Did you know that the word "laser" is actually an acronym? It stands for light amplification by stimulated emission of radiation. The laser is amongst the newest inventions in physics; the first laser was invented only in 1960. Yet, lasers have found a huge number of applications. Examples of laser applications include the CD player, CD-ROM, laser printers, fiber optic communication, entertainment, holography, laser cutting, laser hardening of materials, surgery of various kinds, UPC code readers (at sales counters), laser sorting of biological cells, laser induced nuclear fusion, laser ranging and remote sensing. The laser has also proved to be a revolutionary tool in physics, chemistry, bio-medicine and engineering research. Military applications include range finding, guiding of munitions with laser designators and countermeasures against enemy sensors. Contrary to what you may have seen in movies, lasers have not proven practical as shooting weapons.

How lasers Work	The Drama and History of the Invention
Common Lasers, Characteristics & Applications	Further reading

How Lasers Work

Lasers are simple devices and the basic laser is extremely simple -- at least conceptually. A mirror (typically 1-inch diameter), a "gain" material and a partially transmitting (see through) mirror. That's it! The two mirrors, separated by the gain medium, are held highly parallel to one another. When the gain medium is "excited" (e.g. by passing electricity or by firing a flash lamp next to it), the medium emits light (a subclass of electromagnetic "radiation"), which bounces back and forth between the mirrors while stimulating the medium into emitting more of the light (or radiation). A kind of an avalanche of light emission sets in. The light oscillation between the mirrors then intensifies and a strong beam of light makes its way out through the partial mirror. The real trick in building a laser lies in finding the material that is capable of being excited to a sufficient degree to sustain the oscillation.

Because of the way the device operates, it has been suggested that a more descriptive name would be: "light oscillation through stimulated emission of radiation" or the LOSER. But that acronym never gained any favor for a device that is such a winner!

The laser beam is only a well ordered beam of light. It does have characteristics that distinguish it from "ordinary" light such as light from the sun or a light bulb or even LEDs. The laser beam tends to be highly monochromatic (pure color). It tends to be highly coherent i.e. its component waves are in phase or lock step. The laser beam tends to be very parallel i.e. it can travel tremendous distances with very minimum fanning out. The laser beam can be focused extremely tightly such that even a low energy beam can produce tremendous energy densities in the focused region and can cut through any material -- even diamonds.

Lasers vary widely in size, wavelength, power and pulse duration. The smallest lasers (such as those found in a CD player) can be smaller than a pin head; the largest can take up a few buildings. In wavelength, they vary from UV through visible to infra-red. In pulse duration they commonly vary all the way from continuous wave (cw) to femto-seconds (10^-15 sec)  duration. The highest peak or instantaneous powers achieved have been trillions of watts.

The Drama and History of the Invention

1916: The theory of stimulated emission formulated by Albert Einstein in 1916. But the effect normally is so small that it was essentially forgotten until the 1950's.

1950's -- First Half: Theory of super-radiant emission developed. Charles Townes at Columbia University develops theory for a maser (microwave amplification by stimulated emission of radiation). Townes builds first maser. Independently, Basov and Prokhorov of Soviet Union also develop theory. Basov also builds a maser. Townes, Basov and Prokhorov later share the 1964 Nobel Prize.

1960: Calculations had indicated that ruby would not lase. Even so, Ted Maiman of Hughes Research persists and succeeds at achieving lasing action in ruby at red wavelength. He used a 2-centimeter long rod as the gain material and pumped it with a high energy electronic flash. The most prestigious journal of physics, Physical Review Letters, fails to appreciate the significance and rejects the paper submitted by Maiman. British journal Nature publishes Maiman's paper reporting the world's first laser!

Once the ruby laser was invented, flood gates opened to inventions of other lasers. Once it was discovered that the lasing action is possible, a whole lot of other materials were quickly discovered capable of lasing. Ruby, it turns out, is really a hard material to lase.

Common Lasers, Their Characteristics and Applications:  

Helium-Neon Laser: Invented in 1960 by Ali Javan at Bell Labs. Red wavelength, continuous wave (cw), about a foot long, low power (only about one-thousandth of a watt but can still hurt the eye). Cost about $500. Common applications: Reading the universal product code (UPC) at check out counters, holography and holographic interferometry (useful for nondestructive testing in engineering), as a laser pointer, and in surveying and alignments.

Semiconductor Lasers: Invented in 1962 by Robert Hall at General Electric. Much smaller than a pin head (not including the mount or power source). Widely used in CD/DVD players, laser printers and telephone communication. Commonly at infra-red wavelengths. Red wavelength ones used as laser pointers. In recent years, engineers have succeeded in mass producing shorter wavelength blue lasers that are making high-density DVDs possible.

Carbon-dioxide lasers: Invented in 1963 by Chandra K. N. Patel at Bell Labs. Real industrial workhorse laser. Long infra-red wavelength (10.6 micron). Generally continuous wave, several watts to a thousand watts. Widely used for drilling and cutting applications. Also for laser hardening of engine cylinders and pistons. Used in medicine for surgery especially of blood vessels -- it cuts and coagulates at the same time. Widely used in molecular spectroscopy and pollution monitoring.

Neodymium lasers: A solid state laser. May be the single most commonly utilized laser in research laboratories. Typical size three feet long, cost $50,000 to $100,000. Available as cw or pulsed (nanoseconds to picoseconds). Output wavelength 1 micron (infra-red). Can be "frequency converted" to obtain visible, UV or IR wavelengths. Applications: Research, medicine (surgery), electronics (trimming, drilling, soldering), military (laser guiding of bombs & missiles, ranging, sensor blinding), nuclear fusion.

Argon Laser: Invented in 1964 by William Bridges of Hughes Research Lab. Every laser show features argon (blue and green wavelengths) and krypton (yellow and red) lasers. Typically six feet long, power 10 watts, cw. The argon laser emits several wavelengths in the green and blue regions while krypton emits yellow and red. In labs, the argon laser is for Raman spectroscopy, and as a pump (i.e. power source) for tunable lasers. In medicine, used for eye surgery.

Dye Lasers: Invented in 1966 by Peter Sorokin at IBM. Tunable in wavelength, very popular in spectroscopy labs (chemistry, physics). Certain dyes dissolved in solvents and pumped by an argon laser or a frequency-doubled or -tripled neodymium laser, can lase themselves. Recently, dye impregnated solid state materials have become practical for lasing.

If you want to know more ...

Monthly trade journals such as Laser Focus World and Photonics Spectra report on lasers, allied components and applications. These can be subscribed to free. Popular research journals include Optics Letters, IEEE Journal of Quantum Electronics and Applied Physics Letters. Be aware that laser science and technology is sometimes referred to as "quantum optics", "quantum electronics" and “electro-optics”. May the laser shine in and brighten your path.

The author, Suresh Chandra, is a physicist with SAIC. He has several laser related inventions to his credit, including the Gatlin gun laser.

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