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162 Fiber Optic Communications

MZM field transmission A Field envelope 0 1 0 1 1 1

–V –V /2 V /2 V π Message voltage t
π
π
π
–A 0

1
0
1
1
1
t
Figure 4.23 MZM field transmission. V = V .
bias

Table 4.3 Voltage levels of the message signal.
Binary data 1 0 1 1 1
m(t) V ∕2 −V ∕2 V ∕2 V ∕2 V ∕2

Optical phase 0 0 0 0
pulses is a constant power signal and there is a reversal of phase at the bit boundaries when the data changes
from ‘0’ to ‘1’ and vice versa. RZ-PSK can be generated using a RZ pulse carver in series with a MZM,
similar to RZ-ASK.
PSK can also be generated using a phase modulator. When the message signal corresponds to bit ‘1’, the
phase modulator provides no phase change and when it corresponds to bit ‘0’, the phase modulator changes
the carrier phase by (see Table 4.3). However, the performance characteristics of a phase-modulator-based
PSK are worse than those of a MZM-based PSK [9]. This is because, in the case of the MZM, imperfections
in driving conditions are translated into optical power variations, but the information-bearing optical phase
is intact, whereas when a phase modulator is used, waveform imperfections distort the optical phase as well,
which degrades the performance [9].

4.7.3 Differential Phase-Shift Keying
The DPSK signal generation is analogous to PSK generation except for a precoder which provides differential
coding as discussed in Section 4.5.4. The differentially encoded signal m(t) is used to drive a PM or a MZM
as shown in Fig. 4.24.