Helium-Neon Laser

Helium-Neon Laser

  • The Helium-Neon laser was the first continuous laser
  • It was invented by Ali Javan et. al. in 1961.
  • Is an atomic laser employs four level pumping scheme.
  • Its usual operation wavelength is 632.8 nm, in the red portion of the visible spectrum.
  • It operates in Continuous Working (CW) mode.
  • Active medium is a mixture of 10 parts of Helium and one part of Neon.
  • Ne atoms are active centers and have energy levels suitable for laser transitions while, He atoms help in efficient excitation of Ne atoms.
  • The setup consists of a discharge tube of length 35 cm and bore diameter of 2mm.
  • The gain medium of the laser, as suggested by its name, is a mixture of helium and neon gases, in a 5:1 to 20:1 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 few kilo 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. 
  • Since the cavity window is outside the tube, Brewsters windows may be used at the ends of tube to minimize reflection loss.
  • 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.

  • When voltage is applied to the electrodes it ionizes the gas, the electrons and ions thus produced are accelerated towards anode and cathode respectively.
  • Electrons acquire higher velocity due to its smaller mass when compared to the others. They transfer K.E to He atoms through inelastic atom-atom collision.
  • He atoms are readily excited by electrons impact because of its fairly light mass.
  • Thus He atoms are excited to F2 and F3 states which lie at 19.81 and 20.61 eV respectively.
  • These are meta stable states and these atoms cannot return to ground state by spontaneous emission. 
  • These atoms returns to ground state by transferring energy to Ne atom in the state which has identical energy. Such transfer is called resonant transfer of energy.
  • Ne energy levels E6 and E4 nearly coincide with F3 and F2 so resonant transfer can occur.
  • The additional energy 0.05 eV is provided by the K.E of the He atom
  • 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.
  • When Ne atom in the E6 level comes to E4 level it emits a photon parallel to the axis of the tube 
  • 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 with the stimulating photon.
  • In reality neon energy levels E6, E5, E4,E3,E2 are not single but a group of lines. Consequently several laser transitions are possible.
  • Three main laser transitions are
    •  E6 to E3 – generates laser beam of red colour at 6328Å
    • E4 to E3 – IR beam at wavelength of 1.15 mm
    • E6 to E5 – light in Far IR region at 3.39 mm
  • Level E2 is a metastable state, the neon atoms tend to accumulate at this level, if they are not removed the population in the ground state continuously decreases restricting lasing action.
  • E2 to E1 transition can be induced by collision with the walls of the discharge tube.

  • 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. 

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