Page 245 - Introduction to Information Optics

230 4. Switching with Optics

3 dB Coupler

Fig. 4.22. Top view of a Mach- Zehnder interferometric intensity modulator.

modulators operating at various wavelengths with a bandwidth of up to several
tens of GHz and an operating voltage of a few volts have been demonstrated
[28]. High-speed EO modulators using superconducting electrodes have also
been also studied [29].
Although LiNbO 3 modulators have been successful in some practical
applications, they have several limitations. The main disadvantage is the
devices' strong polarization dependence, since light with different polarizations
will "see" different elements of the EO coefficients. This means that a simple
LiNbO 3 cannot simultaneously switch inputs with different polarizations.
Other disadvantages include limited optical bandwidth, difficulty in integration
with semiconductor lasers and amplifiers, and periodic light output versus
control voltage.

4.3.2.2. Electroabsorptive Modulator
Another way to modulate an optical field with an electric field is through
electroabsorption in semiconductors. With proper design, this type of modula-
tor can be polarization insensitive. These modulators are based on effects of
absorption change induced by an external field. Most commonly used effects
are the Franz-Keldysh effect and the quantum-confined Stark effect (QCSE).
The FK effect can occur in all semiconductors; the QCSE occurs only in
quantum-well (QW) semiconductors.
In the FK effect, the absorption edge of a semiconductor shifts towards the
long-wavelength direction in the presence of an electric field. Figure 4.23 shows
the absorption spectrum for a semiconductor sample with and without an
applied field. Without the field, the absorption coefficient shows the typical
increase for optical energies that equal or exceed the bandgap of the material.