106 Fabrication of Bragg Gratings each section of the grating can be driven into saturation (for a given UV flux), although if the beam profile has a Gaussian average intensity profile, the grating will appear chirped. The regions with low flux will see a smaller index change and hence a smaller change in the effective index of the mode than the central region of the beam. Since the Bragg wavelength is proportional to the effective index of the mode, the beam profile imparts a Bragg wavelength profile propor- tional to the intensity profile, leading to chirp [124]. It is for this reason that there is a concern over the use of a laser with "hot-spots" in the beam. The grating may show fine structure in the reflection spectrum due to the nonuni- form refractive index modulation of the inscribed grating. This has so far not been reported in the literature but may well be a problem for long gratings. Another aspect of low coherence sources, which must be taken into account when designing an interferometer, is the effect of the coherence length. These laser sources are generally better suited to direct printing using a phase mask with the fiber immediately behind the phase plate. Alternatively, a one-to-one imaging system may be used to form the interferometer such that the paths of the two beams are equalized and overlapped. If, however, the paths are not equal or the beams do not overlap, degradation in the visibility can result in poor grating reflectivity and/or spectral profile. The limiting factor for all lasers is the divergence of the beam, which determines how far away the fiber may be placed from the phase mask as (see Fig. 3.1) dl df ( l 2 cosðy m =2Þ ; 2L ð3:4:1Þ where dl is the source bandwidth, df the source angular divergence, l the source wavelength, y m /2 the half diffraction angle, and L the distance of the fiber from the phase mask. The physical significance of Eq. (3.4.1) is that as the diffracted beams are brought together, the divergence causes a dephasing of the interfering beams, reducing the visibility. The contact method is therefore ideally suited for use with low-coherence sources. However, it must be remem- bered that the phase mask is more likely to be damaged owing to contamination from the fiber, i.e., dust, etc., using high-intensity pulses. High peak-power laser sources do allow the writing of Type II gratings, which depend on physical damage to the core region [125]. This aspect is discussed in Section 3.3. 3.5.2 High Coherence Sources Lasers with good spatial and/or temporal coherence fall in this category. Examples include CW intracavity frequency doubled argon-ion lasers operating at 257 nm/244 nm [26,126], QS frequency quadrupled YLF [127], spatially