Lasers and the Space Elevator

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Lasers and the Space Elevator

• Adaleena Mookerjee
• August 16, 2006
• Center for Structures in Extreme Environments,
  Rutgers University

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What are Lasers?
LASER (light amplification by stimulated emission
of radiation) A device that controls the way
energized atoms release photons
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Laser History

• Conceptually developed by Albert Einstein in 1917
• Actually built by Theodore Maiman in the 1960s
  (Ruby laser)
• Previously, was not important at all
• Today, it is all around us from printers to
  barcode readers at the bookstore or mall

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Basic Phenomena of Light

• Wave duality of matter light is capable of
  behaving like a wave and particle

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Light The Wave

• Any electromagnetic radiation with a wavelength
  visible to the eye
• 3 properties of light
• Intensity (amplitude) how bright something
  appears to the human eye
• Frequency (wavelength) color of light produced
• Polarization angle of vibration of light

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Light the Photon

• Photons quanta of electromagnetic radiation
  which can be light
• Bohr Model
• Made up of 3 subatomic particles
• Protons (positively charged subatomic particle)
• Neutrons (neutrally charged subatomic particle)
• Electrons (negatively charged subatomic particle)
• Protons and neutrons are located in nucleus
• Electrons are located in a hypothetical region
  surrounding the nucleus

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Producing Light

• Step 1 Light is emitted and photons are
  released, colliding with the orbiting electron
• Step 2 Collision causes change in velocity and
  position, making the electron absorb photons
  energy
• Step 3 Electron moves to a higher energy level
  or position around nucleus (excited!)
• Step 4 To return, electron releases energy from
  photon, producing light

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Wave and Photon Correlation

• Speed of light (c) 300,000 km/s
• Relationship between photon and wave
• Where ? wavelength and f frequency

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Terminology Photons

• Population inversion when a system of either a
  group of molecules or molecules exist in a state
  where more electrons are in an excited state than
  in the lowest energy level possible
• Absorption photon with a particular frequency
  hits an atom at rest excites it to a higher
  energy level while photon is absorbed
• Spontaneous emission atom in an excited state
  emits a photon with a particular frequency
  returns to ground state
• Stimulated emission photon with a certain
  frequency hits excited atom releases two
  photons of same frequency while electron returns
  to ground state

• Population inversion when a system of either a
  group of molecules or molecules exist in a state
  where more electrons are in an excited state than
  in the lowest energy level possible
• Absorption photon with a particular frequency
  hits an atom at rest excites it to a higher
  energy level while photon is absorbed
• Spontaneous emission atom in an excited state
  emits a photon with a particular frequency
  returns to ground state
• Stimulated emission photon with a certain
  frequency hits excited atom releases two
  photons of same frequency while electron returns
  to ground state

http//perg.phys.ksu.edu/vqm/laserweb/index.html
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Terminology -- Waves

• Scattering when atoms of a transparent material
  is not smoothly distributed over distances
  greater than the length of a light wave
• Reflection light normally collides with the
  boundary of 2 materials
• Objects contain free electrons which jump from
  one atom to another within it
• Energized electrons vibrate ? sends back of
  object as light wave with same frequency of
  incoming wave
• Does not deeply pierce material

• Refraction bending of light when it passes from
  one kind of material to another
• Frequency of incoming light matches natural
  frequency in electrons
• Penetrates deeply into material causing
  vibrations in electrons
• Waves slow down and outside it maintains original
  frequency

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Examples
scattering
reflection
refraction
refraction
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Goals of Presentation

• Discuss what exactly a laser is
• Discuss how a laser works and how to build your
  own
• Discuss the types of lasers available today
  (solid, gas, liquid, semiconductor, excimer, free
  electron)
• Propose the best laser for the space elevator

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What is a Laser? (in words)

• Did you know? Actually, it is an acronym LASER
  (light amplification by stimulated emission of
  radiation)
• A device that controls the way energized atoms
  release photons
• Laser light is very intense, highly directional
  pure in color
• Not very safe to look directly at laser light
• 4 types of lasers
• 1) solid state lasers
• 2) gas lasers
• 3) liquid lasers
• 4) semiconductor lasers

Other forms also are excimer and free electron
lasers, but they dont fall into any of these
categories.
Classified based on gain medium (that will be
defined in one slide) used
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Parts of a Laser
Gain Medium (excitation mechanism) --transfers
external energy to beam --excites particles
--keeps laser at desired wavelength --absorbs
energy in the laser --maintains the laser
Energy source --begins the lasing
process --examples include electrical
discharges, flashlamps, arclamps, lights from
another laser, lights from chemical reactions,
lights from explosions
Output Coupler --where light is allowed to come
out --semitransparent mirrors --controls
effective output of light produced
Optical Resonator (feedback) arrangement of
optical components allowing beam to circulate
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Laser Function Construction

• As you noticed, light is commonly used as the
  energy source and the gain medium
• Energy source applied to system
• Gain medium transfers energy to beam, energizing
  electrons
• Electrons give off light energy to return back to
  its original energy level
• Resonators produce more laser light

• Need to have knowledge in glassblowing,
  fabricating small parts, operating a vacuum
• Use a solid, liquid or gas medium (best gas is
  nitrogen)
• Two resonating mirrors are used to reflect light
  formed
• Energy source applied to the system
• Emission of photons will result in light

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Solid-State Lasers

• Uses a gain medium which is a solid (not
  semiconductor)
• Energy source flashlamp or arclamp
• Optical resonator two mirrors in parallel
• Produces power ranging from milliwatts to
  kilowatts ? light lasts for short durations

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The Ruby Laser (a solid state laser)

• 1st laser by Theodore Maiman
• Used a synthetic ruby and made it in shape of
  cylinder
• Wrapped it around a high intensity lamp
• The blue and green wavelengths from the white
  light triggered an excitement within electrons of
  chromium atom
• When returned to stable state, they released
  energy in form of ruby light
• Phenomena continued until critical level reached
  and pulse released

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Implementation on the Space Elevator

• High powered solid state lasers may be successful
  with providing power
• Obtaining solids may be a problem
• Most acquirable item today is NdYAG (neodymium
  doped yttrium aluminum garnet)
• Produces very limited power

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Gas Lasers

• Active Medium pure gas, mixture of gases, metal
  vapor
• Energy Source electrical discharge, flashlamp,
  arclamp, light from other laser, chemical
  reaction or explosion
• Optical Resonator 2 mirrors in parallel to each
  other
• Power Generated 50 watts to 4 kilowatts of power

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CO2, N2 and HeCd Lasers
CO2 Lasers --uses CO2 to begin lasing
process --Active medium 1 carbon dioxide, 1
nitrogen gas, 1 helium --Process 1) nitrogen
added, exciting carbon dioxide 2) helium
added to remove electrons from lowest energy
level (population inversion) 3) tube sealed and
voltage added exciting system
HeCd Lasers --metal is cooked and
vaporized --helium excited by collisions with
excited electrons --pass on to cadmium
atoms --cadmium heated and added to helium
gas --helium fills cavity while cadmium goes to
cathode
N2 Lasers --uses N2 as active medium --high
voltage power added to system --creates
electrical discharge population
inversion --laser acts for short time --good for
scientific research, pumping other
lasers --minimal damages
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Implementation on the Space Elevator

• Possibly a good idea
• Could utilize gases available on Earth in excess,
  reducing pollution or its harmful effects
• Negative effect too much gas or some gases could
  corrode elevator, cause possible explosions and
  potentially damage the Earth
• Moderate amounts of gas needed for use

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Liquid Lasers

• Active Medium liquid
• Energy Source light from another laser
• Optical Resonator 2 mirrors in parallel with one
  another
• Power Generated few watts covering radius of 20
  micrometers
• Tunable over a wide range and produces a broad
  range of colors in visible spectrum

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Dye Lasers

• Uses organic liquid dyes as active medium
• 1 cm long quartz glass tube
• Dye cell inside of the tube which consists of
  partially reflective mirrors on the front
  diffraction grating on rear

• Laser Action
• Energy added by a light source such as
  flashlamp/laser
• Dye absorbs wavelength of light shorter than what
  it emits and input energy in forms of energy
  heat
• Absorbed energy creates a population inversion
  (electrons excited)
• Vibrational energy loss causes dye molecules to
  go into lowest energy state
• Emission occurs when vibrational levels reach
  ground state.

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Implementation on the Space Elevator

• Not practical
• Provides too little power
• Not feasible to use organic dye (harder to
  maintain)
• Dye could get stale over time (requires
  replacement every few days)
• Could cause malfunction of the laser if dye is
  not replaced
• Dependent on another laser for starting, so two
  lasers would be necessary

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Semiconductor Lasers

• Active Medium semiconductor solid (solid which
  conducts electricity) needs to confine carrier
  take up small volume
• Energy Source electrical impulse
• 1st semiconductor 1962
• Coherent electromagnetic radiation produced by a
  p-n junction using GaAs

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Semiconductor Terminology

• p-type semiconductor semiconductor in which
  electrical conduction is due chiefly to the
  movement of positive holes
• n-type semiconductor electrical conduction due
  chiefly to the movement of electrons
• p-n junction where p-type semiconductor is
  adjoined with the n-type semiconductor

• Valence band where highest energy level has full
  number of electrons
• Conduction band where lowest energy level has no
  electrons
• Band gap (energy gap) space between valence band
  and conduction band
• Minority carrier contains few mobile electrons
  in p-type semiconductor region free holes in
  n-type semiconductor region
• Majority carrier free holes in p-type region
  electrons in n-type region

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Semiconductor Lasing

• Population Inversion charge carriers (electrons)
  cross p-n junctions ? minority carriers
• Minority carriers mix with majority carriers
• Photon is absorbed by electrons (gives energy to
  jump from valence to conduction)
• Leads to stimulated emission, releasing a photon
• Optical resonator reflects the light out
  sometimes back into the solid

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Implementation on the Space Elevator

• Power Generated few milliwatts
• Will not produce enough power
• Small in size wont create enough light
• Option use a system of semiconductor lasers ?
  pretty costly

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Excimer Lasers

• Active Medium noble gas (argon, krypton, xenon)
  halogen (fluorine, chlorine, bromine, iodine)
• Exists for 10 nanoseconds during excited state
• In ground state, exists as separate atoms
• Energy Source UV light
• Optical Resonator 2 mirrors in parallel to each
  other

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Excimer Lasers

• Chemical Composition
• 0.1-0.2 halogen
• Little noble gas
• 90 of helium or neon
• Laser Action
• When electrical discharge or energy is added to
  noble gas, can bind to halogen (excited)
• Gives up additional energy through stimulated
  emission, forming ground state molecule
• Within picoseconds, can separate into 2 atoms ?
  population inversion

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Implementation on the Space Elevator

• FACT sun is composed of hydrogen, helium,
  oxygen, carbon, iron, neon, nitrogen, silicon,
  magnesium sulfur
• Use solar radiation as energy source for the
  laser (excess sunlight)
• Lasing process is for few nanoseconds, but power
  generated few watts to few hundreds of watts
• Radiation exposure will be minimal, but effective
  for the space elevator

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Free Electron Lasers

• Best laser according to Edwards Westling
• Device which emits high powered electromagnetic
  radiation at any wavelength
• Contains an array of magnets in magnetic field to
  excite free unbound electrons
• Tunable over broad range of wavelengths
• Class IV lasers capable of starting fires, burn
  flesh and cause eye damage

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Free Electron Lasers

• Beam of electrons accelerated to relativistic
  speeds (electron accelerator)
• Electrons pass through periodic, transverse
  magnetic field
• Magnetic field causes electrons to travel at a
  sinusoidal path
• Electrons move at higher speeds, releasing
  photons
• Optical mirrors lengthen process

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Implementation on the Space Elevator

• Efficiency of 65
• Can emit radiation at any wavelength (tunable)
• Accelerated electrons release x-rays at hazardous
  levels
• Produces high quantities of power
• Electron accelerator is big
• Very expensive

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Conclusions

• Best Laser gas or excimer laser
• Why? can use gases in the solar system or
  atmosphere
• Other lasers would require further research
• Other Factors to Study
• Threshold of maximum gas needed
• Overall harmful effects of lasers for necessary
  protection
• Some type of radiation shielding
• What should be done if gas runs out?
• Process of the laser
• Strength to endure effects of nature

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Acknowledgements

• At this time, I would like to thank
• Professor Benaroya for giving me this opportunity
  to learn about lasers
• Yuriy Gulak for setting me up here and
  familiarizing me with the technology available
  here
• Everyone of you for teaching me about your
  research making me feel comfortable here