The fiber optic sensor is embedded at the mid-plane of graphite/epoxy laminates to measure the strain in composites. The fiber optic sensor consists of Michelson interferometer using a single-mode bidirectional coupler. Under the tensile loading, the strain measured by the fiber optic sensor is in good agreement with that by the conventional strain gauge. The gauge length of the fiber optic sensor is not affected by the curing process. The effect of resin-rich region near the optical fiber can be ignored in a linear elastic range. The periodic change of amplitude in interferometric fringes is caused by the change of polarization axis. The constructed fiber optic sensor is sensitive enough as a technique for precise measurement. It has a tendency to be influenced by the environment such as air flow in the room, temperature, and vibrations etc.
