"Hey...those writeups on lasers don't say much!"

How a laser works!
Nowadays there are lasers everywhere - CD-players, eye-scanners and in the industry. But how does it really work ? Let me tell you...

Theory
Ok, first you have to understand a simple classical atom model, such as Bohr model of the atom. That's not enough, though.
You also have to have a tiny tiny bit of understanding for quantum physics - read quantum mechanics.
Briefly what one have to understand is this:



    --------         
   /  ____  \------- outer orbit  
  /  /    \  \
 /  /      \  \
/  |   OO   |------- inner orbit
\  |   OO----------- nucleus
 \  \      /  /
  \  \____/  /
   \        /
    \------/

  • The atom consist of the nucleus and the electrons.
  • The electrons can be said to exist in orbits on a constant distances from the nucleus.
  • Distance to the nucleus equals energy.
  • The closer to the nucleus, the lower the energy of the electron.
  • In order to change orbit the electron need to change its energy.
An atom can absorb energy - from heat, electricity etc, and the atom will the get excited. Yes, excited.
The electrons will then get more energy, and will therefore move to orbits further away from the nucleus.
One way to lose energy for an electron, is to send out light - photons. When the photon is emitted,
the electron will then drop back to the an orbit closer to the nucleus. This previous sentence is the principle behind a laser!
Energy
  |
  |------------------------ Excited atom state
  |                |
  |                | _   _   _
  |                |/ \_/ \_/ \-> photon
  |                |
  |                V
   ------------------------ Normal atom state
An excited atom looses energy by sending out light!

The key to the laser is that the electron transitions from an outer orbit to an inner orbit, always result in the same photon.
The orbits always has the same distances between them, and therefore the energy gap between them is constant.
When the photon is released, it will always be of the same energy, which gives the same wavelength - colour.

The Laser
This is how a laser looks like:


               ________________________
              |    o o    o          --|---- Ruby crystal rod
              |         o       o o    | XXXXXXXXXXXXXXXXXXXXXXXXX   Laser light
  Mirror------|      o                 |---Mirror
              |      o  o--- Ruby atoms|  (Semi-reflective)
               ------------------------
It consists of a laser material, for instance ruby crystal, and two mirrors, one of which lets through some portion of the light.
Because of this, only photons - light - that are perpendicular to the mirrors will get through, the rest will be absorbed by the walls.
And this is why the light beam gets very directed and high power !

Finale
So basically, the laser works by first pumping atoms to a high energy state.
Then the excited electrons will fall back to lower orbits, emitting photons of a specific wavelength.
The construction of the laser is in such a way that all light emitted from it is in a very precise and concentrated direction.

The type of laser discussed by bigmouth strikes is what is called a "solid state" laser. There are several other important types of laser. The common thread between them is that (a) they have the full mirror at one end and the half mirror at the other (b) to a lesser or greater extent, they produce a coherent (i.e. in-phase with one another), monochromatic (i.e. single colour) light source.

As you can imagine, a ruby laser isn't exactly cost-effective. Far more common (and cheap) is the semiconductor laser, also known as a diode laser. Diode lasers are closely related to LEDs; in fact at their core, are really just an LED with the mirrors whacked on either end. These are the lasers in your CD-ROM, in those freaking annoying laser pointers; in fiber optic network equipment and so on. They run at low voltages (say around 5 volts, compared with ruby lasers in the hundreds) and they're much much much more compact. But they are not perfect: they are not purely monochromatic or coherent. They're limited in power though.

Another type of laser is the gas laser. These include helium-neon combinations; and they're much the same idea as the solid state ones; but with gas as the core.

A Laser is also a type of sailboat made by Vanguard Sailboats USA. It was designed in the 1970s by Bruce Kirby. It is a single-handed boat with a cat rig. Its specs are as follows:

The Laser is the most popular dinghy in the world. It is used as an Olympic class and is the standard for single-handed racing. Some of the worlds best sailors have come out of Lasers.
LASER - Light Amplification by the Stimulated Emission of Radiation. A wonderful and catchy name that was devised for the device, althought it isn't quite accurate. LASERs do not amplify light at all. The misunderstanding here may come from the fact that the light is conglomerated in the laser cavity until the light that is bouncing back and forth between the mirrors is in large enough quantity that a noticeable amount passes through the partial mirror usually at one end of the laser cavity.

The predecesor to the LASER was the MASER which emmitted a beam of microwave energy. The concept that operates a laser, that being the emission of photons from atoms excited by other photons (which happen to be coherent), is closely related to Einstein's nuclear reaction concept, meaning that the photons, like neutrons in atomic fission, end up causing the release of more of their own kind, pushing the effect on.

There are many different sorts of lasers, and many lasing mediums within these categories. There are solid state/semiconductor lasers, gas lasers, dye lasers, and others. The first lasers used rubies as the lasing medium. A flash tube was used to pump the ruby with energy, photons in this case. Later, other lasing mediums and pumping devices were discovered. Gas lasers tend to use electricity to excite the atoms.

Some common gas lasers are Carbon Dioxide and Helium-Neon (HeNe) lasers, which emit light in the infrared and red-orange regions respectively. CO2 lasers are some of the most efficient lasers, yielding around 10% of their input energy as laseer light.

Solid state lasers made of silicon and other semiconductors are the most efficient lasers known as they use small multiples of the wavelengths of light to create the laser cavity little energy is lost.

Dye lasers are an interesting development in lasers. The lasing medium can be in a solid or liquid form as long as it contains the dye with the actual lasing properties. These lasers are usually pumped with other lasers, making them very inefficient. One of the most interesting features of dye lasers is that some of them are tunable to various frequencies of light. Most lasing dyes are poisonous, but there are a couple that are safe for human consumption, and since gelatin, including the sort that we all may enjoy eating, is a suitable substance to hold the dye, it is possible to make your dessert lase.

One of the best resources on the web for information on LASERs is Sam's Laser FAQ and contains a massive amount of information on the subject, including concepts and do it yourself instructions.

Lasers are rated by their gross output power into a class system. In the United States these standards are regulated by OSHA, and in Europe by CE, though the standards are generally the same in specifcation for both bodies.

Class I - Limited to gross output of .5 mW or less and will not emit radiation at known hazard levels. These are the most common type of laser, used widely in CD/DVD pickup assemblies. Class I Lasers are exempt from any excise controls and their manufacture, purchase and distribution is generally unlimited.

Extended expose to Class I laser is generally considered benign, even direct viewing -- in fact, many Class I lasers are so low power that the human eye won't even register them. Still, don't look directly into one, just in case.

Class I.A - A special designation for laser of the same type as Class I, but having a maximum power output of 4 mW. These are commonly seen in Laser Pointers and barcode scanners. Class I.A Laser are commonly marked as Class I, though they will cause corneal damage after 1000 seconds of direct exposure.

Class II - Low-power visible lasers that have gross output above that of Class I/.A levels but at a radiation power still below 1 mW where eye damage will only occur with extended exposure, generally 1/4 to 1/2 seconds. The concept is that the human aversion reaction to bright light (e.g. It hurts so you don't look into the beam) will protect a person from prolonged viewing.

Class IIIA - These will be lasers in the 1-5 mW range (just a little bit more oomph than a class I.A), but that have radiation characteristics that make intrabeam viewing (e.g. Looking into the laser beam) immediately hazardous regardless of the length of time viewed; Immediately hazardous means your eyes will burn out before you can look away.


Class IIIA are further classified as to their danger level: beams that are only dangerous when projected directly in the eyes and beams were the radiance of the dispersion area, where the beam hits something, is also harmful after extended periods. These are given the classifications of Caution and Danger respectively. This means that when working with lasers in the Danger category, which means a beam irradiance of more than 2.5mW cm2, eye protection is mandatory at all times regardless of you likely hood of looking directly into the beam.

Class IIIB - We're getting into the heavy lifters now. These are lasers with outputs of 5 to 500 mW and cause immediate eye damage as well as skin damage with extended (1000-3000 second) exposure. These are one of two kinds of lasers along with Class IV (below) that actually require physical lockouts as they can be used to hurt people.

Class IV - Big daddies. Devices of these types exceed 500mW or 10 Jcm2 and are classified as munitions. These are lasers that are immediately hazardous for any kind of exposure usually due to their UV or raw Lumen output. These are the lasers that are capable of burning things -- even strong things like wood, metal and stone in some special cases.

Ever see Real Genius? The 6 MegaWatt laser they had was waaaayyy over the baseline power of a Class IV and those dinky welders goggles they were wearing wouldn't have been enough. With a device that big, the area of diffusion is dangerous to even have in your line of sight, and the beam itself ionizes any atmosphere it comes in contact with. (But it was a movie, and I liked it, so I'm not going to to hold that against it).

As a public service, even though you may think a laser "isn't that bright", if you look directly into it "you're not to bright either". Looking into any laser is considered dumb and if you do, you deserve everything you get,

The actual emitter material of the laser can be made of a variety of substances and the laser will have differing characteristics depending on the substance used. The most common difference is the length of the wavelength, and wave length is very useful in many fields such as communications where the shorter wavelength means more information can be transmitted in a given impulse. Some laser materials and their associated wavelengths (in µmeters) are:

Argon fluoride (UV) -- .193
Krypton chloride (UV) -- .222
Krypton fluoride (UV) -- .248
Xenon chloride (UV) -- .248
Xenon fluoride (UV) -- .308
Helium cadmium (UV) -- .325
Nitrogen (UV) -- .337
Helium cadmium (purple) -- .441
Krypton (blue) -- .476
Argon (blue) -- .488
Copper vapor (green) -- .510
Argon (green) -- .514
Krypton (green) -- .528
Double-Pumped Nd:YAG(*) -- .532
Helium neon (green) -- .543
Krypton (yellow) -- .568
Copper vapor (yellow) -- .570
Helium neon (yellow)(**) -- .594
Helium neon (orange) -- .610
Gold vapor -- .627
Helium neon (red) -- .633
Krypton (red) -- .647
Rohodamine 6G dye -- .570 to .650 (tunable)
Ruby -- .694
Gallium arsenide (NIR(***)) -- .840
Nd:YAG (NIR) -- 1.064
Helium neon (NIR) -- 1.15
Erbium (NIR) -- 1.504
Helium neon (NIR) -- 3.39
Hydrogen fluoride (NIR) -- 2.70
Carbon dioxide (FIR(****)) -- 9.6
Carbon dioxide (FIR) -- 10.6

* - Thinkgeek sells a green laser pointer based in this system
** - Most common laser for CD and DVD pickup assemblies, though now supplanted buy laser diodes of the same wavelength
*** - Near-Spectrum Infrared, 0.700-1.400 µm
**** - Far-Spectrum Infrared, >1.400 µm

Information culled from OSHA Laser standards manual (section 3, chapter 6), World Book Encyclopaedia and an ex Bell Labs Employee

Laser is a spice known to the Ancient Rome from near the Greek colony of Kyrene in modern-day Libya, used extensively as a flavor enhancer (the word "umami" comes to mind). It was said to be from the fennel family, somewhat like asafoetida in strength, but tastily pungent and milder -- it was also used for birth control, and as a tea. Pliny gives detailed instructions for identification and harvesting, but cautions the last known stalk of it was presented to the Emperor Nero. The bane of all who seek to emulate Apicius, efforts are now underway to identify and rebreed this valuable commodity.

In my own form of reconstructed Roman cookery, I use Worchestershire sauce, or pine nuts soaked in it.

This write up is about the physics behind laser operation.

The word LASER is a rare gem: an acronym that not only sounds way cool, but also describes the device nicely. It stands for Light Amplification by the Stimulated Emission of Radiation. I'll discuss how this thing works based on this acronym. Disclaimer: This is a slightly dumbed down version of the truth. Some lasers work a bit differently than described here, and a few thorny details are left out for the convenience of the reader.

Light: Lasers used to work with light. Nowadays, however, we can make lasers that work with electromagnetic radiation other than light, such as ultra-violet and infrared. Brave souls are also researching the possibility of X-ray lasers. The laser was preceded by the MASER, Microwave Emission by the Stimulated Emission of Radiation, which works with microwaves, another form of electromagnetic radiation. While similar in principle, it's technically a very different machine.

Now, we skip the amplification to go to the crux of the matter: Stimulated Emission. We'll start with a bit of basic atomic physics. An atom exists of a positive nucleus with negative electrons spinning around it. Normally, this atom is settled down with all its electrons in the lowest possible energy state. It is, however, possible that an atom is excited by an interaction with another particle-maybe a photon, phonon, ion, or an electron, to a higher energy state. This is called absorption. An atom does not like being in an excited state, and after some time, will drop down to a lower energy state, emitting a photon. This is called emission. The key to laser working is a process described by Einstein in 1917 called stimulated emission. In this process, we start out with an excited atom. Now, a photon, with an energy exactly corresponding to the energy the atom gained in being excited, interacts with the excited atom. Through the wonders of quantum mechanics, this photon stimulates the atom to emit. But this emission is very special: the emitted photon is a clone of the original photon. Same energy, same momentum, same direction, coherent, everything. So, now we have a mechanism of breeding similar photons.

The technical challenge lies in the amplification part. We need to have some way of making sure that we get more and more of these identical photons. This is achieved by placing the lasing medium between two highly reflective mirrors, say 99.99%. This device is known as a Fabry-Perot Interferometer. The photons will start to bounce between the mirrors, creating friends by stimulated emission, and becoming stronger and stronger, forming a standing electromagnetic wave between the mirrors. The 0.01% that leaks out becomes a laser beam.

If only it were this simple. Remember I mentioned absoption? Well, it's linked to stimulated emission via the Einstein relations. If you don't do anything special, the absorbtion will kill so many photons that you will never get significant stimulated emission. The only way to make sure stimulated emission can become dominant is making sure there are more particles excited state than in the lower state. Unfortunateley, statistical physics teaches us that it's impossible to have more particles in the excited state than in the lower state if we are in thermal equilibrium. (For the pedants: this is not strictly true if we also consider degeneracy; however, because it also effects the rates of emission and absorption, the effect cancels). This situation is called population inversion

So, we are now left with making sure we somehow break thermal equilibrium between the lower and higher state. There is a wide variety of tricks to do this, like:

  • In a helium neon laser2, helium is excited. This excited helium transfers its energy to neon, populating an excited state of neon. Of this effect is strong enough, you can get laser action.
  • In an excimer laser3, the excited state consists of molecules of excited atoms, called excimer. However, in the ground state, these molecules cannot exist because the atoms don't bond if they are not excited. So, no ground state population.
  • In many pulsed lasers, flashing lights are used to pump the higher level
I think the Radiation in the word laser was mainly added for coolness.

Because the photons emitted by the lasers are pretty much clones, a laser beam is highly monochromatic, very coherent, and well collimated5. These properties make the laser suitable for a very wide variety of applications.

The first laser was a solid-state ruby laser demonstrated by Maimain in 19606, altough there is some controversy whether Gould was earlier. Gould was awarded a patent for lasers in 19771. Interesting bit of trivia: There is a well-known picture of Maiman posing with a large flash lamp and ruby rod. This is not the first laser; the reporter taking the picture suggested using it because it looked better than the smaller tube and rod Maiman was actually using. More than a few physicists tried to replicate Maiman's laser using such a big flashtube7.

In summary, a laser is a device that creates a collimated, coherent, monochromatic bundle of light using stimulated emission of radiation. A good way of causing population inversion is essential for laser operation.


Sources:

  1. http://inventors.about.com/library/inventors/bllaser.htm
  2. http://www.campusprogram.com/reference/en/wikipedia/h/he/helium_neon_laser.html
  3. http://www.iupac.org/goldbook/E02242.pdf
  4. http://www.colorado.edu/physics/2000/lasers/lasers2.html
  5. http://www.electronicsforu.com/efylinux/efyhome/cover/jan2001/elec.htm
  6. http://scienceworld.wolfram.com/physics/Laser.html
  7. http://www.sff.net/people/Jeff.Hecht/pioneers.html

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