A precision fiber optic displacement sensor based on reciprocal interferometry

A fiber optic nanometer range displacement sensor has been developed based on reciprocal interferometry, a concept widely used in fiber optic gyroscopes. Its configuration is similar to a Michelson interferometer but with only one of the two arms used. The principle of operation is the interference between the reflected light wave from the fiber end and that from a reflective object. As both the reference and the sensing light waves travel through the same optical path except for the air gap which is the distance to be sensed, the system is thus reciprocal and insensitive to perturbations introduced to the fiber path. While the system is very simple, it has demonstrated substantially improved immunity against environmental perturbations over the conventional Michelson interferometer. Our experimental results have shown that the interference behavior agrees well with its mathematical model. The system has demonstrated a resolution of 5 nm. The influence of temperature change and PZT induced phase shift to the fiber has been studied and the results have shown that the system is indeed insensitive to these perturbations.