Lasers
Last updated: May 8, 2007.
Lasers used in a NASA experiment.
Picture courtesy of Great
Images in NASA.
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 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?
Ordinary light and laser 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
incoherent (out of step with one another). But in a laser, all the
light rays have the same wavelength and they are coherent
(in
step).
This is what makes laser light such a powerful concentration of energy.
How atoms make light
Before you can understand how a laser works, you need to know how an atom
can make light.
Atoms are the tiny particles from which all things are made.
Simplified greatly, they look a bit like our solar system.
Most of the atom's mass is concentrated in the nucleus
at the
center (red),
made from protons and neutrons packed together.
Electrons (blue) are arranged around the
nucleus in shells
(sometimes called orbitals, or energy levels).
The more energy an electron has, the farther it is from the nucleus.
Atoms make light in a three-step process:
- They start off in their stable "ground state" with electrons in
their normal places.
- When they absorb energy, one or more electrons are kicked out
farther from the nucleus into higher energy levels. We say the atom is
now "excited."
- However, an excited atom is unstable and quickly tries to get
back to its stable, ground state. So it gives off the excess energy it
originally gained as a photon of energy
(yellow wiggly line).
A photon is simply a small packet of light.
A laser is effectively a machine that makes billions of atoms pump out
trillions of photons all at once so
they line up to form a really concentrated light beam.
How lasers work
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, travelling 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.
Examples of uses
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.
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 are currently working to produce
the world's most powerful laser, the National Ignition Facility (NIF),
for nuclear research. Costing $1.2 billion, it will be able to generate
temperatures of up to 100,000,000 degrees.
