Loss rates of laser cavities

The loss rate of a laser cavity can differ from the loss rate of the same c avity 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 r ate through the out-coupling mirror. This affects the calculation of the eq uivalent Schawlow-Townes linewidth of an unstable laser cavity from experim entally accessible quantities, and thereby the determination of the excess noise factor of the set-up. (C) 1999 Elsevier Science B.V. All rights reser ved.