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 two levels, simultaneous pump absorption and signal amplification can not occur.)

An example of a three-level laser medium is ruby (Cr³+:Al₂O₃), as

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