Page 239 - Fiber Bragg Gratings

216 Chapter 5 Apodization of Fiber Gratings

Figure 5.14: Reflectivity and delay characteristics of the chirped moire grat-
ing (from: Ibsen M., Durkin Michael K., and Laming R. L, "Chirped Moire gratings
operating on two-wavelength channels for use as dual-channel dispersion compen-
sators," IEEE Photon. Technol. Lett. 10(1), 84-86, 1998.)

the average index remains unchanged. This prediction has not yet been
confirmed. In order to avoid this problem, it is necessary to choose the
bandwidth of the grating and the envelope period judiciously.

5.2.7 The symmetric stretch apodization method
The methods for apodization that are most flexible also require active
management under the control of a computer. This inevitably means
synchronization of the grating inscribing UV pulse and the position of
the fiber (cf. MPF as well as the MPM methods). While the flexibility is
desirable for a number of applications, simpler methods such as the apod-
ized phase mask are intrinsically faster and probably better suited to
mass production. However, as has been discussed, the apodized phase
mask is fragile in its reported implementation and perhaps less predict-
able in fabrication. It also has the severe limitation of allowing an apodized
grating that is only as long as the phase mask. This requires a selection of
phase masks, unless a tunable interferometer is used; this, unfortunately,
counters the argument for using the phase mask, since it defines the