Page 347 - Fiber Bragg Gratings

324 Chapter 7 Chirped Fiber Bragg Gratings

Figure 7.9: The effect of too few sections in a grating of length 8 mm with
a bandwidth of 5 nm. For 6 sections and a small refractive index (A, 0.75 sections/
4
mm, An = 4 X 10~ ) the spectrum breaks up into individual peaks. The minimum
number of sections for a continuously chirped grating is 3 sections/mm. For
3
stronger refractive index modulation (B, An = 1 X 1(T ), the grating "appears"
continuously chirped. However, neither the delay (not shown) nor the reflectivity
spectrum matches those of the continuously chirped gratings, C.

the band, it is an entirely different story. The edges of a grating have a
large impact on the reflection ripple as well as the delay ripple, as seen
in Figs. 7.4 and 7.5. Consequently even small broadband reflections can
change the ripple in the delay spectrum.

7.2.1 Effect of apodization

In Chapter 5 we saw the effect of apodization on gratings; the immediate
effect was the dramatic reduction in the side-lobe levels in the reflection
spectrum. Chirped gratings tend to have lower side-mode structure in
their reflection spectra to begin with, and the advantage of apodization
is in the reduction of internal interference effects that cause the group
delay to acquire a ripple. We consider here the properties of chirped
gratings, which have reflectivities of the order of 10 dB, suited to the
compensation of linear dispersion in fibers, and study the influence of
apodization. While the details of both the reflection and group delay
spectra change with the strength of the coupling constant of the grating,
general observations remain essentially unchanged. The chirped grating