7.2. PRECOMPENSATION SCHEMES 285
Figure 7.3: Streak-camera traces of the 16-Gb/s signal transmitted over 70 km of standard fiber
(a) with and (b) without SOA-induced chirp. Bottom trace shows the background level in each
case. (After Ref. [26];©c1989 IEE; reprinted with permission.)
10-Gb/s signal could be transmitted over distances 30 to 40 km longer by replacing
binary coding with duobinary coding [19]. The duobinary scheme can be combined
with the prechirping technique. Indeed, transmission of a 10-Gb/s signal over 160 km
of a standard fiber has been realized by combining duobinary coding with an external
modulator capable of producing a frequency chirp withC>0 [19]. Since chirping in-
creases the signal bandwidth, it is hard to understand why it would help. It appears that
phase reversals occurring in practice when a duobinary signal is generated are primarily
responsible for improvement realized with duobinary coding [20]. A new dispersion-
management scheme, called thephase-shapedbinary transmission, has been proposed
to take advantage of phase reversals [21]. The use of duobinary transmission increases
signal-to-noise requirements and requires decoding at the receiver. Despite these short-
comings, it is useful for upgrading the existing terrestrial lightwave systems to bit rates
of 10 Gb/s and more [22]–[24].
7.2.3 Nonlinear Prechirp Techniques.................
A simple nonlinear prechirp technique, demonstrated in 1989, amplifies the trans-
mitter output using a semiconductor optical amplifier (SOA) operating in the gain-
saturation regime [25]–[29]. As discussed in Section 6.2.4, gain saturation leads to
time-dependent variations in the carrier density, which, in turn, chirp the amplified
pulse through carrier-induced variations in the refractive index. The amount of chirp
is given by Eq. (6.2.23) and depends on the input pulse shape. As seen in Fig. 6.8, the
chirp is nearly linear over most of the pulse. The SOA not only amplifies the pulse
but also chirps it such that the chirp parameterC>0. Because of this chirp, the input
pulse can be compressed in a fiber withβ 2 <0. Such a compression was observed in
an experiment in which 40-ps input pulses were compressed to 23 ps when they were
propagated over 18 km of standard fiber [25].
The potential of this technique for dispersion compensation was demonstrated in
a 1989 experiment by transmitting a 16-Gb/s signal, obtained from a mode-locked