A loop topology based white light interferometric sensor network for perimeter security has been designed and demonstrated. In the perimeter security sensing system, where fiber sensors are packaged in the suspended cable or buried cable, a bi-directional optical path interrogator is built by using Michelson or Mach-Zehnder interferometer. A practical implementation of this technique is presented by using an amplified spontaneous emission (ASE) light source and standard single mode fiber, which are common in communication industry. The sensor loop topology is completely passive and absolute length measurements can be obtained for each sensing fiber segment so that it can be used to measure quasi-distribution strain perturbation. For the long distance perimeter monitoring, this technique not only extends the multiplexing potential, but also provides a redundancy for the sensing system. One breakdown point is allowed in the sensor loop because the sensing system will still work even if the embedded sensor loop breaks somewhere.