Unstable-resonator diffraction losses and the excess-noise factor

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Abstract
We present a simple geometrical-optics model for the excess-noise factor of a hard-edged unstable-resonator laser. When we ad hoc incorporate the numerically calculated diffraction losses in this model we find similar behavior to results from the more complex mode-nonorthogonality theory. Under certain conditions the predictions of our model are in reasonable agreement with mode-nonorthogonality theory; phenomena such as the resonant behavior of the excess-noise factor with Fresnel number and the dependence of the excess-noise factor on the transverse resonator 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 deviations occur for values of the round-trip magnification close to unity or much larger than unity and for higher-order modes. This illustrates the profound consequences of mode-nonorthogonality theory, which are apparently much more subtle than can be described by a model directly based on losses.