Fiber optic white-light interferometry for self-calibrated absolute-strain measurements

We present modifications in the design and operation of the conventional optical fiber extrinsic Fabry-Perot interferometric (EFPI) sensor to obtain real-time, self-calibrated, on-line and absolute strain measurements. The absolute EFPI (AEFPI) system utilizes the concept of white light interferometry to interrogate a fiber Fabry-Perot cavity and demodulation of the output signal may be carried out using a number of simple techniques like path matching or optical spectrum analyzer detection. The limitations of the conventional EFPI strain sensors are listed and it is shown that the modified AEFPI system overcomes most of these drawbacks by virtue of the fact that the information is wavelength-encoded. The AEFPI system is used to determine the strain during loading tendons that are commonly utilized in pre-stressed concrete. Other major applications include strain measurements in high-performance aerospace materials and structures under extreme mechanical vibrations and temperature variations. Preliminary experimental results are presented and applications to smart structures are proposed.