240 CHAPTER 6. OPTICAL AMPLIFIERS
Figure 6.8: Frequency chirp imposed across the amplified pulse for several values ofEin/Esat.A
Gaussian input pulse is assumed together withG 0 =30 dB andβc=5. (After Ref. [19];©c 1989
IEEE; reprinted with permission.)
by a SOA. The temporal and spectral changes depend on amplifier gain and are quite
significant forG 0 =30 dB. The experiments performed by using picosecond pulses
from mode-locked semiconductor lasers confirm this behavior [18]. In particular, the
spectrum of amplified pulses is found to be shifted toward the red side by 50–100 GHz,
depending on the amplifier gain. Spectral distortion in combination with the frequency
chirp would affect the transmission characteristics when amplified pulses are propa-
gated through optical amplifiers.
It turns out that the frequency chirp imposed by the SOA is opposite in nature com-
pared with that imposed by directly modulated semiconductor lasers. If we also note
that the chirp is nearly linear over a considerable portion of the amplified pulse (see
Fig. 6.8), it is easy to understand that the amplified pulse would pass through an initial
compression stage when it propagates in the anomalous-dispersion region of optical
fibers (see Section 2.4.2). Such a compression was observed in an experiment [19] in
which 40-ps optical pulses were first amplified in a 1.52-μm SOA and then propagated
through 18 km of single-mode fiber withβ 2 =−18 ps^2 /km. This compression mecha-
nism can be used to design fiber-optic communication systems in which in-line SOAs
are used to compensate simultaneously for both fiber loss and dispersion by operating
SOAs in the saturation region so that they impose frequency chirp on the amplified
pulse. The basic concept was demonstrated in 1989 in an experiment [20] in which a
16-Gb/s signal was transmitted over 70 km by using an SOA. In the absence of the
SOA or when the SOA was operated in the unsaturated regime, the system was dis-
persion limited to the extent that the signal could not be transmitted over more than
20 km.
The preceding analysis considers a single pulse. In a lightwave system, the signal