Biomedical Engineering Reference
In-Depth Information
improve a hybrid opto-electronic processor speed by a factor of 100-1000. The
following examples are representative of this emerging device technology.
3.10.1 Introduction
All optical signal processing systems for both optical communications and
optical computing have tremendous potential for dramatically improving
the information handling capacity and data rate in various signal process-
ing systems by keeping the information in optical form during most of the
processing path [33]. Major improvements in real-time signal analysis can
be obtained through the implementation of all-optical circuits due to the
advantage of low power consumption, high speed, and noise immunity.
Additionally, all optical systems lend themselves directly to implementation
in highly parallel, high throughput architectures.
Dynamic nonlinear optical processes lie at the heart of these optical logic
devices. The index of refraction of a semiconductor can be significantly var-
ied through the creation of electrons, holes, and excitons or by exposure to
high intensities of light. Changes in these properties can induce absorptive
or dispersive bistability in the semiconductor system [34]. A system is said to
be optically bistable if it has two output states for the same value of the input
over some range of input values. Relaxation properties of the excited semi-
conductor state dictate the characteristics of these optically induced optical
changes.
Gibbs et al. [35] in 1979 were among the first to apply excitonic effects to
optical bistability in GaAs. Bistability was demonstrated just below the exci-
ton resonance in a cryogenically cooled, 4 μm thick, GaAs sample utilizing
approximately 200 mW of optical power. The device exhibited a switch-off
time of approximately 40 ns. This bistability was due to the change in refrac-
tive index resulting from nonlinear absorption in the vicinity of the exciton
resonance. One limitation to further progress in this area was that excitonic
resonances are generally not seen in room-temperature direct-gap semicon-
ductors due to temperature broadening effects.
It has been found, however, that multiple quantum well (MQW) structures
show strong exciton resonances at room temperature due to the QW con-
finement and consequential increase in binding energy of the excitons [36].
These resonances show saturation behavior similar to that observed at low
temperature in conventional semiconductors. As such, the MQW structure
represents a preeminent class of device to be researched for implementation
in optical logic function units.
Bulk and integrated bistable devices have demonstrated the ability to
perform a number of logic functions [37]; however, various device param-
eters such as the physical size, power requirements, and speed and tem-
perature of operation have generally proved to be impractical. Two devices
that show promise as optical processing components are the Double-Y
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