Page 90 - Fiber Bragg Gratings

3.1 Methods for fiber Bragg grating fabrication 69

the phase mask nearly in contact with the fiber are the dangers of contami-
nation and permanent damage of the phase mask. A different phase mask
is required for each specific Bragg wavelength. This need not a problem,
since several gratings can be written on a single phase-mask plate, each
at the required wavelength [41]. Alternatively, a tunable interferometer
can be used with a single phase mask; however, it does require careful
calibration and alignment.

3.1.4 Slanted grating
If the fiber is tilted out of the plane of Fig. 3.5, the grating inscribed in
the fiber will be slanted in the direction of propagation of the mode. This,
however, requires the interfering beams to have a large cross-sectional
area so that the beams may overlap, as shown in Fig. 3.10a. This is
inconvenient for most interferometers, since the cylindrical lens focuses
the beams in the plane of the figure, unless the unfocused beam intensity
is already high. An alternative and simple method for inscribing slanted
gratings is to tilt the fiber in the plane of the figure, as shown in Fig.
3.10b [41]. In this case, the coherence properties of the laser will determine
the visibility of the fringes at the fiber. Since the fiber is at an angle to
the incoming beams, the inscription of the grating depends on the overlap
of the two beams and is slightly shortened; the depth D of the fringes for
the interferometer shown in Fig. 3.1 Ob is

where W is the width of the normally incident UV beam and 0 m/2 is the
diffraction angle shown in Fig. 3.3. Figure 3.11 shows the depth of the
fringes and the overlap of the beams. For small tilt angles a, the period
A s of the slanted grating in the direction of propagation varies as

The fringe pattern shown in Fig. 3.3 is unchanged; however, since the
fiber core is at an angle to the fringes, a grating, which is blazed with
respect to the propagation direction of the mode, is formed. These gratings
have special applications as lossy filters and are discussed in Chapters
4 and 8. In Fig. 3.14, the extent of the fringes formed in the overlap region