Round-trip loss

In laser physics, the round-trip loss or background loss $~\beta ~$ determines how much of the energy of the laser field becomes unusable during each round-trip. The loss can be due to some of the laser-field energy being either absorbed, scattered, or both.

The round-trip loss is an important parameter of a laser that affects the self-pulsation. The self-pulsation may take place while the gain takes some time to respond to the variation of the number of photons in the cavity, and the number of photons in the cavity, in its turn, takes some time to respond to 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 the main parameters of the equivalent oscillator with an anharmonic potential as proposed by M. Toda.

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

$~g=\beta +\theta ~$

Such a relation is used in analytic estimates of the performance of lasers . In particular, the round-trip loss $~\beta ~$ may be one of the important parameters that 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 increasing the thickness of the slab of the gain medium; at a certain thickness, overheating prevents efficient operation .

For the analysis of processes in an active medium, the sum $~\beta +\theta ~$ can also be called "loss" . This notation leads to confusion when one wishes to know how much of the energy is absorbed and scattered, and how much of such a "loss" is actually wanted and useful output of the laser.