Biomedical Engineering Reference
In-Depth Information
Mach-Zehnder Interferometric Optical Logic Gate (Double-Y) and the
MQW Oscillator.
3.10.2 Interferometer and Quantum Well Devices
Individual MZIs have been demonstrated in various materials and have
been used as modulators and switches [38]. The MQW Oscillator makes
use of the nonlinear optical effects unique to the MQW structure. Both the
Double-Y, which is an extension of the MZI concept, and the MQWO devices
lend themselves directly to fabrication in GaAs material. Both devices can
overcome the limitations of speed and operational temperature mentioned
previously. In its present configuration, the Double-Y is about 2 cm long and
requires rather large optical powers for switching. On the other hand, the
MQW is already a small, low-power device.
The Double-Y and MQW oscillator devices utilize a total of three optical
effects to perform their logic and switching functions. These are the linear
electro-optic, or Pockels effect, the nonlinear index of refraction, and the
Quantum Confined Stark Effect (QCSE). Derivations of the first two effects
are included at the end of Chapter 2. The Double-Y device has been studied
and fabricated in LiNbO 3 [39] and in AlGaAs [40]. The MQW Oscillator can
be implemented in several forms, one of which has been studied by Wood
et al. [41].
A schematic diagram of the Double-Y device is shown in Figure 3.16. The
signal of interest is input at the center or control leg. The signal splits at the
Y and then recombines. The output depends on how this signal recombines.
If the path lengths down the two legs are identical (or differ by an integral
number of wavelengths) then the control signal recombines in phase and is
referred to as a “one.” If the path lengths differ by an integral number of half
wavelengths, the signal recombines destructively, resulting in a “zero” output.
The Double-Y device utilizes two nonlinear optical effects to change the
path length of either or both legs and thus change the output state between
“one” and “zero.” The Pockels effect provides a phase shift proportional to an
electric field applied across the waveguide. This field is supplied by applying
a voltage to the electrodes over the waveguide. The circuit is completed with
a ground plane below the guide. These contacts are indicated in Figure 3.16.
In the Double-Y device, this effect is used to tune the recombination of the
signal. Tuning is needed to correct for any differences in the physical lengths
of the two legs and to set the initial output state of the device to either “one”
or “zero.” Calculations indicate that for the geometry shown in Figure 3.16,
a phase change of 180° can be obtained by applying a potential of approxi-
mately 1.8 V.
The other effect used to change the path length is the nonlinear or inten-
sity dependent index of refraction. By coupling a high intensity of light into
either of the signal legs, “A” or “B,” the path length of the control pulse will
be changed. It is seen, therefore, that after setting the initial state by using
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