A Design Of Interferometric Fiber Optic Perimeter Security System

Perimeter security refers to the protection of the border to a specific area to prevent illegal intrusion or escape. Traditionally, physical defense is generally used as perimeter security system, such as walls, fences and barbed wire and so on. To improve the performance, inspection on duty is often requires. Nowadays, more technological solutions have been binding to modern perimeter security systems, making it more intelligentize, such as vibration cable, tension cable, infrared shooting, high-voltage grid and video surveillance. However, there are some obvious shortcomings in the perimeter security technological solutions mentioned above, such as susceptible to weather and environmental impact, false positives, false negatives and on-site power supply need.The fiber perimeter security technology, using the optical signal as carrier, inherits all the advantages of optical fiber sensing, like real-time monitoring, strong detection capability, low false alarm rate and the nature of the system security. It not only still works in harsh environments, but also fits to explosive situations, has the possibility to replace the existing perimeter security technology.According to the difference of zone layout, fiber optic perimeter security can be divided into multiple-zones and locating categories. The former divides the whole area into several independent sub-zones using physical partitioning method, and the host patrols each sub-zone to achieve detection of the whole entire region. While the latter uses only one optical fiber monitoring the whole entire region, based on principle of distributed optical fiber sensor, and the disturbance information is obtained by demodulating the optical signal to locate invasion.After comparing of principles and locating technologies of existing fiber optic perimeter system, we designed and built a multi-zones and a locating fiber optic perimeter systems based on Mach-Zehnder interferometer technology, using single-mode fiber. By analyzing the modulation format, signal demodulation, digital processing and measurement error of the interferometric signal, the feasibility and reliability of the interference technology using in fiber perimeter security system have been proved, and the experimental results verify the theoretical analysis correctness.The laser plays a very important role in interferometry, which coherence length is a significant indication. According to the requirements of coherence length and output power of the laser, the design selects a 300 M / 4mw and a 3M / 10 mw laser diode as the multi-zones and locating system light source respectively. And the structure and driver design of laser diode are also present in this paper.The photoelectric conversion of multi-zones and locating systems is also being made an intensive study, from component selection, circuit structure to noise analysis. On this basis, a PIN photodiode and an avalanche photodiode are selected as photoelectric conversion components for multi-zones and positioning systems respectively. For the reverse bias circuit for the avalanche photodiode, this paper also presents a specific design.Since the signal demodulation technique of multi-zones system is simple relatively, and the sampling frequency is low, a microprocessor integrated with ADC is selected to design the acquisition front-end and the host, making the reuse of ADC easier. However, for locating system, the demodulation techniques is more complex, and locating accuracy depends on the sampling frequency. Thus this design uses FPGA technology and high-speed ADC to build the acquisition front-end of locating system, while a DSP digital processor is used in the host design to achieve the high-speed communication with the FPGA.According to the testing result, the experimental prototype basically achieves the expectation result, and further optimize the adjustment will make it capable of replacing the existing perimeter security products.