Sagnac interferometer design for differential rotation measurement

We have demonstrated a differential setup of two fiberoptic gyroscopes in series for measuring the difference in rotation between two parallel axes. The sensor is insensitive to global rotations affecting the two axis equally. The system can be used in structural monitoring of large sized objects such as ships and airplanes. The application at hand is a direct measurement of the torsion vibrations in ships. The idea is to put a Sagnac interferometer in each end of the ship with axis parallel to the alongside axis. This can be done using two independent gyroscopes, but the system developed here reduces the two independent gyroscopes to one interferometer.