FBG-based vibration measurement of rotating structure using optical fiber rotary joint

We construct an intensity-based FBG sensor for a vibration measurement of a rotating structure by using an optical fiber rotary joint (OFRJ) which enables free rotation of fiber while maintaining low-loss coupling efficiency. Although a number of FBG sensors have been proposed for use in various applications, they have not yet been applied to a rotating structure especially for dynamical measurement. In the proposed sensor system, an SOA-based dualwavelength fiber laser is employed as an optical source and an additional reference FBG sensor is multiplexed with the intensity-based FBG sensor by use of a wavelength division multiplexing (WDM) technique. The reference sensor enables to compensate the insertion-loss variation due to the rotating OFRJ so that a real-time vibration measurement of a rotating structure is achieved in the intensity-modulation scheme. In the experiment, the operation principle is successfully confirmed, and the vibration measurements of a rotating cantilever beam and a propeller-like plastic blade are demonstrated.