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This document about LASER, Spontaneous emission, Stimulated emission, Stimulated Absorption or Absorption, Main components of the LASER, The Ruby LASER.
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Light Amplification by Stimulated Emission of Radiation According to Einstien, there are three ways in which an atom can interact with electromagnetic radiation.
Atoms in the energy state E 2 can make a (spontaneous) transition to the energy state E 1 with the emission of frequency ω= (E 2
1 )/ћ
where ћ= h/2π ~ 1.0546 x 10
Stimulated absorption is the process in which the electromagnetic radiation of an appropriate frequency (corresponding to the energy difference of the two atomic levels) can pump the atom to its excited state. The radiated light waves are exactly in phase with the incident ones. So the result is an enhanced beam of coherent light. The rate of stimulated emission depends both on the intensity of the external field and also on the number of atoms in the excited state.
The rate stimulated absorption depends both on the intensity of the external field and also on the number of atoms in the lower energy state.
If we are able to create a state of population inversion (i.e. optical pumping ), in which, there are large number of atoms in the upper state then the number of stimulated emissions would exceed the number of absorption resulting the optical amplification of the beam. The amplification process due to stimulated transitions is phase coherent.
There are three main components of any LASER are (i). The active medium (ii). The pumping source (iii). The optical resonator
The ruby rod is a crystal of aluminum oxide (Al 2
3 ) doped with 0.05% chromium oxide (Cr 2
3 ), so that some of the aluminum atoms in the crystal lattice are replaced by Cr 3+ ions. The energy level diagram of these chromium ions is shown in figure.. The chief characteristic of the energy levels of a chromium ion is the fact that the bands labeled E 1 and E 2 have a life time of ~ 10
sec.- the lifetime represents the average time an atom spends in an excited state before making a transition to a lower energy state. A state characterized by such a long lifetime is termed a metastable state.
The Cr 3+ ion in its ground state can absorb a photon (whose λ ~ 6600 ang. ) and make a transition of the state in the band E 1 : it could also absorb a photon of λ ~ 4000 ang. and make a transition to one of the states in the band E 2
The He – Ne LASER was first fabricated by Ali Javan and his co-workers at Bell Lab. The He – Ne LASER consists of a mixture of He and Ne in a ratio of about 10 :1, placed inside a long narrow discharge tube at a high pressure (~ 1 Torr).
The fist few energy levels of He and Ne atoms are shown in Fig
Similarly He atom in the excited state F 2
The population in levels E 4 and E 6 to be much more than those in the lower energy levels E 3 and E 5
. The transition from E 6 to E 5
4 to E 3 , and E 6 to E 3 result in the emission of radiation having wavelengths 3.39 m, 1. m and 6328 ang., respectively. Only 6328 ang transition corresponds to the well known red light of He – Ne Laser.
Gas LASERS are, in general, found to emit more directional and more monochromatic light. This is because of the absence of such effects as crystalline imperfection, thermal distortion and scattering, which are present in solid state LASERS. Gas Lasers are capable of operating continuously without need for cooling
When the forward bias is applied to the LASER diode, a current flows. Initially at low current , there is spontaneous emission in all directions. As the bias is increased, eventually a threshold current is reached at which the stimulated emission occurs and a monochromatic and highly directional beam of light is emitted from the junction.
The advantage of semiconductor lasers are that they are compact, efficient and can be fabricated with ease. However, The semiconductor lasers have inferior monochromaticity, coherence and directionality than the other lasers.
