3.5. LASER CHARACTERISTICS 113
Figure 3.22: Simulated modulation response of a semiconductor laser to 500-ps rectangular
current pulses. Solid curve shows the pulse shape and the dashed curve shows the frequency
chirp imposed on the pulse (βc=5).
side near the trailing edge of the optical pulse [71]. Such a frequency shift implies
that the pulse spectrum is considerably broader than that expected in the absence of
frequency chirp.
It was seen in Section 2.4 that the frequency chirp can limit the performance of
optical communication systems, especially whenβ 2 C>0, whereβ 2 is the dispersion
parameter andCis the chirp parameter. Even though optical pulses emitted from semi-
conductors are generally not Gaussian, the analysis of Section 2.4 can be used to study
chirp-induced pulse broadening [72] if we identifyCwith−βcin Eq. (2.4.23). It turns
out that 1.55-μm lightwave systems are limited to distances below 100 km even at a
bit rate of 2.5 Gb/s because of the frequency chirp [71] when conventional fibers are
used (β 2 ≈−20 ps^2 /km). Higher bit rates and longer distances can only be realized by
using a dispersion management scheme so that the average dispersion is close to zero
(see Chapter 7).
Since frequency chirp is often the limiting factor for lightwave systems operat-
ing near 1.55μm, several methods have been used to reduce its magnitude [73]–[77].
These include pulse-shape tailoring, injection locking, and coupled-cavity schemes. A
direct way to reduce the frequency chirp is to design semiconductor lasers with small
values of the linewidth enhancement factorβc. The use of quantum-well design re-
ducesβcby about a factor of about 2. A further reduction occurs for strained quantum
wells [76]. Indeed,βc≈1 has been measured inmodulation-dopedstrained MQW
lasers [77]. Such lasers exhibit low chirp under direct modulation. The frequency
chirp resulting from current modulation can be avoided altogether if the laser is contin-
uously operated, and an external modulator is used to modulate the laser output [78].
In practice, lightwave systems operating at 10 Gb/s or more use either a monolithically