All-Optical Frequency Shifting - Optical Signal Processing by Silicon Photonics - page 83

based on silicon and using concentrations of free-carriers have been demonstrated
which require high powered pump light beams to gain modulation [ 20 - 23 ]. The
reason behind this phenomenon is the weak complex-refractive-index drift from
the mean position as a function of concentration of free carriers [ 24 ]. However, the
use of resonators having strong optical light signal confinement can alleviate the
manufacturing of these devices.
The optical modulators at bit rates of 10 Gb/s or higher, the frequency chip
imposed by direct modulation becomes large enough that direct modulation of
semiconductor lasers is rarely used. For such high speed transmitters the laser is
biased at a constant current to provide the CW output [ 25 ]. The optical modula-
tor is placed next to the laser converter, the CW light into a data-coded pulse train
with the right modulation format.
The two types of optical modulators developed for the light wave system appli-
cations are:

1. LiNbO 3 modulator in the Mach- Zehnder configuration and
2. Semiconductor modulator-based on electro absorption.

(a)

Electrical Contacts

Slab Waveguide

Metal Strips

Input Signal

Output Signal

Electrical Contacts

LiNiO 3

(b)

p-contact

n-type substrate

Fig. 7.3 a LiNbO 3 modulator in the Mach-Zehnder configuration (external modulator),
b Semiconductor modulator based on electro-absorption [ 27 ]