IGNOU PHE 9 2018 - Compare and contrast three level and four level pumping schemes.

Question

Compare and contrast three level and four level pumping schemes.

27 Jan 2018

Answer :

Word Count : 376

Three-Level Systems

In a three-level system, the laser transition ends on the ground state. The unpumped gain medium exhibits strong absorption on the laser transition. A population inversion and consequently net laser gain result only when more than half of the ions (or atoms) are pumped into the upper laser level; the threshold pump power is thus fairly high.

The population inversion can be achieved only by pumping into a higher-lying level, followed by a rapid radiative or non-radiative transfer into the upper laser level, because in this way one avoids stimulated emission caused by the pump wave. (For transitions between only

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