Novel method for the demodulation of wavelength shift of fiber Bragg gratings

A novel method to detect the small wavelength shift of the fiber Bragg grating used in an electric current sensor with a comb like fiber filter is proposed in this paper. The wavelength filter was constructed by jointing a piece of hi-bi fiber in the loop of a normal fiber Sagnac interferometer, which may result to a periodically varied spectrum in the reflection or transmission light. Therefore the small wavelength shift of the fiber grating caused by the change of the applied magnetic field can be detected by monitoring the intensity of the light traveling through the comb like filter. With a 0.8nm spacing filter used in the measurement, a sensitivity better than 0.01nm for the Bragg wavelength shift of the fiber grating (limited by the noise of the electronic circuit and detection instruments) has obtained in the experiment. The proposed demodulation scheme provides a new approach to the detection of small wavelength shift of fiber grating.