Helium-Neon Laser Construction and Working

Helium-Neon Laser

Helium-Neon Laser or He-Ne Laser is a gas laser. That uses Helium and Neon. It operates at a wavelength of 632.8 nm in the red part of the visible spectrum. And it is the first continuous wave (CW) laser constructed.

Helium Neon Laser construction and working

A continuous and strong laser beam can be produced by gas lasers. Helium-Neon Laser Consists of a quartz discharge tube with helium and neon gas in the ratio of 1:4. That is One part of Helium with four parts of Neo gas. There is total pressure of 1 torr insides a small electrical discharge.

The gases filled in the quartz tube at the ratio of 1mm of mercury (Hg). Similar to a ruby laser, one end of the tube is a fully reflecting mirror and another end of the tube is partially reflecting error.

A strong radiofrequency generator is used to produce a discharge in gas. This is done to excite helium atoms to a higher energy level. The energy level diagram for the Helium-Neon laser system is shown in Figure.

Working of Helium Neon Laser

energy level diagram of he ne laser

When an electric discharge passes via the gas, the electron in the quartz discharge tube strike with the Helium and Neon atoms. This excites the Helium and Neon atoms and makes them move to the metastable states of energy 20.61 eV for Helium and 20.66 eV for Neon, above the ground state.

Here high voltage DC is used a pump source. This high voltage DC produces high energetic electrons that travels through the gas mixture.

Some of the exciting Helium atoms pass their energy to unexcited  Neon atoms by collision. Thereby Helium atoms induce a population inversion in Neon atoms.

When an exited Neon atom falls suddenly from the metastable state at the energy level of 20.66 eV to the ground state at the energy level of 18.70 eV it emits radiation of 6328 Å photon in the visible spectrum.

This photon travel via the gas mixture, it gets reflected again and again inside the quartz tube due to mirrors placed at the end.It stimulates excited neon atoms and emits a fresh 6328 Å  photon that is in phase with the stimulating photon. The transition from 20.66 eV to 18.7 eV level is the stimulated laser transition.

In Helium-Neon laser, the gain is an inverse function of the radius of the gas tube. This is due to the lower level of the laser transition is depopulated by the collisions of neon atoms with walls.

The part of output radiation escapes from the partially transparent (reflecting) mirror on the other side. The neon atoms fall from 18.7 eV level to lower state E via sudden emission of incoherent light.

For this phase E, the Neon atoms are taken down to the ground state by colliding with the wall of the tube. Thereby the end transition becomes radiationless.

The visible output of the He-Ne Laser has a long coherence length and excellent spatial quality which makes it a good source for holography and also as a wavelength reference for spectroscopy.

Applications

He-Ne lasers have a large number of industrial and scientific uses. It is used in lab demonstrations in optics. It is because of its low operation cost compared to other lasers, and also it produces beams of similar quality in terms of spatial coherence and long coherence length.

It is once used in Pioneer LaserDisc players and discontinued later. The barcode scanners in the supermarket once used red He-Ne lasers and were later discontinued.

Also, a stabilized Helium-Neon laser is used as a benchmark system for the definition of the meter.

Conclusion

This article is for the topic ‘Laser’ for the Tnpsc exam. We have discussed the Construction and working Helium-Neon Laser. And also its applications in daily life. Also, check its continuation from the below links.

References

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* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.
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