Helium-Neon Lasers

[Note to reader: topic sentences are in green; remaining weakness in red.]

ABSTRACT

The design of the helium-neon laser is not complex by modern standards. They consist of only three essential components and operate by the process of stimulated emission and light amplification. Because of their many advantages over other types of lasers, helium-neon lasers are used for many applications in research and industry.

The typical helium-neon laser consists of three components: the laser tube, a high-voltage power supply, and structural packaging. The laser tube consists of a sealed glass tube which contains the laser gas, electrodes, and mirrors. Depending on the power output of the laser, the tube may vary in size from one to several centimeters in diameter, and from five centimeters to several meters in length. The laser gas is a mixture of helium and neon in proportions of between 5:1 and 14:1, respectively. Electrodes, situated near each end of the tube, discharge electricity through the gas. Mirrors, located at each end of the tube, increase efficiency. The power supply provides the high voltages needed (10kV to start laser emission and 1-2kV to maintain it.) The structural packaging consists of mounts for the laser tube and power supply. The laser may also include safety shutters to prevent random exposure and external optics to fine-tune the beam.

[Paragraph below this is my revision of it.] The acronym, LASER, stands for Light Amplification by the Stimulated Emission of Radiation. The processes of light amplification and stimulated emission make the helium-neon laser work. Stimulated emission occurs when electricity is discharged into the laser gas. Electrons in the discharge collide with gas atoms imparting energy to them. These energized atoms are left in an unstable state in which some of their electrons have moved to a higher energy level. Excited atoms will quickly return to their ground state as their electrons drop to their normal levels. Each time an electron drops in level, it will emit a photon equal in energy to the difference between the levels. This type of emission is referred to as spontaneous emission. Stimulated emission occurs when a photon of the proper energy strikes an already excited atom, creating an identical photon. These photons will travel through the laser gas causing even more stimulated emission. This ever-increasing reproduction of photons is called light amplification. Using this process, the laser can effectively generate large numbers of photons from relatively few spontaneous emissions.

[Note revision, in navy incorporates the following concepts: (1) Familiar material is put at the beginning of each sentence; unfamiliar at the end. (2) In particular, each scientific term is mentioned after it is explained, at the end of a sentence. (3) The end of one sentence tends to lead to the subject of the next sentence.] The basic processes in a helium-neon laser are mirrored in the acronym laser, Light Amplification by Stimulated Emission of Radiation. In the laser tube, the electrodes discharge energetic electrons which subsequently transfer energy to the laser-gas atoms. These energized atoms, necessarily unstable, quickly return to their ground state. In the simplest case of a single excited level the excited atom spontaneously emits a photon equal to the energy difference between the excited and ground states; this process is called spontaneous emission. More importantly, such a spontaneously emitted photon can itself stimulate another excited atom to emit a photon. This "stimulated emission of radiation" is the second half of the acronym. Moreover since the energy difference between the excited and ground state of the atom equal the photon's energy, these photons are particularly effective at stimulating the emission of additional photons. As the spontaneously emitted photons travel through the gas encountering exited atoms. they strongly stimulate the emission of photos, thus leading to an amplification in the number of photons. This second process is denoted "light amplification" in the acronym laser.

In helium-neon lasers, the neon atoms are the source of laser light. Because stimulated emission only takes place when there are excited neon atoms available, the process will quickly come to an end unless the neon atoms are replenished with energy. The helium atoms in the laser gas carry out the process of re-energizing the neon. Helium is perfect for this task because it has a meta-stable state (does not decay as quickly) corresponding to the energy required to re-energize the neon. Therefore, not only do the helium atoms have the proper energy to re-energize the neon, they can hold onto that energy long enough to transfer it.

The amount of radiation that the neon atoms can emit is insufficient to produce a powerful beam without using some form of amplification. Much like a light bulb, the photons in the laser gas travel in random directions making it impossible to create a focused beam. The randomness of the photon paths also makes the laser inefficient because many photons may escape the tube before stimulating further emission. This problem is solved by placing mirrors at either end of the laser tube. Although many photons continue to escape the tube without being productive, those photons that are emitted parallel to the axis between the mirrors will be reflected many times. Each time the photons are reflected through the laser gas, they can cause more photons to be emitted in the same direction. In a short period of time, the dominant direction of emission will be along the axis between the mirrors. In standard configurations, one of the mirrors is totally reflective while the other can transmit one percent of all incident light. The beam is formed by the photons that escape through the partially transparent mirror.

While not the most powerful or efficient laser, the helium-neon laser has many advantages over other types of lasers. Most lasers have an efficiency of about 1 percent, about ten times the efficiency of the typical helium-neon laser. Most lasers are capable of delivering power far in excess of the helium-neon laser's 75 milliwatt limit. The advantages of helium-neon lasers are that they can emit visible light, are affordable and have good beam quality. While most lasers cannot efficiently emit visible light, helium-neon lasers usually emit at 632.8nm, producing a red beam. Helium-neon lasers do not require any consumables (sapphire rods or cryogenic gases for example), nor do they generate enough heat to require special cooling devices. They also have good beam quality, that is, their beams stay tightly focused even over long distances.

Helium-neon lasers are versatile devices that have many useful applications. They are often found in integrated bar code readers (the hand-held bar code readers use red semiconductor lasers or red LEDs.) Because they can emit visible light, helium-neon lasers are used in laser surgery to position the powerful infrared cutting beams. Surveyors take advantage of the helium-neon laser's good beam quality to take precise measurements over long distances or across inaccessible terrain. Red helium-neon lasers are also used in holography.

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References


[1]  Jeff Hecht,The Laser Guidebook, 2nd ed.  (Tab Books, 
     Blue Ridge Summit, PA, 1992), pp. 101-119.
[Single-source reference necessarily bring a single point of view to presentation. Always use more than one reference, especially ones with different points of view.]
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Edited by: wilkins@mps.ohio-state.edu [September 1997]