What is a LASER? What are the types and properties of LASER?

A LASER is a device which produces monochromatic i.e (single wavelength), unidirectional powerful light beam. It depends on the phenomenon of "stimulated emission" which was first predicted by Einstein in 1916. 

Einstein considered the equilibrium between matter and electromagnetic radiation in a black body chamber at a constant temperature in which the exchange of energy takes place due to absorption and spontaneous emission of radiation by the atoms.

Einstein observed that the usual absorption and emission processes alone are not sufficient to explain the equilibrium. He then predicted that there must be a third process also now known as stimulated emission. Maiman in 1960 built a first LASER.

LASER plays a key role in the field of Medical Science, factories and various engineering fields. LASER is used to create three dimensional photograph known as holograms.

What is LASER?

The term LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.
It is a device which is used to produce intense, monochromatic, coherent and unidirectional beam.

Principle of LASER

Principle of Laser is based on quantum theory. In this, there are three possible ways in which atom can interact with a photon. The principle of Laser is based on these three phenomenon.

(a) Absorption

(b) Spontaneous Emission

(c) Stimulated Emission

(a) Absorption : An atom has a number of possible quantised energy states characterised by integral numbers. If it is initially in a lower state 1, it can rise to a higher state 2 by absorbing a quantum of radiation (Photon) of energy given by;

hv = E₂ - E₁

Where, E₁ and E₂ are the energies of the atom in the states 1 and 2 respectively. This is absorption of radiation.

When an atom residing in the lower energy state absorbs a photon, it is raised to higher excited state, provided that the energy of the photon equals the difference of the energy levels. This process is known as absorption.

(b) Spontaneous Emission : The atom lying in the upper energy level 2 (E₂) tries to come to the lower level 1 (E₁) by itself after remaining there for 10^-8 seconds, to have stability. The energy is emitted in the form of a photon, which has energy given by;

hv = E₂ - E₁

This process is known as Spontaneous Emission. The photons emitted are not in the same phase, and the process is random in nature.

(c) Stimulated Emission : When the atom lies in the upper energy level, the incident photon may induce transition from E₂ to E₁ and the atom comes to the lower level. In this process, a photon is emitted. This is known as stimulated emission of light. Both the incident photon and the emitted photon have the same energy and in the same phase. This process is controllable.

In the stimulated emission process, an incident photon forces the excited atom to emit another photon of the same frequency in the same direction and the same phase. The two photons go on together as coherent radiation.

The stimulated emission process may be represented by equation.

A* + hv = A + 2hv

Types of LASERS 

Following are the important types of lasers, depending upon lasing medium :-

(a) Solid State Lasers

(b) Gas Lasers

(c) Semiconductor Lasers

(a) Solid State Lasers : The laser in which the lasing medium uses is Solid in nature is known as Solid State Lasers.

For example - Ruby Laser, YAG Laser.

(b) Gas Lasers : The laser in which the lasing medium uses is Gaseous in nature is known as Gas Lasers. 

For example - He-Ne Laser, N₂-CO₂ Laser.

(c) Semiconductor Lasers : The lasing material used in these Lasers is Semiconductor. A semiconductor diode forms the active medium or laser medium.

For example - Gallium Arsenide Laser.

Properties of a LASER Beam

The laser beam has certain characteristics properties which are not present in beams derived from other light sources :

(i) The Laser beam is completely spatially coherent, with the waves all exactly in phase with one another. An interference pattern can be obtained not merely by placing two slits in a Laser Beam but also by using beams from seperate lasers.

(ii) The Laser light is monochromatic.

(iii) The Laser rays are almost perfectly parallel. Hence a Laser Beam is very narrow and can travel to long distances without spreading. It can be brought to an extremely sharp focus.

(iv) The Laser Beam is extremely intense. It can vaporize even the hardest metal. Because of its high energy density and directional property a Laser Beam can produce temperatures of the order of 10°C at a focussed point.