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A phase mask is a quartz plate on which
a periodic corrugation has been engraved. The period and depth of the mask
grating are optimized to maximize the first order of the Bragg reflection for a
given light wavelength. The superposition of the ± 1 order generates an
interference pattern with a period that is half the mask period. The
interference can only occur if the illumination light source exhibits a
sufficient coherence length (temporal and spatial). If the fiber is placed in
the interference region, a FBG with half the phase mask period can be written.
Typical values of 40 % energy in each first order of diffraction are
observed. The remaining zero order (less than 1 % to 5 % of the
incident beam) reduces the fringe visibility of the interference pattern and
induces a constant refractive index change. The realized writing set-up is
presented in Fig. 2-4.
Fig. 2-4 FBG writing set-up with the phase mask technique
The maximal photosensitivity depends
partly on the laser fluence and for this reason the laser beam height is
reduced by a factor three. The beam reducer system is composed of the convex
and concave cylindrical lens that can be seen in the side view of Fig. 2-4.
The beam is preferably kept parallel to protect the phase mask from beam
focusing that could damage the grating, but some experiments have required
focalizing the beam to reach the maximal fluence on the fiber.
The illumination source is a pulsed
excimer laser operating at 193 nm (ArF) with energy ranging from a few
millijoules to 240 mJ per pulse. In addition, another cylinder lens can be
introduced in the system to enlarge the beam width from 6mm to 3cm and thus
increasing the possible grating length. The laser beam width is limited by a
slit limits placed before the phase mask. The phase masks can be changed or
removed very easily. A CCD video system is used to align the fiber in front of
the phase mask with respect to the laser beam. The setup shows a good mechanical
stability since grating erasure was observed only at high total dose. Without
the phase mask, the setup allows homogeneous post-exposure of the fiber to
increase the mean refractive index and permits hence a wavelength fine tuning.
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