Page 98 - Fiber Bragg Gratings

3.1 Methods for fiber Bragg grating fabrication 77

Figure 3.16: Length of a grating normalized to the width of the UV beam
as a function of prism apex angle a. The writing angle 6 ml2 is fixed at 10° for a
Bragg wavelength of —1500 nm. Also shown is the external writing angle of
incidence on the surface of the prism, $. Note that since the beam is folded about
the center, the grating length is approximately half the width for the Lloyd mirror,
whereas with the prism, the grating can be longer.

reduce this loss. Dispersion in the silica prism may also be an issue when
writing gratings longer than a few millimeters.
Other techniques for the production of gratings with a single prism
in a slightly modified version of that shown in Fig. 3.15 have also been
reported [53,54]. These methods require precision-fabricated prisms and
are restricted in tunability of the Bragg wavelength but may prove to be
useful in cases where a rudimentary inscription procedure is necessary.

3.1.7 Higher spatial order masks
According to Equation (3.1.4) the period of the mask may be integer
multiples of the Bragg wavelength. It is therefore possible to use a coarser
grating period than the fundamental period required for the reflection
wavelength. As the period gets larger, it is not practical to use the phase
mask as a diffraction element, and it is necessary to use an amplitude
mask for direct replication of the grating. The larger periods also allow
the photoreduction of the mask using imaging to imprint the correct period