What are Helium-Neon (He-Ne) Lasers?

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- GoPhotonics

Oct 15, 2022

Helium-Neon lasers are gas lasers whose gain medium consists of a mixture of helium and neon gases in the ratio 10:1 at a pressure of about 1 torr. They deliver an output wavelength of 632.8 nm which falls in the infrared region of the electromagnetic spectrum. However, other wavelength emissions ranging from 100 µm in the far infrared region to 540 nm in the visible region are possible by using mirror coatings that reflect at these wavelengths. The He-Ne laser is a continuous wave laser that is typically 0.15-0.5 m long and operates for lifetimes of up to 50,000 hours. The optical power out of this laser ranges from 0.5 to 50 mW.

He-Ne lasers are pumped using an electric discharge where energetically excited electrons collide with the gas mixture in the active medium. Since these collisions are inelastic, there occurs an energy transfer from the pump electrons to the Helium-Neon gas mixture. In a He-Ne laser, photon emission occurs due to the atomic transitions of the neon gas as they have energy levels suitable for the laser transition, and the He gas acts as a buffer gas that provides sufficient energy to the Ne atoms to produce laser output.

The He-Ne gas mixture mainly comprises He atoms which absorb most of the energy supplied by the electrodes present in the laser cavity through electric discharge. The energetic electrons produced through electric discharge collide with the He atoms and this collision results in the transfer of energy to the He atoms. This inelastic collision excites the He atoms to higher metastable energy levels given by23S1 and 21S0. Since the metastable states of He atoms are long-lived energy levels, the excited atoms present there cannot return to the ground state by spontaneous emission. So, the He atoms transfer their energy to the Ne atoms by collision because the excited energy levels (4s and 5s) of Ne atoms are identical to the metastable energy levels of the He atoms.

The collision between He and Ne atoms leads to the complete transfer of energy from the He atoms to the Ne atoms. As a result, the Ne atoms get excited to the upper energy levels 4s and 5s which are metastable states and the He atoms lose their energy and return to the ground state. The metastable states of Ne atoms are long-lived and so as this process continues, there results in a condition of population inversion between the energy levels of Ne atoms. Thus He atoms help Ne atoms to achieve population inversion.

The electrons in the upper laser level of Ne atoms return to lower energy levels through stimulated emission of laser radiations at 1.15 μm (corresponding to the 4s to 3p transition), 3.39 μm (corresponding to the 5s to 4p transition) and 632.8 nm (corresponding to the 5s to 3p transition). Among these, 632.8 nm is always dominated and other wavelengths are produced only if desired, using suitable cavity mirror coatings. The 3p level is efficiently emptied by fast radiative decay to the 3s state, eventually reaching the ground state.

Typical He-Ne laser parameters


He-Ne laser has numerous applications. It is used in interferometry, where it provides a very stable, single-transverse-mode reference beam necessary to identify optical properties of materials such as surface figure and smoothness. It is used in laser printing, where the well-characterized beam is used as a writing source on photosensitive material to provide detailed print patterns. Most super­ markets and other stores now use He-Ne lasers as check-out or inventory scanners to read the digitally encoded bar codes located on products. Pointing applications include reference beams for the alignment of sewer pipes, three-dimensional right-angle reference beams in the construction industry, reference beams for surveying, and target-aiming devices for guns. The 1.523-µm laser is used for measurements of optical fiber transmission lines, which have a minimum loss in that wavelength region.