Monitor-outside-a-monitor effect and self-similar fractal structure in theeigenmodes of unstable optical resonators

A novel mechanism is proposed for the generation of self-similar structure over a limited range of length scales. Our mechanism, which we call the mon itor-outside-a-monitor effect, comprises repeated magnification and additio n of small-scale structure. We invoke this mechanism to explain recent obse rvations of fractal structure in the eigenmodes of unstable optical resonat ors [G. P. Karman et al., Nature (London) 402, 138 (1999)].