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158 Fiber Optic Communications

where P is the input power; the extinction ratio is
0
P max exp (− L)
0
= = . (4.72)
P min exp (− L)
1
To obtain the optimum performance, a high extinction ratio is desirable. To achieve this, typically InP-based
semiconductors are used for 1300-nm or 1550-nm applications.
The absorption coefficient can be changed significantly by applying a relatively lower driving voltage.
Therefore, the EAMs are very effective and the size could be quite small. The length of EAMs is typically
200 mm, whereas that of electro-optic modulators is a few centimeters. EAMs can easily be integrated with
the laser diode, since both are based on similar semiconductor materials. The drawbacks of EAMs are as
follows. (i) They have residual chirps similar to directly modulated lasers. The interaction of the chirp and
fiber dispersion could lead to enhanced pulse broadening. (ii) The extinction ratio is typically ≤ 10 dB, which
could lead to a power penalty [8].

4.7 Optical Realization of Modulation Schemes

4.7.1 Amplitude-Shift Keying
The optical ASK signal can be generated using a MZM, as shown in Fig. 4.19. The optical power of the MZM
output may be written as (Eq. (4.56))
[ ]
m(t) V bias
2
P = P cos − . (4.73)
out 0
V 2V

Let the message signal be a polar NRZ,
{
+V for bit ‘1’
m(t)= . (4.74)
−V for bit ‘0’

Voltage 1 10 1 0 1 NRZ data Power 1 10 1 0 1

t
t

ASK signal

Laser 1 1 0 1 0 1
Field
MZM
t

Figure 4.19 Generation of ASK signal using a MZM.