Lecture 7: Lasers and their applications (II)

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Lecture 7 Lasers and their applications (II)
Requirements for Laser action (YF 38.6)

• Theoretical laser
• We want a three level system in which the
  intermediate state is metastable (ie. the time
  for transition is much longer than the time for
  transition to other states).
• If we can optically pump a three level system
  such that electrons are stimulated from the
  bottom E1 level to the top level E3, and these
  decay to a metastable intermediate state E2 then
  we obtain population inversion

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• Requirements for laser action
• Pumped light must not match E2-E1 hf12, since
  it would deplete the N2 population by stimulated
  emission.
• Need intense pumping action to achieve
  population inversion N2gtN1, with state N2 being
  metastable.
• Use partially silvered mirrors to re-circulate
  the initial laser light to boost stimulated
  emission (create an optical cavity).

• Methods of pumping
• Optical pumping (eg. flash light).
• Gas discharge (eg. gas lasers).
• Chemical excitation (eg. dye lasers).
• Electron excitation (eg. semiconductor lasers).

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3 and 4 level lasers

• The ruby laser
• First laser constructed in 1960 by T.H. Maiman.
• Ruby Al2O3 crystal (sapphire) containing
• 0.05 Cr3.
• Example of a 3 level pulsed laser.
• Pumping is pulsed with a Xe flash lamp (10-3 s)
• Ruby laser only emits short pulses (10-6 s)
• since metastable state depletes quickly and is
  
• not replenished.
• Inefficient process needs large input power
• since excitation is from ground state.

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• The helium-neon laser (YF 38.6)
• First constructed in 1961.
• Mixture of He and Ne (10-3 atm) sealed in a
  glass enclosure with two electrodes to create
  ionisation by continuous gas discharge.
• Laser action not from ground level so the
  population in the lower state is small. It is
  easy to obtain N2gtN1 so it can operate
  continuously.
• He-Ne laser output small (few mW) and gain low
  (1.05 per metre) due to losses in mirrors,
  walls, etc.

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• The helium-neon laser (cont)
• Example of a 4 level continuous laser.
• Collisions between the He atoms and Ne atoms
  cause excitations. The 2s state of He is
  metastable, so collisions of He atoms in this
  high state cause excitations in the Ne atoms.
  These decay to intermediate states

(red)

• Other transitions 543 nm (2.29 eV, green), 1.15
  mm (1.08 eV, infrared) and 3.39 mm (0.36 eV,
  infrared).

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• Modes of vibration of lasing cavity
• Resonant system at optical frequencies.
• Light has to be in phase after round trip
  (dlength of optical cavity)
• Spacing between frequencies
• Could have atomic transitions with a width of
  1000 MHz many longitudinal modes are
  transmitted.

• Example Line width of He-Ne laser is 1.5 x 109
  Hz. The mirror separation is 0.3 m. Calculate Df
  and estimate the number of longitudinal modes
  that resonate in the system.

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Applications of lasers (YF 38.6)

• Helium-neon lasers
• Characteristics l633 nm and a few mW in power.
• High coherence of light is useful for measuring
  length accurately (by interferometry or surveying
  by triangulation).
• Accurate machining.
• Bar-code readers.
• Argon ion laser
• Characteristics blue (l488 nm) and green
  (l514 nm) with 30 W power.
• Used in eye surgery to weld detached retinas.
• Interferometry in gravitational wave detectors.
• CO2 laser
• Characteristics kW of power at 10.6 mm
  (infra-red).
• Used for metal cutting and drilling precision
  holes, shapes.

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• Nd-YAG (neodynium doped ytrium-aluminium-garnet)
  laser
• Characteristics pulsed laser at 1.06 mm
  (infra-red).
• Used in medicine to clear blocked arteries and
  to destroy tissue
• (e.g. tumors).
• Nd-glass laser used in induced fusion
  experiments.
• Semi-conductor laser (e.g. gallium arsenide)
• Characteristics mainly at 800 nm but also at
  680 nm.
• Used for optical communications through optic
  fibres.
• CD players.
• Supermarket checkouts.
• Pump for Nd-YAG laser.
• Laser pointers.

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• Holography
• Make 3D images by recording amplitude and phase
  information from
• object.
• How do you make a hologram? Make interference
  pattern between
• laser light reflected from object and reference
  beam. Coherence of
• laser light is vitally important for holograms.

• How do you reconstruct the image of the
  hologram?
• Reconstruct hologram by passing laser light of
  same
• wavelength through interference pattern.