Physics:Round-trip loss

In laser physics, the round-trip loss, or background loss [math]\displaystyle{ ~\beta~ }[/math] 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 Toda .

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

[math]\displaystyle{ ~g=\beta+\theta~ }[/math]

Such as relation is used in analytic estimates of the performance of lasers . In particular, the round-trip loss [math]\displaystyle{ ~\beta~ }[/math] may be one of important parameters which limit the output power of a disk laser; at the power scaling, the gain [math]\displaystyle{ ~G~ }[/math] should be decreased (in order to avoid the exponential growth of the amplified spontaneous emission), and the round-trip gain [math]\displaystyle{ ~g~ }[/math] should remain larger than the background loss [math]\displaystyle{ ~\beta~ }[/math]; 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 [math]\displaystyle{ ~\beta+\theta~ }[/math] 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.