Page 83 - Fiber Bragg Gratings

62 Chapter 3 Fabrication of Bragg Gratings

to produce holographically, owing to problems with uniformity of illumina-
tion and the requirement for large mirrors. Lenses can be used to alter
the phase front of one of the interfering beams to allow the fabrication
of continuously chirped gratings, as opposed to step-chirped gratings by
e-beam fabrication [231. Since the fabrication of the holographic phase
mask depends on geometrical alignment of interfering beams, the mass
production of identical phase masks may remain a problem.

3.1.3 The phase mask interferometer
UV lithographic replication has been used extensively to fabricate phase
masks directly in silica plates using e-beam writing and plasma etching
[24], to function as lenses and complex spatial elements. This technique
has also been applied successfully to fiber Bragg grating inscription and
reported in the literature by several laboratories at around the same time
[25-281. There are several methods of using the phase mask: as has
been stated, it may perform the function of simple beam splitting in the
interferometer in Fig. 3.1. So why is it such a useful element, when a far
cheaper dielectric beam-splitter can be used instead? Its primary aim is
to be used simply as a wavelength-defining element in an interferometer
(as shown in Fig. 3.5); used as a beam splitter with the beam-combining
mirrors (Fig. 3.5) to adjust the wavelength of the fiber grating.
The change in the Bragg wavelength as a function of the change in
the mutual angle between the two interfering beams as shown in Fig.
3.5 is found by substituting Eqs. (3.1.3) and (3.1.4) into Eq. (3.1.1) and
differentiating with respect to fr.

Figure 3.6 shows the Bragg wavelength in the fiber as a function of
the half-writing angle. With the diffraction angle fixed at ~10° (phase
mask for ~1550 nm), a change of 5° alters the Bragg wavelength by —800
nm [29]. The enormous tunability of this interferometer, as well the ability
to find a reference position for the phase-mask Bragg wavelength, makes
it highly flexible. It requires a single-phase mask, which is used as both
a beam splitter and a Bragg wavelength reference. It can also be used to
replicate chirped phase masks, and a tunability of the Bragg wavelength
of ~250 nm has been demonstrated [29].