Laser | Types of Emission | Principle of LASER

What is Laser ?

A laser is a device by which one can produce a unidirectional, monochromatic, intense, and coherent beam of radiation. LASER is an acronym stand for Light Amplification by Stimulated Emission of Radiation. The emitted light from a laser has some special characteristics. Some of them are:-

• DIRECTIONALITY: It means that when light emitted from the laser, then its divergence is very less due to diffraction. The actual divergence of light is 10^-5 radian. By using this characteristic of lights of laser it has a wide application like surveying, lidar, etc.

• HIGH POWER: As we know that the emitted light waves are continuous,  due to this continuity it has a power lever 10⁵w. due to this characteristic, the laser has wide applications like cutting, welding, etc.
• SPECTRAL PURITY:  Its means that the emitted laser beam has an extremely small spectral width. Due to this characteristic, the laser has many application like optical communication, holography, etc.

Because of this special characteristic of laser light, its have very wide application in several areas, one can say that due to this discovery of laser, the optic physic has become a most important field of study. The working of laser is based on the phenomenon of stimulated emission, which was discovered by Albert Einstein in 1917. We will discuss stimulated and spontaneous emission, the main component of laser, application, in detail later on. There are basically two types of laser which are RUBY LASER and He-Ne LASER.

PRINCIPLE OF LASER 

The working of the laser is described on the basis of the interaction of radiation with matter. There are basically three principle on the basis of which working of laser is describe. These are:-

• Absorption or Stimulated Absorption or Induced Absorption.
• Spontaneous Emission.
• Stimulated Emission.

Let us consider in detail one by one.

1)     Absorption or Stimulated Absorption

Let us consider a material medium which have identical atoms and atoms are described by two energy level  which are E₁ and E₂. Where E₁ is energy of lower or ground state and E₂ is energy of upper or excited state. Further, we assume that N₁ and N₂ be the number of atoms in the lower state (E₁) and the upper state (E₂). Let α(v ) is the energy density of incident radiation. Now, when incident radiation is absorbed by an atom of the ground state. Then it raised to the upper state. This process is called Absorption or Stimulated Absorption. 

Or in other words absorption or stimulated absorption is a process in which electromagnetic radiation of appropriate frequency is absorbed by atom of the ground state and due to absorption of energy, it raised upper state or excited state.

  Fig1: Absorption

The effect of absorption is that the intensity of incident radiation is decreased. Thus, the resultant intensity obtained from medium after the absorption phenomena is decreased. When the atom of the lower state raised to upper state by the absorption process, then this state of atom are highly unstable, the atom will stay for very small time, equal to the lifetime of the excited state. This is order of 10^-8 sec.

2)    Spontaneous Emission

When an atom of the upper or excited state of medium return back to lower state or ground state without any external radiation. Then such a process is called Spontaneous Emission. Since this process can occur even in the absence of any radiation.

Fig2: Spontaneous Emission

Where, E₁ and E₂ are the energy of excited or upper state and energy of ground or lower state.N₁ and N₂ are the number of atoms in-ground or lower state (E₁) and the number of atoms in excited or upper state (E₂).

Where, E₁ and E₂ are the energy of excited or upper state and ground or lower state of the atoms.N₁ and N₂ are numbers of atoms in-ground or lower state and number of atom in the excited or upper state.

The photon or radiation emitted during the spontaneous emission is non-correlated with each other and highly random in nature in direction of propagation, phase, polarization but have identical energy. Therefore, the emitted radiation is intense but not correlated. In spontaneous emission the intensity of emitted radiation (photon) is increased.

3)    Stimulated Emission

In stimulated emission, a radiation or photon of energy equal to energy difference of two energy level involved (energy=E₂-E₁) interact with the atom of the excited state and force the atoms of excited state to return back or came back to the lower state by emitting two photons. This process is called Stimulated Emission.

Fig3: Stimulated Emission

This emitted radiation (photon) during the stimulated emission process are highly correlated in term of direction,phase, polarization, and energy. Such photon that are identical in nature involving direction, phase, polarization, and energy are known as identical photon or coherent or correlated radiation or photons.

Thus, the intensity produced by stimulated emission is increased. the intensity produced in stimulated emission is highly increased. the photon emitted during the stimulated emission are highly coherent due to same phase. The radiation produces by the stimulated emission is unidirectional, intense, monochromatic and highly coherent.    

Look Out for more such articles on light and laser at : Click Here !