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Lasers

by Chris Woodford. Last updated: July 3, 2013.

Lasers are amazing light beams powerful enough to zoom miles into the sky or cut through lumps of metal. Once the stuff of science fiction, they have proved themselves to be among the most versatile inventions of modern times. The miniaturized laser beam that reads music in a CD player can also guide missiles, send emails down fiber-optic telephone lines, and barcode scan goods at the supermarket checkout.

The basic idea of a laser is simple. It's a tube that concentrates light over and over again until it emerges in a really powerful beam. But how does this happen, exactly? What's going on inside a laser? Let's take a closer look!

Photo: Lasers used in a NASA experiment. Picture courtesy of Great Images in NASA.

How is laser light different from ordinary light?

Laser beams follow precise paths between mirrors in a laboratory experiment.

Lasers are more than just powerful flashlights. The difference between ordinary light and laser light is like the difference between ripples in your bathtub and huge waves on the sea. You've probably noticed that if you move your hands back and forth in the bathtub you can make quite strong waves. If you keep moving your hands in step with the waves you make, the waves get bigger and bigger. Imagine doing this a few million times in the open ocean. Before long, you'd have mountainous waves towering over your head!

A laser does something similar with light waves. It starts off with weak light and keeps adding more and more energy so the light waves become ever more concentrated. The "white" light produced by an ordinary flashlight contains many different light rays of different wavelengths that are out of step with one another (scientifically, that's known as "incoherent"). But in a laser, all the light rays have the same wavelength and they are coherent (absolutely in step). This is what makes laser light such a powerful concentration of energy.

Before you can understand how a laser works, you need to know how an atom can give off light. If you're not sure how this happens, take a look at the box how atoms make light in our introductory article about light.

Photo: It's much easier to make laser beams follow precise paths than ordinary light beams, as in this experiment to develop better solar cells. Picture by Warren Gretz courtesy of US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).

How lasers work

A laser is effectively a machine that makes billions of atoms pump out trillions of photons (light particles) all at once so they line up to form a really concentrated light beam.

A red laser contains a long crystal made of ruby (shown here as a red bar) with a flash tube (yellow zig-zag lines) wrapped around it. The flash tube looks a bit like a fluorescent strip light, only it's coiled around the ruby crystal and it flashes every so often like a camera's flash gun.

How a laser works

How do the flash tube and the crystal make laser light?

  1. A high-voltage electric supply makes the tube flash on and off.
  2. Every time the tube flashes, it "pumps" energy into the ruby crystal. The flashes it makes inject energy into the crystal in the form of photons.
  3. Atoms in the ruby crystal (large green blobs) soak up this energy in a process called absorption. When an atom absorbs a photon of energy, one of its electrons jumps from a low energy level to a higher one. This puts the atom into an excited state, but makes it unstable. Because the excited atom is unstable, the electron can stay in the higher energy level only for a few milliseconds. It falls back to its original level, giving off the energy it absorbed as a new photon of light radiation (small blue blob). This process is called spontaneous emission.
  4. The photons that atoms give off zoom up and down inside the ruby crystal, traveling at the speed of light.
  5. Every so often, one of these photons hits an already excited atom. When this happens, the excited atom gives off two photons of light instead of one. This is called stimulated emission. Now one photon of light has produced two, so the light has been amplified (increased in strength). In other words, "light amplification" (an increase in the amount of light) has been caused by "stimulated emission of radiation" (hence the name "laser", because that's exactly how a laser works!)
  6. A mirror at one end of the laser tube keeps the photons bouncing back and forth inside the crystal.
  7. A partial mirror at the other end of the tube bounces some photons back into the crystal but lets some escape.
  8. The escaping photons form a very concentrated beam of powerful laser light.

What do we use lasers for?

Cutting tools

Lasers produce such intense and precisely focused energy that they can cut through metals, ceramics, plastics, and cloths. They have become popular in many industrial operations because high-precision computer-controlled lasers are much more accurate than human-operated cutting tools and, unlike traditional tools, laser beams never become blunt. A typical application involves simultaneously cutting hundreds of thicknesses of cloth according to a preprogrammed garment pattern.

Eye surgery

The pinpoint precision of lasers makes them particularly suitable for "welding" detached retinas and sealing broken blood vessels in the eye. The procedure is painless because the laser light passes straight through the patient's eyeball. Laser surgery can also help to correct eye problems such as short sight. Read more in our main article on laser eye surgery.

Scientific research

In the 60 years or so since lasers were developed, they've become smaller, more precise, and more powerful. At Lawrence Livermore National Laboratory in California, scientists have developed the world's most powerful laser, the National Ignition Facility (NIF), for nuclear research. Costing $1.2 billion, it's housed in a 10-story building occupying an area as big as three football fields, uses 192 separate laser beams that deliver 60 times more energy than any other laser, and it can generate temperatures of up to 100,000,000 degrees!

An overhead view of one of the laser bays at the National Ignition Facility An artist's impression of how laser beams from the NIF are concentrated on a fuel pellet to produce nuclear fusion

Photo: Left: One of the twin laser bays at the National Ignition Facility (NIF) in California. Right: How it works: Beams from the laser are concentrated on a small pellet of fuel in a chamber to produce intense temperatures (like those deep inside stars). The idea is to produce nuclear fusion (make atoms join together) and release a massive amount of energy. Photo credit: Lawrence Livermore National Laboratory.

What other kinds of lasers are there?

The tiny laser beams used in small electronic devices such as CD players work a bit differently. Read all about them in our article on semiconductor lasers.

Who invented lasers?

Maser patent drawing by Arthur Schawlow and Charles Townes from US patent 2,929,922 granted March 22, 1960.

Lasers evolved from masers, which are similar but produce microwaves and radio waves instead of visible light. Masers were invented in the 1950s by Charles Townes and Arthur Schawlow, both of whom went on to win the Nobel Prize in Physics for their work (Townes in 1964 and Schawlow in 1981). They applied to protect their invention on July 30, 1958 and were granted US Patent #2,929,922 (Masers and maser communication system) on March 22, 1960 (you can see one of the drawings from it here).

But did they invent the laser? In 1957, one of Townes' graduate students, Gordon Gould, sketched in his lab notebook an idea for how a visible light version of the maser could work, coining the word "laser" that we've used ever since. Unfortunately, he didn't patent his idea at the time and had to devote the next 20 years of his life to legal battles, eventually gaining a patent for part of the laser invention (Method of energizing a material) and substantial back royalties in 1977.

Laser patent drawing by Gordon Gould from US patent 4,161,436 granted July 17, 1979.

Although Townes and Schawlow are often credited with inventing lasers, the first person to build a working, visible light laser was actually Theodore Maiman, who has never really gained the recognition he deserved: his original writeup of his work was rejected by the journal Physical Review Letters and, despite twice being nominated for the Nobel Physics Prize, he never won the ultimate accolade.

Artwork: Top right The original maser designed in the late 1950s by Arthur Schawlow and Charles Townes, taken from their US Patent 2,929,922, which I've colored to highlight the main components. You can see how closely it resembles the laser in my artwork in the box up above. Bottom left: Gordon Gould's alternative laser is a very different design, but does essentially the same job: amplifying light. Patent artworks courtesy of US Patent and Trademark Office.

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Text copyright © Chris Woodford 2000, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2000/11) Lasers. Retrieved from http://www.explainthatstuff.com/lasers.html. [Accessed (Insert date here)]

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