Unstable-resonator-mode derivation using virtual-source theory

An intuitive technique has been developed in which the lowest- and the higher-order modes for unstable resonators having rectangular or circular mirrors are derived. Instead of two mirrors, it considers the resonator to consist of a train of diffractive edges that are virtual images of the edge of the feedback mirror. Circular-mirror unstable resonators are shown not to exhibit the exact geometrical mode even at high Fresnel numbers.