Helium - Neon Gas Laser

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The figure shows the elements of a type of laser that is often found in student laboratories. The glass discharge tube is filled with a 80%-20% mixture of the inert gases helium and neon. Helium is the "pumping" medium and neon is the "lasing" medium. The above figure is a simplified version of the level structures for these two atoms. Note that four levels, labeled E₀, E₁, E₂, and E₃, are involved in this lasing scheme, rather than three levels.

The basic elements of a Helium - Neon gas laser

pumping is accomplished by setting up an electrically induced gas discharge in the helium-neon mixture. Electrons and ions in this discharge occasionally collide with helium atoms, raising them to a level E₃ which is metastable. Spontaneous emission to the ground state level E₀ is very rare. Level E₃ in helium (= 20.61 eV) is, by chance, very close to level E₂ in neon (= 20.66 eV). So, during collisions between helium and neon atoms, the excitation energy of the helium can be readily transferred to the neon. In this way level E₂ can become more highly populated than level E₁. This population inversion is maintained because (1) the metastability of level E₃ ensures a ready supply of neon atoms in level E₂ and (2) level E₁ decays rapidly to the neon ground state, E_0. Stimulated emission from level E₂ to level E₁ predominates, and red laser light of wavelength 632.8 nm is generated.

Most stimulated emission photons initially produced in the discharge tube will not happen to be parallel to the tube axis. These will be quickly stopped at the walls. Stimulated emission photons that are parallel to the axis, can move back and forth through the discharge tube many times by successive reflections from mirrors M₁ and M₂. These photons can in turn cause other stimulated emissions to occur. A chain reaction thus builds up rapidly in this direction. This results in the inherent parallelism of the laser light.

Rather than thinking in terms of the photons bouncing back and forth between the mirrors, it is perhaps more useful to think of the entire arrangement of the above figure as an optical resonant cavity that, like all organ pipe for sound waves, can be tuned to be sharply resonant at one (or more) wavelengths.

The mirrors M₁ and M₂ are concave, with their focal points nearly coinciding at the centre of the tube. Mirror M₁ is coated with a dielectric film whose thickness is carefully adjusted to make the mirror as close as possible to totally reflective at the wavelength of the laser light. Mirror M₂, on the other hand, is coated so as to be slightly "leaky," so that a small fraction of the laser light can escape at each reflection to form the useful beam.

The windows W, W in the above figure, which close the ends of the discharge tube, are slanted so that their normals make an angle q_p, the Brewster angle, with the tube axis, where

tan q_p = m,

m being the refractive index of the glass at the wavelength of the laser light. Slanted windows transmit light without loss by reflection, provided only that the light is polarized with its plane of polarization in the plane of the above figure. If the windows were square to the tube ends, beam loss by reflection (about 4% from each surface of each window) would make laser operation impossible.