Extreme Science
Lasers
Lesson
From Star Wars and Star Trek, to Mass Effect and Halo, we have all become extremely
familiar with laser technology as it continues to play a vital role in video games, movies,
tv shows, and books. While it may be easy to quickly think of lasers as being fired from
Master Chief's arsenal, cutting away blast doors in the hands of Luke Skywalker, or
defending the Starship Enterprise, we sometimes easily forget that lasers are found in
countless places in the world, with many of their uses making our daily lives much easier.
They show up in an awesome range of products and technologies being used in
everything from cd players, to dental drills, to high-speed metal cutting machines.
Tattoo removal, hair replacement, eye surgery, and even high tech "measuring
tape," each of these items use lasers in various ways. Let's take a look at some of those as
well as how lasers work in general.
But what is a laser? Isn't a laser beam just a bright high-tech form of flashlight? The
answer of course to that is a resounding "NO!" So, what exactly makes a laser light
different from other kinds of light? Well to start out, a bit of trivia... did you know that
the word laser is actually an acronym? That's right, L.A.S.E.R. actually stands for
something. It stands for Light Amplification by the Stimulated Emission of Radiation.
First, let's start out with a few of the fundamentals of laser technology.
Atom's Family Values
When you think about how many things there are in the universe, it really begins to
boggle the mind that there are only around 100 known different kinds of atoms in the
entire universe. Basically everything we see is made up of these 100 types of atoms in an
virtually unlimited number of combinations. It really all depends on how these atoms are
arranged and bonded together to decide whether the atoms make up a cup of water, a
piece of metal, a tree, or the piece of fuzz that comes out of your pocket!
One must keep in mind that atoms are constantly in motion; always vibrating, moving,
and rotating. Even the atoms that make up the chairs that we sit in and the beds we
sleep in are moving around. Gases, liquids, and solids are actually all in motion! Atoms
can be found in different states of excitation, which just means that they can have
different energies. By applying a lot of energy to an atom, it can actually leave what is
called its ground-state energy level and go to an excited level. The same thing can
happen to people, but that's a different class! The exact level of excitation that the atom
achieves depends on the amount of energy that is applied. This energy is typically
applied via heat, light, or electricity. Although more modern views of the atom do not
depict specific discrete orbits for the electrons, it can still be helpful to think of these
orbits as the different energy levels of the atom. For example, if we apply some heat to
an atom, we might expect that some of the electrons that are currently existing in the
lower-energy orbital would move to higher-energy orbitals farther away from the
nucleus. The image above of course represents a highly simplified view of things, but it
actually reflects at the very least the core idea of how atoms work in regards to lasers.
Once an electron moves to a higher-energy orbit, it will eventually want to return to the
ground state. Once it does this, it releases its extra energy as a photon, otherwise known
as a particle of light. In fact, you see atoms releasing energy as photons all the time.
Take for example, your toaster. When the heating element in your toaster gets, well, hot,
it turns bright red or orange. This color that you see is caused by atoms which are
releasing red wavelength photons because they have been excited by the heat. When you
see a picture on a standard TV, what you are seeing is phosphor atoms being excited by
high-speed electrons, and emitting different colors of light. Anything that produces light
does it through the action of electrons changing orbits and releasing photons.
Now how does this connect to lasers? Well, a laser is a device that basically controls the
way that energized atoms release these photons. While there are many different types of
lasers, all of them have specific essential features. In a laser, the lasing medium is
“pumped” to get the atoms into an excited state. Usually, very intense flashes of light or
electrical discharges pump the lasing medium and create a bunch of excited-state atoms.
It is necessary to have a large number of atoms in the excited state for the laser to really
work efficiently. Generally, the atoms are excited to a level that is two or three energy
levels above the ground state. This increases the degree of population inversion, which
is the number of atoms in the excited state versus the number in ground state. Those
excited electrons that we just mentioned earlier have energies greater than the more
relaxed ones. Just as the electron absorbed some amount of energy to reach this excited
level, it can also release this energy. Basically, the electron can simply "relax", and by
doing so, rid itself of some energy. This emitted energy comes in the form of photons,
or light. Are you starting to see the picture now? These photons that are emitted have a
very specific wavelength, in other words, color. The wavelength depends on the state of
the electron's energy when the photon is released. Two identical atoms with electrons in
identical states will release photons with identical wavelengths. Laser light is very
different from normal light, as you can see from the following properties:
•
Laser light is monochromatic, which means it contains only one specific
wavelength of light (one specific color). Like we just stated, the wavelength of
light is determined by the amount of energy released when the electron drops into
a lower orbit.
•
Laser light is highly directional. A laser light has an extremely tight beam and is
very strong and concentrated, as opposed to a flashlight, which releases light in
many different directions, making the light very weak and easily diffused.
•
Laser light is coherent. This means essentially that it is “organized.” Basically
each photon moves "in step" with the others. This means that all of the photons
have wave fronts that launch in unison.
To make a laser, you need something called stimulated emission. Again, this is not your
ordinary flashlight! In a flashlight, all of the atoms release their photons randomly. It's a
bright mess! In stimulated emission however, photon emission is organized. Like we
said before, the photons that any atom releases have certain wavelengths that are
dependent on the energy difference between the excited state and the ground state. If one
of these photons possessing a certain energy and phase should come into contact with
another atom that has an electron in the same excited state, stimulated emission can
occur. The first photon can stimulate atomic emission, making the subsequent emitted
photon from the second atom vibrate with the same frequency and direction as the
incoming photon. (Still with me?)
The other important key to making a laser is a pair of mirrors, which are placed one at
each end of the lasing medium. Photons reflect off the mirrors to travel back and forth
through the lasing medium. As they do this, 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 domino effect occurs, and soon you have many, many photons
of the same wavelength and phase. One of the mirrors acts simultaneously as an "exit".
This mirror is "half-silvered," which means that it reflects some light and lets some light
through. The light that ends up making it through is the laser light. Ta-Da!
There are many different types of lasers. The laser medium can be a solid, gas, liquid or
semiconductor. The type of lasing material employed often designates lasers. Here are a
few examples...
•
Solid-state lasers have lasing material distributed in a solid matrix (such as the
ruby lasers, like pictured above).
•
Gas lasers (helium and helium-neon, HeNe, are the most common gas lasers)
have a primary output of visible red light. CO2 (carbon dioxide) lasers emit
energy in the far-infrared wavelengths, and are typically used for cutting hard
materials.
•
Excimer lasers use reactive gases, such as chlorine and fluorine, mixed with inert
gases such as argon, krypton or xenon. When electrically stimulated, a pseudo
molecule (called a dimer) is produced. These lasers produce light in the
ultraviolet range. The name "excimer actually comes from the terms excited and
dimers.
•
Semiconductor lasers are also sometimes called diode lasers. These are not
solid-state lasers. These electronic devices are generally very small and use low
power. You know them best as the primary component in some laser printers and
cd players.
A ruby laser (like shown above) is a solid-state laser and emits at a wavelength of 694
nm. Depending on the desired emission wavelength, the power needed, and pulse
duration, other lasing mediums can be also be chosen. Some lasers are extremely
powerful, such as the CO2 laser, which can cut through solid steel (sweet!). The CO2
laser is so dangerous and powerful because it emits laser light found in the infrared and
microwave region of the spectrum. Infrared radiation is heat, meaning this laser
essentially melts through whatever it is focused upon!
On the other hand, diode lasers are very weak and are used in modern pocket laser
pointers or laser pens. Usually these lasers emit a red beam of light that has a wavelength
between 630 nm and 680 nm. Like we mentioned earlier, lasers are often utilized in
industry and research to do many things, including using intense laser light to excite other
molecules just to observe what happens to them!
Here are some typical lasers and their emission wavelengths. Keep in mind that a
nanometer (nm) is 1x10-9 meters.
Laser Type/Wavelength (nm)
•
Krypton fluoride (UV)/248 nm
•
Xenon chloride (UV)/308 nm
•
Nitrogen (UV)/337 nm
•
Argon (blue)/488 nm
•
Argon (green)/514 nm
•
Helium neon (green)/543 nm
•
Helium neon (red)/633 nm
•
Ruby (CrAlO3) (red)/694 nm
•
Carbon dioxide (FIR)/10600 nm
Lasers are classified into four fairly broad groups depending on the potential for causing
biological damage. When you see a laser, it is supposed to be labeled with one of these
four class designations:
•
Class I (aka. "am I supposed to feel anything?") - These lasers cannot emit
laser radiation at known hazard levels.
•
Class I.A. (aka. "for barcodes' eyes only") - This 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 - Low-power visible lasers that emit above Class I levels but at a radiant
power not above 1 mW.
•
Class IIIA (aka. "I've got one of those at home!") - Intermediate-power lasers
(cw: 1-5 mW), and are hazardous only for intrabeam viewing. Most pen-like
pointing lasers are in this class.
•
Class IIIB - These are moderate-power lasers.
•
Class IV (aka. "The force is strong with this laser") - These are high-power
lasers (cw: 500 mW, pulsed: 10 J/cm2 or the diffuse reflection limit), which are
hazardous to view under any condition (directly or diffusely scattered), and are a
potential fire hazard as well as a skin hazard. Significant controls are required of
Class IV laser facilities.
So now you know a bit more about lasers. Make sure you watch the video again to make
sure you have a good grasp on how they work, and maybe even get your parents
permission to get a simple laser pointer and try some experiments of your own. Just
make sure you don't try to take over the world with one...