Working of lasers

 LASERS - HOW DO THEY WORK?

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To make a laser, all you need to do is give a big collection of atoms enough energy so they're excited and ready to emit light. Once one of them spontaneously emits a photon, it'll stimulate some of the others to do so and you get a nice cascade of illumination. But instead of letting all the light escape, it's more powerful to trap it between two mirrors and let it to bounce back and forth through the atoms. All that passing light will stimulate them to emit even more light, and as long as you keep on re-exciting the atoms, they're happy to go on emitting light forever.

A Laser

Lasers are monochromatic. This that the laser is a very orderly form of light. Unlike the light bulb which emits light of a variety of wavelengths and in all directions, light from a laser has only one wavelength, that is , one colour and moves in only one direction.

It is the electrons in the atoms that make the difference. There are different energy levels within the atom that the electrons can be in. The energy levels are like floors in a tall building. The electron can choose to be in whichever floor, depending on the amount of energy it has. The most stable system is one with the lowest energy. This means that the electrons in any material are all in their lowest energy states.

Electrons in different energy levels

Occasionally, an electron might get excited to reach a higher energy state. The electron does not stay in the excited state for long. It readily releases energy to return back to its stable, low energy state. The electrons release the energy in any random direction and at any time after it gets excited. At any particular time, some electrons get excited, while others lose energy, so the system on average, remains in the lowest energy stable stable state. This is just like a bunch of office workers taking the lift up and down a building - nothing special !

What's interesting is when most electrons are already mainly in the excited state. Just imagine what happens when all the office workers start taking the lift down when it's time to go home - something interesting is going to happen. By bumping energy into the system, we can achieve what is known as Population Inversion, that is, there are more excited electrons than those in lowest energy state.

Electrons releasing energy

When the electrons start releasing energy, something weird happens. As one electron releases energy, it seemingly communicates with another excited electron to release its energy too. This is known as Stimulated Emission. In other words, when we have a population inversion, there is chain reaction that takes place. When one electron returns back to its lowest energy state and releases energy, it incites other electrons to do the same. Then we get plenty of energy released at the same time. The only problem now is that the energies are released in random directions.

By strategically placing mirrors within a laser, you'll be able to make sure the energies emitted are in the same direction. And this energy packets are the photons. Photons always want to be like other photons - to have the same phase, polarization and go in the same direction. What's more amazing is that if a solitary photon passes by an excited atom that could emit another photon, there's a good chance that it will emit one. Because the two photons want to be together, even before the second one exists. So as the photons bounce back and forth between the mirrors, they seem to keep communicating with the excited atoms within the material, causing more stimulated emission. These photons actually correspond to light of a particular wavelength.

By achieving these properties:

1. Population Inversion

2. Stimulated Emission

3. Strategic planting of mirrors

Working of a laser

we are able to obtain monochromatic, directional and coherent form of light.

So once you have all these friendly photons bouncing around between the mirrors, you can just open up a little hole at the end and let out a blinding stream of coherent light, a laser beam.