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The first successful operation of a continuous wave helium neon laser was observed by Javan, Bennett, and Herriott at Bell Labs in 1962. The helium neon laser uses a low pressure mixture of helium and neon gases. The mixture is predominantly helium with only 10% neon.
Most commercial helium neon lasers use a quasi-hemispherical cavity configuration, where a plane mirror is placed just short of the center of curvature of a concave spherical mirror. The disadvantage of this cavity configuration is that it only utilizes about one-third of the available plasma volume in the cavity. This limitation in power is more than outweighed by its ease of adjustment and stability once aligned.
Because of the rapid motion of atoms or molecules in the laser plasma tube, a gas laser transition does not consist of a sharp line. The Doppler effect causes it to broaden to a smooth Gaussian profile. A Doppler broadened line has a width dependent on the velocity of the atoms (temperature) and the wavelength of the transition. Red HeNe lasers typically have a full-width-half-max (FWHM) of about 1,400Mhz. Superimposed on the Doppler broadened gain curve is the cavity resonance function. For a 0.5 meter cavity, the mode spacing is 300Mhz. The sharpness of these modes is a function of the multipass nature of the cavity and is typically about 1Mhz. For such a cavity, laser output therefore consists of five or six single sharp frequencies (FWHM = approx. 1Mhz) separated by 300Mhz. The relative intensity of the cavity modes is defined by the Doppler broadened gain curve. In a very short cavity (less than 0.15 meter), only one mode will exist and the laser output will then consist of a single frequency.
Mode Sweeping / Mode Hopping:
The multiple longitudinal mode structure described above, of all but the shortest helium neon laser cavities (less than 6 inches) causes a power fluctuation phenomenon termed mode sweeping or mode hopping. The overall amplitude fluctuations are typically a few percent.
Melles Griot helium neon lasers are sealed with a truly zero-leak hard-seal. A special preformed ring of the frit sealing material is placed around the parts during initial assembly. Both the cathode retaining end-cap and the anode tube are manufactured of Kovar (a nickel/iron/cobalt alloy which is thermally compatible with the hard borosilicate glass).Using radio frequency induction heating techniques, the Kovar cathode and end-cap are welded to the plasma tube envelope. Both cavity mirrors are welded to the plasma envelope. The preformed frit material flows fully onto both parts creating a proven and reliable seal. This use of glass-metal seals throughout the tube and the complete absence of epoxy bonds results in the elimination of tube leakage. The overall volume of the plasma tube provides a sufficient gas reservoir so that the gas mixture in the plasma tube is invariant with time and is precisely known. The result is reliable, consistent operation and a long shelf life.
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