What Is Laser?

'Laser' stands for Light Amplification by Stimulated Emission of Radiations. Three level laser works in three energy levels. Consider an atomic system with three energy states E, E' and E''. E is a ground state, E' is a metastable state and E'' is an excited state. Firstly, the atoms are pumped from the ground state to E''. The electrons will jump down to E' causing spontaneous emission. After sometime, the number of electrons in E'' becomes greater than the number of electrons in E. This is called population inversion. Now, a photon of energy difference of two states is incident on this system. When the photon hits this system, it stimulates an electron to move from metastable state to the ground state. As a result of de-excitation, another photon is released. These two photons will cause stimulated emission producing four photons and these photons produce 16 photons and so on. In this way, the number of photons increases. These photons are confined between mirrors which reflect then back and forth. On of the mirror is 5 to 10% transparent. From this transparent portion, parallel photons of same energy, phase and and direction will come out giving laser.