Page 242 - Fiber Bragg Gratings

5.2 Basic principles and methodology 219

The apodization scheme is independent of the length of the grating,
the only requirement being that the fiber be stretched by half-a-period
in each direction, so that for a chirped grating, one end of the fiber is
stretched slightly more than the other, by adjusting the stretch on that
side. For uniform period gratings, no adjustment is necessary when chang-
ing the wavelength of the phase mask in the same spectral window (e.g.,
1500 nm). Figure 5.16 shows the experimental setup of the equipment
used in SAM.
The required stretch to form perfectly apodized gratings as a function
of length is shown in Fig. 5.17. Even for relatively short gratings, the
strain is easily applied. Another possibility with this method is to write
two gratings under static strain to form moire gratings.
By switching off one stretcher, the grating will be apodized only on
the stretched side. As a result, left- and right-hand-end apodization may
be performed independently. For super-step-chirped gratings [26], this
feature allows apodization of each end of the grating. For the first, short-
est-wavelength grating, the short-wavelength end is apodized; other inter-
mediate gratings are printed sequentially without apodization, except
for the last, longest-wavelength grating, in which the right-hand, long-
wavelength end is apodized by switching on the RH piezoelectric stretcher.

Figure 5.16: The symmetric stretching apodization method (SAM). The
phase mask may be replaced by any interferometer. The displacement of the
piezoelectric stretchers is monitored by position sensors to set the required stretch
[25].