Laser Physics: Methods and Sources for Achieving Population Inversion, Cheat Sheet of Physics

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LASER PHYSICS

“ Light amplification by stimulated emission of radiation "

Methods of Achieving Population Inversion  Under normal conditions, more electrons are in a lower energy state than in a higher energy state.  Population inversion is a process of achieving more electrons in the higher energy state than the lower energy state.  In order to achieve population inversion, we need to supply energy to the laser medium.  The process of supplying energy to the laser medium is called pumping.

 The source that supplies energy to the laser medium is called pump source.  The type of pump source used is depends on the laser medium.  Different pump sources are used for different laser mediums to achieve population inversion. Some of the most commonly used pump sources are as follows:  Optical pumping  Electric discharge or excitation by electrons  Inelastic atom-atom collisions  Thermal pumping  Chemical reactions

 When light source provides enough energy to the lower energy state electrons in the laser medium, they jump into the higher energy state E3.  The electrons in the higher energy state do not stay for long period.  After a very short period, they fall back to the next lower energy state or meta stable state E2 by releasing radiation less energy.  The meta stable state E2 has greater lifetime than the lower energy state or ground state E1.  Hence, more electrons are accumulated in the energy state E than the lower energy state E1.  Thus, population inversion is achieved.  Optical pumping is used in solid-state lasers such as ruby lasers.

 Electric Discharge or Excitation by Electrons Electric discharge acts as the pump source A high voltage electric discharge is passed through the laser medium or gas. The intense electric field accelerates the electrons to high speeds and they collide with neutral atoms in the gas. As a result, the electrons in the lower energy state gains sufficient energy from external electrons and jumps into the higher energy state.