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.
[clarification needed] 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
Then, assuming that the geometrical optics is applicable the round-trip gain
; the integration is performed along the whole ray, which is supposed to form the closed loop.
In simple models, the flat-top distribution of pump and gain
is length of the cavity; the laser light is supposed to go forward and back, this leads to the coefficient 2 in the estimate.
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).
determines, what part of the energy of the laser field becomes unusable at each round-trip; it can be absorbed or scattered.
At the self-pulsation, the gain is late 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.
exactly compensate both, the output coupling and losses: Assuming, that the gain is small (
should be decreased (in order to avoid the exponential growth of the amplified spontaneous emission), and the round-trip gain
[5] For the analysis of processes in active medium, the sum
[1] This notation leads to confusions as soon as one is interested, which part of the energy is absorbed and scattered, and which part of such a "loss" is actually wanted and useful output of the laser.