The feasibility analysis of monitoring falling stones by means of a fiber optic interferometer

There are many areas in Taiwan which does not have solid rocks. Suffering from the debris flow induced by the earthquakes and many flood disasters in recent years, the geology even shows the flimsiness. It is very frequent to find lots of falling stones on the highway; sometimes disasters take place. Because the optical fiber has the features of low loss and wide bandwidth; it has replaced the coaxial cable as the mainstream of the communication system in recent years. Because of its high sensitivity characteristic, the interferometer is usually applied to long distance, weak signal detection. In general, if the vibration is located at far away places, the weak signal makes it uneasy to monitor. The configuration of our sensing system is made of an interferometer and fiber Bragg Gratings. A demodulation circuit was used as the signal processor to measure the phase difference caused by vibration. Compare to that of a traditional accelerometer, the fiber optic sensor is more easier to implement in field applications. The later is an all fiber system with good accuracy for low frequency vibration measurement. According to our preliminary test, the dynamic range of the system is 45 dB and the noise level is 7.3 × 10-3 rad. Hence, the object of this paper is to study the feasibility of a monitoring system, which is compatible with the existed optical fiber communication and a high sensitivity interferometer as a falling stone monitoring system.