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A sensitivity-enhanced fiber-optic strain sensor based on interference of higher order modes in circular optical fibers is reported. It has been demonstrated that by utilizing the interference of the higher order modes excited in a segment of multimode fiber (MMF), a strain sensor with an enhanced strain sensitivity which is almost twice that of the popularly used fiber-Bragg-grating-based strain sensors can be constructed. The features of the proposed strain sensor include an extremely simple structure, easy fabrication, and low cost. The modal interference in a circular step-index MMF is analyzed and then the sensing principle, fabrication process, and strain measurements are described.