Loss rates of laser cavities

The loss rate of a laser cavity can differ from the loss rate of the same cavity mode in the absence of gain. It is shown that this difference arises from longitudinal inhomogeneities in the gain, the loss or the energy flow density. A similar ambiguity arises in the definition of the partial loss rate through the out-coupling mirror. This affects the calculation of the equivalent Schawlow-Townes linewidth of an unstable laser cavity from experimentally accessible quantities, and thereby the determination of the excess noise factor of the set-up. (C) 1999 Elsevier Science B.V. All rights reserved.