Laser - Cenni storici

Any of a class of devices that produces an intense beam of light of a very pure single colour. This light beam may be intense enough to vaporize the hardest and most heat-resistant materials. The word laser is an acronym derived from "light amplification by stimulated emission of radiation."

Ruby laser being used in a Q-switch, a special switching device that produces giant output pulse . . .

Fundamental principles

Atoms and molecules exist at low and high energy levels. Those at low levels can be excited to higher levels, usually by heat, and after reaching the higher levels they give off light when they return to a lower level. In ordinary light sources the many excited atoms or molecules emit light independently and in many different colours (wavelengths). If, however, during the brief instant that an atom is excited, light of a certain wavelength impinges on it, the atom can be stimulated to emit radiation that is in phase (in step) with the wave that stimulated it. The new emission thus augments or amplifies the passing wave; if the phenomenon can be multiplied sufficiently, the resulting beam, made up of wholly coherent light (i.e., light of a single frequency or colour in which all the components are in step with each other), will be tremendously powerful.

Albert Einstein recognized the existence of stimulated emission in 1917, but not until the 1950s were ways found to use it in devices. The American physicists Charles H. Townes and A.L. Schawlow showed that it was possible to construct such a device using optical light. Two Soviet physicists proposed related ideas independently. The first laser, constructed in 1960 by Theodore H. Maiman of the United States, used a rod of ruby. Since then many types of lasers have been built.

Of the several different types of lasers produced by different means and used for different purposes, the following are most important.

Optically pumped solid-state lasers

One way to achieve the excitation of atoms to the higher energy level for laser action to take place is by illuminating the laser material with light of a frequency higher than that which the laser is to emit. This process is called optical pumping; the light pump must be of high intensity, as the process is usually rather inefficient.
An optically pumped solid-state laser consists of a rod of the material chosen, with its ends polished flat and parallel and coated with mirrors to reflect the laser light. The sides are left clear to admit the light from the pumping lamp, which may be a pulsed gas discharge, flashing on and off like a photographer's electronic flash bulb. It may be wound around the laser rod, positioned alongside, or focused on it by a mirror (see Figure). The first operating laser employed a rod of pink ruby, an artificial crystal of sapphire (aluminum oxide). Many other rare-earth elements have since been employed, the most widely used being neodymium. Power outputs in the form of brilliant flashes of light of thousands of watts can be obtained.

Liquid lasers

Solid-state lasers have the disadvantage of occasional breakdown and damage at higher power levels because of the intense heat generated within the material and by the pumping lamp. The liquid laser is not susceptible to such damage; the crystalline or glassy rod is replaced by a transparent cell containing a suitable liquid, such as a solution of neodymium oxide or chloride in selenium oxychloride. Such cells can be made as large as desired to increase power output. Only a small number of inorganic liquids, however, will function as lasers.

Dye lasers

Certain organic dyes are capable of fluorescing--i.e., re-radiating light of a different colour. Though the excited state of their atoms lasts only a small fraction of a second and the light emitted is not concentrated in a narrow band, many such dyes have been made to exhibit laser action, with the advantage that they can be tuned to a wide range of frequencies.

Dyes such as rhodamine 6G, which emits orange-yellow light, can be made to lase (provide laser action) by excitation by another laser. Rhodamine 6G was the first dye for which continuous, rather than pulsed, operation was achieved, making possible the production of a continuous beam of tunable laser light. Another dye, methylumbelliferone, with the addition of hydrochloric acid, can be made to lase at wavelengths varying across the light spectrum from ultraviolet to yellow, producing laser light of almost any desired frequency within this range.

Gas-discharge lasers

Atoms in a gas discharge can be excited to radiate and produce light, as in a neon sign. Occasionally, a particular energy level will cause an exceptionally high number of atoms to accumulate within it. If mirrors are positioned at the ends of the discharge tube, laser action results. Though the conditions are unusual and occur for only a few of the many wavelengths at which the discharge emits, most gases can be made to exhibit laser action at some wavelength under certain discharge conditions. Gas-discharge lasers commonly use a helium-neon mixture, though those designed to produce laser action at infrared wavelengths employ such gases as carbon monoxide and hydrogen cyanide.

Gas dynamic lasers

If a hot gas is allowed to cool rapidly, the number of molecules in a low-energy state may decrease more rapidly and fall below the number in a higher energy state, thus permitting laser action. This condition can be achieved by expanding burning carbon monoxide mixed with nitrogen through jet nozzles. High power outputs of more than 30,000 watts can be obtained.

Chemical lasers

Certain chemical reactions produce enough high-energy atoms to permit laser action to take place. Laser action can occur in carbon dioxide, for example, if it is present when the elements hydrogen and fluorine are reacting to produce hydrogen fluoride. Large amounts of energy can be released when only moderate amounts of the appropriate materials react.

Semiconductor lasers

A semiconductor laser consists of a flat junction of two pieces of semiconductor material, each of which has been treated with a different type of impurity. Aluminum gallium arsenide and gallium arsenide typically are used in lasers of this type, though pairs of other so-called III-V compound semiconductors may be employed (see semiconductor device). When a large electrical current is passed through such a device, laser light emerges from the junction region. Power output is limited, but the low cost, small size, and comparatively high efficiency make these devices suitable for use as light sources in optical fibre communications systems (see below) and in compact digital audio disc players.

Free-electron lasers

Lasers of this type are more efficient than any other variety in producing beams of very high power radiation. Furthermore, these devices are tunable, so that they can be made to operate at microwave to ultraviolet wavelengths. (Theoretically they have the potential of generating laser radiation of X-ray wavelength, though present technology is still incapable of such short wavelengths.) In a free-electron laser, free electrons (i.e., those not bound to nuclei) from a particle accelerator or some other source are passed through an undulator (commonly called a "wiggler"), a device consisting of a linear array of electromagnets. An alternating magnetic field in the undulator bends the electrons into a spiral path around the lines of force, whereby they are accelerated to velocities approaching the speed of light and emit energy in the form of synchrotron radiation (q.v.). The intensity and wavelength of this radiation can be adjusted by modifying certain parameters of the magnetic field. Because of this ability to produce laser light tunable over a broad range of wavelengths and high efficiency, researchers believe that the free-electron laser, with further development, will prove especially suitable in such applications as isotope separation, semiconductor research, and ballistic missile defense (namely, as a laser beam weapon).

Lasers producing short, intense pulses

A shutter placed between the amplifying column and the end mirrors of a laser can prevent laser action as long as it is closed. If conditions are otherwise correct for laser action and the shutter is suddenly opened, the stored energy is released as a giant pulse of light lasting only a tiny fraction of a second and having a peak power capacity that may be as high as several hundred thousand kilowatts. This is known as Q-switching. The Q-switch may be a mechanical shutter or, more usually, a liquid or solid optical shutter that is normally opaque but can be made transparent by the application of an electrical pulse. The shutter may also be an opaque dye which becomes transparent when exposed to laser light.

Normally a laser oscillates in several modes--i.e., at several different frequencies. By synchronizing these modes, a process called mode-locking, even shorter, more powerful pulses can be obtained. Such pulses are useful in scientific investigations and in puncturing holes so rapidly that the surrounding material is not affected.

Laser applications

The light produced by lasers is in general far more monochromatic, directional, powerful, and coherent than that from any other light sources. Nevertheless, the individual kinds of lasers differ greatly in these properties as well as in wavelength, size, and efficiency. There is no single laser suitable for all purposes, but some of the combinations of properties can do things that were difficult or impossible before lasers were developed.

A continuous visible beam from a laser using a gas, such as the helium-neon combination, provides a nearly ideal straight line for all kinds of alignment applications. The beam from such a laser typically diverges by less than one part in a thousand, approaching the theoretical limit. The beam's divergence can be reduced by passing it backward through a telescope, although fluctuations in the atmosphere then limit the sharpness of a beam over a long path. Lasers have come to be widely used for alignment in large construction--e.g., to guide machines for drilling tunnels and for laying pipelines.

A pulsed laser can be used in a light radar, sometimes called LIDAR, and the narrowness of its beam permits sharp definition of targets. As with radar, the distance to an object is measured by the time taken for the light to reach and return from it, since the speed of light is known. LIDAR echoes have been returned from the Moon, facilitated by a multiprism reflector that was placed there by the first astronauts to land there. Distances can be measured from an observatory on Earth to the lunar mirror with an accuracy of several centimetres. Simultaneous measurements of the mirror's distance and direction from two observatories on different parts of the Earth could give an accurate value for the distance between the two observatories. A series of such measurements can tell the rate at which continents are drifting relative to each other.

A vertically directed laser radar in an airplane can serve as a fast, high-resolution device for mapping fine details, such as the contours of steps in a stadium or the shape of the roof of a house. With a pulsed laser radar, returns can be obtained from dust particles and even from air molecules at higher altitudes. Thus air densities can be measured and air currents can sometimes be traced.

The high coherence of a laser's output is very helpful in measurement and other applications involving interference of light beams. If a light beam is divided into two parts that travel different paths, when the beams come together again they may be either in step so that they reinforce each other or out of step so that they cancel one another. Thus the brightness of the recombined wave changes from light to dark, producing interference fringes, when the difference in path lengths is changed by one-half of a wavelength. Such devices are called laser interferometers. Very small displacements can be detected, and larger distances can be measured with precision. With lasers, these measurements can be carried out over extremely long distances. Laser interferometers are used to monitor small displacements in the Earth's crust across geological faults. In manufacturing, such devices are employed to gauge fine wires, to monitor the products of automated machine tools, and to test optical components.

Lasers can be so monochromatic that a small shift in the light frequency can be detected. Light reflected from an object that is moving toward the laser is raised in frequency by an amount depending on the velocity of the object (Doppler effect). For a receding object, the frequency is lowered. In either case, if some of the original and the shifted light are recombined at a photodetector, a signal at the difference frequency (the difference in frequency between the original and the shifted light) is observed, and even small velocities can be measured.

The brightness and coherence of laser light make it especially suitable for visual effects and photography that simulate third dimensional depth--e.g., holography (q.v.).

The light from many lasers is relatively powerful and can be focused by a conventional lens system to a small spot of great intensity. Thus even a moderately small pulsed laser can vaporize a small amount of any substance and drill narrow holes in the hardest materials. Ruby lasers, for example, are used to drill holes in diamonds for wire drawing dies and in sapphires for watch bearings. For biological research, a finely focused laser can vaporize parts of a single cell, thus permitting microsurgery of chromosomes.

Strong heating can be produced by a laser at a place where no mechanical contact is possible. Thus one of the earliest applications of lasers was for surgery on the retina of the eye.

Lasers are also used for small-scale cutting and welding. They can trim resistors to exact values by removing material and can alter connections within integrated arrays of microcircuit elements. A pulse of light from a laser can vaporize a sample of a substance for analysis by suitable instruments. By this method an extremely small sample can be analyzed without introducing contaminants.

The high brightness, pure colour, and directionality of laser light make it ideally suited for experiments on light scattering. Even a small amount of light that is scattered with a change of wavelength or direction can be readily identified. In particular, a type of scattering known as the Raman effect produces characteristic wavelength shifts by which molecular species can be identified. With laser sources and sensitive spectrography, small samples of transparent liquids, gases, or solids can be analyzed. It is even possible to measure contaminants in the atmosphere at a considerable distance by the Raman scattering of light from a laser beam.

Laser beams can be used for communications. Because the light frequency is so high (around 5 10{sup 14} hertz for visible light), the intensity can be rapidly altered to encode very complex signals. In principle, one laser beam could carry as much information as all existing radio channels. Laser light can, however, be blocked by rain, fog, or snow so that, for reliable communications on Earth, the laser beam would need to be enclosed in a protective medium. Optical fibres made of glass and covered with a cladding material are employed for this purpose. Waveguides of this kind have been used increasingly in long-distance telephone systems since the early 1980s (see fibre optics).

Laser technology is integral to optical disc recording and storage systems. In such a system, digital data are recorded by burning a series of microscopic holes, commonly referred to as pits, with a laser beam into thin metallic film on the surface of a small-diameter disc. In this manner, information from magnetic tape is encoded on a master disc, which is replicated by a process known as stamping. In the read mode, laser light of low intensity is reflected off the disc surface and is "read" by light-sensitive diodes. The amount of light received by the diodes varies according to the presence or the absence of the pits, and this input is digitized by the diode circuits. The digital signals are subsequently converted to analog information on a video screen. Compact audio disc players work in much the same way except that the digital signals are transformed into sound impulses.

Lasers also are used in a major type of computer printer. Laser printers employ a laser beam and a system of optical devices to etch images on a photoconductor drum. The images are carried from the drum to paper by means of electrostatic photocopying.

(Tratto dall'Enciclopedia Britannica)