A novel fiber Bragg grating(FBG) demodulation system is proposed,which is composed of two cascading Sagnac interferometer loops based on the 3 dB coupler and the polarizing beam splitter(PBS) respectively.Theoretical calculations show that the extinction ratio of the system is doubled compared with the one with a single loop,when two sections of high birefringence photonic crystal fiber(HBPCF) have equal length in each loop.Experimental results show that the extinction ratio of interference is 4...