Page 232 - Fiber Bragg Gratings

5.2 Basic principles and methodology 209

Figure 5.8: Simulated reflection spectrum of a sine function approximated
by including 22 zeroes for a grating 4 mm long. The peak-to-peak index difference
3
is 6 X 10~ , and the grating has 1000 sections. These gratings demand better
step resolution than is possible with the MPF scheme.

Figure 5.9: The refractive index modulation profile showing 31 zeroes along
the length of the grating. Figure 5.8 shows a top-hat reflection spectrum for a
grating with 22 zeroes. The modulation amplitude must have positive and negative
components in order to faithfully reproduce a top-hat spectrum. Truncation leads
to a limiting out-of band rejection of 20 dB.