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An in-fiber Fabry-Perot interferometer with fiber Bragg grating (FBG) mirrors (FBG-FPI) yields extremely narrow transmission peaks within the FBG reflection wavelength range. Periodical scanning of the laser light source wavelength produces a train of optical pulses and the occurrence time of the pulse is modulated as the FBG-FPI is under influence of strain. When dynamic strain due to mechanical vibration is applied to the FBG-FPI, the detected signal of the pulse train is Fourier transformed and processed to reproduce the waveform of the vibration-induced strain. Furthermore, an inexpensive DFB laser can be utilized as a light source because the narrowness of the transmission peaks does not require broad wavelength scanning.