Optical Devices An Overview of Terms from Optical

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