All-fiber grating sensors employing many-cycle directional couplers

Analysis and results of experiments concerning all-fiber passive demodulation of fiber Bragg grating sensors using a many-cycle directional coupler are reported here. The principle of operation is based on the fast wavelength response of a fused tapered 2 by 2 directional coupler pulled through many coupling cycles. The coupler's splitting ratio is then sensitive to small changes in the reflected Bragg wavelength. A design rule to allow the selection and fabrication of a coupler with the desired response has been developed, and the coupler splitting ratio linearity with wavelength is also examined. Because such directional couplers may be readily tailor-made by choosing the number of coupling cycles the coupler is pulled through during fabrication, this approach makes possible a range of sensitivities and wavelengths. Problems encountered with highly sensitive devices due to polarization dependent splitting are also discussed.