The active laser medium or gain 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.
Examples of active laser media include:
In order to lase, the active gain medium must be in a nonthermal energy distribution known as a population inversion. The preparation of this state requires an external energy source and is known as laser pumping. Pumping may be achieved with electrical currents (e.g. semiconductors, or gases via high-voltage discharges) or with light, generated by discharge lamps or by other lasers (semiconductor lasers). More exotic gain media can be pumped by chemical reactions, nuclear fission, or with high-energy electron beams..
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, neither gain nor refractive index depend on 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 relative concentrations can be defined as and .
The rate of transitions of an active center from ground state to the excited state can be expressed with and The rate of transitions back to the ground state can be expressed with , where and are effective cross-sections of absorption at the frequencies of the pump and the signal.
and are the same for stimulated emission;
is rate of the spontaneous decay of the upper level.
Then, the kinetic equation for relative populations can be written as follows: ,
However, these equations keep .
The absorption at the pump frequency and the gain at the signal frequency can be written as follows: , .
The steady-state solution can be written:
The dynamic saturation intensities can be defined: , .
The absorption at strong signal: .
The gain at strong pump: , where is determinant of cross-section.
Gain never exceeds value , and absorption never exceeds value .
At given intensities , of pump and signal, the gain and absorption can be expressed as follows: , ,
where , , , .
The state of gain medium can be characterized with a single parameter, such as population of the upper level, gain or absorption.
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. The 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.