Pseudoheterodyne demodulation technique for fiber Bragg grating sensors using two matched gratings

A pseudoheterodyne, open-loop demodulation technique for detecting wavelength shifts in wavelength encoded fiber Bragg grating sensors is presented. The scheme uses a processing Bragg grating that is identical to one used as a sensor. When the processing fiber grating is stretched 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 immune to fluctuation of optical power in the system and is independent of the source spectral profile characteristics. A resolution of /spl ap/1 μ/spl epsiv///spl radic/(Hz) for static axial strain was achieved.