  A high speed quasi-distributed demodulation method based on the microwave
photonics and the chromatic dispersion effect is designed and implemented for
weak fiber Bragg gratings (FBGs). Due to the effect of dispersion compensation
fiber (DCF), FBG wavelength shift leads to the change of the difference
frequency signal at the mixer. With the way of crossing microwave sweep cycle,
all wavelengths of cascade FBGs can be high speed obtained by measuring the
frequencies change. Moreover, through the introduction of Chirp-Z and Hanning
window algorithm, the analysis of difference frequency signal is achieved very
well. By adopting the single-peak filter as a reference, the length disturbance
of DCF caused by temperature can be also eliminated. Therefore, the accuracy of
this novel method is greatly improved, and high speed demodulation of FBGs can
easily realize. The feasibility and performance are experimentally demonstrated
using 105 FBGs with 0.1% reflectivity, 1 m spatial interval. Results show that
each grating can be distinguished well, and the demodulation rate is as high as
40 kHz, the accuracy is about 8 pm.
