Page 369 - Fiber Bragg Gratings

346 Chapter 7 Chirped Fiber Bragg Gratings

Figure 7.30: The received signal eye diagram at a transmission bit rate of
40 Gb/sec (1550 nm) after 80 km of optical standard fiber and a 200-mm-long
unapodized SSCG dispersion compensating grating (courtesy D. Nesset, BT Labo-
ratories).

7.6 Other applications of chirped gratings

As we have seen, pulses propagating in a fiber are broadened by disper-
sion. There is a broadening if the pulse is not transform limited and it is
chirped. Thus, the individual parts of the spectrum arrive at different
times and can skew the pulse. If the chirp is a time-varying function,
then jitter is also introduced. On the other hand, in a dispersion-free
system, neither the chirp introduced by any component nor the source
bandwidth causes the pulse to be broadened. If a chirped grating compen-
sates for the dispersion of a fiber, then the dispersion and jitter induced
in chirped pulses is automatically compensated [75]. Further, pulses can
be compressed or dispersed depending on the sign of the chirp, by launch-
ing the pulse in a fiber with either anomalous or normal dispersion. If a
pulse is chirped, then it is possible to compensate for the chirp using a
chirped fiber grating and to compress the pulse [14,76-80]. The grating may
also be used for nonlinear pulse compression [81] of pulses that have been
spectrally broadened and linearly chirped through self-phase modulation
(SPM) and propagation in normally dispersive optical fiber [82].