Researchers Demonstrate Fractal Light from Lasers

A team from the University of the Witwatersrand (Wits) and the University of Glasgow has demonstrated that fractal light can be created from a laser, verifying a prediction made two decades ago. The researchers provide experimental evidence for fractal light from simple lasers and add a new prediction, namely that the fractal light should exist in 3D and not just 2D, as previously thought.

A cross section of a fractal pattern, created by a laser in the Wits Structured Light Laboratory. Courtesy of Witwatersrand University.

The researchers observed a variety of fractal shapes in transverse intensity cross sections through the lowest-loss eigenmodes of unstable canonical laser resonators, demonstrating the controlled generation of fractal light inside a laser cavity. They predict 3D self-similar fractal structure around the center of the magnified self-conjugate plane and showed quantitatively that intensity cross sections are most self-similar in the magnified self-conjugate plane.

In explaining the fractal light discovery, Wits professor Andrew Forbes emphasized the importance of knowing where to look for fractals in a laser. “Look at the wrong place inside the laser and you see just a smeared-out blob of light,” he said. “Look in the right place, where the imaging happens, and you see fractals.”

Several patterns of fractal light, created by a laser in the Wits Structured Light Laboratory. Courtesy of Witwatersrand University.

“What is amazing is that, as predicted, the only requirement to demonstrate the effect is a simple laser with two polished spherical mirrors,” said professor Johannes Courtial of the University of Glasglow. “It was there all the time, just hard to see if you were not looking at the right place.”

Fractals are complex objects with a “pattern within a pattern” so that the structure appears to repeat as you zoom in or out of it. In a similar way, the light inside lasers cycles back and forth, bouncing between the mirrors on each pass, in what looks like a recursive loop. Each time the light returns to the image plane created by the mirrors, it is a smaller (or larger) version of what it was — a pattern within a pattern within a pattern.

The laser instrument setup used to create fractal patterns of light. Courtesy of Witwatersrand University.

The work advances the understanding of a fundamental symmetry of nature as found in lasers. The research was published in Physical Review A (https://doi.org/10.1103/PhysRevA.99.013848).

Several patterns of fractal light, created by a laser in the Wits Structured Light Laboratory. Courtesy of Witwatersrand University.

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