What Determines the Wavelength of a Laser?
So why are (most) helium-neon (HeNe) lasers red, argon ion lasers green and blue, and CO2 lasers IR? There is no way to provide a complete answer in a brief discussion without fancy math (quantum mechanics, etc., which would put you to sleep) but it is possible to outline some of the requirements for laser output to be possible at a particular wavelength. Consider the lasing medium – for example, such as the 7:1 mixture of helium and neon used in a HeNe laser. If the gas mixture is excited by an electrical discharge, it will produce a bright line spectrum similar to what is shown in Bright Line Spectra of Helium and Neon. Each of the colored lines represents a particular energy level transition in helium or neon (separate in this case, the combined mixture will differ slightly). One might think that the brightest and thus strongest spectral lines are the most likely to result in laser action. This is not necessarily the case.
So why are (most) helium-neon (HeNe) lasers red, argon ion lasers green and blue, and CO2 lasers IR? There is no way to provide a complete answer in a brief discussion without fancy math (quantum mechanics, etc., which would put you to sleep) but it is possible to outline some of the requirements for laser output to be possible at a particular wavelength. Consider the lasing medium – for example, such as the 7:1 mixture of helium and neon used in a HeNe laser. If the gas mixture is excited by an electrical discharge, it will produce a bright line spectrum similar to what is shown in Bright Line Spectra of Helium and Neon. Each of the colored lines represents a particular energy level transition in helium or neon (separate in this case, the combined mixture will differ slightly). One might think that the brightest and thus strongest spectral lines are the most likely to result in laser action. This is not necessarily the case. For the HeNe case, *none* of the lines in the helium spectra
