Page 231 - Fiber Bragg Gratings

208 Chapter 5 Apodization of Fiber Gratings

Figure 5.7: The sequence of pulses is staggered so that a fringe is spread
symmetrically around the mean position of the fringe already printed in the first
printing.

There are some points that need to be considered in the use of this
method. The minimum step size determines how smoothly the grating
can be apodized. The step size is of the order of 0.01 mm, so that typically
in a grating length of 100 mm, full symmetric apodization can be achieved
with a maximum of ~ 10,000 steps, which is more than adequate for a
smooth profile. These parameters are discussed in Section 5.3.
This technique has been applied to the fabrication of apodized chirped
gratings as well. In this case, not only does the mark-space ratio have
to be changed but also the period.

5.2.5 Position-weighted fabrication of top-hat
reflection gratings

The MPF method is suited to writing gratings with a "top-hat" (TH)
reflection spectrum, shown in Fig. 5.8, along with the spatial profile of
the refractive index modulation of the grating required for these character-
istics (Fig. 5.9). For a perfect TH, an infinite number of cycles of a rect
or sine function are needed. This is not possible for practical gratings,
since the continuous sine function can only be approximated in a discrete
number of steps. This task is demanding in any case, but a good approxi-