Fractals are mathematical series that exhibit replicating patterns at every scale. If the repeated patterns are identical at every scale, the fractal is termed self-similar. Fractals have found their way into applications such as communication and cosmology. Theoretical simulations showed that the eigenmodes of unstable laser resonators possess a fractal character, in contrast with the well-known stable-cavity eigenmodes. Unstable laser resonators have a special plane, called self-conjugate, in which the eigenmodes not only have the same pattern, but are also magnified copies of themselves. Here, we show a novel optical resonator that is capable of generating eigenmodes with self-similar fractal features. Our novel resonator is considered as an analogue to both the monitor-insidemonitor effect and monitor-outside-monitor effects. The fractal feature is proved by finding a typical image of the eigenstate at different scales. More quantitatively, we measured the pattern dimension which had a non-integer value, as is characteristic of self-similar fractals.
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