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The time multiplexing design OLCR has
also been used to measure directly the complex spectral response of FBGs. To
achieve this, the mirror is placed at a position corresponding to an inner
point of the grating in order to have a strong low coherence signal. Instead of
moving the mirror to scan the OPLD, the light frequency is scanned by the
tunable laser. The low coherent signal phase is used to compensate the phase
drifts. This measurement method is known as optical frequency division
reflectometry (OFDR). The obtained complex signal corresponds to the complex
reflection amplitude r(n) and not to the reflection intensity. For this reason, the dynamic
range in dB is twice the one obtained with a direct intensity measurement. The
dynamic range is nevertheless limited by the spontaneous light emission of the
laser source. For the tunable laser we have used, the spontaneous light
emission is under -60 dB. This dynamic limitation can be overcome for an amplitude
measurement by placing the mirror at a position where the low coherence signal
is canceled as the same will occur with the spontaneous part of the laser
light, but the phase signal is lost in this case
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