PSEUDOHETERODYNE DEMODULATION TECHNIQUE FOR FIBER BRAGG GRATING SENSORS USING 2 MATCHED GRATINGS

A pseudoheterodyne, open-loop demodulation technique? for detecting wa velength shifts in wavelength encoded fiber Bragg grating sensors is p resented, The scheme uses a processing Bragg grating that is identical to one used as a sensor, When the processing fiber grating is stretch ed periodically, the system of two gratings produces a carrier at this frequency with its phase modulated by the measurand signal applied to the sensing grating, The demodulation technique is intrinsically immu ne to fluctuation of optical power in the system and is independent of the source spectral profile characteristics. A resolution of approxim ate to 1 mu epsilon/root Hz for static axial strain was achieved.