Optical Devices
An Overview of Terms from Optical
Computing
Review of processors
• 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
realistic systems (speed, noise, fan-out)
• “Three terminal,” ie. one signal affects output state
How is Light Better?
• FASTER
– nothing can travel faster than light in vacuum
– Extremely high bandwidth
• COOLER (transfer, not nec. processing)
– Less loss from scattering as light travels through fibers
than electrons through wires
• FOURIER TRANSFORMS (clever parallel
processing)
– 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
– material with optical properties (e.g., index of
refraction) depending on intensity
– small change in intensity must produce large
change in output
• Utilize properties of interference and
standing waves
Fabry-Perot Interferometer
• 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
• 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
• MULTI-FUNCTION - same device can be
– 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
Why Aren’t Fabry Perot Devices
Front Page Now?
• 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
• BIG!!!
Another Option: Excitons
• 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
• Self-Electro-optic Effect Device (uses feedback)
• Set stage:
– 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
– 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
Problems with
Optical Computers
• Using only part-optical computers (i.e.,
interconnects) requires adapters
• Much research already in semiconductors - hard
for light to beat that
– Light doesn’t interact because it’s not charged
Before the Next Class
• Do Lecture 27 Evaluation by Midnight
Friday (why not now?)
• Complete Reading Quiz 28 by Monday
• Finish project by Monday