Lasers
by Chris Woodford. Last updated: January 9, 2012.
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?
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 do the flash tube and the crystal make laser light?
- A high-voltage electric supply makes the tube flash on and off.
- 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.
- 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.
- The photons that atoms give off zoom up and down inside the ruby crystal, traveling at the speed of light.
- 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!)
- A mirror at one end of the laser tube keeps the photons bouncing back and forth inside the crystal.
- A partial mirror at the other end of the tube bounces some photons back into the crystal but lets some escape.
- 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
Since the laser was patented in 1958, lasers have 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!
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.
Further reading
On this website
On other websites
- Laserfest: A superb website set up to mark the 50th anniversary of the laser's invention. Includes excellent videos by laser pioneers, including Charles Townes and Theodore Maiman, and many other useful articles and resources.
Articles
- Lasers scan future possibilities by Jonathan Amos, BBC News, 12 May 2010. Explains how lasers became an indispensable part of everyday life—and where the technology is heading next.
- Lasers lift dirt of ages from artworks by Doreen Walton, BBC News, 26 February 2010. Lasers can be used to blast away dirt, restoring paintings to their former glory.
- Laser vision fuels energy future by Jonathan Fildes, BBC News, 6 June 2007. How lasers could be used to produce energy through nuclear fusion.
- Wired: This Day in Tech: May 16, 1960: Researcher Shines a Laser Light by Randy Alfred, Wired, May 16, 2011. Explains how laser pioneer Theodore Maiman created the first ruby crystal laser.
- Wired: Lasers Search for more articles about lasers at Wired magazine.
- The Race to Make the First Laser by Jeff Hecht. A short account of laser history from Hecht's book Laser Pioneers (see below).
Books
For younger readers
- Routes of Science: Light by Chris Woodford. Blackbirch, 2004. One of my own books, this is the story of how scientists unraveled the mysteries of light. Ages 9–12.
- Light by David Burnie. DK Books, 1998. What is light and how can we use it?
For older readers
- Lasers by Anthony E. Siegman. University Science Books, 1990. One of the best introductory books on lasers for college-level students, written by a Stanford Professor.
- Principles of Lasers by Orazio Svelto. Springer, 1998. Another detailed student text for undergraduates.
- Laser Pioneers by Jeff Hecht. Academic Press, 1992. The story of the laser, including interviews with 15 of the key pioneers.
