A helium–neon laser or HeNe laser, is a type of gas laser whose
gain medium consists of a mixture of helium and neon inside of a
small bore capillary tube, usually excited by a DC electrical
discharge. History of HeNe laser development  The first HeNe
laser emitted at 1.15 μm in the infrared and was the first gas
laser. However a laser that operated at visible wavelengths was
much more in demand, and a number of other neon transitions were
investigated to identify ones in which a population inversion
can be achieved. The 633 nm line was found to have the highest
gain in the visible spectrum, making this the wavelength of
choice for most HeNe lasers. However other visible as well as
infrared lasing wavelengths are possible, and by using mirror
coatings with their peak reflectance at these other wavelengths,
HeNe lasers could be engineered to employ those transitions;
this includes visible lasers appearing red, orange, yellow, and
green. Lasing transitions are known from over 100 μm in the far
infrared to 540 n