Page 97 - Fiber Bragg Gratings

76 Chapters Fabrication of Bragg Gratings

Figure 3.15: The Lloyd prism interferometer. The interfering beam paths
are within the bulk of the prism; however, there is a path difference introduced
between them due to the refractive index of the prism. The UV beam must be
spatially and temporally coherent with a uniform intensity for the production of
high-quality gratings.

but because of refraction, the side of the prism at which total internal
refraction occurs is smaller than the length of the Lloyd mirror [see Section
3.1.3, similar to Eq. (3.1.6)]. For a given beam width, the prism interferom-
eter expands the length of the grating, and this is shown in Fig. 3.16.
Using simple geometry, the length of the grating L g may be shown to be

the parameters for which are defined in Fig. 3.15.
At large angles of incidence (small apex angles), the grating length
increases rapidly but reflection losses increase at the same time. The
polarization useful for writing a grating with low birefringence (p-polar-
ized) is reflected more than the unwanted polarization (s-polarized) [40]
[see Section 3.1.8]. Antireflection coating of the surface will naturally