LASER- How and Why they work?

Let’s understand how LASER actually work.

LASER stands for : Light Amplification by Stimulated Emission of Radiation

I’m not going to throw all the big boring theory of LASER at you. Its boring to learn that way .

We will think together and understand it in baby steps (along with building virtual laser in our mind) .

Let’s get started!

First we need to know what makes laser light different from any torch light.

  1. Monochromatic : Torch light have photons of many different wavelengths altogether. Laser light is monochromatic i.e it contains photons of same wavelengths (i.e frequency range is really low) and thus appears to be of one single colour.
  2. Coherent : Light emitted by torch is not in phase and don’t have constant path difference.
  3. Directionality : Photons from a torch light travel in all different directions while photons of Laser light travel in one particular direction.
  4. High Intensity : Laser light is highly intense and diverge extremely less unlike torch light.

So now we need to do 2 things:


1) Understand how we can produce any light (photons) at first place.

2) And then will check how we can alter our experimental setup to produce photons with desired properties. (and by desired properties, I mean the properties of laser light)

Let’s just check we are on the same page.

Spontaneous Absorption and Emission

The Basic Physics is :

When an atom is supplied with energy , it’s electron jumps from lower to higher energy level. This is called Absorption.

After the election jumps to higher energy level, sooner or later, it falls back down to the ground state releasing photon (packet of light) of energy equal to the energy difference of two energy levels. This is called Spontaneous Emission. Lifetime of electron in excited state is 10^-8 .

Thus , light is produced. The material to which energy is supplied is called GAIN MEDIUM. Okay, so our 1st step is complete. Let’s head on to 2nd one. We now know how to produce light.

But the photons produced (by spontaneous emission) are not in phase, not of same frequency and also not travelling in one direction.

Our task is to check how we can alter our experimental set-up to produce photons which are in phase, of same frequency, and travelling in one direction. In other words, we want photons which have same properties.

Here comes the Albert Einstein to our help!

In 1916, when Einstein was investigating spontaneous emission and laying the statistical ground rules for Quantum theory, he realized that there was another possibility – called Stimulated Emission.

Stimulated Emission

This is the key phenomenon behind LASER. (Read it Carefully)

Consider an electron in excited state E2. If a photon of specific energy  (E2-E1) strikes/interacts (without itself being changed) with this excited electron, the electron is triggered to fall back to lower energy state E1 releasing another photon with phase, frequency, and direction of travel identical/similar to the striking photon. 

Thus, two photons- Impinging one and emitted one- have same frequency, same direction of travel and are in phase. 

Yay! We got two photons with similar properties i.e with the characteristic of LASER light. But how do we make so many photons like this? For that we need so many electrons in excited state i.e the favourable conditions of stimulated emission to occur again and again.

To have more electrons in excited state is called Population Inversion.

It is achieved by a process called pumping.

Out of many, most common is optical pumping . External Source like Xenon flash light provides energy to the electrons which then jumps to excited state.

Now here’s the catch. For the gain medium of laser, such materials are used whose atoms have a third energy level called Metastable state.

Electrons in the metastable state of energy have longer life-time, so electrons remain in excited state for longer time; and thus more favorable for stimulated emission to occur. 

Alright! We have now achieved population inversion. 

So stimulated emission will occur at a good rate and produce identical photons.

But this is not the end of story. We want laser light to be intense and highly coherent.

How to do that? Use mirrors. 

Two mirrors are placed on the either side of the cavity.

One mirror is fully reflective while other is partially reflective which allows some photons to pass through which we get as output- the Laser light.

I’ll explain how mirrors are useful. Due to mirrors, light travels back and forth in the optical cavity. This is useful in two ways:

  1. It traps the photon in the gain medium as long as possible for stimulated emission to occur.
  2. The light returned to the gain medium acts a feedback to maintain the properties like phase, wavelength, polarization and direction. 

Thus it makes the laser light as intense and monochromatic as possible.

Parallel mirrors force photons to bounce between them and cause chain reaction. those photons not travelling perpendicular to mirrors will miss one mirror and not be amplified. 

Wola! Our LASER device is ready.

Just one last question to make everything clearer. Why does Laser light stay narrow and doesn’t spread out like every other light?

Answer lies in the Coherence. Laser light is highly monochromatic and thus have photons of same wavelength (i.e frequency range is really low).

While normal light have photons of all different wavelength and thus scatter vividly (as bending and scattering of light depends upon the wavelength).

Congrats! you now understand the basics of Laser Physics.

Lasers are everywhere!

Ever since the discovery of Laser physics, it has become ubiquitous, finding implications in almost every sector of contemporary society. It would take at least a big book to mention all its applications. 

It has seen a very rapid growth both in Industrial and research field. Many different kinds of laser have been developed which have  different intensities and different output power. The pulsed laser has significantly brought revolution in the field of Technology.

Lasers range in size from microscopic diode lasers to the size of 3 football fields used of Inertial Confinement Fusion (ICF).

3 of the Biggest Experiments using LASER.

The upcoming blogs are about the 3 of the biggest experiments using LASER

  1. National Ignition facility (NIF) : It aims to bring Star Power to earth. It uses laser to fuse hydrogen nuclei together producing enormous clean energy by nuclear fusion, the same way the stars do.Know more about it here.
  2.  Laser Cooling : It is a technique to cool down atoms and molecules to the temperature near to absolute zero. Such conditions are required for experiments related to Quantum Physics.Know more about it here.
  3. Laser Interferometer Space Antenna (LISA) : It comprises of 3 spacecrafts millions of miles away from each other in space using Laser for detecting Gravitational Waves.Know more about it here.

Lasers are the future.

Stay Curious!

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