Optical Devices
An Overview of Terms from Optical
Computing
Review of processors
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All computer operations can be constructed from
series of on/off gates
MOSFET allows “large” current when small
voltage applied to gate; no current when no
voltage applied to gate
This NON-LINEAR effect is necessary for fast
operations
Must have one signal affecting output state (other
signal)
How is Light Better?
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FASTER
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COOLER (transfer, not nec. processing)
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nothing can travel faster than light in vacuum
even in materials, accelerating electrons to nearlight speeds requires much energy
Less loss from scattering as light travels through
fibers than electrons through wires
FOURIER TRANSFORMS
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Light traveling through a lens performs a Fourier
transform automatically
Can Light Do Logic?
Yes - e.g., the Fabry-Perot interferometer
 Need non-linear optical material
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material with optical properties (e.g., index
of refraction) depending on intensity
 small change in intensity must produce
large change in output
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Utilize properties of interference and
standing waves
Fabry-Perot Interferometer
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Ends of cavity like open ends of string: wave not
inverted when it is reflected
Standing wave set up if cavity length integer
number of half-wavelengths
Can’t just change frequency, since that affects
other devices too
More Fabry-Perot
Interferometer
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Index of refraction determines wavelength
Intensity affects index of refraction
If intensity inside cavity high enough, wavelength
will change - from destructive to constructive
This is a resonant process - a large effect occurs
very quickly
Can amplify a signal by keeping a constant
intensity near the critical value
Advantages of FP
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MULTI-FUNCTION - same device can be
NOT – high constant signal in resonance –
input takes over resonant region (?)
 AND – low constant signal – need both
inputs to produce resonance
 OR – medium constant signal - either input
strong enough to produce resonance
 Amplifier – medium constant signal – small
input leads to resonance
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Why Aren’t Fabry Perot
Devices Front Page Now?
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High intensity used to change n also produces heat
- materials (usually) expand when heated - throws
off interference effect
Can switch on faster than off
Need wide bandgap to operate at room T
Another Option: Excitons
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Hole and electron are attracted, lowering energy in a bound
state
Photons emitted when hole and electron pair (exciton)
combine has therefore slightly less energy than when hole
and electron are not bound
Can maximize this effect by forcing electron and hole into
close proximity (quantum well)
Applying a voltage means energies are closer together, but
might break bond
Can minimize bond breaking by quantum well
SEEDs
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Self-Electro-optic Effect Device (uses feedback)
Set stage:
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Shine light of exciton energy on quantum well in
middle of p-n junction
Light is absorbed and produces excitons
Apply reverse bias which slightly separates
excitons but “significantly” lowers the energy and
reduces absorption
If light intensity is increased, absorption increases
slightly
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can produce more excitons and raise their energy
brings energy back to absorption peak
Pros and Cons of SEEDs
 Needs
only low power (FP needs high
power)
 Easier to manufacture – don’t need
fine-tuned cavity length
 Lower operating speed than FP
Why Aren’t Optical Computers
Front Page Now?
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Using only part-optical computers (i.e.,
interconnects) requires adapters
Much research already in semiconductors - hard
for light to beat that
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Light doesn’t interact because it’s not charged
Light doesn’t obey exclusion principle (not
fermion)