still have certain defects. The perimeter fence alarm system made of new fiber optic sensing technology in optoelectronic technology has very obvious technical advantages. This article introduces the defects of traditional perimeter fence alarm systems and the advantages of fiber optic sensing, the principles and advantages of fiber Bragg grating sensors, the composition and working principle of intelligent perimeter fence alarm systems based on fiber Bragg grating sensors, the solution to the system's multiple intrusion positioning alarms and its application and market prospects.

1. Comparison between Fiber Optic Sensing Technology and Perimeter Fence Alarm System

Over the years, traditional perimeter security or fence alarm systems, such as active infrared counter-radiation, microwave counter-radiation, leakage cable, vibration cable, electronic fence, power grid, etc., have made certain contributions to security technology prevention. However, due to some objective technical conditions and other factors, there are still certain defects: For example, the fence alarm system of active infrared counter-radiation is susceptible to environmental restrictions such as the height, twists, turns, and bends of terrain conditions, and they are not suitable for harsh climates, and are easily affected by natural climates such as high temperature, low temperature, strong light, dust, rain, snow, fog, and frost, and are prone to false alarm rates; for example, fence alarm systems such as leakage cables, vibration cables, electronic fences, and power grids are all active electrical sensors, and the system power consumption is very large; and electronic fences, power grids, etc. are harmful to a certain extent; they are also susceptible to electromagnetic interference, signal interference, crosstalk, etc., which reduces sensitivity and increases false alarm rates and missed alarm rates.

Compared with the above perimeter security or fence alarm system, the perimeter security or fence alarm system made by the new optical fiber sensing technology in photoelectric technology has very obvious technical advantages:

(1) Anti-electromagnetic interference, good electrical insulation, safe and reliable, corrosion-resistant, and stable chemical properties, so it is completely unaffected by lightning and can work in harsh chemical environments, field environments, and places with strong electromagnetic interference;

(2) Small size, light weight, flexible geometry, low transmission loss, large transmission capacity, and excellent reliability and stability;

(3) It can not only detect external disturbances, but also determine the location of external disturbances. The system has low cost, simple structure, easy expansion and installation;

(4) No radiation, no flammable or explosive materials, waterproof and environmentally friendly;

(5) Low energy dependence, which can greatly save the cost of power supply equipment and lines and is suitable for long-distance use;

(6) Different detection methods can be selected according to the conditions of the object being tested. In addition, it has little impact on the medium being tested, so it is very beneficial for application in fields with complex environments such as structural testing.

Perimeter security or fence alarm systems can be realized using two types of fiber optic sensors in fiber optic sensing technology: one is the fiber Bragg grating distributed fiber optic sensor; the other is the fiber interferometric fiber optic sensor. This article discusses the former, and the latter has been introduced in another article.

In recent years, fiber Bragg grating (FBG) in fiber optic sensing technology is one of the fastest growing and most widely used fiber passive devices. One of the main advantages of fiber Bragg grating sensing is that it is easy to construct a distributed sensing system, and one of the most critical technologies for constructing a distributed sensing system is multiplexing technology, including wavelength division multiplexing (WDM), time division multiplexing (TDM), space division multiplexing (SDM) and their combined multiplexing technology. Since its sensitive variable parameter is the wavelength of light, it is not affected by the changes in light intensity caused by factors such as light source and transmission line loss. In addition, fiber Bragg grating has the characteristics of simple production, small size, stable and reliable performance, and easy connection with the system and other fiber optic devices. If it is used as a sensing component, real-time measurement and distributed measurement can be achieved.

Since the fiber Bragg grating has a reflective effect on light of a specific wavelength, and its reflection center wavelength changes with the changes of physical quantities such as temperature and stress, it has excellent temperature and strain response characteristics, so it has a very broad application prospect in the field of sensing. With the widespread application of fiber Bragg grating sensing technology in measurement, it provides a broad opportunity for the research of security technology prevention systems. Obviously, the strain and temperature sensing characteristics of fiber Bragg grating can be used to make perimeter security and fence alarm systems, so its research has great practical and social significance.

The above briefly describes the defects of traditional perimeter fence alarm systems and the advantages of fiber optic sensing. The following introduces the principles and advantages of fiber Bragg grating sensors, the composition and working principle of perimeter fence alarm systems based on fiber Bragg grating sensors, the system's solution to multiple intrusion positioning alarms, its applications and market prospects, etc.

2. Fiber Bragg Grating Sensor Principle

Fiber Bragg Grating (FBG) is a development of the concept of diffraction grating, and its diffraction is achieved by the change of the refractive index inside the optical fiber. FBG was introduced in 1978. It uses the photosensitivity of doped optical fibers (such as germanium, phosphorus, etc.) to make the external incident photons interact with the doped particles in the fiber core through ultraviolet writing, resulting in periodic or non-periodic permanent changes in the refractive index of the fiber core along the fiber axis, forming a spatial phase grating in the fiber core, as shown in Figure 1. In the figure, the period Λ of the fiber Bragg grating is generally less than 1μm.

Figure 1. Uniform period fiber Bragg grating structure

The basic principle of fiber Bragg grating (FBG) sensing is shown in Figure 2.

Figure 2. Fiber Bragg grating sensing principle

The principle of fiber Bragg grating sensing is that when a beam of light is sent into a Bragg fiber grating, according to grating theory, when the Bragg condition is met, total reflection will occur, and its reflection spectrum will peak at the Bragg wavelength. When the grating is affected by an external physical field (such as stress, strain, temperature, etc.), its grating pitch Λ changes accordingly, thereby changing the wavelength of the back-reflected light. The change in the corresponding physical quantity of the measured part can be determined based on the size of the change in ΔλB.

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FBG is like a narrow-band mirror, reflecting only one wavelength and transmitting the rest. The reflected wavelength is called the Bragg wavelength, which satisfies the Bragg equation of the fiber grating, that is, it satisfies the condition

(1)

In the formula, ∧ is the Bragg grating period; neff is the effective refractive index of the reverse coupling mode. This equation provides a theoretical tool for the sensing response of the Bragg wavelength of the fiber Bragg grating under external influences, that is, any process that changes these two parameters will cause the shift of the grating Bragg wavelength. Therefore, the common FBG sensor realizes the detection of the measured value by measuring the drift of the Bragg wavelength.

Among all the external factors that cause the Bragg wavelength shift of the grating, the most direct ones are stress and strain parameters. Because whether the grating is stretched or squeezed, it will cause the grating period ∧ to change, and the elastic-optical effect of the optical fiber itself makes the effective refractive index change with the change of the external stress state. Based on this, the fiber Bragg grating can be used to make a sensitive optical fiber sensor. Among them, the shift of the Bragg wavelength of the grating caused by stress can be uniformly described by the following formula

(2)

In the formula, ΔΛ is the elastic deformation of the optical fiber itself under stress; Δneff is the elastic-optical effect of the optical fiber. Different external stress states will lead to different changes in ΔΛ and Δneff. Therefore, as long as the shift ΔλB of the grating Bragg wavelength in the reflected signal is detected, the change of the sensor quantity to be measured can be detected.

From the perspective of the photoelastic effect, the fiber Bragg grating is more sensitive to longitudinal pressure than transverse pressure. Combining the photoelastic and waveguide effects, the fiber Bragg grating is less sensitive to uniform transverse stress than longitudinal expansion, so under complex stress conditions, the wavelength shift caused by longitudinal pressure will dominate.

If only the axial strain (i.e. longitudinal pressure) is considered, the relative change in the displacement of the central wavelength is

(3)

In the formula, is the fiber Bragg grating strain sensitivity coefficient, is the axial strain. From formula (3), we can get

(4)

It can be seen from formula (4) that the change of reflection wavelength is proportional to the strain. In other words, the corresponding strain can be obtained from the change of reflection wavelength.

Changes in external temperature will also cause the Bragg wavelength of the fiber Bragg grating to shift. From a physical point of view, the main reasons for the wavelength shift are: fiber thermo-optic effect, fiber thermal expansion effect, and elastic-optic effect caused by thermal stress inside the fiber. Starting from the Bragg equation (1), when the external temperature changes, the equation (2) is expanded to obtain the shift of the Bragg wavelength of the fiber Bragg grating caused by the temperature change ΔT. It is confirmed through theoretical derivation that when the material is determined, the sensitivity coefficient of the fiber Bragg grating to temperature is basically a constant related to the material coefficient. Therefore, for pure fused silica optical fiber, when the influence of external factors is not considered, its temperature sensitivity coefficient basically depends on the refractive index temperature coefficient of the material, and the elastic-optic effect and waveguide effect will not have a significant effect on the wavelength shift of the fiber Bragg grating. Therefore, the following expression can be obtained, that is,

(5)

Where αn is the thermo-optical coefficient and αΛ is the linear thermal expansion coefficient. For fused silica optical fiber, αn=0.86×10-5/oC, and αΛ=5.5×10-7/oC.

It can be seen from equation (5) that the change in reflected wavelength is proportional to the temperature change ΔT. That is, the corresponding temperature can be obtained from the change in reflected wavelength. For a wavelength of 1.55μm, the wavelength shift caused by a unit temperature change is 10.8pm/oC.

3. Advantages of Fiber Bragg Grating Sensors

There are many types of fiber optic sensors, which can measure many physical parameters with high resolution. Compared with traditional electromechanical sensors, they have many advantages: small size, light weight, flexibility, convenience, inherent explosion-proof, anti-electromagnetic interference, corrosion resistance, high temperature resistance and no grounding requirements, so their application range is very wide. In addition to the advantages of general fiber optic sensors, fiber grating sensors also have the following advantages:

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(1) Stronger anti-interference ability, high reliability and stability

FBG sensors use the wavelength of light as the smallest unit of measurement. They only need to detect the movement of the grating wavelength in the optical fiber, and are not related to the light intensity. They are insensitive to the fluctuation of light intensity, and therefore have higher anti-interference ability than general optical fiber sensors. FBG sensors are wavelength-encoded sensors. The fluctuation of light source intensity, random fluctuation caused by the microbending effect of optical fiber, coupling loss, etc. cannot affect the wavelength characteristics of the sensing signal. Therefore, the sensing system has high reliability and stability.

(2) High measurement sensitivity, high resolution, high accuracy and good repeatability

The fiber Bragg grating (FBG) sensor is significantly superior to ordinary fiber sensors in that its sensing signal is wavelength modulated, so its measurement signal is not affected by factors such as light source fluctuations, fiber bending loss, connection loss and measuring instrument aging, so the measurement results have good repeatability. In addition, since it uses the wavelength of light as the smallest unit of measurement, and the detection of FBG wavelength movement has reached a high resolution of pm, it has higher measurement sensitivity and accuracy than traditional fiber sensors.

(3) Large dynamic range, good linearity, self-calibration, and can be used for absolute measurement of external parameters

Fiber Bragg grating sensor, because tensile and compressive stress can produce changes in its Bragg wavelength, so the sensor has excellent deformation matching characteristics in structural detection, with a large dynamic range (up to 10000×10-6ε) and good linearity.

In addition, the fiber Bragg grating sensor avoids the unclear phase measurement and the need for a fixed reference point in general interferometric sensors. After self-calibration of the fiber Bragg grating, it can achieve long-term absolute measurement of external parameter changes.

(4) Multiple sensors can be integrated into the same optical fiber for multiplexing, making it easier to construct various forms of optical fiber sensor networks

Fiber Bragg grating sensors are very suitable for making multiplexed and distributed fiber optic sensors because they are easy to integrate multiple sensors in the same fiber. Figure 3 shows an example of fiber Bragg grating sensors realizing multi-point measurement in one fiber. For example, the FBG application system Si425 developed by MICRON-OPTICS in the United States can measure up to 4 channels of 512 FBG sensors at the same time, with a scanning range of 50nm, a resolution of 1pm, and a measurement frequency of up to 244Hz.

Figure 3: Distributed measurement of multiple points using a single optical fiber

The FBG type distributed sensing system has unique advantages in the multi-point distributed measurement of stress, and can simultaneously complete the dual parameter measurement of temperature and stress, opening up a broader prospect for FBG application. Figure 4 introduces the topological structure of the FBG array using WDM/TDM demodulation. Obviously, this fiber grating sensor is convenient for forming various forms of fiber optic sensing networks.

Figure 4. Topology of FBG array using WDM/TDM

(5) It is convenient to monitor bridges and other buildings at a long distance (more than 5 km), and can provide early warning/alarm to make the system intelligent.

In the fiber grating strain test system, the stable and high-precision strain signal obtained by the fiber grating sensor is sent to the modem through the optical cable remote transmission, and then directly input into the computer information processing system. In this way, the expert system for evaluating the structural condition of buildings such as bridges can be used to make safe (normal) and unsafe warning/alarm evaluations of structures such as bridges, making the system intelligent. At the same time, the assessment report or health information of bridges can be transmitted to the management department of bridges and other facilities through the Internet in a timely manner, so as to realize the information management of online structural health monitoring. In addition, only one optical cable is required between the bridge site and the demodulator, and the distance can reach more than 5km, which can realize the distributed measurement and centralized monitoring and processing of bridges.

(6) Simple structure, long life, easy maintenance, easy expansion and installation

The sensor probe has a simple structure and small size. Since its outer diameter is the same as that of the optical fiber itself, it is easy to expand and install and is suitable for various applications. In addition, the sensor system itself operates reliably and the sensor element has a long life. Its demodulator and subsequent processing equipment can be placed in a centralized monitoring room, avoiding the disadvantage that the instrument is difficult to protect on site and facilitating maintenance and repair, thereby improving the reliability and maintainability of the monitoring system.

(7) The grating writing process is relatively mature and easy to form large-scale production

At present, the method of writing fiber Bragg gratings through ultraviolet light has become relatively mature. This ultraviolet writing allows external incident photons to interact with the doped particles in the fiber core, resulting in periodic or non-periodic permanent changes in the refractive index of the fiber core along the fiber axis, making it easier to form a spatial phase grating in the fiber core, which is also convenient for large-scale production.

(8) Easy to make into smart sensors, widely used

There are many areas where fiber grating sensors can be expanded, such as embedding distributed fiber grating sensors into materials to form smart materials, which are convenient for making smart sensors. Smart materials refer to the embedding of sensitive elements into the body and materials of the components to be measured, so as to achieve real-time monitoring of their safe operation, failures, etc. while the components or materials are working normally. Among them, the effective combination of optical fibers and electrical conductors with a variety of materials is one of the key issues, especially the realization of automated weaving with textile materials. Figure 5 shows a vest embedded with optical fibers and electrical conductors. The embedding of optical fibers and electrical conductors has been automated, solving another problem for the commercialization of smart clothing.

Fiber Bragg grating sensors can conduct real-time and safe monitoring of parameters such as load, stress, temperature and vibration of large components. Gratings can also replace fiber optic sensors of other types of structures and be used in chemical, pressure and acceleration sensing.

At present, fiber Bragg grating sensors are generally considered to be ideal devices for realizing "fiber smart structures" and "fiber sensitive materials". They are widely used in aerospace, petrochemical industry equipment, power equipment, ship structures, building structures, bridge structures, medical equipment, nuclear reactor structures, etc.

Figure 5: Smart vest

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4. Composition and principle of fiber Bragg grating sensing intelligent perimeter fence alarm system

The Fiber Bragg Grating Sensing Intelligent Perimeter Fence Alarm System is a security alarm system built using high-tech technologies such as laser, fiber optic sensing and optical communication. It is a modern defense system that monitors and alarms emergencies that threaten public safety. It is a new system based on Fiber Bragg Grating distributed fiber optic sensing technology applied to perimeter monitoring and protection.

The composition and principle of the fiber Bragg grating sensing perimeter fence alarm system are shown in Figure 6.

Figure 6. Composition and principle of the fiber Bragg grating sensing perimeter fence alarm system

As can be seen from Figure 6, this system uses a single optical fiber (optical cable) as a two-in-one sensor and transmission device. It monitors various disturbances that directly touch the optical fiber (cable) or are transmitted to the optical fiber (cable) through carriers such as covering soil, barbed wire, fences, pipelines, etc., so as to carry out continuous and real-time monitoring at any point in the entire process around the clock.

The working principle of the system is that after collecting the wavelength modulation data of the intrusion disturbance, it is sent to the wavelength mobile demodulation device at the back end of the monitoring room through the transmission optical fiber for demodulation. After analysis and intelligent identification by the signal processing system, different types of external interference can be judged: for example, non-hazardous environmental interference such as thunder, firecrackers, car horns, rain sounds, etc. can be identified and harmless judgments can be made; it can also identify climbing barbed wire, pressing walls, running or walking in prohibited areas, and mechanical construction that may threaten perimeter buildings. Therefore, system early warning or real-time alarm can be realized, so as to achieve the purpose of early warning monitoring of threatening behaviors that invade the perimeter of the fortified area. In order to accurately locate, it is only necessary to obtain the accurate length of the optical fiber, and then convert the optical fiber length distance into the actual distance according to the on-site situation, and obtain accurate and reliable positioning accuracy in the alarm information, so as to realize the positioning alarm function of the long-distance security system. Through the location of the intrusion site provided by the system, CCTV camera monitoring can also be linked to sound and light alarms or personnel can be dispatched to the site for processing.

For local high-risk areas, the system can also monitor and record voice. This function does not require electrical or metal sensors, but can be achieved with optical fiber, thus enriching the functions and protection level of the monitoring system using a single optical fiber.

5. Solution and application of multiple intrusion positioning alarm of intelligent perimeter fence alarm system

(1) Solution to multiple intrusion location alarms in the intelligent perimeter fence alarm system

When a threatening intrusion is detected, the system can locate the point of intrusion in real time based on the analysis of optical signal modulation, so that security personnel can take effective measures in a timely manner to prevent subsequent incidents of intrusion.

In order to solve the problem that in the optical fiber fence alarm system based on FBG sensors, when multiple FBG sensors are disturbed at the same time, it is difficult to locate the alarm signal and cannot effectively identify and judge the alarm signal, an analysis method based on empirical mode decomposition (EMD) and wavelet packet characteristic entropy algorithm can be used to solve the problem. This method uses the empirical mode decomposition method to be sensitive to and effectively retain the mutation of the signal, as well as its unique adaptive decomposition characteristics. First, the alarm signal is empirically decomposed, and then combined with wavelet packet decomposition to obtain the wavelet packet coefficient to extract the energy distribution of its signal, and then normalized to obtain the energy distribution feature vector of the signal. Finally, correlation analysis is used to realize the recognition and judgment of the alarm signal. In this way, by establishing an experimental model and analyzing the collected alarm signals, the effectiveness of this method in solving the cascade judgment alarm signal of FBG sensors in the optical fiber fence alarm system is proved.

(2) Application of Fiber Bragg Grating Sensing Intelligent Perimeter Fence Alarm System

The fiber Bragg grating sensor intelligent perimeter fence alarm system can monitor any point along the entire fiber optic laying area in real time around the clock. It can adopt different fiber optic cable laying methods according to the different needs of various perimeters: such as concealed underground burial, fence, wall laying, open laying, and concealed laying.

①Used as a long-distance border warning system

Such as important national borders, bonded area isolation zones, customs ports and other long-distance and large-scale places to prevent intrusion or crossing the border. Especially in the mountains, deserts and places where manual patrol and inspection are very difficult, this kind of optical fiber cordon is laid.

②Used for perimeter security defense of important facilities

◆ Military sites and national defense facilities: such as troops, airports, military ports, missile/rocket launch bases, radars, and communication sites;

◆Important facilities of the national economy: power stations, substations, museums, financial warehouses, and sports stadiums;

◆Defense of flammable and explosive places: oil depots, gas stations, oil and gas storage tank areas, explosives depots, etc.;

◆Important protection places: prisons, schools, reservoirs, industrial plants and mines, important residential areas, etc.

③ Used for protection of oil and gas pipelines (anti-excavation, anti-theft, anti-destruction)

④Used for protection of communication power cables, etc.

From the above introduction, it can be seen that the fiber optic fence based on long-distance quasi-distributed FBG sensors, as a new type of security monitoring system, not only has the characteristics of anti-electromagnetic interference, anti-corrosion, and easy reuse, but also has the advantages of mature FBG technology, low cost, and accurate alarm positioning. It has important application prospects in the security field and is also the mainstream development direction of intelligent security monitoring in the future.

When the fiber Bragg grating sensor fiber is invaded by the outside world, the wavelength of the fiber Bragg grating changes, causing a sudden change in light intensity, thus triggering an alarm. Of course, in the actual application of security technology prevention systems, it is best to simultaneously verify the video surveillance system linkage before driving the sound and light alarm to improve the reliability of the security system.

Quasi-distributed FBG sensing technology has been used in health monitoring of bridges and buildings. Its sensing principle is achieved by modulating the Bragg center wavelength through external parameters such as temperature and stress. When the external environment causes changes in the FBG temperature and stress, the grating period and refractive index will change, thereby causing changes in the reflection wavelength. Compared with other technologies, the advantages of using FBG include: strong multiplexing capability, which can realize long-distance quasi-distributed perimeter monitoring; strong anti-electromagnetic interference and corrosion resistance, suitable for harsh environments; simple structure, easy to combine with other materials to realize intelligent structures; intrusion monitoring with optical fiber as the sensing medium is highly concealed, etc. Moreover, the wavelength demodulation and the cost are getting lower and lower, laying a technical foundation for the practical application of the system.

The characteristics of the intelligent perimeter fence alarm system based on fiber Bragg grating sensors are simple and efficient, easy to install, simple to maintain, low cost, and the sensitivity can be adjusted according to the actual installation environment, which is very convenient for users. Therefore, after it is transformed into finalized production, it will be very suitable for large, medium and small perimeter fence deployment users, and will be rapidly popularized and promoted in the market.