122 CHAPTER 3. OPTICAL TRANSMITTERS
Figure 3.27: Two kinds of external modulators: (a) LiNbO 3 modulator in the Mach–Zehnder
configuration; (b) semiconductor modulator based on electroabsorption.
temperature. The threshold current also increases with aging of the transmitter because
of gradual degradation of the semiconductor laser.
The driving circuit shown in Fig. 3.26 adjusts the bias level dynamically but leaves
the modulation current unchanged. Such an approach is acceptable if the slope ef-
ficiency of the laser does not change with aging. As discussed in Section 3.5.1 and
seen in Fig. 3.20, the slope efficiency of the laser generally decreases with an increase
in temperature. A thermoelectric cooler is often used to stabilize the laser tempera-
ture. An alternative approach consists of designing driving circuits that usedual-loop
feedbackcircuits and adjust both the bias current and the modulation current automat-
ically [112].
3.6.3 Optical Modulators
At bit rates of 10 Gb/s or higher, the frequency chirp imposed by direct modulation
becomes large enough that direct modulation of semiconductor lasers is rarely used.
For such high-speed transmitters, the laser is biased at a constant current to provide the
CW output, and an optical modulator placed next to the laser converts the CW light
into a data-coded pulse train with the right modulation format.
Two types of optical modulators developed for lightwave system applications are
shown in Fig. 3.27. The electroabsorption modulator makes use of the Franz–Keldysh
effect, according to which the bandgap of a semiconductor decreases when an electric
field is applied across it. Thus, a transparent semiconductor layer begins to absorb
light when its bandgap is reduced electronically by applying an external voltage. An
extinction ratio of 15 dB or more can be realized for an applied reverse bias of a few
volts at bit rates of up to 40 Gb/s [113]–[120]. Although some chirp is still imposed
on coded pulses, it can be made small enough not to be detrimental for the system
performance.
An advantage of electroabsorption modulators is that they are made using the same
semiconductor material that is used for the laser, and thus the two can be easily inte-