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We present a novel grating modulation technique for photonic bandgap fiber (PBF) gas sensors. The grating modulation technique involves modulating the Bragg wavelength around a specific gas absorption line and measuring the second-harmonic signals for optical gas sensing. The responses of the PBF gas sensors employing the grating modulation technique to different acetylene concentrations ranging from 0% to 10% agree well with the prediction. Both 0.8- and 1.3-m-long PBF gas sensors are demonstrated.