Types of Lasers

Based on the laser medium used, lasers are generally classified as solid state, gas, semiconductor, or liquid.

Solid-State Lasers

The most common solid laser media are rods of ruby crystals and neodymium-doped glasses and crystals. The ends of the rod are fashioned into two parallel surfaces coated with a highly reflecting nonmetallic film. Solid-state lasers offer the highest power output. They are usually operated in a pulsed manner to generate a burst of light over a short time. Bursts as short as 12 × 10-15 sec have been achieved, useful in studying physical phenomena of very brief duration. Pumping is achieved with light from xenon flash tubes, arc lamps, or metal-vapor lamps. The frequency range has been expanded from infrared (IR) to ultraviolet (UV) by multiplying the original laser frequency with crystal-like potassium dihydrogen phosphate, and X-ray wavelengths have been achieved by aiming laser beams at an yttrium target.

Gas Lasers

The laser medium of a gas laser can be a pure gas, a mixture of gases, or even metal vapor and is usually contained in a cylindrical glass or quartz tube. Two mirrors are located outside the ends of the tube to form the laser cavity. Gas lasers are pumped by ultraviolet light, electron beams, electric current, or chemical reactions. The helium-neon laser is known for its high frequency stability, color purity, and minimal beam spread. Carbon dioxide lasers are very efficient, and consequently they are the most powerful continuous wave (CW) lasers.

Semiconductor Lasers

The most compact of lasers, the semiconductor laser usually consists of a junction between layers of semiconductors with different electrical conducting properties. The laser cavity is confined to the junction region by means of two reflective boundaries. Gallium arsenide is the most common semiconductor used. Semiconductor lasers are pumped by the direct application of electrical current across the junction, and they can be operated in the CW mode with better than 50 percent efficiency. A method that permits even more-efficient use of energy has been devised. It involves mounting tiny lasers vertically in such circuits, to a density of more than a million per square centimeter. Common uses for semiconductor lasers include compact audio digital disk (CD) players and laser printers.

Liquid Lasers

The most common liquid laser media are inorganic dyes contained in glass vessels. They are pumped by intense flash lamps in a pulse mode or by a gas laser in the CW mode. Tunable dye lasers are a type for which frequency can be adjusted with the help of a prism inside the laser cavity.

Free Electron Lasers

Lasers using electrons unattached to atoms and pumped to lasing capacity by an array of magnets were first developed in 1977 and are now becoming important research instruments. They are tunable, as are dye lasers, and, in theory, a small number could cover the entire spectrum from infrared to X rays. Free electron lasers should also become capable of producing very high-power radiation that is currently too expensive to produce.

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