Page 107 - Fiber Bragg Gratings

86 Chapter3 Fabrication of Bragg Gratings

although the quality of the grating was not perfect. It is important to
eliminate stitching errors between the imprinted fields, as in the case of
the phase mask, requiring a positional accuracy of better than 0.1 A over
the length of the grating. As an example, this implies maintaining an
overall positional accuracy of less than 50 nm over the entire length of
the 200-mm long grating, a demanding task.
Figure 3.21 shows the apparatus used for writing long gratings by
the multiple printing in fiber (MPF) technique [73]. The fiber is held in
a glass V-groove along its entire length and translated along the pulsed
interference fringes in sychronym with the pulses. By moving the fiber
at a constant velocity with a linear motor, the vibrations common in
stepper-motor-driven systems are eliminated. Mounting the fiber carriage
on an air bearing further helps this. The critical features of the technique
are the requirement of a long precision glass V-groove to hold the fiber
in position with submicron accuracy, high beam quality of the pulsed
laser, an accurate control system for fiber translation, and above all,
stability of the pulsed source for imprinting gratings of a well-determined
index of modulation. Since the fabrication process imprints overlapping
gratings, it is possible to change the period or the modulation index locally
or continuously along the length of the grating. A detail of the printing
process is shown in Fig. 3.22. This system is flexible and is able to cater
for any type of grating, including those with phase steps, chirp, apodiza-
tion (see Section 3.1.9 and Chapter 5).
A slight modification of the MPF scheme is shown in Fig. 3.23. Here
not only the fiber is allowed to move, but also the interferometer [491. In
this case, the interferometer is the phase mask that moves at a velocity

Figure 3.21: The multiple printing in fiber (MPF) grating technique. The
substantial carriage is potentially capable of movements of up to 1 meter (courtesy
R. Stubbe).