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A novel approach to interrogating in real time a linearly chirped fiber Bragg grating (LCFBG) sensor based on chirped
pulse compression using a Sagnac loop interferometer (SLI) with improved pulse compression performance is proposed
and experimentally demonstrated. The proposed system consists of a mode-locked laser (MLL), a SLI incorporating an
LCFBG, which makes the SLI have a spectral response with an increasing or decreasing free spectral range (FSR), a
dispersive element and a photodetector. The significance of using an SLI incorporating an LCFBG is its capability of
providing equal dispersion for two pulses traveling along the clockwise and counter-clockwise paths, which would
effectively avoid a non-complete temporal interference, and improves the pulse compression performance. When the
fiber sensor is experiencing a strain, the strain information would be conveyed to a wavelength shift caused by the Bragg
wavelength change, which is further transferred to the change of the FSR. An ultra-short pulse train generated by the
MLL would be spectrum shaped by the SLI, and the shaped spectrum would contain the information of the wavelength
change. The demodulation is performed in the time domain by mapping the spectrally shaped waveform to the temporal
domain using a dispersion compensating fiber (DCF) as the dispersive element. The generated temporal waveform is
then correlated with a special reference waveform, with the location of the correlation peak indicating the wavelength
change which reflects the strain or temperature change. A theoretical analysis is carried out, which is validated by an
experiment. The experimental results show that the proposed system can provide an interrogation resolution as high as
0.22 με at a speed of 48.6 MHz with a correlation peak to sidelobe ratio of 2.5.
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