Page 176 - Fiber Optic Communications Fund

Optical Modulators and Modulation Schemes 157

4.6.2.3 Electroabsorption Modulator (EAM)
Electroabsorption refers to the dependence of the absorption coefficient of a semiconductor on the applied
electric field. The band gap of a semiconductor decreases as the applied field increases. Consider an optical
carrier with frequency f < E ∕h, where E g0 is the band gap in the absence of the applied electric field. Let
g0
c
the driving voltage V(t) vary from 0 to V volts, as shown in Fig. 4.17. When V(t)= 0, the photon energy
0
is less than the band gap and the optical carrier is not absorbed. When V(t)= V , the band gap decreases.
0
Now the photon energy could be larger than the band gap and the optical carrier is absorbed, which generates
electron–hole pairs. Thus, the information in the electrical domain is translated into the optical domain.
Fig. 4.18 shows the typical dependence of the absorption coefficient on the wavelength. Suppose the carrier
wavelength is .Let and be the absorption coefficients at V(t)= 0 and V(t)= V , respectively. If L is
1
0
0
0
the length of the modulator, the optical power exiting the modulator is
{
P max = P exp (− L) when V(t)= 0
0
0
P out = (4.71)
P = P exp (− L) when V(t)= V
min 0 1 0
V(t)
V(t)
V 0
t
Light signal EA
modulator

Field envelope

t

Figure 4.17 Amplitude modulation using an EAM.

Absorption coefficient α 1 V = 0 V = V 0

α 0
Wavelength, λ
λ 0
Figure 4.18 Typical dependence of absorption coefficient on wavelength.