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We report a fiber strain sensor based on a single fiber Bragg grating (FBG) with a minimum detectable strain of 280 femtostrain/√Hz in the 20-kHz range. This breakthrough was made possible by operating the FBG on one of its slow-light peaks, and utilizing a FBG with a particularly low loss, fabricated using ultrafast pulses, to maximize the sensitivity. A theoretical and experimental noise analysis shows that the sensor noise is limited by laser frequency noise and not fiber phase noise, which suggests that even greater performance can be expected with a more stable laser frequency. © 2012 Optical Society of America.