HOW LASERS WORK
~ by Davin Flateau
Lasers come in many shapes and sizes, and perform a myriad of functions ranging from surgery to video recording. In this document, we’ll explain how laser light is produced, why it’s so useful, and some of its most common applications.
In 1917, Albert Einstein introduced the field of physics to the concept of the laser, which stands for Light Amplification by the Stimulated Emission of Radiation.
What’s So Special About Laser Light?
Laser light has several properties that, together, make lasers useful.
Laser Light is Highly Monochromatic
Light from the sun, or a light bulb, is generally seen as “white”, and contains many wavelengths of light (seen as different colors when white light is put through a prism). Laser light, on the other hand, is generally monochromatic, meaning that it contains one specific wavelength of light. This wavelength of light can be seen as one single, intense color (red, blue, green, or yellow, etc., depending on the laser) or invisible (ultraviolet or infrared). Lasers can, and do, produce more than one color, but these colors are discrete individual wavelengths of light, as opposed to the broad spectrum of sunlight or fluorescent light.
Laser Light is Highly Coherent
Laser light wavelengths can be thought of as “organized”. The photons of laser light all “move in step” with one another. Light from a light bulb, for instance, has wavelengths that are not nearly as organized, with most photons’ waves traveling chaotically and interfering with one another. It’s the coherent, organized property of laser light that makes it capable of delivering a high amount of energy in a small beam. In the case of visible lasers, this makes the laser beam very bright and intense.
Laser Light is Highly Directional
Because of the way laser light is produced (described below), beams of laser light are very small, tight, and bright. Photons in a laser beam are traveling almost exactly parallel to each other. For instance, if a flashlight and a laser beam were shone on a building across the street from your home, the flashlight beam would appear several feet wide, while the laser beam would be only be inches across.
Laser Light Can Be Sharply Focused
Due to the laser light’s parallelism, it can be focused very efficiently compared with other types of light. Focused laser beams can deliver very high amounts of energy over a very tiny space.
How does it work?
There are many ways to produce laser light. There are lasers that operate with gas, crystals, and diodes; lasers can be as small as a pinhead, or be large enough fill an entire room. However, they all operate on the same general principle. Light Amplification (generating more light) by the Stimulated Emission of Radiation (by stimulating atoms with radiation — that is, light). We’ll explain the operation of one common type of laser, the gas ion laser, that is used in science, industry, as well as Laser Fantasy laser shows.
Gas Ion Lasers
Gas ion lasers use a tube filled with a gas. Often, this gas is a noble (or inert) gas (such as Neon, Argon, or Krypton, or a combination of noble gases).
This tube is applied with a high voltage electric current, which travels down the length of the tube. This discharge creates collisions between the electrons from the electricity, and the atoms of gas in the laser tube.
The interaction between the electrons and the atoms of gas affects the gas atoms; the gas atoms become ionized, and some gas ions that interact with more electrons are excited to a higher energy state.
The atoms quickly return back to a lower state of energy, but in going from the excited state to a lower energy state, a photon of light is generated. This is the general principle behind a neon light. Lasers, however, go one step further.
The photon that is released can then interact with other atoms of gas. If the atom happens to be excited, a second photon is generated when the atom returns to its “ground” state of energy. This second photon is in every way identical to the original photon in direction, polarization and energy.
A “chain reaction” takes place, where photons continually collide with gas atoms, generating more photons, and therefore more light. The direction of this light is random, with some photons going up, down, and just about every way possible. To produce a single concentrated beam, a mirror is placed at each end of the laser tube. Photons that happen to travel in the direction along the mirrors will reflect back down to the other mirror, and so on. During these reflections, the photons interact with more atoms in the process described above, creating more photons traveling between the mirrors. Soon, many atoms along the mirror axis are emitting light through this stimulated emission of photons. All the while the electricity keeps the gas atoms primed, and ready to emit photons. The light traveling between the mirrors (at the speed of light) can be thought of as an optical resonant cavity that, like an organ pipe for sound waves, can be tuned to resonate one or more wavelengths (or colors) of light. One of the mirrors at one end of the laser tube is only partially reflective, letting a tiny part of the laser light out. This tiny amount of light is the laser beam that can be used for scanning a bag groceries, reading and writing audio and video to or from a CD, performing delicate surgery, and much more.
Lasers themselves don’t magically perform surgery, read compact discs, or weave laser light concerts. Lasers only produce a unique source of light and energy for these various applications. A variety of optics, mechanical motors, electronics and optical detectors, and good engineering (people!) come together to produce all of these amazing feats with this unique light. Lasers are the heart of these applications, making them possible. Here are just a few of the many users of lasers:
The small, intense bright beam of a laser can be focused with lenses and other optics to provide a point of energy intense enough to burn through living tissue. Because “laser scalpels” are so small, they can very delicately reach difficult places. The burning action of laser surgery also instantly clots the incision, reducing bleeding dramatically. Reattaching detached retinas and using fiber optics to burn away ulcers in the stomach are just a couple of the medical uses of lasers. Lasers used in surgery include Nd:YAG crystal lasers (Neodymium and yttrium aluminum garnet), argon gas ion lasers, and excimer lasers.
Laser Welding, Cutting & Blasting
Once again, the laser’s intense energy when focused makes it ideal for providing concentrated welding and cutting. Laser cutting and welding can be extremely precise. Clothing manufacturers can use lasers to cut precise fabric patterns. Laser welding can allow the easy welding of two different kinds of metals and alloys, making the resulting product significantly stronger than other techniques. Many car manufacturers use laser welding performed by industrial robots to assemble vehicles.
The intense color of laser light has opened up a whole new world for laser artists to weave a new kind of art. Laser shows are usually performed in planetarium domes, and set to music ranging from new age to rock and roll. Laser shows began using gas ion lasers, including Argon, Krypton-Argon, and Helium-Neon lasers. Colors used to be changed by using multi-wavelength lasers (such as Argons or Krypton-Argons) and sending the laser through crystals which vibrate with sound waves (AOMs Acousto-Optic Modulators), providing full-color imagery. Modern lasers are solid state or diode lasers, which are far more simple and efficient. Sets of high-speed vibrating mirrors called scanners move the laser beams in different patterns. Abstract imagery or full-motion animation can be displayed in laser shows.
Laser-powered fusion holds hope of generating tremendous amounts of electricity through the use of lasers. Highly focused, powerful lasers “zap” tiny fuel pellets from all sides, triggering thermonuclear fusion. In experiments at the Lawrence Livermore National Laboratory, laser pulses deliver close to 200 kJ (kilojoules) of energy to each pellet in less than a nanosecond. This single pulse delivers approximately 2 X 10^14 W – about 100 times the world’s installed electric power! The feasibility of a working reactor is still the subject of ongoing research.
Lasers are at the heart of some of the fastest methods of information transfer yet devised. Using fiber optic bundles to carry them, modulated laser beams can transfer huge amounts of information. The internet is just one information technology taking advantage of laser fiber optics. In fact, the words you are reading now were most likely transferred most of the way to your computer via lasers in this manner. Lasers in compact disc players and video discs players read tiny reflections on CDs and laserdiscs to play back audio and video. Soon, your home may be fitted with fiber optics to carry cable TV and phone service.