10'4 Rare Earth Lasers

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10.4 Rare Earth Lasers Amplifiers
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10.4.1 General Considerations

• Rare Earth Elements
• Elements with atomic numbers from 57-71.
• Nd(60) and Er(68) are the most important
  ones.
• A Common Solid State Laser Material
• A Common Solid State Laser material is made by
  doping rare earth (Nd), in to a variety of
  optically transparent host materials such as
  glass (mostly SiO2),YAG (Y3Al5O12) and Sapphire
  (Al2O3).
• Active atom is the Rare Earth.

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Spectroscopic Notation

• The characteristic of a quantum state is
  specified by the notation,
• 2S1LetterJ
• S Spin angular momentum
• J Total angular momentum
• Prescription for the Letter symbol

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E.g. Ground State of Neodymium

• Spectroscopic Notation 4 I 9/2
• Degeneracy 2J1 10
• There are 10 different wave functions describing
  this state.

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Description

• In rare earths (RE), the active e- participating
  in the optical transition is one of the 4f e-
  that is shielded by n5 and n6 orbits.
• Energy levels of the rare earths are only weakly
  dependent on the host lattice.
• Changes in the wavelength of the RE laser
  depending on the host lattice are very small and
  those changes appear in the 3rd or 4th
  significant figure.

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Stark Effect Energy Splitting

• These active atoms (RE) experience a local E
  field generated by the host lattice and hence
  degenerate energy levels are shifted (Stark
  effect).
• Since stark effect is proportional to E (not to
  the direction), the positive negative fields
  yield the same shift.
• For an example consider NdYAG, One can reduce
  the degeneracy in to (2J1)/2 doubly degenerate
  states.

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10.4.2 NdYAG Lasers

• Laser Material Y3Al5O12 (or YAG) Small
  amount of Nd
• (host
  material) (RE)
• Active atoms participate as if they were triply
  ionized, Nd 3 depend on the host lattice.
• The Nd3 substitutes for Y3 in the cubic crystal
  of YAG, and thus each dopant atom sees more or
  less identical environment.
• Since active atoms are in well defined
  environment, the energy levels are well defined
  and narrow.

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Pumping

• Flash Lamp
• Semiconductor laser (808 nm)

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Energy Level Diagram
Source www.star.le.ac.uk/rw/courses/lect4313_fig
14.jpg
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Laser Output

• NdYAG laser system can be operated either CW or
  pulsed mode, mode locked or Q-switched.
• By using frequency doubling techniques O/P
  wavelength can be converted to green 1.064
  /2 532 nm with diffraction limited TEM00
  mode.

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Source Joseph T. Verdeyen, Laser Electronics,
3rd edition (prentice-Hall, 1995)
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Source Joseph T. Verdeyen, Laser Electronics,
3rd edition (prentice-Hall, 1995)