Page 91 - Fiber Bragg Gratings

Figure 3.10: (a) The fringes formed at the point of intersection need a large
cross-sectional area for a slanted grating to be written in the fiber. Shown in (a)
is view of the fringes in a plane normal to the zero-order, and a is the rotation
angle in that plane, (b) shows another method for writing slanted gratings, in
the plane of the incoming beams [41]. The fiber is rotated by an angle a within
the plane of the beams such that it overlaps with the interference fringes; the
coherence properties of the UV source as well as the depth of fringes determine
this.

is shown. The maximum possible fringe depth D is indicated in Fig. 3.11.
Coherence, both temporal and spatial, limits D to less than this value,
as described in Eq. (3.1.8). For a phase mask used in near contact with
the fiber, the depth of the fringes is less than D/2.