Principle of direct pumping



Energy diagram of laser upper and lower levels in Nd:YAG for 1064nm oscillation.

Optical radiation emitted from bound electrons can be described by the transition between 2 electrinic levels. However, at least one more electronic level is necessary for making population inversion in order to obtain the stimulated emission. This scheme called "3-level lasers," by which Dr. Maiman invented laser oscillator in 1960. 3-level lasers involves following 3 levels: a ground state (4I9/2 in above figure), an excited state (4F5/2 in above figure), a laser upper state (4F3/2 in above figure).

However, the existence of one more additional level surprisingly improve laser performance: this is called as "4-level lasers." 4-level lasers involves 4 levels: a ground state (4I9/2 in above figure), an excited state (4F5/2 in above figure), a laser upper state (4F3/2 in above figure), and a laser lower state (4I11/2 in above figure). By distinguisha laser lower level from ground state, effective energy storing become possible.

Even though the direct pumping into the emitting level requires only 3 levels (a ground state of 4I9/2, a laser upper state of 4F3/2, and a ground state of 4I9/2, this scheme can reduce the stokes shift with no reduction in other laser performances compared to 4-level lasers.


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