It is four level LASER in which the population inversion is achieved by electrical discharge.
A mixture of about 7 parts of the He & 1 part of Ne is contained in glass tube at a pressure of about 1 mm Hg. At both ends the tubes are fitted with optically plane and parallel mirrors one of them be partially silvered and other is completely silvered
An electrical discharge is produced in gas mixture by electrodes connected to a high frequency electric source..
|Fig. Experimental setup of|
The electrons from the discharge collide and pump to a meta-stable states 20.61 eV & 20.66 eV respectively above their ground state some of the excited He atoms transfer their energy to the ground state Ne atom by collision with the 0.05 eV of additional energy been provided by the K.E of the atom.Thus He atom helps in acheiving a population inversion in Ne atom.
When on excited Ne atom passes spontoniosly from the metastable state at 20.66 eV to state at 18.70 V it emits a 6328 A photon this photon travels through the gas mixture and it is moving parallel to the axis of the tube it is reflected back & forth and it stimulated excited Ne atom & cause it to emit a fresh photon in phase with the stimulating photon
This stimulated transition from 20.66 eV level to 18.70 eV level is the LASER transition This process is continued and a beam of coherent radiation beats up in the tune.
When the beam becomes sufficently intense a portion of it escapes through the partially silvered ends.
From the 18.70 eV level the Ne atom passes down spontoneously to a lower metastable state emitting incoherent lights & finally to the ground state through collision with the tube walls. This transition is Radiationless transition.
|Fig. Energy diagram of |
He-Ne laser is better then Rubi laser because in the Laser pumping process is quite easier.The excited He-atom gives their energy to the Ne atom in ground state Hence excited.