the round-trip loss may be one of important parameters which limit the output power of a disk laser; at the power scaling, the gain should be decreased (in order to avoid the exponential growth of the amplified spontaneous emission), and the round-trip gain should remain larger than the background loss ; 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 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. References Laser science