Imaging through a kaleidoscope

Kaleidoscopic effect illustrated here with two candles located at the output of an open box made of mirrors.

The first kaleidoscope was developed in the early 1800s by Sir David Brewster, who was seduced by the beauty of the generated patterns, which are symmetrical and, at the same time, very complex. In our study, we demonstrate that the kaleidoscopic effect, beyond its artistic function, can be exploited by scientists working with optical fibers.

The development of optical endoscopes is motivated by a number of biomedical applications such as brain imaging. Multimode fibers are excellent candidates to minimize the invasiveness of these procedures. However, the propagation of light in multimode fibers is very complex. Indeed, in this type of fiber, light propagates in an unpredictable way, producing patterns that are difficult to interpret at the fiber output. Techniques currently exist to reconstruct images from such patterns, but these techniques are very sensitive to all deformations applied to the fiber, which strongly limits the applicability of these techniques in biology.

In our study, we demonstrate that it is possible to produce images with multimode fibers even if they are deformed. To this end, we were inspired by Sir David Brewster's kaleidoscope, and we traded our usual fibers, which have a circular core, for square-core fibers. Indeed, the symmetry properties of these fibers generate a remarkable kaleidoscopic effect, which transports information in a way robust to deformations. We tested this method by reconstructing images of fluorescent sources through the fiber, even when this fiber was randomly deformed, thus reproducing the dynamic aspect of the perturbations that typically occur when studying living organisms. Despite these perturbations, we successfully managed to reconstruct images of the sources through the fiber. This new technique is thus promising for the development of miniature endoscopes for biomedical imaging.

This study has been published in PNAS (Bouchet et al., 2023).

Speckle-correlation imaging techniques are widely used for noninvasive imaging through complex scattering media. While light propagation through multimode fibers and scattering media share many analogies, reconstructing images through multimode fibers from speckle correlations remains an unsolved challenge. Here, we exploit a kaleidoscopic memory effect emerging in square-core multimode fibers and demonstrate fluorescence imaging with no prior knowledge on the fiber. Experimentally, our approach simply requires to translate random speckle patterns at the input of a square-core fiber and to measure the resulting fluorescence intensity with a bucket detector. The image of the fluorescent object is then reconstructed from the autocorrelation of the measured signal by solving an inverse problem. This strategy does not require the knowledge of the fragile deterministic relation between input and output fields, which makes it promising for the development of flexible minimally invasive endoscopes.