Fig.1. Construction of He-Ne laser

The first such 1аsег was built in I960 at Bеll Telephone Laboratories. It consists of a discharge tube 100 cm long with an inside diameter of 1,5 cm filled with helium at 1 torr pressure and with neon at 0.1 torr. Flat reflector plates, which must be adjusted parallel within a few seconds of arc, are included in the gas filled section of the tube.

A simplified diagram of this laser is given in Fig.1.

The power required to excite the laser action is in the tens-of-watts range. The output роwer is in the 1/100-th watt range. At this level of operation, the tube is so cool that it can be held in the hand without discomfort.

Thе great value of helium-neon lasers is their remarkable monochromaticity and stability under carefully controlled experimental conditions.

To communication engineers such a continuously operating source is an important device.

Molecular Lasers

Molecular lasers are the most high powered and most efficient type of gas laser. They work by the transfer of vibrational energy from one type of molecule to another. The atoms making up the molecule, when it is excited, vibrate relative to one another, and the molecule has energy levels similar in form, although of different value, to the energy levels of isolated atoms. But the process does not involve the movement of orbiting electrons to more distant orbits.

The type of structure used in molecular lasers is very different from that used in neutral atom lasers. In one type of molecular laser, flowing nitrogen is excited by electrical discharge and then flows into the tube between the end mirrors which contains the active gas. This is usually carbon dioxide. The vibrational energy of the nitrogen molecules is transferred to the carbon dioxide molecules by collision, and the carbon dioxide atoms later return to the ground state giving up the energy to the laser beam.

An efficiency of about 15 per cent was obtained with the carbon dioxide type, which is far greater than the 0.1 per cent achieved with the neutral atom lasers. Apart from the fact that it is more efficient than the other molecular gases, carbon dioxide also has the advantage that it is chemically stable and can, if necessary, be excited directly by an electric discharge.

The path between the end mirrors, that is, the length of the carbon dioxide laser tube, must be as long as possible if large amounts of power are to be obtained. Tubes 20 m long have been used and if such lasers are to be conveniently mo­bile they must be folded in some way. Two, three and four tubes placed parallel to each other and optically coupled have been used in some carbon dioxide lasers.

Laser Applications

The use of lasers is restricted only by imagination. Lasers have become valuable tools in industry, scientific research, communication, medicine, military technology, and the arts.

Industry

Powerful laser beams can be focused on a small spot with enormous power density. Consequently, the focused beams can readily heat, melt or vaporize material in a precise manner. Lasers have been used, for example, to drill holes in diamonds, to shape machine tools, to trim microelectronic components, to heat-treat semiconductor chips, to cut fashion patterns, to synthesize new material, and to attempt to induce controlled nuclear fusion. The powerful short pulse produced by a laser also makes possible high-speed photography with an exposure time of several trillionths of a second. Highly directional laser beams are used for alignment in road and building construction.

Lasers are used for monitoring crustal movements and for geodetic surveys. They are also the most effective detectors of certain types of air pollution. In addition, lasers have been used for precise determination of the earth-moon distance and in tests of relativity. Very fast laser-activated switches are being developed for use in particle accelerators, and techniques have been found for using laser beams to trap small numbers of atoms in a vacuum for extremely precise studies of their spectra.