專利摘要：
An on-line Sagnac interferometric distributed fiber-optic leakage detection device comprising a light generator, an optical fiber coupler, a hollow pipeline, a Faraday rotator mirror, a sensing optical fiber, a delaying optical fiber, a photo detection device and a spectrum analyzer. Signals resulting from a leaky pipeline are obtained by the linear leakage detection system. The signals are analyzed by the spectrum analyzer to discover any null frequency shift so that exact location of the leakage point along the hollow pipeline can be computed.