Page 125 - Fiber Bragg Gratings

104 Chapter 3 Fabrication of Bragg Gratings

beam, which determines how far away the fiber may be placed from the
phase mask as (see Fig. 3.1)

where d\ is the source bandwidth, 8$ the source angular divergence, A
the source wavelength, 6 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 remembered 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 grat-
ings, which depend on physical damage to the core region [125]. This
aspect is discussed in Section 3.2.

3.4.2 High coherence sources

Lasers with good spatial and/or temporal coherence fall in this category.
Examples include CW intracavity frequency doubled argon-ion lasers op-
erating at 257 nm/244 nm [26,126], QS frequency quadrupled YLF [127],
spatially filtered, line-narrowed frequency doubled dye lasers [13]. The
first laser has excellent spatial and temporal coherence, being derived
from the argon ion laser mode. Even with this laser, there can be fine
structure in the transverse beam profile of the UV mode due to the low
walk-off angle in the frequency doubling crystal, BBO. The beam is ellip-
tical, but this is an advantage because the shape is suited to writing
fiber gratings. The latter two lasers can be made to have good temporal
coherence by line narrowing, but requires good spatial filtering to generate
a satisfactory Gaussian mode profile. The pinholes used for filtering re-
quire careful alignment and have a tendency to burn after a short time
owing to the high peak power; they require frequent replacement.
The quadrupled Nd:YLF and the intracavity frequency doubled argon
ion lasers belong to the class of turnkey systems. These are reliable lasers
and need little attention other than routine maintenance; for example,
mirrors need occasional cleaning. The frequency doubling crystals tend
to be KTP for the YLF followed by a BBO crystal enclosed in an airtight
holder with silica windows. Although the transverse mode profile may