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. The best known form operates at a wavelength of 632.8 nanometer (nm), in the red portion of the optical spectrum or visible spectrum.
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 He at 1 torr and Ne at 0.1torr (an average 50 pascal unit (Pa) per cm of cavity length) in a glass envelope. The energy or pump source of the laser is provided by a high voltage electrical discharge through an anode and cathode at each end of the glass tube. A current of 5 to 100 mA is typical for Continuous wave (CW) operation. 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 normally small, with cavity lengths of around 15 cm up to 0.5 m, and optical output powers ranging from 1 m watt (W) to 100 mW.
The red HeNe laser wavelength is usually reported as 632 nm. However, the true wavelength in air is 632.816 nm, so 633 nm 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 drift|Frequency stabilized versions enable the wavelength to be maintained within about 2 parts in 1012 for months and years of continuous operation.
The laser process in a HeNe laser starts with collision of electrons from the electrical discharge with the helium atoms in the gas. This excites helium from the ground state to the 23S1 and 21S0 long-lived, metastable excited states. Collision of the excited helium atoms with the ground-state neon atoms results in transfer of energy to the neon atoms, exciting neon electrons into the 3s2 level. This is due to a coincidence of energy levels between the helium and neon atoms.
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