Spectral amplitude or intensity measurements can be performed in
reflection or in transmission. We have used a tunable laser or a broadband
light source to measure the intensity responses. Fig. 2-14 shows the
measurement set-up based on the tunable laser. The polarized light of the laser
is launched in one arm of a coupler (eventually goes through a polarization
controller to excite in a defined manner the polarization modes if a PM fiber
is used). The reflected intensity from the grating is collected by the detector
DR and the transmitted intensity by the detector DT. It
should be noted that the remaining spontaneous light of the laser source limits
the maximal dynamic range of the measurement (60 dB for the tunable laser
available during this work). In the second method, the tunable laser is
replaced by a broadband light source and the detectors by an optical spectrum
analyzer or a monochromator.
Fig. 2-14 Reflection and
transmission intensity measurement set-up with a tunable laser; tunable laser
(TL), coupler (CPL), polarization controller (POLA), fiber Bragg grating (FBG),
detector for reflection intensity (DR) and for transmission
intensity (DT)
The time multiplexed OLCR set-up developed for the measurement of
the grating impulse response has also been used as a spectral measurement
system (§4.3.7). In this case, the reflection amplitude (not the intensity) and
the reflection phase are collected. We note that the Fourier transform of an
OLCR measurement gives also the complex reflection amplitude.
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