Page 196 - Fiber Optic Communications Fund

Optical Modulators and Modulation Schemes 177

NRZ data, m I (t)
A m I (t)cos(2πf c t)
X
–A

~ cos(2πf t)
c
QPSK signal
+
–π/2 m I (t)cos(2π f c t) + m Q (t)sin(2π f c t)
NRZ data, m (t)
Q
sin(2πf t)
c
A
X
–A m (t)sin(2π f c t)
Q

Figure 4.48 Generation of QPSK signal using two BPSK signals.

Electrical message signal
m j (t)
Acos(2πf c t) Ap(t)cos(2πf c t)
Ap(t)cos[2π f t + k p m j (t)]
Pulse Optical c
Laser
carver phase modulator
Figure 4.49 Optical QPSK generation using a phase modulator.

101 1 1
m I (t) +1 t
–1
Ap(t)cos(2π f c t)/ 2 Ap(t)m I (t)cos(2πf c t)/ 2

Acos(2π f c t) Ap(t)cos(2π f c t) I-arm
MZM QPSK signal
3-dB coupler 3-dB coupler
Q-arm MZM
Laser Pulse carver
–π/2
Ap(t)cos(2π f c t)/ 2 Ap(t)m Q (t)sin(2πf c t)/ 2
000 1 0 Optical IQ modulator
+1
m Q (t) –1 t

Figure 4.50 Optical QPSK generation using two MZMs.