Page 225 - Fiber Bragg Gratings
P. 225
202 Chapter 5 Apodization of Fiber Gratings
Figure 5.4: The self-apodized fringe profile of a two-wavelength source. The
parameters have been chosen for illustration purposes only [9]. The grating length
must be chosen to exactly match a single period of the envelope. Since this process
has not modified the beam intensity profile, the change induced in the effective
index of the mode by the UV dose is the same all along the fiber.
2
There are other methods of generating moire patterns for apodiza-
tion. An optical wedge placed in the path of one of the beams of the UV
interferometer will change the Bragg wavelength of the grating being
written. Removal changes the Bragg wavelength; if the wedge angle is
chosen such that the wavelength difference between the two Bragg periods
is exactly one period more or less over the length of the grating, then
apodization will occur. In this case, the difference in the Bragg grating
period is given by
The wedge required for this purpose must impart a TT phase change
from one end of the grating to the other. Thus, for ~10-mm-long gratings,
a wedge angle of the order of 5' to 10' of arc is required at a Bragg
wavelength of ~1550 nm [9].
Now that the principles have been set out whereby apodization can
be performed without altering the intensity of illumination across the
length of the grating, other practical implementations are discussed in
the following sections.
2
Etymology: Fr., watered silk; referring to pattern formed on it.