Basics of laser action

Laser Action
The combination of spontaneous emission first, and then stimulated emission, causes the laser to "lase," which means it generates a coherent beam of light at a single frequency.

1. Basics of laser action Ms.A.Vivetha , MSc, MPhil, BEd , Asst Prof of Physics. Bon Secours College for Women

2. Laser The word 'Laser' is an acronym for Light Amplification by Stimulated Emission of Radiation.   

3. Characteristics of laser  The laser beam ( i ) is monochromatic. (ii) is coherent, with the waves, all exactly in phase with one another, (iii) does not diverge at all and (iv) is extremely intense

4. Conditions to achieve laser action i )  There must be an inverted population i.e. more atoms in the excited state than in the ground state. (ii) The excited state must be a metastable state.

5. Absorption Atoms in the lower energy state absorb energy from the incident photon and moves to the higher energy state. Probability of absorption depends on: Number of atoms present in the lower energy state. Intensity of incident light

6. Population inversion   Atoms in the excited state jumping to the lower state under the influence of another photon emit a photon of the same frequency as the incident photon. Probability of Stimulated emission depends on: Number of atoms available in the excited state Intensity of the incident light The direction of propagation of energy, phase and state of polarization of energy of the emitted photon is exactly the same as those of the stimulating photon. Thus photons are coherent.

7. Spontaneous emission Atoms in higher energy state jumps to the lower energy state with emission of a photon at random (i.e. without influence from a photon) Probability of Spontaneous emission depends on: Number of atoms available in the excited state.

8. Stimulated emission Atoms in the excited state jumping to the lower state under the influence of another photon emit a photon of the same frequency as the incident photon. Probability of Stimulated emission depends on: Number of atoms available in the excited state Intensity of the incident light The direction of propagation of energy, phase and state of polarization of energy of the emitted photon is exactly the same as those of the stimulating photon. Thus photons are coherent.

9. Pumping The phenomenon of achieving population inversion, i.e the process which raises the atoms from lower energy state to higher energy state in the active medium. Methods: Optical Pumping: a light source is used to supply luminous energy and create population inversion by optical photon. Electrical Pumping : electrical discharge converts the gas medium into plasma which liberates electrons which, in turn, are accelerated by the strong electric fields present in the tube. These electrons, on collision with neutral gas atoms, makes some atoms jump to excited state. Chemical Pumping : An exothermic chemical reaction is used to produce energy.

10. Number of atoms available in the excited state Intensity of the incident light The direction of propagation of energy, phase and state of polarization of energy of the emitted photon is exactly the same as those of the stimulating photon. Thus photons are coherent. Probability of Stimulated emission depends on: