Helium-neon laser is one of the earliest, most mature and most widely used lasers. The gain medium of the laser, as suggested by its name, is a mixture of helium and neon gases, in approximately a 10:1 ratio, contained at low pressure in a glass envelope. The gas mixture is mostly helium, so that helium atoms can be excited. The excited helium atoms collide with neon atoms, exciting some of them to the state that radiates 632.8nm.
Therefore, the laser produced by helium-neon laser is a pure monochromatic light with a wavelength error of only a few nanometers, and has a great coherence length, which is not affected by temperature fluctuations, plus the effect of resonant cavity, which ensures a good collimation of laser output (the divergence Angle is only a few milliradians).
Helium-neon lasers are widely used in the field of precision measurement, where good monochromatic light, coherence and collimation are needed.
The main drawback of helium-neon lasers is that they require thousands of volts of high-voltage dc power, and the power supply is prone to failure. In addition, helium-neon lasers have only a single red light and no other wavelengths of light, so their options are limited.
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