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NOTE: the following explanation of how laser works is one that is typically repeated throughout science textbooks. However, I recently learned that this explanation is incompatible with modern theories about quantum physics and light propagation. I am working on a better explanation that is compatible with accepted optical theories. For the time being, I will leave you with this typical explanation.
LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.
You are probably already familiar with the Rutherford-Bohr model of the atom, where electrons are illustrated as orbiting the nucleus of an atom like the moon orbits the earth.
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| The Rutherford-Bohr model |
The Rutherford-Bohr model illustrates energy distribution among electrons. Electrons cannot be pinned down so easily as to say they fall into orbits. The "orbits" are actually averages of probabilities of where the electron energy is distributed (welcome to the world of quantum physics). When an electron absorbs energy, it is illustrated as rising to a higher orbit.
Electrons cannot exist between energy states. If an electron absorbs some energy, but not enough to achieve the next higher orbit, it will emit that energy immediately and drop back to its original energy state. This is important to understanding why glass is transparent. The energy states in glass are very far apart. If some energy in the form of light strikes glass, the electrons in the glass absorb this energy. However, under most conditions, the light doesn't have enough energy to push the electrons into the next higher energy state and the electrons immediately fall back to their original state and emit the new-found energy. This energy continues in the same direction it was traveling when it encountered the glass. When the energy exits the glass, it creates new light to continue on the original journey (scientists don't agree as to whether the energy can still be considered light while it is in the glass).
In some other materials, electrons can absorb enough energy to reach the next higher energy level. When they do, they will remain for some time. At some random time the electron will release this energy and fall back to the original state. When it does the energy will be released as electromagnetic radiation. This radiation will be of a specific quantity with a specific amount of energy and is illustrated as a particle called a photon. Having a specific amount of energy, it will thus have a specific wavelength (if it is visible light it will have a specific color). The wavelength is unique to the amount of energy difference between the two energy states that the electron passed between. A certain atom will be able to emit electromagnetic radiation of certain specific wavelengths unique to that atom. This is how astronomers can tell what chemicals are in stars and nebulae that are many light years distant.
This emission of radiation is called spontaneous emission. Its properties are: it is emitted at a random time and in a random direction. The wavelength is unique to the atom that emitted the radiation and unique to the two energy states the electron passed between when it lost energy to emit the radiation. The packet of radiation is illustrated, in quantum physics, a particle called a photon.
When an electron is in a high energy state, it is capable of dropping to a lower stat and producing a photon of a specific energy level. If a photon of that same energy level encounters the atom, it will cause the electron to release its energy immediately. The photon it produces will have exactly the same energy as the photon that caused the emission. It will also be traveling in exactly the same direction and will be lock-stepped in spin phase with the photon that caused the emission. This is called stimulated emission.
The following illustrations of stimulated emission and laser are from a video provided by toutestquantique.fr:
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| An atom with electrons in low energy state. |
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This atom has absorbed energy and now has one electron in a high energy state. |
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A photon (a bundle of light energy) with just the right amount of energy encounters the atom. |
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This causes the electron to drop to a low energy state and to emit exactly the same amount of energy as contained in the photon that encountered the atom. This emitted photon is identical to the original photon. This is called stimulated emission. |
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If many atoms are placed between mirrors and stimulated emission is initiated… |
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the effect will cascade until a large percentage of the light produce is stimulated emission trapped between the mirrors. |
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| If some of this light is allowed to escape, the escaping light constitutes a laser beam. |
In reality, light (both from spontaneous emission and stimulated emission) goes in all directions and only a small percentage is trapped between the mirrors. After a short time this trapped light cascades to become the majority of light. Also, the laser beam is created by making the coating on one mirror thin enough that some of the photons escape. These escaping photons constitute the laser beam. There is no hole in one of the mirrors.
Many lasers use gas as the lasing medium. These are essentially neon tubes with mirrors at each end. The gasses commonly used are a Helium-Neon mix, argon and CO2.
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| A helium-neon laser |
Solid state lasers are made of solid crystals. They are energized (pumped) with bright lights such as Xenon flash tubes (like camera flash tubes). The crystals are rod-shaped with the ends parallel and coated to form the laser mirrors. Certain impurities are introduced into the crystals that have the properties to produce the laser light.
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| A ruby solid state laser |
Light-Emitting Diodes can be shaped and coated to form lasers. These are used in CD, DVD and Blu-Ray equipment as well as red laser pointers.
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| A laser diode |
A typical green laser uses an infrared laser diode (808 nm) that is fired through a neodymium-doped yttrium orthovanadate (Nd:YVO4) crystal. This crystal is another lasing medium that produces laser light at 1064 nm. This is directed through a potassium titanyl phosphate (KTP) crystal that doubles the frequency resulting in a wavelength of 532 nm. An infrared filter blocks the 808 nm and 1064 nm radiation.
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| A green laser pointer |
Gas lasers are either on or off. There are no levels in between. Solid state lasers emit light in pulses. Laser diodes can be amplitude modulated; their brightness can be varied over a continuous range. This is useful for analog television over fiber optic cable as well as other analog signals.
Vertical cavity surface-emitting lasers (VCSELs) are semiconductor lasers whose emission direction is perpendicular to the surface of the wafer. VCSEL devices typically have a more circular output beam than conventional laser diodes. VICSELs are common in multimode fiber optic systems.
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