Page 356 - Fiber Bragg Gratings

7.3 Super-step-chirped gratings 333

Figure 7.17: A schematic of the super-step-chirped grating (SSCG). Each
grating is of nominally an identical integer number of periods [47].

each other, without a perturbation. In reality, unless care is taken, there
is a small error 8l/A 2 at the join. This error will ultimately have an effect
on the performance of the grating.
We now compare this with a uniform period grating with a phase
step. A phase step of a quarter wavelength, as we have seen in Chapter
6, introduces a bandgap in the reflection spectrum. The effect is similar
in gratings, which have a small chirp, since the Bragg wavelengths of two
adjacent sections are only slightly different, and the reflection spectrum is
strongly overlapped. This is indeed the case with SCGs and is shown in
Fig. 7.18. The reflection spectrum centered at the wavelength of the join
shows the quarter wavelength dependence of the bandgap. However, the
dependence changes with increasing gap, but the join is also invisible at

Figure 7.18: The reflection spectrum of the join region of 2 X 100 mm long
SCGs as a function of the gap at the join in fractions of the Bragg wavelength.
The gratings are 100 mm long with a chirped bandwidth of 0.75 nm each.