The Audio Control of Laser Displays

The Laser

V/L III

Fig. 1. VIDEO/LASER III

The scientific discoveries that led to the development of the laser came in the 1950s and 1960s. In 1981, a well–publicized suit was being argued in the courts regarding a patent application filed by Gordon Gould in 1959 that apparently disclosed techniques that could have been applied to construct a laser. However, Theodore H. Maiman, a researcher at the Hughes Aircraft Company, conclusively demonstrated the first operational laser in 1960, using a ruby rod as the lasing medium.

Charles H. Townes (U.S.), Nicolai G. Basov (U.S.S.R.), and Alexander M. Prokhorov (U.S.S.R.) won the 1964 Nobel Prize in physics for their basic research investigations in quantum electronics that resulted in the engineering breakthroughs of Maiman and others. Lasers were originally called “optical masers,” maser being the acronym for an earlier device, which produced Microwave Amplification by Stimulated Emission of Radiation. The acronym laser comes from Light Amplification by Stimulated Emission of Radiation. The microwave spectrum is 3 x 109 to 3 x 1011 Hz; the infrared, 3 x 1011 to 4 x 1014 Hz; the spectrum visible to the human eye, 3.95 x 1014 Hz to 7.9 x 1014 Hz with the ultraviolet, X–ray, gamma ray, and cosmic ray spectra above the visible in the total electromagnetic spectrum. The visible spectrum can be expressed as wavelengths occupying the range of 700 nanometers (or 7 x 10–7 meter: red) to 400 nm (4 x 10–7 meter: violet) — less than one “octave.”

The fundamental property of lasers is the coherent nature of the light emitted. This coherence of the light simply means that all corresponding points on the wavefront are in phase. The atoms in the lasing medium emit light waves (or photons, if one prefers) that are not only in phase, but are all eventually propagated together in the same direction. Gas lasers—helium–neon, argon, krypton, and mixed–gas—are types most often used in laser light shows. They incorporate a plasma tube (a gas discharge tube not unlike a fluorescent lamp or a neon sign), which is surrounded by a strong electromagnetic field and placed within an optical cavity. The coherent, in–phase propagation of the photons results from several simultaneous actions occurring within the laser head: electrons are accelerated through the plasma tube and “spun” by the magnetic field, causing ionization and “population inversions” of the gas atoms, the stimulation of which results in the emission of the photons as light energy. The optical cavity in a typical argon or krypton ion laser is about one meter in length, within which standing waves on the order of 106 in magnitude occur, at the visible wavelengths of light. Resonances, standing waves, and oscillations are produced in the cavity in the 3.95 x 1014 to 7.9 x 1014 Hz range. This optical cavity is defined as the distance between two mirrors at either end of the laser head, often outside of the plasma tube.

The rear mirror is almost totally reflective, while the front transmitting mirror is partially reflective and allows the output beam to exit from the optical cavity. This beam of light exhibits very special properties owing to its coherence: it is usually quite intense (very concentrated in cross–sectional area), collimated (the light beam consists of almost–parallel rays, diverging very slightly over great distances), and monochromatic (totally saturated in hue). This last property can be ascribed to the single wavelength nature of the photon emission, which defines a discrete frequency in the visible spectrum. By comparison, all other forms of light are emitted with phase relationships of the photons distributed in a random, non–coherent manner. The analogy of white light, such as sunlight, to random “white” noise in the audio spectrum is both appropriate and meaningful.

In addition to solids such as the ruby, and gases such as krypton, lasing media can include liquids. Exotic liquids containing dyes have been used to construct lasers whose wavelengths are tunable over a wide range in the visible spectrum. However, gas lasers, especially krypton or mixed–gas krypton–argon, offer the most attractive possibilities for laser art, because they produce several specific wavelengths of pure spectrum colors simultaneously. The plasma tubes of mixed–gas lasers contain a mixture of these two “noble” gases (each usually having a valence of 0), which become ionized to a higher positive valence when the plasma tube is operating. The laser produces the spectral lines of krypton and argon, all of which are bound up in a single output beam approaching white. The total beam may be refracted into its colored component beams by a prism. An example of a mixed–gas laser is the Coherent Inc. model CR–MG, used in the VIDEO/LASER III deflection system at The University of Iowa.

In 1981, Coherent Inc. and a major competitor, Spectra–Physics, sold ion lasers like the CR–MG beginning at $15,500 (plus shipping). These “scientific” or “industrial” lasers require careful setup and maintenance procedures; a certain level of competence is expected of the user. Such lasers do not operate on ordinary house current (208 V, 3 phase, 10 kW AC service is required); an adequate supply of water for cooling must flow through both the laser head and its separate regulated power supply, and precise operating conditions of voltage, current, and pressure within the plasma tube must be maintained.

As many as thirty discrete wavelengths from the infrared through the visible to the ultraviolet may be generated from ionized krypton and argon gases. Following are the ones most useful for kinetic laser art displays.

Wavelength
(nanometers)
ColorLasing MediumCoherent Inc.
CR–MG laser
647.1*redkrypton250
568.2*yellowkrypton100
530.9yellow–greenkrypton80
520.8greenkrypton100
514.5*greenargon250
488.0*blue–greenargon250
476.5*blueargon100
468.0*deep blue (violet)kryptonless than 50
*Used in the VIDEO/LASER systems. The 647.1, 514.5 and 476.5 nm wavelengths are found very close to the “corners” of the chromaticity diagram of CIE (Comité International d’Éclairage). They were used as the red, green, and blue primary colors in a laser–generated projection color television system exhibited in the Hitachi Pavilion at Expo ’70, Osaka, Japan.

Since all of these wavelengths are available together, the total (unrefracted) output beam of a mixed–gas laser can measure up to 2 watts. An ordinary light bulb with an output rating of 100 watts can deliver only about 15 to 20 watts of light power output. Even this would appear to be much more than the 2 watts from the laser, yet at 30 meters from either source, the laser beam is on the order of 1,000,000 times more intense. Because of this concentration of power in collimated laser beams, stringent safety precautions must be observed when lasers are operated in public.[1] However, with the proper safeguards, viewing a laser event is as safe for the eyes of the audience members as watching a motion picture.


[1] See Deborah Van Brunt: Laser Light Show Safety: Who’s Responsible? Rockville, Maryland: U.S. Department of Health and Human Services, Bureau of Radiological health. 1980. FDA Publication No. 80-8121