Demodulation of FBG sensors embedded in a fiber-optic Sagnac loop

For condition monitoring of large scale electrical power transformers, a fiber-optic multi-stress sensor system was constructed by combining fiber-optic acoustic sensors and fiber Bragg grating temperature sensors in a fiber-optic Sagnac interferometer. To separate the grating signals from the interferometer output, an attenuator was placed at an asymmetrical position in the Sagnac loop. By balancing the counter propagating light intensities with the attenuator, the background noises could be suppressed to obtain grating signals with enough signal-to-noise ratio. With the preliminary experiments, the temperature and the vibration information at multiple locations could be measured simultaneously with single optical circuit and signal processing unit.