CHARACTERISTICS OF LASER

LASERs play a very vital role in many fields and industries.

What makes it so special?

It's characteristics.

Lasers have:

1.High directionality

2.High monochromaticity

3.A high degree of coherence

4.High brightness

So let's understand each one in detail.

1. High directionality:

The lasers are made of active medium placed in between two reflecting resonator mirrors one which can partially reflect and the other which can fully reflect the light.
Any photon travelling in a direction away from the cavity axis reflects away by the mirrors within a few reflections and is thus not allowed to propagate further, thus the beam drawn from the output mirror is highly parallel and directional. The degree of directionality is expressed as the divergence. 
Minimum spot point: The curvature of the mirror confines the light within the cavity and causes the beam to narrow down to a radius wo  and this is called a minimum spot point.

The beam of divergence phi is equal to 1.22*wavelength / 2*wo

The divergence tells us how rapidly the beam spreads when its emitted.

For a laser, the beam divergence is 1milliradian.

The angle of divergence = (a2-a1) / 2*(d2-d1)

where d1 and d2 distances from the laser window and a1 and a2 are the diameters of the spot.

HIGH MONOCHROMATICITY:

The light emitted from a laser is monochromatic, that is, it is of one wavelength (colour).

The emission or absorption of the photons whose frequency lies between f and f+df which is denoted as spectral broadening.

The three important mechanisms which rise to the spectral broadening are :

1. Doppler broadening: The atoms emitting and absorbing photons are not at rest. They move with some velocity in a probable region within the atom. Thus the frequency of emitted radiation changes slightly and this is called doppler broadening.

2.Collision broadening: During the emission or absorption if the atoms get collided then resulting frequency gets effected and this is known as collision broadening.

3.Natural broadening: In solid materials emitting photons leads to damping of the amplitude of the wave train and this is known as natural broadening.

Spectral width or line width is a quantity which is used to find the degree of monochromaticity of light.        

                                change in wavelength =                                                        - (c* change in frequency) / ((frequency)^2)

      For lasers change in wavelength is approx. 0.001nm and can be made much narrower.

Theoritical Understanding Behind the LASERS

LASER stands for Light Amplification through Stimulated Emission. The theoretical basis was given by Albert Einstein and the first laser was developed by T.H. Mainmann.

LASERs work on 3 main processes.

They are Stimulated Absorption, Spontaneous Emission and Stimulated Emission.

Stimulated Absorption:

When a photon with energy hf, is incident on an electron of an atom present in the ground state, the electron excites to higher energy levels by absorbing the photon. This process is directly proportional to the number of electrons in the ground state. 

Spontaneous Emission:

The excited electron stays for 100 nanoseconds in the excited state and then emits a photon i.e, releases energy which is equal to the difference between energies of the higher energy level and the minimum energy level, to get back to ground state. This process is directly proportional to the number of electrons in the excited state.

Stimulated Emission:

When the electron moves to a metastable state from the excited state by releasing no radiation but some ignorable amount of energy it stays for more than 100 nanoseconds. So then when we incident a photon towards the electron in the metastable state it emits a photon which has a similar phase with that of the electron incident. This creates coherent light waves with high directionality.

This process is directly proportional to the number of electrons in the excited state or in the metastable state.

The principle behind the working of the principle is population inversion.

 

POPULATION INVERSION: 

 

In general, an atom has more number of electrons in the ground state than in the higher energy levels. But after excitation of more number of electrons into the higher energy levels they settle down in the metastable for a longer time and that is known to be population inversion. In simple words, there will be more number of electrons in higher energy level than in lower energy levels.

As the Stimulated Emission is directly proportional to the number of electrons in the metastable state this process enhances and accelerates the process.

This population inversion can be achieved by pumping mechanisms or techniques. They are :

 1.Optical pumping

 2.Electrical discharge pumping 

 3.Chemical pumping

 4.Injection current pumping

Einstein's coefficient (EC):

Let the number of electrons in the ground state is N1 and the excited state is N2.

Let energy density p(f)= nhf where n is the number of photons per unit volume at frequency f such that E2-E1=hf.

Absorption = B12*N1*p(f)

where B12 is EC of absorption.

Spontaneous Emission = A21*N2

where A21 is EC of spontaneous emission.

Stimulated emission = B21*N2*p(f)

where B21 is EC of stimulated emission.

Absorption is an upward transition and the other two are the downward transition processes of the electron.

In equilibrium both the transitions become equal i.e,

 B12*N1*p(f) = A21*N2 +  B21*N2*p(f)

 B12*N1*p(f) -  B21*N2*p(f) = A21*N2

 p(f) [ B12*N1 - B21*N2] = A21*N2

 p(f)*B21 [ N1 - N2] = A21*N2

 p(f) = (A21/B21)*(N2/N1-N2)

 p(f) = (A21/B21)*(1/(N1/N2)-1)

 

 From Maxwells Boltzmann distribution law :

N1/N2 = e^(E2-E1)/KT

p(f) = (A21/B21)*(1/(e^(E2-E1)/KT-1))

p(f) = (A21/B21)*(1/(e^(h(f))/KT-1))   ---------eqn1

By comparing  eqn1 with Plank's radiation formula we get :

The above relations are referred to as Einstein's relations.

This is the theoretical explanation behind the lasers.

For more interesting and easy explanations of physics comment in the comment section.

Energy is liberated matter, matter is energy waiting to happen

WHAT DOES UNCERTAINTY ACTUALLY MEAN?

       

 HEISENBERG'S UNCERTAINTY PRINCIPLE  

It becomes a bit perplexing and puzzling to know that if you are reduced into a quantum sized body then your position and momentum are not certain. If you are not aware of anything like this before then it surely puts your mind into the blank area where this statement raises numerous questions and some don't even agree with this.

But this statement is the foundation of the quantum theories which have remarkably and marvellously changed the entire world's thinking towards science.

It looks nonsensical to say that you are present everywhere in a probable region and your position is uncertain till someone measures it at a certain time. But it is true no matter what we think.

The statement says that we cannot know both the position and momentum of a subatomic particle simultaneously.

This theory says that the change in momentum times the change in position is always greater than or equal to planks constant times 1/4(pi).

This principle also holds true for conjugate pairs of physical quantities such as Energy and time and also angular momentum and angle.

This principle stands as the base for the whole quantum physics thus this fundamental theory is very important to understand the more complicated theories.

For more interesting topics explanations suggest me in the comments section.

“Look up at the stars and not down at your feet. Try to make sense of what you see, and wonder about what makes the universe exist. Be curious.”