Gain (laser)

In laser physics, gain or amplification is a process where the medium transfers part of its energy to the emitted electromagnetic radiation, resulting in an increase in optical power. This is the basic principle of all lasers. Quantitatively, gain is a measure of the ability of a laser medium to increase optical power.

The gain can be defined as the derivative of logarithm of power ${\displaystyle ~P~}$ as it passes through the medium:

where ${\displaystyle ~z~}$ is the coordinate in the direction of propagation. This equation neglects the effects of the transversal profile of beam.

In the quasi-monochromatic paraxial approximation, the gain can be taken into account with the following equation

where ${\displaystyle ~\nu ~}$ is variation of index of refraction (Which is supposed to be small),

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