He-Ne lasers and laser applications
The design of the helium-neon laser is not complex by modern standards. They consist of only three essential components and operate by the process of stimulated emission and light amplification. Because of their many advantages over other types of lasers, helium-neon lasers are used for many applications in research and industry.
The laser tube consists of a sealed glass tube which contains the laser gas, electrodes, and mirrors. Depending on the power output of the laser, the tube may vary in size from one to several centimeters in diameter, and from five centimeters to several meters in length. The laser gas is a mixture of helium and neon in proportions of between 5:1 and 14:1, respectively.The first HeNe lasers emitted light at 1.15 μm, in the infrared spectrum, and were the first gas lasers. 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 stimulated emission 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.
The mechanism producing population inversion and light amplification in a HeNe laser plasma originates with inelastic collision of energetic electrons with ground state helium atoms in the gas mixture. As shown in the accompanying energy level diagram, these collisions excite helium atoms from the ground state to higher energy excited states, among them the 23S1 and 21S0 long-lived metastable states.
This excitation energy transfer process is given by the reaction equations:
He*(23S1) + Ne1S0 → He(1S0) + Ne*4s2 + ΔE and
He*(21S) + Ne1S0 + ΔE → He(1S0) + Ne*5s2
where (*) represents an excited state, and ΔE is the small energy difference between the energy states of the two atoms, of the order of 0.05 eV or 387 cm−1, which is supplied by kinetic energy.
Red HeNe lasers have many industrial and scientific uses. They are widely used in laboratory demonstrations in the field of optics because of their relatively low cost and ease of operation compared to other visible lasers producing beams of similar quality in terms of spatial coherence and long coherence length (however since about 1990 semiconductor lasers have offered a lower-cost alternative for many such applications). A consumer application of the red HeNe laser is the LaserDisc player, made by Pioneer. The laser is used in the device to read the optical disk.