Abstract

Unstable optical resonators that use a super-Gaussian reflectivity output coupler have been demonstrated to be successful in generating coherent radiation from high gain and large transverse-mode volume lasers. In the past, the design procedure for these resonators has been based on the geometrical-optics solution for the transverse eigenmodes, in which the fundamental mode is also a super-Gaussian of the same order as the mirror. In this work, the limits of validity of the geometrical-optics solution are quantified by numerically solving for the diffractive (Fox-Li) transverse eigenmodes as a function of the Gaussian-aperture-equivalent Fresnel number N_eq. It is shown that for low cavity magnifications (M ≤ 2), the diffractive eigenvalues deviate from the geometrical-optics value for N_eq ≤ 2. At these low equivalent Fresnel numbers, the transverse mode structure is determined by propagation effects (diffractive spreading), in addition to the mirror reflectivity profile. Furthermore, resonators with super-Gaussian mirrors of order n ≥ 6 exhibit transverse mode crossing points when N_eq ≤ 2. Near a mode crossing point, the fundamental mode resembles a higher-order ring mode with a large central depression. The implications for the design of these resonators will be discussed.

© 1992 Optical Society of America