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A practical single-mode birefringent fiber strain sensor is demonstrated. The sensor is based on optical frequency-modulated continuous-wave (FMCW) interference and consists of three lengths of single-mode birefringent fiber. Two fibers are used as the leading fibers, while the third fiber that locates in the middle and has a 45Â° rotation of principle axes is used as the strain sensing fiber. The strain is determined by measuring the phase shift of the beat signal produced by the two beams in the sensing fiber. The advantages of the sensor include high resolution, large dynamic range, long gauge length, long leading fibers, simple configuration, and immunity from the environmental influence.