Round-trip gain

Round-trip gain refers to the laser physics, and laser cavities (or laser resonators). It is gain, integrated along a ray, which makes a round-trip in the cavity.

At the continuous-wave operation, the round-trip gain exactly compensates both the output coupling of the cavity and its background loss.

Generally, the Round-trip gain may depend on the frequency, on the position and tilt of the ray, and even on the polarization of light. Usually, we may assume that at some moment of time, at reasonable frequency of operation, the gain ${\displaystyle ~G(x,y,z)~}$ is function of the Cartesian coordinates ${\displaystyle ~x~}$, ${\displaystyle ~y~}$, and ${\displaystyle ~z~}$. Then, assuming that the geometrical optics is applicable the round-trip gain ${\displaystyle ~g~}$ can be expressed as follows:

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