| Lasers show up in an
amazing range of products and technologies. You will find them in everything
from CD players to dental drills to high-speed metal cutting machines to
measuring systems. They all use lasers. But what is a laser? And what makes
a laser beam different from the beam of a flashlight?
In this edition of How
Stuff Works, we will look at lasers and how they work so you can
completely understand this fascinating technology!
The
Basics of an Atom
There are only about 100 different kinds
of atoms in the entire universe. Everything we see is made up of these
100 atoms in an unlimited number of combinations. How these atoms are arranged
and bonded together determines whether the atoms make up a cup of water,
a piece of metal, or the fizz that comes out of your soda can!
Atoms are constantly in motion. They continuously
vibrate, move and rotate. Even the atoms that make up the chairs that we
sit in are moving around. Solids are actually in motion! Atoms can be in
different states of excitation. In other words, they can have different
energies. If we apply a lot of energy to an atom, it can leave what is
called the ground-state energy level and go to an excited level.
The level of excitation depends on the amount of energy that is applied
to the atom via heat, light, or electricity.
Figure 1 is a classic interpretation of
what the atom looks like. It consists of a nucleus (containing the
protons and neutrons) and an electron cloud. It?s helpful to think
of the electrons in this cloud circling the nucleus in many different
orbits. Although more modern views of the atom do not depict discrete
orbits for the electrons, it can be useful to think of these orbits
as the different energy levels of the atom. In other words, if we apply
some heat to an atom, we might expect that some of the electrons in the
lower energy orbitals would transition to higher energy orbitals further
away from the nucleus. This is a highly simplified view of things, but
actually contains the core idea of how atoms work in terms of lasers.
Figure 1 - An atom, in the
simplest model,
consists of a nucleus and
orbiting electrons
Figure 2 - Absorption of
Energy
An atom absorbs energy in
the form of heat, light, or electricity.
Electrons may move from
a lower energy orbit to a higher energy orbit.
Once an electron moves to a higher-energy
orbit, it eventually wants to return to the ground state. When it does,
it releases its energy as a photon -- a particle of light. You see
atoms releasing energy as photons all the time. For example, when the heating
element in a toaster turns bright red, the red color is caused by atoms,
excited by heat, releasing red photons. When you see a picture on a TV
screen, what you are seeing is phosphor atoms, excited by high-speed
electrons, emitting different colors of light. Anything that produces light
-- fluorescent lights, Coleman lanterns,
incandescent bulbs -- does it through the action of electrons changing
orbits and releasing photons.
The
Laser/Atom Connection
A laser is a device that controls
the way that energized atoms release photons. Laser is an acronym for Light
Amplification by Stimulated Emission of Radiation, which describes
very succinctly how a laser works!
Although there are many types of lasers,
all have certain essential features. In a laser, the lasing medium is ?pumped?
to get the atoms into an excited state. Typically, very intense flashes
of light or electrical discharges pump the lasing medium and create a large
collection of excited-state atoms (atoms with higher energy electrons).
It is necessary to have a large collection of atoms in the excited state
for the laser to work efficiently. In general, the atoms are excited to
a level that is two or three levels above the ground state. This increases
the degree of population inversion. The population inversion is
the number of atoms in the excited state versus the ground state.
Once the lasing medium is pumped, it contains
a collection of atoms with some electrons sitting in excited levels. The
excited electrons have energies greater than the more relaxed electrons.
Just as the electron absorbed some amount of energy to reach this excited
level, it can also release this energy. As Figure 3 illustrates, the electron
can simply relax, and in turn rid itself of some energy. This emitted
energy comes in the form of photons (light energy). The photon
emitted has a very specific wavelength (color) that depends on the state
the the electon's energy when the photon is released. Two identical atoms
with electrons in identical states will release photons with identical
wavelengths.
Figure 3
Laser light is very different from normal
light. Laser light has the following properties:
-
The light released is monochromatic.
It contains one specific wavelength of light (one specific color). The
wavelength of light is determined by the amount of energy released when
the electron drops to a lower orbit.
-
The light released is coherent. The
light is ?organized? and each photon moves in step with the others. This
means that all of the photons have wave fronts that launch in unison.
-
The light is very directional. A laser light
has a very tight beam and is very strong and concentrated. A flashlight,
on the other hand, releases light in many directions and the light is very
weak and diffuse.
To make these three properties occur takes
something called stimulated emission. This does not occur in your
ordinary flashlight -- in a flashlight, all of the atoms release their
photons randomly. In stimulated emission, photon emission is organized.
The photon that any atom releases (Figure
3) has a certain wavelength that is dependent on the energy difference
between the excited state and the ground state. If this photon (possessing
a certain energy and phase) should encounter another atom that has an electron
in the same excited state, a phenomena called stimulated emission
can occur. The first photon can stimulate or induce atomic emission such
that the subsequent emitted photon (from the second atom) vibrates with
the same frequency and direction as the incoming photon.
The other key to a laser is a pair of mirrors,
one at each end of the lasing medium. Photons, with a very specific wavelength
and phase, reflect off the mirrors to travel back and forth through the
lasing medium. In the process, they stimulate other electrons to make the
downward energy jump and can cause the emission of more photons of the
same wavelength and phase. A cascade effect occurs, and soon we may have
propagated many, many photons of the same wavelength and phase. The mirror
at one end of the laser is "half-silvered", meaning it reflects some light
and lets some light through. The light that makes it through is the laser
light.
You can see all of these components in
the following figures, which illustrate how a simple ruby laser works.
The laser consists of a flash tube (like you would have on a camera), a
ruby rod and 2 mirrors (one half-silvered). The ruby rod is the lasing
medium and the flash tube pumps it.
1
2
3
4
5
Figure 5
In scene 1 you have the laser in its non-lasing
state. In scene 2 the flash tube fires and injects light into the ruby
rod. The light excites atoms in the ruby. In scene 3 some of these atoms
emit photons. In scene 4, some of these photons run in a direction parallel
to the ruby's axis, so they bounce back and forth off the mirrors. As they
pass through the crystal, they stimulate emission in other atoms. Monochromatic,
single-phase, columnated light leaves the ruby through the half-silvered
mirror -- laser light!
Three-Level
Laser
Figure 4 illustrates what happens in a
real-life, three-level laser.
Figure 4
Types
of Lasers
There are many different types of lasers.
The laser medium can be a solid, gas, liquid, or semiconductor. Lasers
are commonly designated by the type of lasing material employed:
-
Solid-state lasers have lasing material
distributed in a solid matrix, e.g., the ruby or neodymium-yttrium aluminum
garnet (YAG) lasers. The neodymium-YAG laser emits infrared light at 1,064
nanometers (nm). A nanometer is 1x10-9 meters.
-
Gas lasers (helium and helium-neon,
HeNe, are the most common gas lasers) have a primary output of visible
red light. CO2 lasers emit energy in the far-infrared, and are
used for cutting hard materials.
-
Excimer lasers (the name is derived
from the terms excited and dimers) use reactive gases such
as chlorine and fluorine mixed with inert gases such as argon, krypton,
or xenon. When electrically stimulated, a pseudo molecule or dimer is produced
and when lased, produces light in the ultraviolet range.
-
Dye lasers use complex organic dyes
like rhodamine 6G in liquid solution or suspension as lasing media. They
are tunable over a broad range of wavelengths.
-
Semiconductor lasers, sometimes called
diode lasers, are not solid-state lasers. These electronic devices are
generally very small and use low power. They may be built into larger arrays,
e.g., the writing source in some laser printers or compact disk players.
The first type of laser was called a ruby
laser. A ruby laser is a solid-state laser and emits at a wavelength of
694 nm. Other lasing mediums can be selected based on the desired emission
wavelength (see table below), power needed, and pulse duration. Some lasers
are very powerful, such as the CO2 laser, which can cut through
steel. The reason that the CO2 laser is so dangerous is because
it emits laser light in the infrared and microwave region of the spectrum.
Infrared radiation is heat, and this laser basically melts through whatever
it is focused upon.
Other lasers, such as diode lasers, are
very weak and are used in today?s pocket lasers used to point at things.
These lasers typically emit a red beam of light that has a wavelength between
630 nm and 680 nm. Lasers are used in industry and research to do many
things, including using the intense laser light to excite other molecules
to observe what happens to them.
Here are some typical lasers and their
emission wavelengths:
| Laser Type |
Wavelength (nm) |
|
|
| Argon fluoride (UV) |
193 |
| Krypton Fluoride (UV) |
248 |
| Nitrogen (UV) |
337 |
| Argon (blue) |
488 |
| Argon (green) |
514 |
| Helium neon (green) |
543 |
| Helium neon (red) |
633 |
| Rhodamine 6G dye (tunable) |
570-650 |
| Ruby (CrAlO3) (red) |
694 |
| Nd: YAG (NIR) |
1064 |
| Carbon Dioxide (FIR) |
10600 |
Laser
Classifications
Lasers are classified into four broad
areas depending on the potential for causing biological damage. When you
see a laser, it should be labeled with one of these four class designations:
-
Class I cannot emit laser radiation
at known hazard levels.
-
Class I.A. is a special designation
that applies only to lasers that are "not intended for viewing" such as
a supermarket laser scanner. The upper power limit of Class I.A. is 4.0
mW.
-
Class II are low-power visible lasers
that emit above Class I levels but at a radiant power not above 1 mW. The
concept is that the human aversion reaction to bright light will protect
a person.
-
Class IIIA are intermediate power lasers
(cw: 1-5 mW), which are hazardous only for intrabeam viewing. Most pen-like
pointing lasers are in this class.
-
Class IIIB are moderate power lasers.
-
Class IV are high-power lasers (cw:
500 mW, pulsed: 10 J/cm2 or the diffuse reflection limit)
are hazardous to view under any condition (directly or diffusely scattered),
and are a potential fire hazard and a skin hazard. Significant controls
are required of Class IV laser facilities.
About
the Author
Matthew Weschler holds a MS degree in
Physical Organic Chemistry from Florida State University. His thesis topic
was picosecond laser spectroscopy and he studied how molecules react picoseconds
after being bombarded by laser light. You can reach Matthew at matthew.weschler@howstuffworks.com |
