Page 124 - Fiber Bragg Gratings

3.4 Sources for holographic writing of gratings 103

120 mW average at 244 nm for a 20-Hz system with 12-ns pulses [115].
The frequency-doubled tunable dye-laser source can be line-narrowed to
increase temporal coherence, helping maintain interference despite path-
length differences. However, the UV beam may need to be spatially filtered
if the beam-quality is not good so as to be able to write uniform reflection
gratings. High-quality gratings have been reported using this method
[116], however, by multiple pulse writing.
With low transverse coherence, it is still possible to write good grat-
ings, provided the spatial nonuniformity averages out over the time period
required to write the grating. However, the pulse-to-pulse transverse
beam variation affects single-pulse writing of gratings [117-121] or, for
example, during fiber drawings [122,123], as was originally suggested by
Askins et al. [118]. The power density variation manifests itself in two
ways: first, the reflectivity varies from grating to grating, and second,
the peak reflection wavelength, which is a function of the induced index
change, varies between gratings. Beam profile nonuniformity has a rather
more serious deleterious effect on the grating reflection spectrum, causing
multiple peaks and chirp. The spectral shape is discussed in more detail
in Chapters 4 and 9. Multiple pulses can ensure that 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 propor-
tional to the effective index of the mode, the beam profile imparts a Bragg
wavelength profile proportional 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 nonuniform 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 inter-
ferometer such that the path of the two beams are equalized and over-
lapped. 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