A distributed fiber optic vibration sensor is described, in which two Michelson interferometers are used as phase detectors and two 3×3 couplers are deployed to demodulate the time-varying phase change caused by vibration. The two interferometers are separated by four wavelength division multiplexers. The position of the vibration is obtained by signal correlation, which can be used as a perimeter security sensor to locate the intruder. The experimental results with a 4012 m fiber sensor are discussed.