Round-trip loss

In laser physics, the round-trip loss, or background loss $~\beta ~$ 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 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 with an anharmonic potential as proposed by M. Toda.

At the steady-state operation, the round-trip gain $~g~$ exactly compensate both, the output coupling and losses: $~\exp(g)~(1-\beta -\theta )=1~$. Assuming, that the gain is small ($~g~\ll 1~$), this relation can be written as follows:

$~g=\beta +\theta ~$

Such as relation is used in analytic estimates of the performance of lasers . In particular, the round-trip loss $~\beta ~$ may be one of important parameters which limit the output power of a disk laser; at the power scaling, the gain $~G~$ should be decreased (in order to avoid the exponential growth of the amplified spontaneous emission), and the round-trip gain $~g~$ should remain larger than the background loss $~\beta ~$; this requires to increase of the thickness of the slab of the gain medium; at certain thickness, the overheating prevents the efficient operation .

For the analysis of processes in active medium, the sum $~\beta +\theta ~$ can be also called "loss" . 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.