Fiber Bragg Grating Perimeter Intrusion Alarm System, Characteristics and Application in Subway

With the vigorous development of urban construction, subway construction in various cities is also in full swing. The protection of subway operation sites is the guarantee of safe subway operation. As a special and important place, subway vehicle depots must effectively prevent illegal intrusion and various destructive activities. At present, the mainstream perimeter intrusion alarm systems include vibration optical cables, infrared (microwave) detection, laser targeting, pulse electronic fences, tension fences, etc. In the vehicle depot, it is common to encounter issues such as false alarms and missed alarms caused by external interference, which greatly increases the workload of maintenance and management.

At present, the mainstream perimeter intrusion warning technology is mainly divided into two categories:
1. Wireless mode
Wireless methods include infrared (microwave) detection, laser targeting, etc; The wired methods include vibration optical cables, pulse electronic fences, tension fences, etc. Overall, wireless methods have little impact on the landscape, but have certain requirements for terrain, poor anti-interference ability, and relatively high false alarm rates;

2. Wired mode.
The wired method has poor aesthetics, but has low requirements for terrain, strong anti-interference ability, and low false alarm rate.

The fiber optic grating perimeter intrusion alarm system is a type of vibration optical cable, which utilizes high-tech technologies such as laser, fiber optic sensing, and optical communication to construct an intrusion alarm system. It is a new system based on quasi distributed fiber optic sensing technology applied to perimeter monitoring and protection. It uses optical fibers to cover the defense area, which has many advantages such as no electricity, strong anti-interference, high reliability, lightning protection, easy remote transmission, and precise positioning. It has broad application prospects.

The fiber optic grating perimeter intrusion alarm system mainly consists of fiber optic grating vibration sensors, transmission fibers, fiber optic grating demodulation systems, system analysis and display software platforms, and linkage equipment. Fiber Bragg Grating (FBG) vibration detection fiber optic cable, as the detection front-end for external intrusion behavior, can be deployed arbitrarily in the surrounding area and collect various disturbance signals transmitted directly or indirectly, thus achieving 24-hour online monitoring of various vibration information in the surrounding area; The fiber optic grating signal processor in the backend can demodulate the optical wavelength signal carrying vibration information feedback, and restore the vibration signal in the surrounding area. The system software identifies and judges the characteristics of the vibration signal, sends intrusion alarm signals to vibration information with intrusion behavior characteristics, and accurately locates the intrusion location based on the feedback center wavelength.

The main application characteristics of fiber optic grating perimeter system in subway are as follows:

1. Easy to install. Low requirements for terrain environment, adaptable to different perimeter environments, simple installation, and can be completed without the need for very professional technical personnel.

2. Essentially passive, with low maintenance workload. The front-end equipment is a detection optical cable without active equipment, which will not be affected by external electromagnetic fields. There is no need to consider the damage of external electrical components to wind, rain, high and low temperatures, and ultraviolet radiation, which can greatly reduce maintenance workload and save electricity.

3. The false alarm rate is low. The front end adopts a Bragg grating sensing fiber optic cable, which reflects the corresponding waveform by sensing external vibrations, and distinguishes different alarm forms through the characteristic information of optical signals, rather than intensity signals, thus having better discrimination and improving the accuracy of alarms.

4. High reliability. Each section of the vibration fiber optic cable belongs to an independent zone. When a section is damaged, the other zones are not affected in any way. When maintaining, only single core fiber optic cables need to be fused and do not need to be recalibrated. There is no need to modify software configuration, so they can be connected and used immediately.