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Active Laser Medium
Source of optical gain in a laser
The active laser medium (also called gain medium or lasing medium) is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state previously populated by a pump source.
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 sub-level system, the fast transitions ensure that thermal equilibrium is reached quickly, leading to the Maxwell-Boltzmann statistics of excitations among sub-levels in each system (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.
The state of gain medium can be characterized with a single parameter, such as population of the upper level, gain or absorption.
Efficiency of the gain medium
The efficiency of a gain medium can be defined as
Within the same model, the efficiency can be expressed as follows:
For the efficient operation both intensities, pump and signal should exceed their saturation intensities;
, and .
The estimates above are valid for a medium uniformly filled with pump and signal light. Spatial hole burning may slightly reduce the efficiency because some regions are pumped well, but the pump is not efficiently withdrawn by the signal in the nodes of the interference of counter-propagating waves.