  In the past, a number of heat engine models have been devised to apply the
principles of thermodynamics to a laser. The best one known is the model using
a negative temperature to describe population inversion. In this paper, we
present a new temperature scale not based on reservoir temperatures. This is
realized by revealing a formal mathematical similarity between the expressions
for the optimum power generated by an endoreversible heat engine and the
optimum output-coupled power from a three-level laser resonator. As a
consequence, the theory of endoreversibility can be applied to enable a new
efficiency analysis of the cooling of high-power lasers. We extend the
endoreversibility concept also to the four-level laser model.
