Page 368 - Fiber Bragg Gratings

7.5 Systems measurements with DCGs 345

We note that at the longer-wavelength end of the unapodized grating,
the eye penalty fluctuates because of the GDR frequency becoming closer
to the transmission frequency, since the eye closes as a result of the
additional dispersion. This becomes less important for 40 Gb/sec, since
more of the grating bandwidth is being used for dispersion compensation,
and the fluctuations on the long-wavelength side also become smaller.
This result has been further investigated by Garthe et al. [73] for long
gratings. The conclusions are similar, in that the eye penalty has a maxi-
mum value depending on the position in the bandwidth, because the GDR
frequency induces satellite pulses that may coincide with adjacent time
slots.
Figure 7.29 shows the reflection and relative group delay of a 200-mm-
long SSCG made with two 100-mm-long chirped gratings. Low-repetition-
rate ultrahigh-speed measurements performed on this grating with a
dispersion of 1310 psec/nm and bandwidth of 1.5 nm at the output of 77
km of standard telecommunications fiber [74] shows that a 3.8 psec input
pulse sits on a wide low-level pedestal at the output. The pedestal, which
is limited to approximately two bit periods, has the detrimental effect of
causing the eye to close slightly when the entire bandwidth of the grating
is used. However, systems transmission experiments performed on the
same grating at 40 Gb/sec over 77 km of standard fiber indicate that the
received eye remains open, as shown in Fig. 7.30, which without the DCG
is completely closed.

Figure 7.29: The reflectivity and relative group delay of a 200-mm SSCG
(dispersion of 1310 psec/nm) [74].