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In this paper, we report a novel wavelength interrogation technique to measure the relative wavelength spacing between two individual resonances of a tilted fiber Bragg grating (TFBG) sensor applied as a refractometer. The TFBG refractometer is sensitive to the refractive index changes in its surrounding medium and reflects the perturbation on its transmission wavelength changes. The presented technique is based on an arrayed waveguide grating (AWG) with its space-to-wavelength mapping capability. The spatial position of the input beam is controlled by a closed-loop piezoelectric motor. The absolute transmission wavelength of target resonance is obtained by introducing the actual spatial position of the input beam, provided by a position feedback encoder, to the pre-calibrated space-to-wavelength mapping relationship. Therefore, the wavelength spacing of the two target resonances is obtained as well as the measurement of refractive index. Initial results show that the proposed AWG-based wavelength interrogator has a multi-channel measurement capability and the potential to be packaged into a portable, light weight and cost-effective device.