Page 126 - Fiber Bragg Gratings

3 A Sources for holographic writing of gratings 105

not be uniform shot-to-shot for this laser, it does allow the use of cylindrical
lenses for focusing into the BBO crystal. This scheme overcomes the
transverse walk-off problem in BBO associated with spherical lens focus-
ing. However, it is not possible to use cylindrical focusing in the intracavity
frequency doubling in BBO. A schematic of the Z-folded resonator is shown
in Fig. 3.29. With careful adjustment of the fold and focusing mirrors,
this laser can be operated at a stable CW output in excess of 300 mW at
244 nm. A cylindrical lens can be used at the output to circularize the
beam. Typically, the beam diameter is approximately 2 mm.
The coherence length of the argon-ion laser operating in the single-
frequency mode is more than adequate for use in a beam splitting interfer-
ometer even with path length differences approaching several centime-
ters, although the path length difference is usually not so large. This laser
is also suited for use in the scanned phase-mask interferometer, or for
writing blazed gratings, as discussed in Section 3.1.4.
Recent work on writing grating with near-UV wavelength radiation
[128-130] means that other high-quality laser sources may be used for
grating fabrication. An argon laser operating at 302 nm is one option,
which allows the inscription of gratings directly through the silicone resin
polymer jacket [71] or the use of a novel polymer at a wavelength of 257
nm [70]. The lifetime of the argon ion laser operating in the UV is probably
an issue for the fabricator. However, other sources as yet not demon-
strated, e.g., the intracavity krypton ion laser operating at 647 nm, fre-
quency doubled to 323.5 nm, would be an attractive option, with plenty
of power available at the UV wavelength.
3.1 lists a summary of the different types of lasers operating in the
UV, used for grating fabrication.

Figure 3.29: A schematic of the intracavity frequency-doubled, argon ion
laser. 488-nm wavelength radiation is doubled to generate 244-nm UV emission.
Angle tuning does phase matching of the BBO crystal. It is not necessary to
temperature stabilize the BBO crystal [26].