Helium Neon Laser Basics

A Helium neon laser refers to a kind of gas laser with a gain medium consisting of a mix of neon and helium inside bore capillary tubes, generally excited through the discharge of DC electric current. Also called a He Ne laser, it is used for several purposes, primarily industrial usage.

The first lasers

The very first helium neon lasers had an emission at around 1.15 µm in infrared and were the very first examples of gas lasers. However lasers which functioned at a visible wavelength found favor with a larger number of people, numerous other neon related transitions were researched upon for identifying the ones wherein population inversion could be achieved. As it is, 633 nm had the biggest gain in the visible spectrum, thereby making it the preferred wavelength for most helium neon lasers that were manufactured. However there are other infrared and visible lasing wavelengths that are possible. By making use of a mirror coating with the peak reflectance of the other wavelengths, helium neon lasers can be engineered for using those transitions. Thereby we get visible lasers in red, orange, green and yellow colors.

The lasing transitions

Lasing transitions may range from 100 µm in far infra-red to around 540 nm in visible. Now, since the visible transition at other wavelengths than 633 nm does not have higher gain, such lasers usually have low output powers. Moreover, these are more expensive too. The best and the most popular helium neon laser functions at a 632.8 nm wavelength in the red portion of a visible spectrum.

The gas mixture

The mix of neon and helium gases is in a ratio of approximately 10:1, and is contained at lower pressure within glass envelopes. The energy source of a laser is through the high voltage discharge of electricity that is passed through gas between the electrodes within the tubes. 3 to 20 mA DC current is usually needed for the CW operation. The optical cavity of a laser generally features 2 concave mirrors. It may also feature one concave and one plane mirror, with one having high reflectance while an output-coupler mirror allows approximately one per cent transmission. Gain bandwidth in He Ne lasers is dominated by the Doppler broadening instead of pressure broadening because of lower gas pressure, which makes it quite narrow.

Several industrial and scientific uses

As it is, the helium neon laser has several industrial as well as scientific uses. Such lasers are broadly used for laboratory demonstration in optical research or study because of the comparatively lower cost as well as ease of operation. A consumers application of the red helium neon laser is the laser disc player, by Pioneer. In this device a laser is used for reading the optical disks.

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