Helium-neon laser

A helium-neon laser, usually called a HeNe laser, is a type of small gas laser. HeNe lasers have many industrial and scientific uses, and are often used in laboratory demonstrations of optics. Its usual operation wavelength is 632.8 nm, in the red portion of the visible spectrum.[1]


Schematic diagram of a helium-neon laser


The gain medium of the laser, as suggested by its name, is a mixture of helium and neon gases, in a 5:1 to 20:1 ratio, contained at low pressure (an average 50 Pa per cm of cavity length [2]) in a glass envelope. The energy or pump source of the laser is provided by an electrical discharge of around 1000 volts[citation needed] through an anode and cathode at each end of the glass tube. A current of 5 to 100 mA is typical for CW operation.[3]. The optical cavity of the laser typically consists of a plane, high-reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end.
HeNe lasers are typically small, with cavity lengths of around 15 cm up to 0.5 m, and optical output powers ranging from 1 mW to 100 mW.
The red HeNe laser wavelength is usually reported as 632nm. However, the true wavelength in air is 632.816 nm, so 633nm is actually closer to the true value. For the purposes of calculating the photon energy, the vacuum wavelength of 632.991 nm should be used. The precise operating wavelength lies within about 0.002 nm of this value, and fluctuates within this range due to thermal expansion of the cavity. Frequency stabilized versions enable the wavelength to be maintained within about 2 parts in 1012 [4] [5] [6] for months and years of continuous operation.

The carbon dioxide laser (CO2 laser) was one of the earliest gas lasers to be developed (invented by Kumar Patel of Bell Labs in 1964[1]), and is still one of the most useful. Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available. They are also quite efficient: the ratio of output power to pump power can be as large as 20%.
The CO2 laser produces a beam of infrared light with the principal wavelength bands centering around 9.4 and 10.6 micrometers.

Amplification


The active laser medium (laser gain/amplification medium) is a gas discharge which is air cooled (water cooled in higher power applications). The filling gas within the discharge tube consists primarily of:
Carbon dioxide (CO2) (around 1020 %)
Nitrogen (N2) (around 1020%)
Hydrogen (H2) and/or xenon (Xe) (a few percent; usually only used in a sealed tube.)
Helium (He) (The remainder of the gas mixture)
The specific proportions vary according to the particular laser.
The population inversion in the laser is achieved by the following sequence:
Electron impact excites vibrational motion of the nitrogen. Because nitrogen is a homonuclear molecule, it cannot lose this energy by photon emission, and its excited vibrational levels are therefore metastable and live for a long time.
Collisional energy transfer between the nitrogen and the carbon dioxide molecule causes vibrational excitation of the carbon dioxide, with sufficient efficiency to lead to the desired population inversion necessary for laser operation.

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