Eigenvalues for unstable resonators with slightly misaligned strip mirrors.

Very small misalignments in unstable strip resonators may cause detachment of the low-loss eigenmode at a lower equivalent Fresnel number N(eq) and introduce different periodicities into the eigenvalue curves as a function of N(eq). Using the resonance equation developed previously from waveguide mode theory, this behavior is explained in physical terms by coupling between mode fields with even and odd symmetry, which are uncoupled in the perfectly aligned configuration. A simplified explicit equation derived for the eigenvalues of the detached mode is found to predict correctly the periodicities of the eigenvalue oscillations. The waveguide method therefore continues to provide an explicit and reliable procedure for rapidly locating the maxima and minima of the detached-mode eigenvalues over broad ranges of N(eq).