Page 115 - Fiber Bragg Gratings

94 Chapters Fabrication of Bragg Gratings

Figure 3.25: Writing of a continuously chirped grating by bending the fiber
in the fringe plane. Note that the bending also causes the grating to be blazed
with respect to the fiber axis.

so that even with a large radius of curvature, the grating may be substan-
tially chirped. Gratings with bandwidths of 7.5 nm and peak reflectivity
of 99%, as well as 15-nm bandwidth with a peak reflectivity of 5%, have
been reported with this technique [98].
As in the case of fiber tilted with respect to the fringe planes, bending
has a similar effect of imparting a blaze and consequently radiation loss.
The loss, which manifests itself on the short-wavelength side of the Bragg
wavelength even in unblazed gratings, is increased by blazing and may
not be desirable; for a chirped grating, this can be a serious problem, if
the radiation loss spectrum lies within the chirped bandwidth of the
grating. Loss due to coupling to cladding modes in chirped gratings can
be reduced by using fibers that are strongly guiding, but cannot be entirely
eliminated. It may be substantially reduced, however by the use of special
fibers with a photosensitive cladding [97] (see Chapter 4).
A constant strain along the length of a fiber while a grating is imprinted
merely shifts the Bragg wavelength on strain release. In order to impart a
chirp, a nonuniform strain profile has to be used, and there are several
practical methods for implementing this. If an optical fiber is tapered (e.g.,
by etching) such that the outer diameter varies smoothly in the region of
grating, the application of a longitudinal force leads directly to strain that