Page 92 - Fiber Bragg Gratings

3.1 Methods for fiber Bragg grating fabrication 71

Figure 3.11: The overlap of the two interfering beams forms a diamond fig-
ure, with a depth of the fringes D. The grating length varies depending on the
placement of the fiber within the fringes.

3.1.5 The scanned phase mask interferometer

Figure 3.12 shows how the phase mask may be scanned for inscribing long
gratings into fibers. This technique was first demonstrated by Ouellette et
al. [42]. It was shown that 19-mm long gratings may be faithfully repro-
duced in fibers; a slight nonuniformity in the phase mask was also re-
moved by applying a temperature gradient across the fiber length after
writing the grating. Byron et al. [43] reported a 50-mm long grating in
which the quality of the interferogram was varied by adjusting the inten-
sity of the writing beam along the length of the grating. This method
allows the tailoring of the transfer characteristics of the fiber grating and

Figure 3.12: The phase mask used as a scanned interferometer is a powerful
method of fabricating long-fiber gratings. The quality of the grating is dependent
on the uniformity of the phase mask.