Page 365 - Fiber Bragg Gratings

342 Chapter 7 Chirped Fiber Bragg Gratings

Figure 7.25: The reflectivity of a single and 10 cascaded, 100-mm-long hyper-
bolic tanh apodized gratings (chirped bandwidth of 0.75 nm and peak-peak refrac-
5
tive index modulation of 7.5 X 10~ ).

also has to be made for drift of the grating wavelengths and the signal
source.
For the GDR, cascading of the gratings may have a beneficial effect
if the ripple cancels; alternatively, it may increase where it is in phase.
This is especially important for unapodized or imperfectly apodized grat-
ings. Some of the large-amplitude high-frequency GDR generated in a
cascade of identically apodized gratings is reduced with random variations
in the grating profiles. Figure 7.26 shows the relative GDR of a cascade
of three identical gratings of type B (in Fig. 7.15) and a for a cascade of
all three gratings, B, C, and D.

7.5.1 Systems simulations and chirped grating
performance
The theoretical aspects of DCG in systems applications have been consid-
ered by the several workers [30,66-70]. Of the many indicators of the
performance of a grating, the receiver eye penalty is probably the most
significant. As a number alone, it is not very useful, since the properties
of the DCG are not constant across the bandwidth. One clearly needs to
know the effect of the insertion loss, the change in the reflected power as
a function of detuning, and the influence of the nonlinear dispersion.