Helium-neon Laser

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Helium-neon Laser
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Historical facts

• The Helium-Neon laser was the first continuous
  laser.
• It was invented by Javan et. al. in 1961. But how
  did Javan manage to do this?
• This shows that it is no coincidence that Javan's
  first He-Ne laser oscillated at a
• Wavelength of 1.5µm, since the amplification at
  this wavelength is considerably
• Higher than the 632 nm line which is reached at
  what is now commonly known as the red line, which
  was made to oscillate only one year later by
  White and Ridgen.
• The similarity between the manufacturing
  techniques of He-Ne lasers and electron valves
  helped in the mass production and distribution of
  He-Ne lasers..
• It is now clear that He-Ne lasers will have to
  increasingly compete with laser diodes in the
  future. But He-Ne lasers are still unequalled as
  far as beam geometry and the purity of the modes
  are concerned. Laser diodes will have to be
  improve to a great extent before they pose a
  serious threat to helium-neon laser

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Introduction

• A helium-neon laser, usually called a He-Ne
  laser, is a type of small gas laser. HeNe lasers
  have many industrial and scientific uses, and are
  often used in laboratory demonstrations of
  optics.
• He-Ne laser is a four-level laser.
• Its usual operation wavelength is 632.8 nm, in
  the red portion of the visible spectrum.
• It operates in Continuous Working (CW) mode.

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Construction of He-Ne laser

• The setup consists of a discharge tube of length
  80 cm and bore diameter of 1.5cm.
• The gain medium of the laser, as suggested by its
  name, is a mixture of helium and neon gases, in a
  51 to 201 ratio, contained at low pressure (an
  average 50 Pa per cm of cavity length ) in a
  glass envelope.
• The energy or pump source of the laser is
  provided by an electrical discharge of around
  1000 volts through an anode and cathode at each
  end of the glass tube. A current of 5 to 100 mA
  is typical for CW operation.
• The optical cavity of the laser typically
  consists of a plane, high-reflecting mirror at
  one end of the laser tube, and a concave output
  coupler mirror of approximately 1 transmission
  at the other end.
• HeNe lasers are normally small, with cavity
  lengths of around 15 cm up to 0.5 m, and optical
  output powers ranging from 1 mW to 100 mW.

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He-Ne Energy level diagram

• The left side of the representation shows the
  lower levels of the helium atoms.The energy scale
  is interrupted and that there is a larger
  difference in energy in the recombination process
  than is evident in the diagram.
• A characteristic of helium is that its first
  states to be excited, 21S1 and 21S0 are
  metastable, i.e. optical transitions to the
  ground state 11S0 are not allowed, because this
  would violate the selection rules for optical
  transitions. As a result of gas discharge, these
  states are populated by electron collisions
• A collision is called a collision of the second
  type if one of the colliding bodies transfers
  energy to the other so that a transition from the
  previous energy state to the next higher or lower
  takes place. Apart from the electron collision of
  the second type there is also the atomic
  collision of the second type. In the latter, an
  excited helium atom reaches the initial state
  because its energy has been used in the
  excitation of a Ne atom. Both these processes
  form the basis for the production of a population
  inversion in the Ne system.

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Working of He-Ne laser

• A description of the rather complex HeNe
  excitation process can be given in terms of the
  following four steps.
• (a)When the power is switched on, An energetic
  electron collisionally excites a He atom to the
  state labeled 21So . A He atom in this excited
  state is often written He(21So), where the
  asterisk means that the He atom is in an excited
  state.
• (b) The excited He(21So) atom collides with an
  unexcited Ne atom and the atoms exchange internal
  energy, with an unexcited He atom and excited Ne
  atom, written Ne(3s2), resulting. This energy
  exchange process occurs with high probability
  only because of the accidental near equality of
  the two excitation energies of the two levels in
  these atoms. Thus, the purpose of population
  inversion is fulfilled.

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• When the excited Ne atom passes from metastable
  state(3s) to lower level(2p), it emits photon of
  wavelength 632 nm.
• This photon travels through the gas mixture
  parallel to the axis of tube, it is reflected
  back and forth by the mirror ends until it
  stimulates an excited Ne atom and causes it to
  emit a photon of 632nm with the stimulating
  photon.
• The stimulated transition from (3s) level to (2p)
  level is laser transition.
• This process is continued and when a beam of
  coherent radiation becomes sufficiently strong, a
  portion of it escape through partially silvered
  end.
• The Ne atom passes to lower level 1s emitting
  spontaneous emission. and finally the Ne atom
  comes to ground state through collision with tube
  wall and undergoes radiationless transition.

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Applications of He-Ne laser

• The Narrow red beam of He-Ne laser is used in
  supermarkets to read bar codes.
• The He- Ne Laser is used in Holography in
  producing the 3D images of objects.
• He-Ne lasers have many industrial and scientific
  uses, and are often used in laboratory
  demonstrations of optics.