Page 243 - Fiber Bragg Gratings

220 Chapter 5 Apodization of Fiber Gratings

Figure 5.17: The strain applied to a fiber for perfect apodization as a function
of grating length for the SAM technique.

A schematic of this principle of making ultralong gratings is shown in
Fig. 5.18.
With greater stretch of the fiber, a larger number of cycles of the
apodization profile may be written, for example, a bowtie profile. Turning
off both stretchers may alter the apodization profile after a single pass.
The next over laid grating is left unapodized, thereby building a modified
cosine refractive index modulation profile.
Increasing the stretch further forms a single cosine envelope, shown
in Fig. 5.19. The stretch method has the same effect as the dual frequency
moire grating apodization. At the zero crossing of the envelope, a TT phase

Figure 5.18: The super-step chirped grating, apodized on each end. With a
uniform period phase mask, the chirp is zero so that a long, unchirped apodized
grating can be written. Careful alignment can eliminate the stitch error at the
join, or it may be UV "trimmed" [26].