LASERS!!! Ian Mallett
How They Work (Part 1)
How They Work
1.Take an atom.
How They Work
1.Take an atom. 2.Energize it somehow.
How They Work
1.Take an atom. 2.Energize it somehow. 3.Eventually, the atom will spontaneously drop to a lower-energy state.
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.
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”!
How They Work (Part 2)
How They Work
● Due to more quantum stuff, photons like to be together (same phase, wavelength, etc.)
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.
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.
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”
How They Work (Part 3)
How They Work
1. Get some atoms. ( “gain medium” )
How They Work
( “pumping source”—this is often a 1. Get some atoms. flashbulb of the right color or a diode ) 2. Excite them.
How They Work
1. Get some atoms. 2. Excite them. 3. Atoms emit light (in-sync due to quantum effects).
How They Work
1. Get some atoms. 2. Excite them. 3. Atoms emit light (in-sync due to quantum effects).
How They Work
1. Get some atoms. 2. Excite them. 3. Atoms emit light (in-sync due to quantum effects).
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.
How They Work
1. Get some atoms. ( Sidenote: why is emission in the mirror 2. Excite them. direction increased? ● Photons that are going the “wrong” way hit 3. Atoms emit light (in-sync the sides and are absorbed. due to quantum effects). ● Whereas, photons going the “right” way 4. Bounce the light to reflect and can continue causing more increase emissions in atoms to emit photons.
a preferred direction. ● 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. )
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
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
Types of Lasers
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 (!)
Color and Lasers
Color and Lasers
● This is the space of all max-brightness colors you can see.
(image techmind.org) 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) 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) 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) 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!
● To see a laser is to see new colors (or at least ones you usually don’t see . . . )
(image techmind.org) Color and Lasers
● Your eye is sensitive to different points on the spectral locus by different amounts:
Laser Safety
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!
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.
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.
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.
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!
Buying Lasers
Buying Lasers
● Diode lasers → the internets – Most are 405nm (deep violet), 532nm (green), or 650nm–660nm (deep red). – Important note: ● 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).
● Gas, solid-state, etc. lasers → academic surplus – It’s possible to build your own, too!
● Others → work for the DoD or a few rare labs
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!).
Uses of Lasers
Uses of Lasers
● Instruments – Rangefinders and LIDAR – Levels – Spectroscopes – Guide stars – Laser mice – ...
Uses of Lasers
● Weapons – Blow stuff up – Blinding soldiers (illegal, except in China) – Disorient soldiers and robots – Missile countermeasures – Aiming – Target locks – ...
Uses of Lasers
● Medical surgeries: – Eye – Cosmetic, especially skin – Tumor excision or irradiation
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
Demo
Demo
● Nominal wavelengths:
• 405nm (deep violet)
• 445nm–450nm (blue)
• 510nm (emerald green) (pretty!)
• 515nm (emerald green) (indistinguishable) • 532nm (chartreuse)
• 589nm (yellow, touch of orange) (pretty!)
• 635nm–638nm (red, touch of orange)
• 650nm (deep red) (looks similar)
● Nominal powers all 5mW
Additional Talking Points
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
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 = (1.0000 μm)*(0.9980 m) / ⎷ (1.5885 m)² + (0.7960 m)²) ≈ 4.4800 10⁻ ⁷ m = 448 nm
My cinematic setup (actual experiment more precise) 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.
Credits
● https://vignette.wikia.nocookie.net/sims/images/e/e7/Floor_Mirror.jpg/revision/latest?cb=20110427213536
● https://image.freepik.com/free-icon/atom_318-38283.jpg
● https://2.bp.blogspot.com/-V2ESDXZJuOY/T2ea7n2JzcI/AAAAAAAAACY/ReoTA1l3Y7M/s1600/emission-spectra-1.jpg
● http://www.laserpointersafety.com/news/news/nonaviation-incidents_files/pic-2011-12-19-at-3.21.29-pm.png
● http://www.laserpointersafety.com/news/news/nonaviation-incidents_files/screen-shot-2010-09-03-at-8.41.56-am.jpg
● http://www.laserpointersafety.com/news/news/nonaviation-incidents_files/screen-shot-2010-09-15-at-9.32.47-am.jpg
● http://www.techmind.org/colour/CIE_sRGB_hires.jpg
● Me
● Suggestions, various