Page 244 - Fiber Bragg Gratings

5.3 Fabrication requirements for apodization and chirp 221

Figure 5.19: A single-period cosine envelope moire grating formed by stretch-
ing the fiber by twice the required amount for perfect apodization (as in Fig. 5.15).
The arrow indicates the position of the automatically introduced TT phase shift in
the fringes, equivalent to a 77/2 phase shift at the Bragg wavelength. The length
of the grating has been chosen to be deliberately short to show the occurrence of
the phase shift.

change occurs between the two sections of the fringes, as can be seen in
Fig. 5.19. This effect can be used to automatically introduce multiple,
regularly spaced 77/2 phase shifts at the Bragg wavelength for the fabrica-
tion of a top-hat reflection spectrum and multiple band-pass filters (also
see Section 5.2.6). The difference between stretching or the dual-frequency
multiple-period moire gratings and the MPF technique for writing a simi-
lar grating is that in the latter, a deliberate phase shift has to be written
in, whereas in the former, the phase shift is automatically introduced.

5.3 Fabrication requirements for
apodization and chirp

As has been demonstrated in Section 5.2.6, the maximum chirp that can
be written using the two mini-grating replication methods (MPF and
MPM) is dependent on the length of the subgrating; in the case of the 1-
mm-long subgrating for the MPF method, it is only possible to write a
grating with a chirp of ~1 nm, since it is equivalent to a change of one
period in 1 mm. In order to write larger chirps and apodize the grating