Page 199 - Fiber Optic Communications Fund

180 Fiber Optic Communications

A 2
A 1
Unipolar NRZ data
NRZ data
m 1
m 2 m 3

QPSK
Laser PM MZM
modulator
QPSK 8-PSK 16 QAM

Figure 4.54 Schematic of 16-QAM generation using a QPSK modulator.

4-ASK signal
m I (t)

Ap(t)cos(2π f c t)/ 2

Acos(2π f t) Ap(t)cos(2π f t) I-arm
c
c
MZM
QAM signal
3-dB coupler 3-dB coupler
Q-arm MZM
Laser Pulse carver
–π/2
Ap(t)cos(2π f c t)/ 2
Optical IQ modulator
m Q (t) t
4-ASK signal

Figure 4.55 16-QAM generation using an optical IQ modulator.

which acts as an amplitude modulator. The bias conditions for a MZM to act as an amplitude modulator are
discussed in Section 4.6.2.2. When the message signal m = A and A (> A ) volts, we obtain the inner and
3 1 2 1
outer circles of the constellation, respectively.
A QAM signal may be interpreted as a signal obtained by simultaneous amplitude modulation of in-phase
and quadrature carriers (see Eq. (4.113)). This suggests that a QAM signal can be generated using an optical
IQ modulator as shown in Fig. 4.55 [15]. To obtain a square 16-QAM signal, 4-ASK electrical signals are
used to drive the optical IQ modulator, as discussed in Section 4.9.2.

Example 4.3 A MASK signal transmitted over the fiber channel
The driving voltage to the MZM is adjusted such that the optical field amplitudes are equally spaced. The
MZM output in a symbol slot 0 < t < T is
s
√
A = P (2m + 1), (4.117)
out 0