Page 226 - Fiber Bragg Gratings

5.2 Basic principles and methodology 203

5.2.2 The amplitude mask

Amplitude shading of the intensity profile of the interference pattern (for
example, the natural Gaussian profile of a laser beam) helps reduce the
side lobes in the reflection spectrum. However, a symmetric "chirp" is
also incorporated in the grating such that the blue part of the reflection
spectrum acquires an ugly structure (see Chapter 9). This is not good for
systems in which many such filters may be required to isolate tightly
packed channels. Clearly, simple amplitude shading is not in itself useful
for apodization. However, amplitude masks may be used in conjunction
with corrective measures to alter the waveguide parameters to result in
a constant effective index of the mode. The method developed to apodize
gratings relies on a double exposure: the first to precondition the fiber
with an amplitude mask, followed by the inscription of the grating again
with another amplitude mask in conjunction with a phase mask [10]. The
dose in the preconditioning exposure is adjusted to allow for the inscription
of the grating with a symmetric fringe intensity profile.
In Fig. 5.5 the preconditioning and grating illumination profiles are
shown along with the period-averaged UV intensity. The envelopes of
the precondition and the fringes are orthogonal functions. The result of
apodization on the reflection spectrum due to the double exposure is

Figure 5.5: The preconditioning UV intensity shaped by the amplitude mask,
the fringe profile, and the period-averaged UV intensity is shown. Since the
average UV intensity is constant as a function of position, so is the effective index.