Page 163 - Fiber Optic Communications Fund

144 Fiber Optic Communications

4.5 Digital Modulation Schemes
4.5.1 Amplitude-Shift Keying

A laser is an optical carrier whose amplitude and/or phase can be varied in accordance with a message signal
by means of an optical modulator. Let the laser output be (Fig. 4.5(a))

c(t)= A cos (2f t + ). (4.21)
c
In Eq. (4.21), the amplitude A, frequency f , and phase factor are constants. When the amplitude A is varied
c
in accordance with a message signal m(t) while keeping f and constant, the resulting scheme is known as
c
amplitude modulation. Suppose the amplitude is proportional to the message signal m(t),
A(t)= k m(t), (4.22)
a
where k is amplitude sensitivity. Now, the carrier is said to be amplitude modulated. The modulated signal
a
can be written as
s(t)= k m(t) cos (2f t + ). (4.23)
a c
When the message signal m(t) is a digital signal, such as shown in Fig. 4.5(b), the modulation scheme is
known as amplitude-shift keying (ASK) or on–off keying (OOK). In general, to transmit bit ‘1’, a sinusoid of
certain amplitude A is sent and to transmit bit ‘0’, a sinusoid of amplitude A is sent.
1
2

4.5.2 Phase-Shift Keying
When the phase of the carrier is varied in accordance with the message signal m(t) while keeping the
amplitude A and frequency f constant, the resulting scheme is known as phase modulation. Suppose the
c
phase is proportional to the message signal,
(t)= k m(t), (4.24)
p
c(t)

Optical carrier
t

m(t) 1 1 0 1 (a) 0 1 1 1

Message signal

0
(b) t
s(t)
Modulated signal
t
(c)

Figure 4.5 Modulation of the optical carrier by digital data: (a) carrier, (b) data, and (c) modulated signal.