Page 249 - Introduction to Information Optics

234 4. Switching with Optics

of about 850 nm, in the near-infrared region of the spectrum. At this wave-
length, the AlGaAs material is transparent, so there is no optical loss in the
AlGaAs contact regions. GaAs itself is opaque at these wavelengths, so the
substrate has to be removed to make a transmission modulator. Devices can
be made with various other semiconductor materials for various operating
wavelengths in the infrared region, and some devices have been demonstrated
in the visible region.
In a typical transverse modulator, there are about 50 to 100 quantum wells.
This gives a total thickness of roughly i micron for the quantum-well region.
The entire diode structure will be a few microns thick altogether. A typical
device might have an operating voltage of 2-10 V. An important parameter
characterizing the behavior of this device as an intensity modulator is the
on off ratio R on- 0{ f, expressed as

Kon-off = exp(A«L), (4.43)
where Aa is the maximum achievable change in the absorption coefficient and
L is the light path length through the electroabsorptive material. The values of
5
Aa for MQW modulators are on the order of 10 /cm. Within these parameters,
the optical transmission of the modulator might change from 50 to 20% as the
voltage is applied.
Sometimes, it is more convenient to make a modulator that works in
reflection rather than transmission. This can be done by incorporating a mirror
into the diode structure, so that the light reflects back through the quantum
wells. The advantage of this is that the light makes two passes through the
quantum-well material, which increases the contrast of the optical modulation.
Such reflective modulation is also convenient when mounting the devices on
electronic chips, since it allows conventional chip mounting without having to
make transparent electronic chips and mounts.
Despite the large value of Aa available in MQWs, only limited on-off ratio
values are achieved in transverse modulator geometry, because of the limited
interaction length. The best way to increase the effective interaction length is
to use a waveguide modulator, shown in Fig. 4.27. The propagation direction
of the light lies in the plane of the MQW layers, which is part of the optical
waveguide. The interaction length can be as long as necessary to obtain the
desired on-off ratio. Since the quantum wells usually represent only a fraction
of the overall waveguide, and the rest of the waveguide is composed of material
without electroabsorption effect, the on-off ratio must be modified as follows:

Kon-off = exp(FAaL), (4.44)

where F is the overlap between the quantum wells and the optical mode of the
waveguide, the value of which is in the range 0.01-0.3 for typical waveguide
modulators.