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An evolution of the causal T-matrix
method has been proposed to take account of the losses that can occur in
gratings, for example in the cases of blazed FBGs. This evolution has leaded to
a modified layer-peeling reconstruction method that can be applied on grating
with distributed losses.
We have seen that the reconstruction by
layer-peeling allows to find the grating strength, function of Dnac,
and the chirp function. In order to differentiate the period chirp from the DC
refractive index chirp, at least two reconstructions at different temperatures
(or axial strains) are required.
The simulation of the reconstruction
with different parameters has shown that the required dynamic range of the
starting spectral or impulse response is not fundamental and that the number of
spectral point has to exceed 10 times the number of layers. Observation of the
reconstruction of noisy data has shown that the influence of noise is less important
for the reconstruction starting from the impulse response. Finally, the
reconstruction process by layer-peeling is limited for very strong gratings for
which a spectral bandwidth is depleted before the grating end. Measurements
from both sides and application of a temperature or axial strain ramp can
improve partially the reconstruction of these strong gratings.
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