Optical Devices

1. Optical Devices An Overview of Terms from Optical Computing

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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!!!

9. 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

10. 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

11. 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

12. 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

13. Before the Next Class • Do Lecture 27 Evaluation by Midnight Friday (why not now?) • Complete Reading Quiz 28 by Monday • Finish project by Monday