LASERS!!! Ian Mallett

Transcription

1 LASERS!!! Ian Mallett

2 How They Work (Part 1)

3 How They Work 1.Take an atom.

4 How They Work 1.Take an atom. 2.Energize it somehow.

5 How They Work 1.Take an atom. 2.Energize it somehow. 3.Eventually, the atom will spontaneously drop to a lower-energy state.

6 How They Work 1.Take an atom. 2.Energize it somehow. 3.Eventually, the atom will spontaneously drop to a lower-energy state. 4.The energy difference comes out as a photon.

7 How They Work Sidenote: that photon s wavelength corresponds to the energy gap. Electron shells have characteristic level s they can be at, so the photons emitted are at characteristic wavelengths. This is why different elements have characteristic spectra!

8 How They Work (Part 2)

9 How They Work Due to more quantum stuff, photons like to be together (same phase, wavelength, etc.)

10 How They Work Due to more quantum stuff, photons like to be together (same phase, wavelength, etc.) Hence, when a photon passes by an energized atom, the atom is likely to drop to a lower-energy state, emitting a photon in the same direction.

11 How They Work Due to more quantum stuff, photons like to be together (same phase, wavelength, etc.) Hence, when a photon passes by an energized atom, the atom is likely to drop to a lower-energy state, emitting a photon in the same direction.

12 How They Work Due to more quantum stuff, photons like to be together (same phase, wavelength, etc.) Hence, when a photon passes by an energized atom, the atom is likely to drop to a lower-energy state, emitting a photon in the same direction. stimulated emission

13 How They Work (Part 3)

14 How They Work 1. Get some atoms. ( gain medium )

15 How They Work 1. Get some atoms. 2. Excite them. ( pumping source this is often a flashbulb of the right color or a diode )

16 How They Work 1. Get some atoms. 2. Excite them. 3. Atoms emit light (in-sync due to quantum effects).

17 How They Work 1. Get some atoms. 2. Excite them. 3. Atoms emit light (in-sync due to quantum effects).

18 How They Work 1. Get some atoms. 2. Excite them. 3. Atoms emit light (in-sync due to quantum effects).

19 How They Work 1. Get some atoms. 2. Excite them. 3. Atoms emit light (in-sync due to quantum effects). 4. Bounce the light to increase emissions in a preferred direction.

20 How They Work 1. Get some atoms. 2. Excite them. 3. Atoms emit light (in-sync due to quantum effects). ( Sidenote: why is emission in the mirror direction increased? 4. Bounce the light to increase emissions in a preferred direction. Photons that are going the wrong way hit the sides and are absorbed. Whereas, photons going the right way reflect and can continue causing more atoms to emit photons. The photons going the correct direction also deplete the region of energized atoms that wrong photons could use. The effect is, over time, only the photons going the right way get amplified. )

21 How They Work 1. Get some atoms. 2. Excite them. 3. Atoms emit light (in-sync due to quantum effects). 4. Bounce the light to increase emissions in a preferred direction. Mirror Mirror

22 How They Work 1. Get some atoms. 2. Excite them. 3. Atoms emit light (in-sync due to quantum effects). 4. Bounce the light to increase emissions in a preferred direction. 5. Let some light out. Laser! Light Amplification by the Stimulated Emission of Radiation Mirror Semi-silvered mirror

23 Types of Lasers

24 Types of Lasers Solid-State: gain medium is solid. Gas, Metal-Vapor: gain medium is gas. Dye, Excimer: gain medium is dye or excimer molecules. Plasma laser: gain medium is plasma. E.g. in a gas-dynamic laser, gain medium and pumping source is combustion. Semiconductor (diode): energy comes from PN-junctions quantum effects. Handheld lasers are almost always of this type. (Most other laser types use an arc-lamp or electrical tube for energy.) Chemical: energy comes from a chemical reaction. Free-electron laser: energy comes from particle accelerator (!) Nuclear laser: pumped by nuclear weapons (!)

25 Color and Lasers

26 Color and Lasers (image techmind.org) This is the space of all max-brightness colors you can see.

27 Color and Lasers This is the space of all max-brightness colors you can see. (image techmind.org) The triangle contains the colors that this projector ought (theoretically) be able to display correctly.

28 Color and Lasers This is the space of all max-brightness colors you can see. The triangle contains the colors that this projector ought (theoretically) be able to display correctly. (image techmind.org) Most colors you see in the real world fall somewhere in the middle of this diagram.

29 Color and Lasers This is the space of all max-brightness colors you can see. The triangle contains the colors that this projector ought (theoretically) be able to display correctly. Most colors you see in the real world fall somewhere in the middle of this diagram. (image techmind.org) This outer line is called the spectral locus. This is where lasers are!

30 Color and Lasers This is the space of all max-brightness colors you can see. The triangle contains the colors that this projector ought (theoretically) be able to display correctly. Most colors you see in the real world fall somewhere in the middle of this diagram. This outer line is called the spectral locus. This is where lasers are! (image techmind.org) To see a laser is to see new colors (or at least ones you usually don t see... )

31 Color and Lasers Your eye is sensitive to different points on the spectral locus by different amounts:

32 Laser Safety

33 Laser Safety Usual hazard is eye damage. Frequently exaggerated, but danger is very real: 1 mw (1 milliwatt): probably safe no matter what. 5 mw: considered safe by USA (any damage is very likely to heal, and anyway the blink reflex prevents it). 10mW: possible hazard for temporary or minor injury. Any injuries likely to heal completely and quickly. 50mW: likely hazard for minor or moderate injury. Injuries may not fully recover, or may take a long time in doing so. 100mW: definite hazard for moderate or severe injury. Injuries unlikely to fully recover. 500mW: hazard for severe injury. Only partial recovery can be hoped for. 1000mW = 1W: hazard for severe injury. Only partial recovery can be hoped for. Get goggles when working with high-powered lasers, especially when combined with lenses, mirrors, or anything else that can throw beams unexpected places!!! Beware invisible wavelengths!

34 Laser Safety Case study: High-powered consumer laser pointer (probably cheap ~50mW mislabeled as 5mW) Deliberate exposure in both eyes. Eyes damaged to 20/35 vision. Recovered to 20/20 after six months.

35 Laser Safety Case study: 150mW green laser Deliberate and repeated exposure in both eyes. [S]evere vision loss in left eye, 20/50 vision in right. Left eye eventually healed to 20/25 after four weeks, right to 20/32.

36 Laser Safety Case study: 1 watt blue laser Exposure about 1 second (actually, likely much less) Blood initially covers retina After several months, eye cleared up and vision partially recovered.

37 Laser Safety Additional: Lasers starting around 100mW can burn stuff (like you!). Reflections of lasers can be hazardous too! Reflections from specular ( shiny ) materials can be just as dangerous as direct exposure! Reflections from diffuse ( matte ) materials are safer, but higher-power lasers can still damage if you stare at the spot. At the very least, it will make your eyes ache. Flash blindness and distraction (hazard to vehicle pilots) It is (rightly) a felony to shoot a laser at an aircraft in the USA!

38 Buying Lasers

39 Buying Lasers Diode lasers the internets Most are 405nm (deep violet), 532nm (green), or 650nm660nm (deep red). Important note: Gas, solid-state, etc. lasers academic surplus Cheap ones are often overspec : listed as 5mW, but actually much higher! Especially a problem with 532nm green, which has lots of extra power in invisible infrared (unless it s filtered out, which cheap pointers don t). It s possible to build your own, too! Others work for the DoD or a few rare labs

40 Buying Lasers Much artificial scarcity of consumer lasers outside the common wavelengths. Lots of misleading advertising, too. Selling mislabeled lasers or selling a laser >5mW as a pointer is illegal. Buying / owning such a laser is not. Most laser pointers are mislabeled, so you probably can t resell any laser pointer you have (and you should also be more careful with it!).

41 Uses of Lasers

42 Uses of Lasers Instruments Rangefinders and LIDAR Levels Spectroscopes Guide stars Laser mice...

43 Uses of Lasers Weapons Blow stuff up Blinding soldiers (illegal, except in China) Disorient soldiers and robots Missile countermeasures Aiming Target locks...

44 Uses of Lasers Medical surgeries: Eye Cosmetic, especially skin Tumor excision or irradiation

45 Uses of Lasers Industrial / Commercial: Cutting / welding / drilling / marking / scoring Laser pointers Printers Fiber optic communications CDs / DVDs / Blu-Rays Light shows, laser tag Projectors, holograms

46 Demo

47 Demo Nominal wavelengths: 405nm (deep violet) 445nm450nm (blue) 510nm (emerald green) (pretty!) 515nm (emerald green) (indistinguishable) 532nm (chartreuse) 589nm (yellow, touch of orange) (pretty!) 635nm638nm (red, touch of orange) 650nm (deep red) (looks similar) Nominal powers all 5mW

48 Additional Talking Points

49 Additional Talking Points Laser appearance in space (it doesn t) How does a FEL work? Frequency doubling / tripling / quadrupling How does a common 532nm laser pointer work? Beam divergence, and diffraction limits thereon Measuring wavelength with diffraction gratings Q-switching and laser-induced plasma Physics experiments with lasers. e.g. single/double slit/hair Patron deity of lasers? Laser propulsion Nuclear fusion Nature of light

50 Measuring Wavelength Calculate angle and feed it into diffraction grating formula. Simplifies to: λ = (grating spacing)*(dot separation) / (grating standoff)² + (dot separation)² Example: λ = ( μm)*( m) / ( m)² + ( m)²) ⁷ m = 448 nm My cinematic setup (actual experiment more precise)

51 532nm Laser Pointer 1.AlGaAs 808nm laser diode pumps a 2.Nd:YVO4, Nd:YAG, or Nd:YLF crystal, which lases at 1064nm. This then enters a 3.KTP or LBO crystal, which converts two photons at 1064nm into one photon at 532nm. This then 4.(Should) pass through an IR filter to remove any stray IR. Then 5.Comes out the front as a $5 laser beam.

52 Credits Me Suggestions, various