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Specific vibration frequency detection based on hetero-core fiber optic sensing scheme for security environment monitoring

Fiber optic sensors have a potential to make ideal vibration sensors for smart structures because they can be operated without supplying the power at the sensor itself, have the capability of resistance to corrosion and fatigue, immunity to electrical interference, usage for a long time and safe. Additionally, the fiber optic sensors are practically able to operate the remote sensing due to stable transmission fiber line. Especially, hetero-core optical fiber sensors for environmental monitoring system have several advantages such as high accuracy, repeatability and temperature independency. This paper reports a novel approach to dynamic measurements for vibrations of the window using two types of the hetero-core optical fiber vibration sensors. A first vibration sensor can detect a vibration with high sensitivity due to effectively convert the vibration into change in the curvature of the fiber sensor. A second type of vibration sensor, which is lower sensitivity than the first type sensor, can detect vibrations with a few millimeters in thickness. It is shown that the vibration frequency depends on a fiber length of the two fixed points. A characteristic of spectrum peak shows the relatively wide frequency bands with ranging from 400 to 5000 Hz and from 1000 to 4500 Hz as the fixed points distances in the ranges of 15–45 mm and 10–25 mm for the first and second types of sensors, respectively.