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b)
Source detuning effect
The influence of the detuning
wavelength between the FBG and the broadband source central wavelength has been
studied for a 40 nm bandwidth source. The results are shown in Fig. 3-23
for a detuning of 0, 10, 30 and 50 nm, respectively. The impulse responses
for a detuning smaller than 10 nm are very close to the case without
detuning. For a more important detuning, the impulse amplitude drops consistently
due to the reduced overlap between the FBG and the source. Another effect also
appears : for high detuning, the amplitude signal at the grating input and
output presents an peak due to the refractive index step that acts like a
broadband mirror. This is presented in the zoomed view of the grating output in
Fig. 3-23 (the curves has been shifted in the OPLD axis for clarity). The
phase difference shows a smoothing effect, that is the p-shifts are spread over an
OPLD range proportional to the detuning.
Fig. 3-23 Source detuning effect on the complex impulse response amplitude
(top) and phase difference (bottom) for a 10 mm homogeneous grating with 2×10-4 refractive index modulation; 0 nm (solid
lines), 10 nm (dashed lines), 30 nm (dashed-dotted lines) and
50 nm (dotted lines) detuning values are considered
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