UNSTABLE-RESONATOR DIFFRACTION LOSSES AND THE EXCESS-NOISE FACTOR

We present a simple geometrical-optics model for the excess-noise fact or of a hard-edged unstable-resonator laser. When we nd hoc incorporat e the numerically calculated diffraction losses in this model we find similar behavior to results from the more complex mode-nonorthogonalit y theory. Under certain conditions the predictions of our model are in reasonable agreement with mode-nonorthogonality theory; phenomena suc h as the resonant behavior of the excess-noise factor with Fresnel num ber and the dependence of the excess-noise factor on the transverse re sonator symmetry are recovered. However, in the general case there is no quantitative agreement between the two approaches. We confirm this with an experiment on an unstable-cavity He-Xe laser. Severe deviation s occur for values of the round-trip magnification close to unity or m uch larger than unity and for higher-order modes. This illustrates the profound consequences of mode-nonorthogonality theory, which are appa rently much more subtle than can be described by a model directly base d on losses.