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 \ \ / / \ \____/ / \ / \------/
Energy | |------------------------ Excited atom state | | | | _ _ _ | |/ \_/ \_/ \-> photon | | | V ------------------------ Normal atom state
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) ------------------------
FinaleSo 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.
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.
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
In my own form of reconstructed Roman cookery, I use Worchestershire sauce, or pine nuts soaked in it.
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:
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:
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