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We present a sensitive optical sensor design by integrating a circular-hole defect with an etched diffraction grating (EDG) spectrometer based on amorphous-silicon photonic platforms. The circular-hole defect obtained from the outdiffusion of the hydrogen in the annealing process can induce strong resonant scattering loss at some special wavelengths. The positions of resonant peaks will linearly shift with the refractive index change of the detected sample (the sensitivity is 9200 nm/RIU). In addition, the influences of the defect on the sensing characteristics are numerically analyzed based on a scalar diffraction method and a Green tensor technology.