Page 123 - Fiber Bragg Gratings

102 Chapters Fabrication of Bragg Gratings

grows at approximately half the initial Bragg wavelength, followed by
the growth of the N = 1 grating. The final grating is stronger than the
original and is able to withstand a higher temperature. These gratings
form in most fibers, although they have yet to be observed in hydrogen-
loaded fibers. The gratings are reviewed in Chapters 2 and 9.

3.4 Sources for holographic writing of
gratings

There are several UV laser sources that may be used for inducing refrac-
tive index changes and for fabricating gratings in optical fibers. Methods
for generating UV or deep UV radiation require the use of excimer lasers,
nonlinear crystals for frequency mixing of coherent visible/infrared radia-
tion, or line-narrowed dye laser radiation. UV laser sources may be catego-
rized into two types — low spatial/temporal coherence or spatially coherent
sources. Sources in the first category have been used extensively for grat-
ing fabrication but need to be used with care for high-quality grating
production. The primary advantage is in the high average and peak power
available in the UV region. On the other hand, some frequency mixing
methods produce UV radiation of high temporal and spatial coherence
and are ideally suited for grating formation. Typically, these lasers demon-
strate high average power capability but have the disadvantage of lower
peak power densities. Nevertheless, these have also been shown to be
highly successful for inducing large index changes while maintaining
excellent grating quality.

3.4.1 Low coherence sources
The first source reported for use in grating formation was the excimer-
laser pumped frequency doubled oscillator-amplifier dye laser operating
in the 240-nm window [13]. This source can produce approximately 0.1
W average tunable radiation between 240 and 260 nm using a /3-barium
borate (BBO) crystal by doubling the (CHRYSL 106) dye-laser output.
The laser must be line-narrowed to increase the coherence length, and
longitudinal pumped dye lasers are preferable since they produce a higher
quality beam compared to side-pumped dyes which produce a triangular
beam profile. An alternative pump laser for the dye is the frequency-
mixed output at 355 nm from a YAG laser operating at a wavelength of
1064 nm. Similar output may be achieved with a maximum of around