Helium-neon laser, output

The most familiar gas laser is the helium—neon laser (23,24). Sales of commercial helium—neon lasers exceed 400,000 units per year. The helium—neon laser is a compact package that produces a continuous beam of orange-red light. The inside diameter of the tube is commonly around 1.5 mm. The output of helium—neon lasers available commercially ranges from a fraction of a milliwatt to more than 35 mW. They have many appHcations in the areas of alignment, supermarket scanning, educational demonstrations, and holography. 

Although the first laser demonstrated was a solid state ruby laser, for many years the most common commercial systems were gas lasers such as helium neon lasers and argon ion lasers, or lasers based on organic dyes. Helium neon lasers were frequently limited in output power, argon ion lasers required expensive, sophisticated power supplies and cooling sources, and the dyes used in dye lasers were messy and often toxic. In the past decade, solid state lasers and diode lasers have become the dominant players in the commercial marketplace. 

Another common laser class is that of gas lasers, which includes helium neon (HeNe) lasers, carbon dioxide (CO2) lasers, nitrogen lasers, and so on. The helium neon laser, widely used until the advent of the diode laser, was one of the first types developed and commercialized. As described above, it is a discharge-pumped gas laser, which generally produces an output measuring a few mW in power. 

Gas Lasers. A variety of ga.s lasers is available commercially. These devices are of four types (1) neutral atom lasers such as He-Ne (2) ion lasers in which the aciive species is Ar or Kr (3) molecular lasers in which the lasing medium Is CO, or Nil and (4) excimer lasers. The helium-neon laser is the most widely encountered of all lasers because of its low initial and maintenance costs, its great reliability, and its Kivv H)wcr consumption. The nuist important of its output lines is at 6.32.8 nm. It is generally operated in a continuous mode rather than a pulsed mode. 

The diameter of the helium-cadmium laser beam was substantially less than the diameter of the helium-neon laser beam. The interferometer output was the interference pattern of the polarized beams of the helium-neon laser, against the background of which was observed a periodically time-varying intensity region corresponding to the action of the helium-cadmium laser. 

Fig.13.12, Saturated absorption on the 6328 A transition of a helium-neon laser. (a) Neon absorption tube inside laser resonator. (b) Output power as a function of oscillation frequency showing saturated absorption resonance superimposed on normal Gaussian envelope. (After Lee and Skolnick (1967).)...

Brightness. This is defined as the power emitted per unit area of the output mirror per unit solid angle and is extremely high compared with that of a conventional source. The reason for this is that, although the power may be only modest, as in, for example, a 0.5 mW helium-neon gas laser, the solid angle over which it is distributed is very small. 

This transition is, at present, the shortest wavelength C.W. laser transition available and is widely used in the fields of photochemistry and photofabrication. Unfortunately, lasers pumped by Penning reactions are limited in output power by the saturation of the helium metastable density at high currents, exactly as in the case of the helium-neon system discussed,in section 11.4.2(d). In an attempt to overcome this limitation, considerable effort has been made to develop metal vapour laser systems excited by thermal-energy charge transfer. 

The familiar 632.8 nm output of the He-Ne laser is available in several integrated Raman spectrometers of analytical interest. Unlike ion lasers, the lasing transition in the He-Ne is an atom (neon), so power need not be consumed to create excited ions. Helium ions and electrons carry the current in the He-Ne laser tube, but energy is transferred to Ne atoms before lasing, and the process is much more efficient than that of ion lasers. Ordinary 110 V electrical power and air cooling are sufficient, and He-Ne lasers are generally much smaller (and less expensive) than ion lasers. However, the output optical power is much lower for the He-Ne laser compared to Ar+ lasers, with the common... 

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