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This letter describes a simple optically passive detection scheme for Bragg grating sensors. This detection scheme is based on two cavity lengths in Fabry-Perot read-out interferometers to produce two quadrature phase shifted signals from the Bragg grating sensor. Using the amplified spontaneous emission from an erbium-doped fiber amplifier as a low-coherence and high-power broad-band source, the passive detection technique is demonstrated by the use of Bragg grating sensors in dynamic strain measurements. This scheme is capable of measuring dynamic strain with a resolution of 0.1 μ/spl epsiv///spl radic/(Hz) root mean square at a frequency of 500 Hz for 600 μm cavity length in the Fabry-Perot read-out system.