Excess noise in low Fresnel number unstable resonators

Numerical calculations of excess noise factors in low Fresnel number unstable resonators are described in detail. Computed mode profiles in one transverse dimension together with associated Petermann K-factors are presented; dynamical considerations such as injected wave excitation are also examined. The properties of the zero-order modes are consistent with virtual source theory and with a simple formula for K based on a geometrical optics approximation. While virtual source theory is asymptotic, we find that it can make good predictions for Fresnel numbers as low as unity. Full numerical calculations are however needed to determine accurate mode profiles and K-factors in some regimes. A new technique for calculating accurate higher-order mode profiles is also developed and this is employed to evaluate K-factor dependencies of the first two higher-order even modes. q 1999 Elsevier Science B.V. All rights reserved. PACS: 42.50.Lc; 42.60.Da; 42.60.Jf