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In this work, two OLCR set-ups have
been realized that measure simultaneously the amplitude and the phase OLCR
signals of FBG in the 1300 nm range. Both exhibits very high S/N only
limited by the fiber Raleigh scattering. The essential difference between the
two set-ups concerns the phase reference measurement method. In the first
design, a reference laser at a wavelength different from the FBG wavelength
propagate at the same time in the interferometer. The laser interference phase
signal defines a distance reference where the OLCR phase is free of drifts
(wavelength multiplexing scheme). In the second design, the reference laser is
at the Bragg wavelength of the grating. In this case, the difference between
the laser phase and the OLCR phase varies slowly with the OPLD and is free of
drifts (time multiplexing). The time multiplexing method allows OPLD samplings
as large as several tens of microns.
We first present the time multiplexing
design. The following sections present the details on how to measure complex
OLCR response, with an emphasis on the static method used in our set-ups, the
balanced detection scheme, the polarization problem in all-fiber
interferometers. The wavelength multiplexing design is then described. Finally,
a comparison of the two set-ups is presented.
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