Page 230 - Fiber Bragg Gratings

5.2 Basic principles and methodology 207

discussed. The principle of this method is to write short (4-mm) gratings
that are overlapped, so that at each printing only a few new periods are
printed [16]. This is possible with a pulsed UV laser system but requires
extreme precision in positioning the fiber. To overcome this problem, the
fiber is supported over the entire length of the grating to be fabricated
in a long glass vee-groove. The vee-groove is fabricated using two pieces
of glass assembled together with a small gap at the apex of the vee. This
allows a vacuum system to be used to hold the fiber precisely in position
as it is translated. Figure 3.21 shows the overall fabrication equipment.
Other important issues are the smoothness of the fiber translation system
and the precise timing of the laser pulse. The former problem is overcome
by translating the fiber continuously at a constant speed on an air bearing,
during fabrication, using a linear motor capable of long translation (500
mm). The location of a point on the fiber carriage is measured continuously
by an interferometer, which is modulated by a Pockels cell. This tracking
interferometer has a resolution of —0.3 nm over the translation distance.
Part of the tracking signal is fed back to maintain a constant speed and
to compensate for vibrations. The position of the interferometer is fixed,
and the fiber is translated across the fringes, both backward and forward.
The process is entirely controlled by a computer, which is programmed
to generate a particular function in the firing sequence of the laser, move-
ment direction, and speed of translation.
The schematic of the printing is shown in Figure 5.7. Overlapping
fringes are shown in a sequence. The center shows the first printing of a
grating; the bottom shows the position of the second printing relative to
the first arranged to arrive just ahead of a fringe maximum already
printed by a distance + 8, and the top is the position of the third printing,
which arrives immediately after the second pulse to write on top of the
same fringe maximum but delayed by a distance —8. Also shown is the
fringe in the fiber core at the third pulse, being a combination of the
fringes due to the second and the third pulses only. As can be seen, the
fringe spreads symmetrically around the original maximum. By altering
the sequence of pulses, the fringes can be filled in so that the grating
gradually disappears toward the edges.
Using this method, unapodized gratings with a FWHM bandwidth
of 4.6 pm (L g = 200 mm) and apodised gratings with a bandwidth of 27
pm (L g = 50 mm) have been demonstrated [17], both close to theoretically
predicted values for low-reflectivity gratings (2-3%). The method natu-
rally allows any type of apodization to be programmed in.