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.
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At the continuous-wave operation, the round-trip gain exactly compensates both the output coupling of the cavity and its background loss.
Round-trip gain in geometric optics
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
where
In simple models, the flat-top distribution of pump and gain
In the steady-state continuous wave operation of a laser, the round-trip gain is determined by the reflectivity of the mirrors (in the case of stable cavity) and the magnification coefficient in the case of unstable resonator (unstable cavity).
Coupling parameter
The coupling parameter
Round-trip loss (background loss)
The background loss, of the round-trip loss
At the self-pulsation, the gain lates to respond the variation of number of photons in the cavity. Within the simple model, the round-trip loss and the output coupling determine the damping parameters of the equivalent oscillator Toda.
At the steady-state operation, the round-trip gain
Assuming, that the gain is small (
Such as relation is used in analytic estimates of the performance of lasers. In particular, the round-trip loss
For the analysis of processes in active medium, the sum