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.

The round-trip loss is important parameter of a laser, that affects the self-pulsation. The self-pulsation may take place while the gain takes some time to respond the variation of number of photons in the cavity and the number of photons in the cavity, in its turn, takes some time to respond the variation of gain. Within the simple model, the round-trip loss and the output coupling determine the period of pulsations and their relaxation. These are main 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.