G. H. C. New, G. S. McDonald, and J.P. Woerdman
Glance at the mode patterns of an unstable cavity laser and the immediate impression is of their beauty and complexity. Perform a detailed mathematical examination of their structure and one discovers that the patterns possess fractal character. We have christened the unstable cavity system as a kaleidoscope laser, after the device invented by Sir David Brewster in 1816. Just as the patterns in a toy kaleidoscope vary endlessly as the tube is shaken, so the modes of the kaleidoscope laser change with the shape of the defining aperture and the dimensions of the cavity (represented by the Fresnel number).
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