Example of A Model of Gain Medium
A universal model valid for all laser types does not exist. The simplest model includes two systems of sub-levels: upper and lower. Within each level, the fast transitions lead to the Boltzmann distribution of excitations among sub-levels (fig.1). The upper level is assumed to be metastable. Also, gain and refractive index are assumed independent of a particular way of excitation.
For good performance of the gain medium, the separation between sub-levels should be larger than working temperature; then, at pump frequency, the absorption dominates.
In the case of amplification of optical signals, the lasing frequency is called signal frequency. However, the same term is used even in the laser oscillators, when amplified radiation is used to transfer energy rather than information. The model below seems to work well for most optically-pumped solid-state lasers.
Read more about this topic: Active Laser Medium
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