Page 246 - Introduction to Information Optics

4.3. Fast Electro-optic Switches: Modulators 231

1000
3
E = 0 E=10 V/cm
100

10 -
0.8 0.9 1.0
Wavelength dim)
Fig. 4.23. Absorption spectra for a GaAs sample with and without an external field.

Over a range of wavelengths, the absorption coefficient increases from 10 cnT ]
3
to over 10 cm" ^ An optical signal at wavelength A O (A O = 0.9 fim in Fig. 4.23)
will experience a significant change in absorption when an external field is
applied. The total change in the transmitted signal depends on the change in
the absorption coefficient and the path length through the modulator. Note
that the electroabsorptive effect associated with the FK mechanism in bulk
materials is usually quite limited because the large electric field leads to the
smearing out of the absorption edge as electron and hole wave functions have
diminished spatial overlap with increasing field [30].

4.3.2.2.1. Multiple-Quantum-Well Modulator
The multiple-quantum-well (MQW) modulator is based on the QCSE in
quantum wells (QW) [31, 32, 33]. The EA modulator consists of MQW layers
and, when unperturbed, it is transparent to the light signal. However, under
proper electrical bias, the quantum-well and barrier energies are distorted so
that absorption rises sharply at energies just above the MQW band gap.
Optical absorption change in the quantum well via QCSE is relatively large.
In practice this means we can make small and efficient optical modulators
using quantum wells. The QCSE, like other electroabsorption mechanisms in
semiconductors, is very fast. In fact, there are no intrinsic speed limitations on
the mechanism itself until time scales well below a picosecond. In practice,
speed is limited only by the time taken to apply voltage to the quantum wells,
which is typically limited by resistor-capacitance limits of the external circuit.
There are two commonly used schemes for MQW modulators. One is the
transverse (also called surface-normal) modulator, where light comes in and
out of the surface of a semiconductor chip. The other is the MQW waveguide
modulator. Using a surface-normal scheme, it is possible to make two-dimen-
sional arrays of quantum-well optical modulators.