Active laser medium Totally Explained

Everything about Laser Gain Medium totally explained

The active laser medium or gain medium is the material that exhibits optical gain within a laser. The gain is generated by stimulated emission on electronic or molecular transitions to a lower energy state from a higher energy state previously stimulated by a pump source.
Examples of active laser media include:

Certain crystals, typically doped with rare-earth ions (for example neodymium, ytterbium, or erbium) or transition metal ions (titanium or chromium); most often yttrium aluminium garnet (YAG), yttrium orthovanadate (YVO4), or sapphire (Al₂O₃);
Glasses, for example silicate or phosphate glasses, doped with laser-active ions;
Gases, for example mixtures of helium and neon, nitrogen, argon, carbon monoxide, carbon dioxide, or metal vapors;
Semiconductors, for example gallium arsenide (GaAs), indium gallium arsenide (InGaAs), or gallium nitride (GaN);

Pumping gain media (for example, supply of energy) can be achieved with electrical currents (for example semiconductors, or gases via high-voltage discharges) or with light, generated with discharge lamps or with other lasers (semiconductor lasers). More exotic gain media can be pumped by chemical reactions, nuclear fission, or with high-energy electron beams..

Example of a model of gain medium

A universal model valid for all laser types doesn't exist. The simplest model includes two systems of sub-levels: upper and lower. Within each level, the fast transitions lead to the Boltzman 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

~omega_gg 1~

.
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hole burning may slightly reduce the efficiency because some regions are pumped well, but the pump isn't efficiently withdrawn by the signal in the nodes of the interference of counter-propagating waves.

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