Page 229 - Fiber Bragg Gratings

206 Chapters Apodization of Fiber Gratings

achieved for a reflectivity 10 X greater. It is anticipated that the fabrication
of longer gratings is not only likely to be difficult owing to uniformity of
the grating but also very expensive and time-consuming.
Phase masks fabricated with stepped sections have been demon-
strated [13]. However, one of the problems with e-beam fabrication of
phase masks is the step size, which can be programmed to allow a variable
mark-space ratio of the grooves. An alternative to this approach is to
write a moire grating on the photoresist of the phase mask plate. Writing
two gratings of different wavelengths so that at the edges, the two patterns
are exactly half a period out of phase as shown in Section 5.2.1 easily
does this. The dose delivered by the e-beam for each overlaid grating is
half that required for the resist to be fixed. With two exposures, the correct
dose is delivered [14] and on developing the mask, the moire grating is
revealed [15]. In this phase mask, the diffraction efficiency varies continu-
ously along its length, and it is possible to fabricate phase masks for
long gratings. While it is attractive and convenient to replicate apodized
gratings directly using apodized phase masks, there are issues that need
to be addressed, which are definite drawbacks in the fabrication and use
of such a mask. The feature size of the mask near the edges of the grating
becomes infinitesimal, and as a result, the features have no strength. Nor
is there any guarantee that they will survive the phase mask fabrication
process in a repeatable fashion. The fragility of such a phase mask makes
it impossible to handle. It is likely to be damaged easily, either optically
or mechanically, during the process of fiber grating replication. There is
also the additional problem of the removal of contamination from such a
fragile phase mask. The better option for an apodized phase mask is to
alter the etch depth while keeping the mark-space ratio constant. This
ensures the strength and eases handling and cleaning processes.
The next section explores two techniques based on the application of
a combination of optical and mechanical methods. Both are highly flexible
and capable of produce a variety of gratings, apart from simple apodiza-
tion.

5.2.4 Multiple printing of in-fiber gratings applied to
apodization
The multiple printing in fiber (MPF) method has been discussed in Chap-
ter 3. In this section, the particular attributes and requirements for the
fabrication of (a) apodised gratings and (b) top-hat reflection gratings is