Page 359 - Fiber Bragg Gratings

336 Chapter 7 Chirped Fiber Bragg Gratings

The computed reflectivity and group delay of a 1.3-meter-long SSCG
[48] with a bandwidth of 10 nm and no stitching errors are shown in
Fig. 7.22a. The overall reflectivity is similar to shorter gratings, but as
the bandwidth is increased, the shorter wavelength end group delay suf-
fers less from the broadband reflection of the long-wavelength edge of the
grating. Note that in this simulation, the resolution of the computation
(1 pm) has lost the information on the group delay ripple. Figure 7.22b
shows a high-resolution simulation (0.1 pm) of the reflectivity and group
delay, to highlight the GDR of the first part of a 1-meter-long unapodized
grating. A detail of the group delay ripple of ~ ±60 ps is shown in Fig.
7.22c.
Long gratings with only a small amount of apodization on the long-
wavelength end (normally the input end for dispersion compensation) will
remove a substantial part of the group delay ripple and should be useful
for transmission rates in excess of 10 Gb/sec.

7.4 Polarization mode dispersion in
chirped gratings

An issue that becomes important at high transmission bit rates (>10 Gb/
sec) is the effect of polarization mode dispersion (PMD). As the transmis-
sion rate increases, the bit period reduces. If any component in the trans-
mission path is birefringent, the different pulse arrival times of the two
polarizations can degrade the BER. In long transmission systems, these
two polarization mix stochastically, so that pulse broadening is not easy
to compensate [49]. In a short grating component, however, PMD is not
generally large in transmission, since the pulse arrival times are simply
due to the difference in the propagation constants of the two polarization
states times the length of the grating. In a chirped dispersion compensat-
ing reflection grating (DCG), the effect of birefringence is more severe,
causing a large additional dispersion. PMD, or more correctly, birefrin-
gence induced PMD, in unapodized gratings is more of a nuisance than
in unapodized gratings.
In order to assess the impact on the PMD of birefringence in a fiber,
whether intrinsic or due to the process of fabrication of a grating, we
examine how the Bragg wavelength of a grating is affected by a change