LASERS AND THEIR WORKING MECHANISMS
It is coherent with the stimulating incident radiation. It has the same frequency & phase as the incident radiation.The rate of stimulated emission depends both on the intensity of external field and also on the number of atoms in the excited state.Population Inversion: The number of particles N2, i.e., population of higher energy level is less than the population N1 of lower energy level. Making the number of particles N2 more in higher energy level than the number of particles N1 in lower energy level (N2 > N1) is called as population inversion or inverted population.A system in which population inversion is achieved is called an active system. The method of raising the particle from lower energy state to higher energy state is called as pumping. A more common method of pumping is optical pumping.
Characteristics of a laser:
i) The light is coherent with all the waves exactly in phase with each other.
ii) Laser beam hardly diverges. i.e., The laser rays are almost parallel.
iii) The beam is nearly monochromatic.
iv) The laser beam is extremely intense. The beam can produce a temperature of 104oC at a focused point.
Principle of operation of lasers: Consider a group of atoms all in the same excited state. A passing photon may cause stimulated emission in one of these atoms. This results in the emission of two photons. Each of these photons may cause induced emission in two other excited atoms. This process may continue in a chain reaction. The result will be an intense beam of photons moving in the same direction and all are coherent.