We have shown that for a wavelength
bandwidth where the fiber dispersion is negligible, the complex OLCR response
of a FBG corresponds to the convolution of the complex impulse response of the
grating with the degree of coherence of the light source. We have also shown
that the grating impulse response is less directly connected to the OLCR
response in the case where the dispersion effects are not negligible.
Nevertheless, it is experimentally or mathematically possible to retrieve the
complex impulse response in this case.
We have detailed the new developed OLCR
set-ups, explained the measurement method, the apparatus performances and the
limitations. This instrument measures the amplitude and the phase information
of the FBG. The main results concerns the time-multiplexing OLCR set-up that
exhibits a noise level of -120 dB for optical fiber devices (limited by the Rayleigh
back-scattering) and a large range of allowed OPLD resolution due to the phase
difference measurement method.
The reconstruction process from the
OLCR measurement to the complex coupling coefficient has been presented and the
reconstruction of different FBGs has been shown. The main results are an axial
resolution of 20 mm and a maximal absolute error of the amplitude and the phase of
5 % calculated by comparison between the reconstructions conducted from
both side of the FBG. The reconstruction of a FBG that exhibits loss has also
been reconstructed using the modified layer-peeling method and a good matching
between the reconstructions from both sides is observed.
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