Lecture 8: Laser amplifiers

Three-level systems
An effective three-level system satisfies the following
conditions:
• Population relaxation from level |3> to level |2> is very
fast and efficient, ideally 2 >> 32 ≈ 3
s. t. the atoms excited by the pump quickly end up in level
|2>
• Level |3> lies sufficiently high above level |2> with E 32
>> k B T s. t. the population in level |2> cannot be
thermally excited back to level |3>
• The lower laser level |1> is the ground state, or its
population relaxes very slowly if it is not the ground state.
Under these conditions
R 2 ≈ W p N 1 , R 1 ≈ -W p N 1 , and N 1 +N 2 ≈ N t
1 ≈∞and 21 ≈ 2
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