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We propose an active optical sensor based on a microcavity with gain. Greatly improved sensitivity can be achieved in active microcavities as compared with passive high-Q microcavities. We show that an active sensor using a gain-doped microsphere can provide 104-fold narrower resonance linewidth than does a passive microcavity in the transmission spectrum. Such highly sensitive microcavity optical sensors can be used to detect low concentrations of chemicals or biomolecules in their surroundings. Our analysis shows that this type of compact active microcavity is sensitive to an effective refractive index change of the order of 10-9.