Damped Vibration Sensing Using Optical Fiber Resonator Method

In past few years the demand for fiber optic vibration sensors has increased as they are free from electromagnetic interference (EMI) and chemical corrosion. These sensors have been widely used for monitoring earthquakes, hydroelectric dam, and structural damages in bridges and highways. This research presents a new method which implements amultilongitudinal mode fiber laser (MLMFL) to detect damped vibrations. MLMFL is made up of Single mode fiber (SMF), Erbium doped fiber (EDF), Fiber Bragg grating (FBG) and 3dB coupler forms a resonant cavity. The EDF provides gain for the cavity. When light with a wavelength of 980nm pump power is launched into the resonant cavity, multilongitudinal modes are produced within the reflected bandwidth of the FBG. Several beat frequency signals are produced by beating any two modes in the resonant cavity. When damped vibrations are applied, the strain experienced by the fiber changes the length of the cavity which changes the beat frequency. Thus by measuring the change in beat frequency of the resonant cavity the damped vibration frequencies are measured through proper calibration. Damped vibration frequencies were modeled using analytical simulations to compare with those measured experimentally. From analytical simulations, the damped vibration frequencies are observed from 500 Hz to 3000 Hz while the experimental method shows damped vibration frequencies from 500 Hz to 2000Hz. The difference between simulation and experimental results is due to different methods used for generating damped vibration frequencies and demodulation techniques