Introduction to He-Ne Lasers

He-Ne stands for HeliumNeon. The HeNe laser active medium consists of two gases which do not interact form a molecule. Therefore HeNe laser is one type of atomic gas lasers.

Construction of He-Ne Lasers:

The construction of typical HeNe laser plasma tube can be shown as:


The tube where the lasing action takes place consists of a glass envelop with a narrow capillary tube
through the center. The capillary tube is designed to direct the electrical discharge through its small
bore to produce very high current densities in the gas.

The output coupler and the HR (high reflective) mirror are located at the opposite ends of the plasma
tube. To make laser tubes more economical and durable manufacturers often attach the mirror s
directly to the ends of the capillary tube as shown above. This is very common with small low power
lasers. With high power tubes or when optically polarized output is desired, the capillary tubes ends
are cut at an angle and sealed with glass planes called Brewster windows. When this is done then the
mirrors mush be mounted in mechanically stable but adjustable mounts. This allows the operator to
align the mirror surfaces parallel to each other but perpendicular to the axis of the capillary tube.

The plasma tube has a large cylindrical metallic cathode and a smaller metallic anode. The current is
directed from cathode to anode.In figure shown, the gas reservoir provides a supply of extra gas. This reservoir helps to maintain a uniform pressure over long period of time and provides extra gas to replace any gas that may escape
through the tube or through the seals where the loads pass through the glass envelop. Usually all HeNe plasma tubes have a gas reservoir.

Function of He-Ne Laser:

In the HeNe laser the light is produced by atomic transitions within the Neon atom. The Helium does not directly produce laser light but it acts as a buffer gas, the purpose of which is to assist/help the atoms of the other gas to produce lasing in as manner.

When energy from the pumping source is applied HeNe gas mixture then some of the energy is observed by the Helium atoms. In other words we can say that helium atoms achieve an excited state. Now when the Helium atoms move within the laser tube, they collide with the Neon atoms. At each collision some of the energy within the helium atom is transferred to the Neon atom and so raising it to an excited metastable state. When a sufficient number of Neon atoms reach to this state then population inversion occurs and hence the lasing can take place.

This can be shown by simplified energy level diagram as:


Here upward transition shows the absorption of energy from the pumping source by Helium atom. While down ward transition shows the emission of energy / light or lasing present in the Neon atom only.

In diagram above there are 3 down word energy transitions for Neon that produce lasing. If transition occurs at the relatively small energy step from E5 to E4 then low energy infrared photon is released with a wavelength of 3.391 microns. If transition occurs at E5 to E2 which is much larger energy step then it produces short wavelength more energetic photon at 632.8nm. This gives the red light which is most desirable for HeNe laser applications.

E3 to E2 then it produces a laser output at 1.152microns in infrared portion of the spectrum.

Characteristics of HeNe Laser:

The HeNe laser is a relatively low power device with an output in the visible red portion of the spectrum. The most common wavelength produced by HeNe lasers is 632.8nm, although two lower power (1.152μm and 3.391μm) infrared wavelengths can be produced if desired. Majority of HeNe lasers generate less than 10m watt of power, but some can be obtained commercially with up to 50m watts of power. For HeNe lasers the typical laser tube is from 10 to 100 cm in length and the life time of such a tube can be as high as 20,000 hours.


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