In structural engineering, fiber-optic sensors are very attractive for quality control during the construction and afterwards to monitor the health of the building. The ability to measure strain and temperature simultaneously by multiplexed interferometric optical fiber sensors (IOFS) makes them appropriate for structure health monitoring. The IOFSs can be classified into resonance and nonresonance types. Polarization-maintained fibers are employed in different IOFS such as optical fiber gyroscopes and intruder detection systems. Depending on the application of IOFS, optical fiber interferometers are used in discrete, quasi-distributed, and distributed fiber sensors. In some applications such as intruder detectors, which are based on the distributed or quasi-distributed detection, optical fiber interferometers (OFIs) can be manufactured by commercial optical fiber and other components, which are designed for optical fiber communication applications. In OFIs, the transformation media may be one or more optical fibers. In OFIs utilizing multimode optical fibers such as the Sagnac interferometer, each mode defines an individual optical path.