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What is a LASER

Written by Harpreet

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LASER stands for “light amplification by stimulated emission of radiation”. It is a device that produces a highly intense, unidirectional and coherent beam of light in either the visible or the infrared region with a very narrow spectral width. To understand the action of a LASER, one must first know what a ‘stimulated emission’ is.


When electrons in a matter absorb energy they jump from their ground state to a higher excited state. This energy can be supplied to them in the form of heat or light. After a small period of time, these electrons return back to the ground state and emits out energy in the form of heat or light. This is spontaneous emission in case the energy emitted is in the form of light. This light will be incoherent and scattered out in many directions. Your every day light bulbs are an example of this phenomena.

Now when an electron travels to the excited state, it remains their for a fixed period of time before returning back to the ground state. What if this electron still in an excited state, encounters a photon incident upon it . This electron will then jump back to the ground state by emitting a photon which is in the same direction as the original incident photon and which is in coherence with it. This is stimulated emission and the action of a LASER is based on this very principle.

But for LASER action, we require stimulated emissions on a large scale. This is achieved by first of all, supplying energy to the LASER material from an outside source for excitation of a large number of electrons so that the number

of electrons in the excited stage exceeds the number of electrons in the ground state. This process is known as ‘Population Inversion’.

Initially then, a few spontaneous emissions will take place and photons will be emitted in all directions. But the photons emitted in the direction prependicular to the plane of the material will be made to oscillate back and forth in the LASER by the use of mirrors. These photons will then bring about a large number of stimulated emissions in the LASER material and a highly intense, coherent, directional beam of light with very narrow spectral width will escape out from one of the two mirrors covered in a semi reflective coating.

LASERS are available today as Solid State Lasers (Ruby Laser) , Gas Lasers ( He-Ne Laser, Carbon Dioxide Laser), Semi Conductor Lasers etc. each having their own characteristic output. These find applications in fields of communication, medicine, welding, military etc.

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