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A laser is a device that emits a beam of coherent light through an optical amplification process. LASER is an acronym and the letters in the word "LASER" stand for Light Amplification by Stimulated Emission of Radiation.

How lasers work
Lasers operate on the principle of stimulated emission and the necessity of achieving population inversion in atoms or molecules.

Principals involved in Lasers

 * 1) Stimulated Emission:
 * 2) * When an electron in an atom is excited, it can release a photon when struck by another photon. This process is known as Stimulated Emission.
 * 3) * Not to be confused with Spontaneous Emission, where an excited electron naturally releases energy as a photon.
 * 4) Population Inversion:
 * 5) * Population inversion by definition means that there are more atoms in excited state than in the ground state.
 * 6) * But ordinarily this is not possible. Hence in reality, it refers to having more atoms in a higher excitation state (  ( for eg. three level system including  the ground state) state 2) than in a lower excitation state (state 1)

Acquiring of Photons

 * To form a laser, a large number of photons are required to align in a straight line in a coherent, monochromatic and collimated beam.
 * To Achieve this, we use population inversion, and wait for an atom to spontaneously emit a photon.
 * And when that photon hits another atom in the same  excited state, it causes stimulated emission, resulting in the emission of another photon of  same energy. This is called the amplification of the number of photons.
 * This process repeats until  a large number of photons are produced. In order to produce a high amplification we need to prevent the escaping of photons  and bring them back again into the system.

Prevention of escaping of Photons

 * To form a laser beam, we need to prevent the escaping of photons from the system and multiply the number repeatedly.
 * For that, we put a 100% reflecting mirror on one side of the system, and another mirror of 80% reflectance on the other side of the system  to reflect the photons by 180 degrees at each of these mirrors and bring them back into the system repeatedly.

Formation of a Laser Beam

 * The photons are reflected back and forth repeatedly and when a large amplification takes place through stimulation emission, a part of the beam comes out through the 80% reflecting side.

Excitation of Atoms

 * There are various ways of exciting  atoms, for example- by the application of  electricity, RF,  light etc.
 * For example, if we take the system as a gas filled tube  we can put the electrodes on either side of the tube and connect  an electric cable to pass the current. Or an RF voltage can be applied by means of two terminals coupled to the tube. Or by the application of light for optical pumping.

Types of lasers
There are 5 main types of lasers, which are-

1.Gas Lasers

2.Solid-State Lasers

3.Fiber Lasers

4.Liquid Lasers (Dye Lasers)

5.Semiconductor Lasers (Laser Diodes)

Gas Laser
Gas lasers are used in a wide variety of applications, including holography, spectroscopy, [https://www.cognex.com/what-is/industrial-barcode-reading/laser-scanners#:~:text=Laser%20scanners%20use%20a%20laser,reflected%20light%20from%20the%20barcode. barcode scanning], air pollution measurements, [https://www.ulsinc.com/learn/laser-material-processing#:~:text=Laser%20Material%20Processing%20uses%20laser,the%20quality%20of%20finished%20products. material processing], and laser surgery.

Solid-State Lasers
Solid-state lasers are alused for LIDAR technology as well as various medical applications, including tattoo and hair removal, tissue ablation, and kidney stone removal.

Fiber Lasers
Examples of fiber lasers used for these applications include ytterbium and erbium-doped fiber lasers.

Liquid Lasers (Dye Lasers)
Some of the laser dyes are rhodamine (orange, 540–680 nm), fluorescein (green, 530–560 nm), coumarin (blue 490–620 nm), stilbene (violet 410–480 nm), umbelliferon (blue, 450–470 nm), tetracene, malachite green, and others.

Semiconductor Lasers (Laser Diodes)
One example of semiconductor laser is diode laser, which is currently the most efficient device for converting electrical energy into optical energy.