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PRINCIPLES AND WORKING OF A LASER:In our earlier session we have learnt different characteristic properties of Lasers.Now let us learn about the principles of working of laser.Generally laser works in the form of process which consists of 4 important processes.They are: 1.Absorption2.Spontaneous emission3.Pumping and population inversion and4.Stimulated emission of electromagnetic radiation.Now let us discuss about the processes that takes place in atoms. ABSORPTION:It is well known that, atom consists of different energy states.Let us consider two energy states of an electron.Let us say ground state as Egand excited state as Ex.When electromagnetic energy falls on the atom, in the form of light photon of frequency ‘v’.Whereas electromagnetic energy is equal to the difference of Eg- Ex = hv.When it energy falls on the electron, it absorbs the energy and jumps from ground state Eg to excited state Ex.SPONTANEOUS EMISSION:As we already said, electrons jump from ground state to excited state.But the excited electrons that jumped to higher energy state, doesn’t remain for longer period in that state and come back to their original ground state by losing its energy in the form of photons.The photons don’t have any correlation in phase and thus considered as incoherent light.Thus the release of energy by the electrons on their own is said to be as spontaneous emission.Transition of electrons from one state to another state occurs random in time.By definition,Spontaneous emission is defined as the process in which the electrons in the excited atoms are released on their own from their higher energy state to the ground state.PUMPING AND POULATION INVERSION:As we already said, the transitioned electrons don’t stay for longer time in the excited state.There are electrons of some substances or systems, where they remain in the excited state for longer period.Such systems are called ACTIVE SYSTEMS or ACTIVE MEDIA.These are generally compounds or mixtures of different elements.When electrons turn into active systems, their electronic energy levels are also modified and thus it acquires some special properties.Now let us discuss them in detail.Now let us consider atom has three electronic states E1, E2, E3 in active medium.Generally electrons exist in lower state called as Ground state E1.Let us number of electrons in ground state as N1.As we already learnt., Due to the process of absorption when electromagnetic energy equal to the difference of E3 — E1 are incident on atom, electrons jump from state 1 to the excited state 3.Due to the process of spontaneous emission, the excited electron in the E3 state remains for less period, in order of 10-8sec.Usually most of the electrons loses its energy due to spontaneous emission and returns from E3 state to E1 state. The number of electrons in this state is said to be as N1.But some of the electrons may lose very less energy E3 -E2and jump to state E2.The less emitted energy by electrons is called as Thermal energy. This energy is absorbed by the medium itself.The transition or jumping of electrons from E3 state to E2 state is called as non-radiative transition (or) invisible transition. The electrons in this state E2 remains for longer period compare to state E1, in the order of 3 x 10-3milli seconds.This electronic state is called as Metastable state.