Excess noise in strip off-axis confocal unstable resonators

The lowest-loss mode and higher-order modes in strip off-axis confocal unstable resonators are calculated by the eigenvector method. The moduli of all pairs of the adjacent order eigenvalues exhibit crossing behavior. For the lowest- and next-lowest-loss modes, crossings occur when the equivalent Fresnel number N eq is close to s + 0.08 (s positive integer), and the lowest-loss mode profile calculated by the Fox–Li iteration is no longer self-reproducing, but the superposition of both eigenmodes with unfixed complex weights varies as the number of iterations. Numerical calculations of excess noise factors K in strip off-axis confocal unstable resonators are described in detail. The value of K shows an irregular resonance switching against N eq. The hybrid model presented by van Eijkelenborg et al. is modified in this paper, and shows a good quantitative agreement with nonorthogonality theory.