An efficient way to measure the complex
impulse response of a fiber Bragg grating is based on its analysis with an
optical low coherence reflectometer (OLCR). The OLCR technique was first used
to characterize single mode fibers in the late 80s [1-19 to 1-22].
An OLCR uses a broadband source coupled to an all-fiber Michelson
interferometer. The reference arm contains a broadband mirror, whereas the
interrogation arm contains the device under test. The portion of the test arm
that should be analyzed is selected by balancing its optical path length with
that of the reference arm and can be defined with a micrometer precision [1-23].
The complex OLCR measurement of a FBG corresponds to the convolution between
the complex impulse response of the grating and the degree of coherence of the
light source. The degree of coherence of a Gaussian light source is also a
Gaussian function, for which the time bandwidth corresponds to the light
coherence time (inversely proportional to the spectral bandwidth of the light
source).
The OLCR technique has been used to
find the position, the length and the coupling coefficient of homogenous Bragg
gratings [1-23, 1-24], to demultiplex several gratings in the space
domain [1-25] and to measure the complex spectral response of FBGs [1-26
to 1-28]. The most promising aspect of OLCR is the possibility to
retrieve the spatial information along the grating for distributed measurements
[1-29].
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